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

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?


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