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-   -   KB FB10 "Toaster" (https://www.vintage-radio.net/forum/showthread.php?t=70801)

westcliff 18th Jun 2011 12:40 am

KB FB10 "Toaster"
 
I've acquired this little radio but have some problems to sort out. The bottom of the front of the radio has snapped off, being attached only where it is glued to the grille cloth. A semi-circular piece has also snapped off the base and the tuning knob had seized.

Internal examination revealed the celluloid scale backing plate was missing, the drive cord was just string and one of the dial attachment pillars had snapped off and was nowhere to be found.

I know that the valve line-up on these radios was subject to occasional change, so for the record, my one has the following:

V1 - 6BE6
V2 - 6BA6
V3 - EBC90 in a shielding can
V4 - KT63
V5 - 5Y3GT

The intention is to repair the case using epoxy with fibreglass backing, and car body filler to fill in the spaces. Then it will have to be re-painted.

Is the correct cream colour easily available? If not, it's going to be a custom colour which Mrs Westcliff will choose, as the radio will be hers.

I managed to get the tuning mechanism working with the application of sewing machine oil, but that drive cord will have to go. The wavechange switch has been cleaned and the loudspeaker removed.

Underneath, it's mostly quite clean, although the components at the output valve end are all blackened. Someone has added a Hunts 25uF 25v electrolytic capacitor between V4 and chassis, with the chassis end detached. The wire on V5 pin 8 is (to the OT), is also detached. The mains lead is completely missing. It's not been cut off, it's just not there at all.

Tomorrow I'll make a start replacing the waxies and checking the resistors and I will be using Trader sheet number 969.

If you want to chip in with information, advice or answers, please feel free.

Gus.

DangerMan 18th Jun 2011 1:23 am

Re: KB FB10 "Toaster"
 
Hi Gus,
there were many different valves used, but as far as I know, no FB10's had rectifiers with 5v heaters, and in fact the rectifier has a common heater supply with the other valves, which is connected to chassis and is 6.3v from an autotransformer. A 5Y3 won't work in this situation, as the HT appearing at the filaments will be shorted to chassis and the heater voltage is wrong any case.

A 6X5GT (or EZ35 equivalent) is the correct valve as it has cathodes insulated from the 6.3v heaters.
Even a 6X5G might foul the fibre shroud inside the handle moulding or the dial diffuser backing, and the output valve should be a 6V6GT, a lot smaller than your average KT63, for the same reason. Perhaps that's why the diffuser is missing.
Good news is, EBC90 is the same as a 6AT6.

Unless the mains transformer has been replaced and the set modified, a 5Y3 would be a disaster if switched on. Maybe it's just been put there to make up the numbers.

Bad news, I know, but I hope this helps

Pete

westcliff 18th Jun 2011 12:00 pm

Re: KB FB10 "Toaster"
 
Hi Pete, many thanks for that critical advice. It's not too bad as I have some NOS 6V6GTs I've been hoarding for my Fender guitar amps. I might even have a 6X5 somewhere, although it's probably a G.

I think this radio was more of an ornament than anything else. The complete absence of a mains lead is probably to ensure the set can't be connected to the mains by mistake.

Gus.

westcliff 20th Jun 2011 5:58 pm

Re: KB FB10 "Toaster"
 
All of the electrics are just about done, apart from the mains lead and a tubular waxy capacitor between S2 on the wavechange switch, and the chassis. It is marked "001 uF" which I imagine means 0.001uF. It isn't on the circuit diagram on the trader sheet. C2, C5 and C6 are shown in that area and they are all present.

I know from reading other threads on the forum that these radios sometimes deviated from the published circuit, so I don't know whether to replace it or just get rid of it.

There was a 25uF electrolytic I mentioned earlier which I removed and didn't replace and I wonder if I should do the same with this one. Any advice please?

Gus.

