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

westcliff 12th Nov 2011 3:31 am

Re: KB FB10 "Toaster"
 
Thanks for the information Rich. The output transformer hasn't been touched. It's one of the few components I've not had to work on.

Late tonight, after everyone was in bed, I put the set on the dining table and prepared to test the smoothing capacitor voltages. It was plugged into the lamp limiter, which was plugged into the RCD.

The first capacitor I tested was the one with three connections. I couldn't get the voltage at switch-on as I needed one of my hands for the switch, but I got a reading seconds later. It was at 214 volts DC and slowly went down to just over 206 volts by the time the set had warmed up.

Then I realised I couldn't hear anything. I optimistically tuned into Radio 4 on LW and it sounded just right. The set was switched to MW and the nearest station on the dial, a French one, sounded good too. The FB10 was picking up stations all over the place and there was no buzzing, humming, or crackling, just programmes.

I left the set running, tuned to Gold, while I washed the dishes and tidied the kitchen. Then I switched the lamp out of the circuit for full mains and it continued to work as it should. There was a feeling of warmth above the 6V6 output valve and the rest of the set was cool. The only smell was a faint "old radio" smell.

The smoothing capacitors are definitely okay. Would a bad solder joint at either end of these capacitors cause humming? I'm asking because the capacitors were not easy for me to fit, being axial. I didn't have any tag strips and the capacitors are "floating", with relatively long connections covered in heat-shrink tubing. Possibly not my finest work.

Gus.

Sideband 12th Nov 2011 5:50 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 479589)
The smoothing capacitors are definitely okay. Would a bad solder joint at either end of these capacitors cause humming?.

Yes most definately!

Quote:

Originally Posted by westcliff (Post 479589)
I'm asking because the capacitors were not easy for me to fit, being axial. I didn't have any tag strips and the capacitors are "floating", with relatively long connections covered in heat-shrink tubing. Possibly not my finest work.
Gus.

These should be easy enough to fit? Once the set is removed from the cabinet, remove the speaker and the capacitors are usually mounted in a clip against the chassis. In one of these sets, I did have to remove the wavechange switch to get to the fixing bolt but it was no big deal and probably took no more than half an hour. I restuffed the old can (which took longer than replacing the can) simply because there was nowhere suitable to mount axials neatly.

Sounds like you just need to check your connections around the smoothing caps. Sorted!!


Rich.

westcliff 13th Nov 2011 2:16 pm

Re: KB FB10 "Toaster"
 
I was up past 3 o'clock this morning testing the set. I'd check various connections (the smoothing capacitor connections were all good), and switch on, with good results. Then I'd slide the case on and test again and it would be fine. When I screwed the case on and switched on, the noise would return. Quite frustrating it was.

After ensuring the case screws weren't touching any components or wiring, I came to the conclusion that the final few millimetres where the case and the base were being tightened together was pushing something against something else, with unwanted consequences.

There is very little vertical clearance underneath the chassis and I did find two components protruding. One was the capacitor between V4 and V3 and the other was a resistor between V4 and the chassis. The capacitor could have its wire pushed into contact with an unused pin on the V4 socket and the resistor could press against a number of chassis connections and had in fact separated two of them so that their contact was at best intermittent. These components have now been moved to where the can do no harm. This evening I will re-attach the case and hopefully the set will work.

Gus.

Sideband 13th Nov 2011 9:36 pm

Re: KB FB10 "Toaster"
 
There is no harm in using sleeving on any long leads or any leads that may be close together. Black heatshrink probably looks best.


Rich.

westcliff 14th Nov 2011 7:29 pm

Re: KB FB10 "Toaster"
 
Hi Rich. I've used silicone and heatshrink sleeving while working on this set. Any old wiring that looked okay, I left alone. The only wires I didn't put sleeving on are connections to chassis and really short bits.

Your comment made me think though. I remember reading somewhere on this site that the original sleeving used on '40s and '50s radios could get sticky and when it was sticky, it could conduct. The lead from the transformer to V4 pin 7, has old sticky sleeving on it. I mentioned in post 45 that the lead was too short to route round the edges of the chassis, so it goes directly across the chassis to the pin. It doesn't touch anything, but the mains cable is underneath it. When I fasten the base on to the case, the mains cable may push it upwards, on to the components above.

Is the sticky sleeving story true? I've decided to re-route the transformer lead anyway, but it would be useful to know one way or the other.

The set's status today is "buzzing". Last night it was working before I screwed the case on.

Gus.

Neil Purling 14th Nov 2011 8:30 pm

Re: KB FB10 "Toaster"
 
Please clarify:
Does it buzz on the bench as a naked chassis?
I assume it does not
It is still quiet with the Bakelite case loosely in place (no screws). Yes or No
The buzz returns at some point as the case or the base board is screwed into place.

Sideband 14th Nov 2011 9:43 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 480270)
Is the sticky sleeving story true? I've decided to re-route the transformer lead anyway, but it would be useful to know one way or the other.

The set's status today is "buzzing". Last night it was working before I screwed the case on.

Gus.

Not aware of the sleeving becoming conductive but I suppose if the impregnation had failed and exposed the cloth, it could become damp and slightly conductive. I doubt it will cause your problem though. Possibly something sharp could pierce the cloth insulation when it's all screwed together. If it works OK out of the case and only buzzes when screwed together, something is touching where it shouldn't.

If it was me, I would slide the top case on and loosely fit the bottom plate. Then switch on, check that the set works and then fit the screws one at a time, screwing each one up slowly and see at which point the buzz returns. This may help to isolate where the problem is.

Can you post a good close-up picture of the underside? it may help us to see where the problem is (don't worry if your wiring is less than tidy......)!!;D


Rich.

westcliff 15th Nov 2011 2:18 am

Re: KB FB10 "Toaster"
 
Neil, as a chassis, it works very well. With the case lowered on to the chassis, it works very well too. When the case is screwed on, it buzzes loudly. Usually, it buzzes after I remove the base and then returns to normal after I've poked around the general area of V4.

Rich, I've yet to try screwing the case on while the set is working. I'll do that next time. Sorry I can't post a picture just now; my camera has stopped working. And yes, the wiring is less than tidy. It's better than it was though.

Gus.

westcliff 24th Nov 2011 6:40 pm

Re: KB FB10 "Toaster"
 
There is some progress to report, but not in a good way.

The set will not play properly at all; it doesn't matter if the case is on or off. Furthermore, when switched on, there are no voices or music to be heard, only a loud hum between stations and an almighty buzzing when a station is tuned in.

Prior to this latest calamity, I noticed that the wavechange switch lever was floppy in operation. The reason was that the spacers on which the switch is mounted had some play, so the whole switch could move. I had previously found the floppiness useful when removing the speaker, as the switch lever could be moved just far enough to free it from the bottom mounting hook.

Now that I have tightened up the spacers the switch operates quite stiffly and of course there is the dreadful noise when I switch the set on. The switch still seems to change wavebands. I suspect that my repair work and the unwelcome din may be connected. I am also starting to think that the previous good/no good behaviour could have been caused by the loose switch and not by my atrocious soldering.

Tonight's task will be to test the new theory by loosening the wavechange switch and trying various degrees of looseness to see how the set performs.

Gus.

PJL 24th Nov 2011 7:19 pm

Re: KB FB10 "Toaster"
 
If it's a wafer switch type I do quite often strip the whole switch down and refurbish the wafers.

Once the wafer is out, Viakal (domestic limescale remover) is a brilliant cleaner and will strip the oxides off in seconds. As it is strongly acidic so you need to rinse plenty of times afterwards and finish off with Servisol or a very light oil.

Viakal is a gift to radio restorers (providing you can rinse the item heavily afterwards) and will strip grease and remove oxides on metalwork in seconds. Unless you have tried it you would not beleive how quickly it cleans brass. I tend not use it on painted surfaces for fear the acid would run underneath and erode the metal over time.

westcliff 25th Nov 2011 9:30 am

Re: KB FB10 "Toaster"
 
Last night I removed the bolts holding the wavechange switch in place so that I could clean the switch properly. The first thing to happen was that the central part of the switch fell out. While this made it easier to clean (it was filthy), it took me about an hour to work out how to get it back in place.

We don't have Viakal PJL, not yet anyway, so I used contact cleaner, kitchen roll and a toothbrush. Now the switch is cleaner than it has been for decades.