Sideband 20th Jun 2011 9:45 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 442290)
I know from reading other threads on the forum that these radios sometimes deviated from the published circuit

Understatement!!;D I've never found two the same! Yours has a 6V6, both of mine have a 6BW6 (which is basically the B9A equivalent). One of mine has 3x 8.2K resistors in the HT rail, the other has a large 15K. Another variation is a tapped output transformer for hum-bucking. One of mine has it, the other doesn't. Oh yes and one has a ferrite rod and the other doesn't.....

There's loads more variations as well.....

Don't know about the .001. According to the Trader sheet 969, S2 connects to the LW trimmer which is only 70pF. A .001 here will just about kill any signal. The only .001 listed is the aerial coupling capacitor.

None of the versions have a 25uF capacitor on the output valve cathode so you can do away with that.


SB

westcliff 21st Jun 2011 12:31 am

Re: KB FB10 "Toaster"
 
The 0.001 goes straight to the chassis from S2. Perhaps the switch was noisy in the past and somebody thought that a 0.001 cap would be just the job to fix it. The aerial coupling capacitor is in the right place. That 25uF electrolytic was in parallel with R12.

Tonight I glued a semi-circular lug back onto the base and started to prepare the case for repair and painting. One thing I have to do is clean the grille cloth. What is the safest way to clean it? I'd prefer not to unglue it from the baffle if possible.

Gus.

westcliff 30th Jun 2011 12:27 am

Re: KB FB10 "Toaster"
 
2 Attachment(s)
I spent a couple of hours on the bodywork tonight. It took some time to remove the spring clips securing the dial glass and the back panel. One in fact pinged away and will take some time to find. Two of the studs are missing as are four of the clips. I may have to come up with an alternative way of securing the dial glass.

I gave the case a good clean with foam and a child's toothbrush, so now I can glue on the broken section. The plan is to epoxy the broken parts on, there being one large piece and four tiny fragments. When I've done that, the back of the repair as well as a nasty long crack will be fibreglassed. I have the glass fibre so tomorrow I'll buy some resin and hardener.

Later on I was reading some old FB10 threads and I found this one: https://www.vintage-radio.net/forum/...highlight=fb10

I was thrilled to read in post 17 that Andy (AF024), had the same 0.001uF cap as I do, from S2 on the wavechange switch to chassis. His FB10 ended up working well, so I have decided to replace the old wax capacitor with a new one like Andy did.

Here, hopefully, are a couple of pictures of the radio, as I received it.

Gus.

Attachment 53206

Attachment 53207

westcliff 18th Jul 2011 1:21 pm

Re: KB FB10 "Toaster"
 
I've not been idle since I last posted on this thread, I just had plenty of other things to do.

Having finished my work on the chassis, I glued the case together with my Poundland epoxy. Of the smaller fragments, I used just one, to fill in the gap by the volume control. Over the weekend I fibreglassed behind the cracks and used P38 car body filler on the front of the set. Last night and this morning I sanded it down and now it's ready for a coat of primer, to see if the cracks will be invisible.

I'm in the middle of making a spray booth out of an old box which previously contained a computer monitor. When it stops raining, the spray booth will be installed on the seat of my outside toilet and I'll start spraying.

My friend gave me another FB10 for parts, to aid my restoration. The good news is that I have more than enough dial glass clips. The bad news is that the "scrapper" FB10 is complete and in good condition. It even has a nice tidy ferrite aerial and a 6BW6 output valve. The main problem with it seems to be dust and cobwebs, and a broken dial cord. I've not been renovating radios for long, but it looks like an easy restoration for someone. I never thought that this hobby would lead to a crisis of conscience. :)

Gus.

Herald1360 18th Jul 2011 7:47 pm

Re: KB FB10 "Toaster"
 
Clearly you now need another, really scruffy, FB10 so you can finish your case rebuild one and restore the scrapper and salve your conscience.

westcliff 18th Jul 2011 9:40 pm

Re: KB FB10 "Toaster"
 
3 Attachment(s)
That's what I was thinking Chris. There's no way I can destroy the donor radio. I have decided to "borrow" from it the parts I need and replace them when I can. Sounds like embezzlement, doesn't it? 8-o

Here are some pics of the repaired damage. I was generous with the fibreglass hardener because there is a long crack I want to stay closed.

Gus.

Herald1360 18th Jul 2011 11:43 pm

Re: KB FB10 "Toaster"
 
I think the official term is "cannibalizing". We've all done it. :-]

westcliff 3rd Sep 2011 12:02 am

Re: KB FB10 "Toaster"
 
The body has been finished for a few weeks now. I'm happy with the finish, my first radio respray, although it is still possible to see the cracks from many angles.

I needed a grille cloth as the old one was badly faded. I'm very lucky to have Sid Chaplin only a couple of miles from my house, so I called him and he sorted me out with a lovely cloth, just what I was looking for.

I fitted the cloth by following advice from this forum. The cloth was stretched on a wooden frame and nailed in place. Glue was applied and the baffle board was stuck to the cloth and left weighted down for the night. The next day, I pulled out the nails and trimmed off the excess cloth. I was very pleased with the result as the cloth is perfectly straight. It looks good in the radio.

Tonight I dug out the lamp limiter and started to test the set. At first the lamp glowed dully and the dial lamp did not light up; nor did the 6X5GT I'd recently bought. A dial lamp and rectifier were cannibalised from my donor FB10.

The next time, the lamp limiter glowed more brightly and the dial lamp illuminated. The replacement rectifier also lit up as it should. After a while, LW sound came faintly from the speaker. There were a few channels picked up, then more on MW. The lamp continued to glow with no change in its brightness and there was a buzzing noise accompanying the stations I was listening to. Because of the buzzing and the lamp limiter staying bright, I switched off rather than put full mains through the set.

My inclination is to replace the electrolytic capacitor can with the one from the donor FB10, see if that makes a difference. Comments and suggestions will be most welcome.

Gus.

electrogram 3rd Sep 2011 12:25 am

Re: KB FB10 "Toaster"
 
The smoothing/resevoir cap in these sets is unreliable, unlike that in the Bush dac 90a. It has about 250v across it and is physically small and is just waiting to explode ! I would fit new caps . There is just room below the speaker magnet to fit a three-tag strip to which you can connect two separate new caps

westcliff 3rd Sep 2011 3:56 pm

Re: KB FB10 "Toaster"
 
A new 32uF + 32uF can has been ordered. Cheers Electrogram, your words have saved me the task of removing the can from the donor. I'd become used to these old electrolytics working just fine. I'll post again when the new cap can has been fitted.

Gus.

westcliff 21st Sep 2011 12:33 am

Re: KB FB10 "Toaster"
 
The new 32uF+32uF can was far too big to fit in the space behind the speaker, so I ordered some single 30uF 450v electrolytics. They arrived at the weekend and I fitted a pair last night.

Tonight I plugged the set into the 100w lamp limiter and ran it with the lamp in circuit. It picked up stations all over MW and a good few on LW but there was a faint buzz right across the dial on both wavebands. The lamp stayed a dull orange throughout.

After 40 minutes, I switched the lamp out, to put full mains through the set. There was an immediate flash and crackle and the set died (I saw all of the valve filaments darkening and the dial lamp went out). The lamp limiter is powered through an RCD; this did not trip off.

Of all the sets I've repaired, six in total, this the first one that has gone bang on me after working on it. Normally when this happens, I replace all of the tubular waxies and as many resistors as necessary, then the radio works. This time, I've already done all of that.

Should my fault-finding begin with the components I haven't replaced?

Gus.

Neil Purling 21st Sep 2011 7:42 am

Re: KB FB10 "Toaster"
 
If it is totally dead, no heaters I assume you changed C19 (across the mains) for a X2 type. I also assume the mains switch is not playing up.
Are the replacement electrolytic capacitors OK. Check them on the voltage range of your meter first. If they are fine then they may still hold a charge.
See if there is a short to chassis from either side of the resistor between both capacitors.
if one has gone 'bang' it may be obviously swollen or the end is blown outwards.
If the set was drawing excessive HT the flash may have been the rectifier protesting.
I hope it survived.
Remove all the valves and check the auto-transformer. I would hope there is continuity all along that.
With the limiter in risk a quick on-off, with a meter across the heater chain.
It should give a reading close to what you'd expect and something like 8 volts without the limiter when off-load.
If all is well check by putting any valve in apart from the rectifier and if should glow normally.
I wouldn't expect a LT fault. It really sounds as if the set was drawing excessive HT and the rectifier experienced a flash-over. Let us know what you find.