Re-assembly was a bit of a nightmare, reminiscent of fitting a Bush DAC10 switching unit, except everything is much tinier in the KB. I eventually finished but it was too late to test the set, which I will do tonight.

Gus.

westcliff 30th Nov 2011 12:35 pm

Re: KB FB10 "Toaster"
 
My most recent efforts to get the set working have involved checking continuity, to ensure that all the connections I made are sound. So far, everything seems okay, but I did discover something last night which I don't understand.

The noise the set makes points to the smoothing capacitors as a potential source of the problem. Last night, I was checking their connections and I mistakenly tested continuity from the rectifier to the smoothing capacitor it is not connected to.

The DMM gave out a short "beep" and its display said 946 ohms. Is this indicative of a fault with the capacitor? Or is it just one of these things I've not yet learned about?

Thanks.

Gus.

Station X 30th Nov 2011 12:45 pm

Re: KB FB10 "Toaster"
 
If I understand you correctly, you'll be measuring R11 (Trader Sheet), but the reading is a bit low, so there may be a parallel path due to a fault. I haven't studied the diagram in detail.

westcliff 1st Dec 2011 4:07 am

Re: KB FB10 "Toaster"
 
Graham, I took R11 out and measured it - it was 944 ohms, half what it should be. I don't have any 1.8k resistors apart from some 0.25 watt ones, which I guess are not the right ones to use (the Trader sheet doesn't say).

I did find an old unused carbon comp one, rated at 1.5k ohms, which measures 1866 ohms, so I intend to use it to test the set.

Gus.

Sideband 1st Dec 2011 9:25 am

Re: KB FB10 "Toaster"
 
Not got the circuit here but I'm guessing that is the smoothing resistor? You need a 1 watt at least...I would use a 2 watt.



Rich.

westcliff 1st Dec 2011 11:35 am

Re: KB FB10 "Toaster"
 
Yes it's one of the smoothing resistors. The one I intend to use has a brown body if that's any indication of it's wattage rating. It is the same colour and size as the one I'm replacing.

Gus.

Station X 1st Dec 2011 11:49 am

Re: KB FB10 "Toaster"
 
Looking at the ERT sheet, it's described as a 1/2 Watt 1.8K resistor.

westcliff 1st Dec 2011 6:26 pm

Re: KB FB10 "Toaster"
 
Replacing R11 has made no difference. Smoothing capacitors are the next things to check. The speaker has once again been removed and I am satisfied that all smoothing capacitor connections are good. Is there a way to test electrolytics without applying power to them?

One other thing I noticed is that R2, the oscillator stabiliser resistor, measures 284 ohms instead of the 220 ohms it should be. I will change it, but it's not the cause of the problem, is it?

Gus.

Station X 1st Dec 2011 6:42 pm

Re: KB FB10 "Toaster"
 
Change the resistor, as it's more than 20% off value. I doubt it'll cure the problem. though.

Pull out the valves except for the rectifier. Connect your meter on DC volts range across one of the smoothers. Switch on. As the rectifier warms up the HT should rise and then steady somewhere above the specified HT voltage. Switch off. The reading should now fall very slowly. If it drops quickly the smoother is duff. Repeat for the other smoother.

westcliff 3rd Dec 2011 12:25 am

Re: KB FB10 "Toaster"
 
Thanks for the tip Graham.

The resistor has been changed and the electrolytics tested. One capacitor got to 330 VDC, then took around 10 minutes to drop to 75 VDC and the other reached 335 VDC and diminished slowly as well. They're don't seem to be causing any problems.

I did two other things last night. First, I re-soldered the OT wire to the electrolytic as all the wiggling had disconnected two of its three strands. The other thing was to repair a previous repair where the on-off switch chassis connection was a mostly bare wire joined to the original wire by a big blob of solder without the wires actually touching each other. The bare wire was able to touch one of the volume pot tags.

No change when the set was switched on though, still a droning hum between stations, with added crackling and distortion when a station is tuned in.

I'm running out of ideas. Any suggestions people? I'll try anything if someone tells me how to do it.