Sideband 21st Sep 2011 8:52 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 466622)
Should my fault-finding begin with the components I haven't replaced?

Gus.

Start with the easy bits. Just do some basic resistance measurements when disconnected from the mains. Firstly as the set died, with heaters and pilot light going off, this suggests something at the mains end so firstly, connect you meter, set to read ohms, across the mains lead between L-N and switch on. The meter should read somwhere between 50 and 100 ohms (depends on the actual transformer). Incidentally, if the set has a 13 amp plug fitted, make sure it hasn't taken out the fuse (you did fit a 3 amp didn't you)? If you get no reading, check the plug fuse and the on/off switch (single pole on these). If the fuse and switch are OK then check directly across the transformer. As this set uses an autotransformer, it's just one winding with a tap for the heaters. Somewhere around these parts you should find the problem.

There is very little else left that will make one of these go dead. The worst case is an O/C transformer but I would have thought that unlikely. Maybe the switch didn't like full mains.

SB

westcliff 21st Sep 2011 11:38 pm

Re: KB FB10 "Toaster"
 
Thank you Neil and Sideband for your comments, they are very helpful.

The 3-amp fuse is okay and the resistance on the mains lead between L and N is 162 ohms. I checked my other FB10 and got 171 ohms, so 162 would seem to be acceptable for this model.

While I was checking the mains transformer (142.5 ohms), I saw that a resistor had become detached from its wire. The resistor is R13 I think (the Trader sheet is not clear), and it should have a value of 113 ohms. This one, when removed, measured 9.5 ohms. It is the rectifier's surge limiter and is situated on top of the chassis, between the 6V6 output valve and the aerial.

Can this be the source of my problem, or another sympton? It will have to be replaced in any case. I don't know the power rating of the resistor, but I could use a 5-watt 120 ohm one, or 2 x 5-watt 250 ohm in parallel, for 125 ohms. Would either or both of these be suitable?

Thanks again.

Gus.

DangerMan 22nd Sep 2011 12:26 am

Re: KB FB10 "Toaster"
 
Hi Gus,
I think you've found a likely candidate for the trouble. A 5W resistor should be ok (DAC90 used a 1 watt carbon one) but keep it away from heat sensitive bits or wires.
With a low value surge limiter, the surge as the voltage suddenly stepped up when you switched out the limiter could cause a hefty current pulse.
I'd replace the resistor, then retry using the lamp limiter. Don't just switch it out though, I'd be inclined to switch off and then give the set a minute for the heaters to cool down before switching on again without the limiter. That way there'll be no big voltage step onto hot valves.

Pete

Neil Purling 22nd Sep 2011 6:40 am

Re: KB FB10 "Toaster"
 
A 113 ohm surge-limiter. Is that what the bands on the resistor suggest as well, or is the component burnt too badly to read? Try 120R rated 1 or 2 watts. 1W ought to be enough.
Kolster-Brandes are notorious for making un-documented changes of all kinds in this model, so expect discrepancies between the Trader sheet and what you have in real life.
The rectifier should have survived the abuse. If not, send me a PM.

westcliff 22nd Sep 2011 9:32 am

Re: KB FB10 "Toaster"
 
Neil, the resistor is a green blob with no writing or colour bands on it. Its size suggested a higher power rating, which is why I was considering the use of a 5-watt replacement. Using 1- or 2-watt replacements will be easier as space is at a premium on this chassis.

I should find out tonight if the rectifier has survived.

Gus.

Sideband 22nd Sep 2011 9:56 am

Re: KB FB10 "Toaster"
 
The value of that resistor changed almost with each different model! 120 ohms would be OK as a replacement. However if that failed, it wouldn't make the heaters or pilot lamp go off as well. It feeds the rectifier as a surge limiter and would only cause loss of HT.

The resistance of the transformer seems OK so it's a bit of a mystery why the heaters and pilot lamp went off. Pull the rectifier out and connect the set to the mains via your lamp limiter. See if the remaining valves and pilot light come on. If they do, it proves the transformer and 6V tap as OK. If they don't then it could be a connection from the heater tap or the tap within the transformer (seems unlikely). Also check on full mains (without the rectifier). If it runs on full mains without the rectifier, it could be that the rectifier has developed a cathode-heater short which would effectively short out the HT via the heater tap. I would expect the rectifier to be giving an impressive firework display however!

See how you get on.


SB

westcliff 9th Oct 2011 12:24 pm

Re: KB FB10 "Toaster"
 
I have done a bit of testing to try to identify the problem with the set and the last thing I tested was the lamp limiter. The 100w bulb was fine, but the 3A fuse had blown. What does this tell me about the radio I was testing?

Something else has been puzzling me - when I replaced the capacitor can with two single capacitors, that's all I did. Do these old capacitor cans additionally contain resistors? I read in another thread about a 680K resistor in a can and now I wonder of there was a resistor in my old can.

What I did was to join the -ve ends of the new capacitors together then attach them to the same wire as was connected to the -ve terminal of the old can. Similarly, the +ve ends of the new capacitors were attached in the same way that the two old +ve can terminals had been. The set then did work on the lamp limiter with the lamp on, until I switched the lamp out of the circuit.

Other tests I have done show that the set's plug, 3A fuse, power lead and on-off switch are all fine.

Gus.

Steve_P 9th Oct 2011 2:47 pm

Re: KB FB10 "Toaster"
 
What is the working voltage of the new smoothing caps? Can they possibly handle the voltage from the limiter and fail when the full voltage is applied?

The other point I would think about is that resistor. Try a higher value?

Cheers,

Steve P.

Retired 9th Oct 2011 5:41 pm

Re: KB FB10 "Toaster"
 
Hi,

Good luck with this set Gus. I'm watching the thread with interest because I bought one of these a couple of years ago to restore although mine has a decent case.

Kind regards, Col.

westcliff 10th Oct 2011 1:17 am

Re: KB FB10 "Toaster"
 
Steve, the new capacitors are rated at 450v. The surge limiting resistor being used just now is near enough the correct value. I haven't been able to test the set since I installed that resistor, due to the lamp limiter fuse having blown and me not realising.

Col, thanks for popping in. Good luck with your one when you get round to it.

Now the lamp limiter is fixed, my next step is to fire the set up without the rectifier. All going well, I will fit a replacement 6X5GT, kindly supplied by Neil Purling, and see what happens.

Gus.

Sideband 10th Oct 2011 8:23 am

Re: KB FB10 "Toaster"
 
There are no discharge resistors fitted in the smoothing cans so don't worry about that. The 450v rating is good so all being well, it was the rectifier that failed. Makes sense as this would likely have taken out the fuse in the lamp limiter and also the surge limiter resistor. hopefully everything should be OK now.




SB

westcliff 11th Oct 2011 12:13 am

Re: KB FB10 "Toaster"
 
Looks like you were right Sideband. This evening I tested the set on the lamp limiter, the rectifier being absent. The dial lamp lit up, as did the four remaining valves. I switched off, left it a while to cool, then fitted the rectifier. Using the lamp limiter, there were no bangs or flashes and stations were picked up.

Following advice from Pete (Dangerman), I left the radio to cool after the half-power lamp test. In fact I brought it through to the "office", where I plugged it into the mains via the RCD. It's playing Gold very nicely as I type, albeit with a burning smell which I trust will shortly dissipate. It must be all the goo I couldn't reach with the cotton buds and my little boy's old toothbrush.