Gus.

frsimen 3rd Dec 2011 12:50 pm

Re: KB FB10 "Toaster"
 
Gus,
Reading through your posts, there are a few potential problems that come to mind. The first is the position of wiring within the set. Make sure you keep the mains lead well away from V3 and its wiring, as it would be very easy to introduce unwanted hum by that path. The same is true of your heater transformer wiring. Make sure you route it as it was originally, away from the components.

The other problem could be poor connections to chassis. You have mentioned that you have found some already. In particular, check C9 (the AGC decoupler) and C13, the HT decoupler. It would be a good idea to check all the wiring and component changes you made. Also,check around the wavechange switch in case you have things touching that shouldn't, since you did the work in that area.

You can't be too far off success, as it has been working on and off.

Paula

westcliff 3rd Dec 2011 8:17 pm

Re: KB FB10 "Toaster"
 
Hi Paula, it's good to hear from you again.

The transformer leads are both routed along the edge of the chassis. The power lead's route goes between V3 and V2; it does go close to the 1.8k ohm part of R10.

I have already tested all of the work I've done and at the continuity level at least, everything is okay.

I have turned my attention to the wavechange switch again. The two terminals on S1 show continuity when S1 is closed and also when it is open. I have disconnected the 70pF capacitor which is mounted across both of these terminals, to eliminate it as a suspect. It is innocent so it looks like I will have to remove the switch again.

Gus.

Station X 3rd Dec 2011 8:26 pm

Re: KB FB10 "Toaster"
 
C2, 70pF is not wired across S1.

The reading across S1 will be low when it's open on account of L3.

frsimen 3rd Dec 2011 8:34 pm

Re: KB FB10 "Toaster"
 
Gus,

When S1 is open you will still measure some continuity, around 110 ohms, because L3 is wired across the contacts. If you're measuring a much lower resistance than that with the switch contacts open, there is something amiss.

I wouldn't worry too much about the mains lead running near R10, that's unlikely to be a problem. If it's close to R4-R7, C12 or C14 that could result in hum. Does the noise go away when the volume is turned down? If not, does it change as you tune the radio with the volume turned down?

Paula

Station X 3rd Dec 2011 8:39 pm

Re: KB FB10 "Toaster"
 
I think L3 is 11.0 ohms, not 110 ohms.

westcliff 3rd Dec 2011 9:03 pm

Re: KB FB10 "Toaster"
 
Graham, I know what you're saying is correct, but on my FB10 there is a 70pF flat wax capacitor wired across the two S1 terminals. Again, on my FB10, there is a 0.001 uF tubular capacitor from S2 to chassis. Neither of these feature on the circuit diagram on Trader sheet 969, but the set was working well with them in place.

Paula, the volume control has no effect on the hum, which is fairly loud whatever volume setting is used.

When S1 is open there is continuity at 16 ohms.

The mains lead runs near C12, but not that close.

Gus.

frsimen 4th Dec 2011 12:04 am

Re: KB FB10 "Toaster"
 
Gus,

16 ohms is perhaps a tad higher than it ought to be, but not too far out. Station X is right, it should be 11 ohms not 110 ohms.

The decoupling arrangement on this set is a little unusual in that V1, V2 and V3 all share one decoupling capacitor, C13. Its value (0.1uF) is sufficient to decouple well at RF, but won't be so good at 50Hz. When everything is working properly, that shouldn't matter, but I wonder if you have an AGC line fault, with the control grid of V1 floating and causing 50Hz to appear across C13. The AGC is fed via the switch and your latest trouble started after you worked on that. Try checking the resistance from V1 control grid to chassis. It should be around 2.7M ohms if all is well (assuming the component values on the Trader sheet are correct). If it's much higher, trace the wiring back through the switch S1, L2, R4 and the volume control.

You could also try removing V1 and V2 to see if the hum goes with them unplugged.

Paula

Sideband 4th Dec 2011 12:05 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 485347)
Paula, the volume control has no effect on the hum, which is fairly loud whatever volume setting is used.

Then the fault MUST be in the circuits after the volume control, i.e output stage or power supply. There is very little to check here assuming connections are correct and agree (hopefully) with the circuit.

As the set has worked OK and the hum originally seemed to be associated with fitting the case it still seems likely that there is a short somewhere. Photo's of the underside could be a great help here as I feel that something very fundamental is being missed. These are relatively simple sets...by that I mean there are no odd circuits involved and there are only a few places where hum could be introduced.


Rich.

westcliff 7th Dec 2011 12:25 am

Re: KB FB10 "Toaster"
 
3 Attachment(s)
Thank you again for your helpful comments.

I have taken some pictures of the underside as Rich suggested.

Gus.

DangerMan 7th Dec 2011 1:07 am

Re: KB FB10 "Toaster"
 
Hi Gus,

I can't really see where the coupling capacitor to V4 from V3 is connected, so it's worth checking it's on the right pin of v3 etc..
If all else fails, try disconnecting it from v3 anode, and put it to chassis. All the hum should go, leaving just a small residual. If it doesn't, either the smoothing isn't working properly (wiring or dodgy capacitor) or you may have a heater-cathode short in V4.
HTH

Pete

westcliff 7th Dec 2011 2:06 am

Re: KB FB10 "Toaster"
 
Thanks Pete. As far as I know, that capacitor is correctly connected (V4 pin 5 to V3 pin 7), but I've put the radio away for the night so I'll check tomorrow. It's been six months so far; one more day won't hurt. :thumbsup:

Gus.

Sideband 7th Dec 2011 11:39 am

Re: KB FB10 "Toaster"
 
Can you post some pictures around the smoothing capacitor and rectifier? I know it will probably mean removing the speaker but that only takes a minute or so.

Rich.

westcliff 8th Dec 2011 1:55 am

Re: KB FB10 "Toaster"
 
Pete, that capacitor was attached to the correct pins. I tried attaching it to chassis and the hum was still present. I have replaced it as a precaution. The output valve was also replaced but there was no change.

Rich, I will try to get some pictures tomorrow evening when I get back from my work's xmas drinking session. There's not much to see as the leads from the smoothing capacitors are all covered in sleeving and heatshrink.

Gus.

Sideband 8th Dec 2011 8:54 am

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486470)
Pete, that capacitor was attached to the correct pins. I tried attaching it to chassis and the hum was still present.

If that is the audio coupling capacitor and attaching it (temporarily) to chassis didn't stop the hum then it confirms that the fault is in the output stage or smoothing circuit...it can't be anywhere else!

This is sounding more and more like a wrong connection somewhere. There are several different circuits for the HT decoupling and smoothing on these sets (no surprises there)! Do you know what version you have? One of the versions uses a tap on the output transformer to provide hum cancellation.


Rich.

*edit*. Looking at the pictures you provided, it looks like you have the version with a large 15K resistor providing HT decoupling (the green wirewound replacement in yours). Can you confirm this?

westcliff 8th Dec 2011 3:15 pm

Re: KB FB10 "Toaster"
 
Rich, that large green resistor near the wavechange switch is one of a pair which are supposed to make up 5.9k ohms. Its value is 4.5k ohms and its partner is 1.8k ohms (on the Trader sheet, it is described as 2 x 8.2k ohms in parallel plus 1.8k ohms in series).

There is a connection from a smoothing cap to the OT; is this the hum cancellation? The lead goes to the central of three connections on the bottom of the OT.

Gus.

PJL 8th Dec 2011 8:50 pm

Re: KB FB10 "Toaster"
 
This set seems to have been one problem after another! These are the facts as I have read them:
1. The hum remains if you disconnect the audio coupling cap and connect it to ground
2. Changing the output valve makes no difference
3. You have replaced the 32+32uF reservoir and smoother
4. Someone has been there before and had disconnected the rectifier and added a 25uF capacitor

I think it might be good to have the voltages on V4 anode grid screen and cathode.

At the moment suspects might be:
1. The wiring around the HT supply
2. A faulty rectifier
3. Leakage in the output valve socket
4. Poor earthing
5. Faulty output transformer

I would favour a wiring error but the voltages might help in diagnosis.

There should be two 32uF capacitors, one on the centre tap of the OT (reservoir) and one to the screen of V4 (smoothing).