An hour later...I switched it off because the burning smell persisted, then smoke started to come from the set. The transformer is painfully hot to touch and the chassis is fairly warm too.

Is the overheating anything to do with the output valve control grid resistor (R9 on the Trader sheet, 470k ohms)? I replaced that one and it looks like I made a bad solder joint where it is connected to the chassis. I'll fix that tomorrow.

The good news is that the radio was still playing nicely when I switched it off.

Gus.

Steve_P 11th Oct 2011 12:54 am

Re: KB FB10 "Toaster"
 
It's unlikely to be that, especially if you've changed C15. if the transformer does not get that hot without the rectifier valve in, then check R13, fit a new rectifier and then it is probably the transformer. Unlikely looking at the circuit, but with the number of mods on these dear little sets, anything is possible!!

Cheers,

Steve P.

DangerMan 11th Oct 2011 1:08 am

Re: KB FB10 "Toaster"
 
Hi Gus,

the FB10 transformer doesn't supply anything at all to the HT circuit, and only feeds the heaters. The rectifier anode come from the mains directly via the surger limiter resistor and so no fault here can overheat the transformer.

If you are worried about it, first leave it running with no valves fitted.
It should maybe get warm but no more than warm, no matter how long it runs. If it gets HOT, it's probably duff.
Sounds like it is, if smoke is present and is coming from the transformer :(

Then if all ok, repeat with valves in, but rectifier limiter resistor disconnected. The transfomer will now get hotter/warmer due to heater consumption, note.. you should do this with the case on because the internal heat from the valves will add to the temperature rise.

Then repeat with radio in working condition.... everything connected.
Now the extra internal heat from the HT consumption will make things even warmer/hotter.

This procedure should give you some idea of the "expected" transformer temperature rises or unexpected rises due to faults, for instance if it gets red hot with no valves in.

Hope this helps.

Do you have any way of measuring the actual temperature? If so, and if the above route fails to give an answer, I can run mine and measure its temperature to give you a comparison. This one is in use for hours every day, so I'm pretty certain it's ok!

Pete

DangerMan 11th Oct 2011 8:17 am

Re: KB FB10 "Toaster"
 
Hi Gus,

another quick thought.
If you can remove all the valves and the dial bulb, then switch on while measuring the AC current consumption, this will be the transformer's magnetising current, and will give a measure of the its health or otherwise.
I'll do the same (unless someone here has one in bits and can get there quicker) and we can compare readings.

Pete

Sideband 11th Oct 2011 1:43 pm

Re: KB FB10 "Toaster"
 
Usually if a transformer starts to overheat, the smell is a giveaway! As Pete says, the transformer only supplies heater current so carry out the checks he suggests. I think the transformers run warm but certainly not too hot to touch. I'll dig mine out soon and check it with a thermocouple.


SB

westcliff 11th Oct 2011 5:40 pm

Re: KB FB10 "Toaster"
 
Thank you Sideband and Pete for your comments. Do you measure the transformer current at the live and neutral sides of the on-off switch? It's another of these things that I've not done before.

Gus.

DangerMan 11th Oct 2011 8:24 pm

Re: KB FB10 "Toaster"
 
No Gus, not there!
That would be the place to measure the mains voltage, and will blow up a meter set to measure current.
Connect one of the mains leads (say live) direct to the supply but connect the other to one lead of your meter. Connect the other meter lead to mains neutral.
Set meter to AC current, switch on, and take the reading.
I have just got my meter out... I will be back with the mag current for my set.

Pete

I'm back... it is 38.5mA with all valves and the dial lamp removed.

westcliff 11th Oct 2011 10:30 pm

Re: KB FB10 "Toaster"
 
Just so I know what I'm doing - unsolder the mains lead neutral wire from the radio's on-off switch and insert the DMM between the switch and the mains neutral wire. My DMM has two sockets for testing current, 400mA and 10A. I intend to use the 400mA socket given Pete's 38.5mA.

I won't be doing any testing tonight but I'll have time tomorrow evening. If any of what I've written in this post looks wrong, please don't hesitate to let me know.

Gus.