Station X 8th Dec 2011 9:03 pm

Re: KB FB10 "Toaster"
 
1 Attachment(s)
Here's part of the circuit.

Sideband 8th Dec 2011 9:28 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486562)
Rich, that large green resistor near the wavechange switch is one of a pair which are supposed to make up 5.9k ohms. Its value is 4.5k ohms and its partner is 1.8k ohms (on the Trader sheet, it is described as 2 x 8.2k ohms in parallel plus 1.8k ohms in series).

There is a connection from a smoothing cap to the OT; is this the hum cancellation? The lead goes to the central of three connections on the bottom of the OT.

Gus.


Ahh now that's interesting. One of mine is the same and it has a tap on the output transformer. Now carefully check the wiring around the transformer. The cathode of the rectifier (pin 8) is connected directly to the tap (centre of the three). This means one of the other three connections goes to the anode of the output valve and the last one goes to the smoothing resistor 1.8K. It is important that the transformer primary is connected correctly. The idea of the tap is that it helps to reduce hum to a low level by cancelling out ripple from the rectifier. If the transformer is connected wrongly it can add to the hum rather than reduce it. If the transformer appears to have been undisturbed however, we can assume it's connected correctly. If it looks as if the wiring to it may have been disturbed, it will do no harm to reverse the two outer connections (leaving the tap where it is) and see if the hum increases or reduces. There is the remote possibility that the transformer is faulty. Hopefully your doner set has the same transformer.

If everything seems OK in this area, there is not much left other than the smoothing caps themselves. They should both be 32uF.

What is still puzzling is the fact that the radio has worked OK until the cabinet was fitted......:-/

I've played both of mine recently and there is very little hum.




Rich.

westcliff 9th Dec 2011 1:09 am

Re: KB FB10 "Toaster"
 
1 Attachment(s)
PJL, you've got the facts right but to your list we can add that the aerial to chassis connection was broken, and both the 200-220v and 230-250v power connections were in use at the same time. Hence the smoky power transformer.

Of your list of suspects, I favour poor earthing, then leakage in the output valve socket.

I re-soldered the aerial to the nearest chassis point, which is shared with the V4 grid bias resistor (R12), the V4 heater filament and the V4 control grid resistor (R9).

The V4 socket has been scrubbed with Servisol and my little boy's toothbrush several times on both sides.

Rich, the rectifier cathode is connected directly to the centre of the three terminals on the OT, although it is connected to one of the 30uF capacitors along the way. The other two OT terminals are connected to the 1.8k ohms smoothing resistor R11 (the connection is on a tag on the wavechange switch), and to the V4 anode.

You asked me to post pictures of the smoothing capacitors and the rectifier area. The rectifier is shown in one of the pictures I posted the other day and the capacitors are shown below.

The left hand capacitor is connected to the rectifier, the OT and C18, the tone correction capacitor. The rectifier lead is the red one which passes the power transformer screw and the OT lead is the black one more or less parallel to the red one. The tone cap lead is the orange one coming out from underneath the power transformer.

Gus.

bhamham 9th Dec 2011 10:51 am

Re: KB FB10 "Toaster"
 
1 Attachment(s)
Just an observation... I've restored two of these sets and mounted new smoothing & reservoir caps in the same direction as the original i.e., 90-degrees from how you have yours.

On a set (not an FB10) I did a few years back I mounted a smoother not in the original manner and it played havoc with tuning the set with a lot of hum.

PJL 9th Dec 2011 3:27 pm

Re: KB FB10 "Toaster"
 
Voltages V4 grid, anode, screen and cathode. If it's a poor earth it should show up with a few measurements. Also measure the voltage between grid and cathode.

Sideband 9th Dec 2011 3:56 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 486697)
The V4 socket has been scrubbed with Servisol and my little boy's toothbrush several times on both sides.


Yuk!! I hope he doesn't use it afterwards!!;D

Well there doesn't look too much wrong there but of course it's difficult to see minor errors on a photo.

There really isn't much left to cause hum of that level. I think we've established that the hum is still present with the coupling capacitor temporarily connected to earth. That leaves the output stage and power supply which in this case includes the choke winding on the output transformer. With everything connected correctly, presumably the hum is still present. Identify the control grid of the output valve and short it to earth. If the hum is still present it would be worth changing the output transformer. You will need to check that the one in your doner set is the same type with three connections on the primary.