Herald1360 11th Oct 2011 10:50 pm

Re: KB FB10 "Toaster"
 
Sounds OK, as long as those two ranges are for AC current! Also if you're suspicious of problems, use the 10A range. If that looks OK, switch off and repeat with the meter to 400mA to get a higher resolution reading.

If your DMM has only DC current ranges, then better to connect a 1ohm wirewound resistor where the meter would have been and measure the AC voltage across the resistor.

westcliff 12th Oct 2011 12:35 am

Re: KB FB10 "Toaster"
 
Thanks Chris. My meter can measure AC current. I think I'll start with the 10A range as I am fairly suspicious of the transformer.

Assuming the worst, I do have a replacement transformer in my donor set.

Gus.

Tim 12th Oct 2011 4:22 pm

Re: KB FB10 "Toaster"
 
Easy way to connect in if you want to measure a set's current.

Easy with a single pole switch:
Switch on/off switch to OFF. Connect meter to switch terminals in line of set.

If you have a double pole switch and don't want to go to the effort of bridging the other half of the switch see if there is a voltage selector, if so pull the link and connect in there with the set switched on as normal. The same applies to fuses if fitted.

westcliff 16th Oct 2011 1:33 am

Re: KB FB10 "Toaster"
 
I measured the current with the set switched on and all of the valves and dial lamp removed. This was fairly unsatisfactory, as I didn't get one static value. What I did get was a starting measurement of 412mA, which went down increasingly slowly. After two minutes approximately, it was reading 310mA, but the transformer was too hot to touch and smelled of burning (it smelled of burning right from the start), so I terminated the session.

Does this mean that the transformer is useless and should be replaced?

Gus.

Herald1360 16th Oct 2011 9:55 am

Re: KB FB10 "Toaster"
 
Unless you are lucky enough to find a short somewhere in the heater circuit, yes. Even then the transformer's probably had it, but at least the replacement won't go the same way. A final repeat with the secondary of the TX isolated perhaps?

Sideband 16th Oct 2011 11:05 am

Re: KB FB10 "Toaster"
 
Certainly seems like the transformer has had it. I'll guess that the original fault was the rectifier developing a cathode-heater short which theoretically would short the HT out via the heater winding. If the set was left running in this condition, the rectifier would have been glowing red hot and the transformer heater winding would have been under considerable stress resulting ultimately with insulation breakdown.

A similar problem happend with a Murphy radio I was asked to look at. This used a 7Y3 (I think) loctal based rectitfier that had developed a cathode/heater short. The transformer was completely burned out.

Looking inside the valve, it looked as if part of the cathode was burned away and I confirmed this by carefully breaking the valve open and removing the electrodes.



SB

westcliff 31st Oct 2011 2:14 am

Re: KB FB10 "Toaster"
 
This evening I finished swapping the faulty transformer for the one from my donor set.

There were a couple of differences in the connections. The original transformer was connected to pins 2 and 7 on V4, the 6V6 output valve. On the donor set, these connections were to V3 pin 3 and to chassis. I re-connected to the 6V6 with some difficulty as the "new" leads were not as long as the original ones.

On my original transformer, the two wires on the other side were both (very badly), connected to the 230v-250v connection on the voltage selector. :o

The donor transformer had one connection to the 200v-220v side and the other to the 230v-250v side. When I fitted the donor transformer, I copied these connections, to both sides on the voltage selector.

Tomorrow I will repeat all the testing and fingers crossed, nothing will emit smoke this time.

Gus.

DangerMan 31st Oct 2011 9:10 am

Re: KB FB10 "Toaster"
 
"On my original transformer, the two wires on the other side were both (very badly), connected to the 230v-250v connection on the voltage selector. :o"

That could explain why it was toasting itself. That's rather a lot of shorted turns!

Pete

Neil Purling 31st Oct 2011 6:34 pm

Re: KB FB10 "Toaster"
 
Good luck Westcliff. This set has tried your patience more than enough.

westcliff 1st Nov 2011 1:57 am

Re: KB FB10 "Toaster"
 
Thanks for the good wishes, Neil. Tonight I couldn't wait, so I put all the valves, etc back and screwed the set into its case, switching it on via the lamp limiter.

The good news is that the lamp stayed very dim. The dial lamp was bright and the set warmed up in around half a minute.

That's all the good news. There was a continuous droning buzz which was loud with the volume turned low. While the set could pick up stations, people's voices sounded like Daleks under water. It sounded good the other week, before the transformer overheated, so I must have done something since then to make it sound bad. When crackling started, in addition to the other unwelcome sounds, the set was switched off. It sounded the same on MW and LW.

It was on for around three minutes and there was no noticeable heat from the transformer.

One thing I've not done is to touch the alignment. Okay, one of the little exposed coils attached to the tuning capacitor had fallen off, so I soldered it back on, but the set sounded fine after that.

I mentioned yesterday that the leads on the replacement transformer were shorter than on the original one. One problem I had was the lead to V4, pin 7. This was originally routed round the edge of the chassis. It is now routed straight across several components. Could something be picking up AC by inductance? Nearby components are C12, C15, C18, V3 and of course V4 (Trader sheet 969 references). Please excuse me if this is a daft question.

Gus.

Sideband 1st Nov 2011 8:46 am

Re: KB FB10 "Toaster"
 
Don't touch the alignment, at least not yet. A droning buzz is indicative of smoothing problems and that will give you a 'Dalek' sound in worst cases. I would have checked the set before putting it back into the case. You can't do voltage checks or test anything else when it's all boxed up! Patience! Check first of all that the replacement transformer is indeed OK...easy, take all the valves out and just leave the pilot lamp to give a visual indication. If it's all cool after about 5 minutes then that is one problem definately cured.

You then need to check the wiring to the smoothing caps and output transformer to make sure that everything is OK. I presume you have replaced the 6X5 rectifier?


Rich.

westcliff 2nd Nov 2011 12:58 pm

Re: KB FB10 "Toaster"
 
Last night the transformer was tested following Rich's suggestion. Only the dial lamp was in place when the set was switched on. The dial lamp lit, the set was left for five minutes, then switched off. The transformer was cool after this.

Back in went the valves and the unwelcome noise returned. I started substituting valves one at a time, with no difference to the sound. I have yet to try a different rectifier as the one in place was working well a couple of weeks ago, but I will try a different one next time. I will also check the wiring to the new smoothing capacitors and to the output transformer, and let you know what I find.

Gus.

Sideband 2nd Nov 2011 2:49 pm

Re: KB FB10 "Toaster"
 
You could try removing valves one at a time starting with the mixer and work back. One of the valves may have a heater/cathode short which will cause hum. Try removing the mixer, switch on and check for hum. If it's still there, refit the mixer and remove the I.F amp. Check the set again. Repeat the procedure for the AF amp/detector. If the set still hums at this point, it's either the output valve or rectifier (unlikely the reccy as this would cause other problems with possibly a repeat of the damaged transformer). With just the output valve and rectifier in place, there should be very little hum from the speaker.


Rich.

westcliff 9th Nov 2011 1:49 am

Re: KB FB10 "Toaster"
 
The set has now been tested by substituting the valves one by one, with no difference in the sound. Next thing is to check the DC voltages from the brand new smoothing capacitors. I'll post the results here and hopefully someone can tell me if the capacitors are any good.

Gus.

Sideband 9th Nov 2011 9:21 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 478804)
Next thing is to check the DC voltages from the brand new smoothing capacitors.

It's very unlikely that the new smoothing capacitors will be faulty. Can you post some pictures of the chassis underside, preferably some good close-ups around the smoothing caps. Might help to assess what's wrong. Some of these sets had a 'hum-bucking' arrangement using a tap on the output transformer and some didn't. If this is connected wrongly, it could add hum rather than cancel it.

From my own personal experience with these sets, hum has never been a problem as long as the smoothers are OK of course.

Lets have those voltages as well because it may show where the problem is

Rich.


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