Rich.

westcliff 11th Dec 2011 2:34 am

Re: KB FB10 "Toaster"
 
I spent the whole of Saturday tidying up the chassis connections mentioned in post 89, above. There was a huge blob of solder and old wires to get rid of; my puny soldering irons and I found it hard work. The result was no change, although LW now hums louder than MW. To be honest, I can't remember if this was the case previously.

I had a look at the output transformer and noticed that of the two wires that connect to the speaker, one doesn't move when you pull it and the other can be wiggled up and down. It is still firmly attached inside the transformer. Can this be a problem?

I have not taken any voltage measurements yet, but intend to do that tomorrow. Nor have I shorted the 6V6 control grid to earth; again, I'll have a go tomorrow.

Bruce, thank you for that observation about the orientation of the smoothing capacitors. I will probably try rotating them 90 degrees before I try replacing the output transformer. I have some lovely brand new 32+32uF cans but they are unfortunately too big to fit behind the speaker.

Is it an easy job to change my floppy frame aerial with its broken frame for a ferrite one? Does anything else have to be changed? I'm asking because the donor set has a ferrite aerial attached to the output transformer and if I have to change the transformer, I might as well change the aerial at the same time.

Gus.

Herald1360 11th Dec 2011 11:09 am

Re: KB FB10 "Toaster"
 
To fit a ferrite rod aerial instead of the frame, it will have to have the same inductance for its tuned windings if the tuning is to track properly and the coupling arrangements will need to be the same. You may be able to achieve the inductance by sliding the windings along the rod, but if the frame has an isolated coupling winding and the rod a tapped coil you may need to make some extra windings to suit.

It sounds like an interesting challenge- make sure everything else is working as well as possible first or you could end up in an awful tangle!

PJL 11th Dec 2011 12:47 pm

Re: KB FB10 "Toaster"
 
Frame aerials work extremely well and are fairly tolerant of interference.

westcliff 11th Dec 2011 5:08 pm

Re: KB FB10 "Toaster"
 
PJL, I have measured the voltages you suggested. All are DC volts.

Control grid: varies between 7 and 13 volts.
Anode: 236 volts
Screen: 206 volts
Cathode: 9.26 volts
Control grid/cathode: -8.92 volts

The voltages at the anode, screen and cathode look fine. I don't know what the control grid voltage should be, nor the control grid/cathode voltage.

The hum lessened as I took some of the measurements, but was still present.

Gus.

Top Cap 11th Dec 2011 6:18 pm

Re: KB FB10 "Toaster"
 
Don't give up on it Gus, if you get too dis-enchanted with it I am always happy to take a second look for you. You only need to send the chassis and you would need to wrap the valves well in bubble wrap then bubble wrap again if sending by post. Not sure where in Essex you are, I have no transport of my own but might be able to persuade someone to come over and pick the chassis up. I do have a soft spot for FB10's as they bring back a lot of happy memories working in a village radio shop in my young teens. I am located not far from the M25 in Watford. Les

Sideband 11th Dec 2011 6:51 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff (Post 487296)
Control grid: varies between 7 and 13 volts.

:o If those voltages are correct, then there is something seriously wrong around the output valve! Firstly and most importantly the control grid shouldn't have any DC voltage on it at all. The control grid should measure zero or at least very little (maybe a few millivolts) with respect to chassis. Please tell us how you measured this voltage.

Quote:

Originally Posted by westcliff (Post 487296)
The hum lessened as I took some of the measurements, but was still present.

That may indicate an earthing problem somewhere


Rich.

westcliff 11th Dec 2011 7:13 pm

Re: KB FB10 "Toaster"
 
Rich, I crocodile-clipped the black lead of my DMM to the chassis and applied the other lead to pin 5 when the set had warmed up.

Gus.

Station X 11th Dec 2011 7:54 pm

Re: KB FB10 "Toaster"
 
Quote:

Originally Posted by westcliff
Control grid: varies between 7 and 13 volts.

Disconnect the end of C15 (Trader Sheet) which is connected to pin 5. This is "THAT CAPACITOR" in the FB10. Check the voltage on pin 5 wrt chassis again. With any luck it will now be 0V. Check the voltage on the disconnected end of C15. If you see a voltage here, then C15 is definitely faulty.


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