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Old 16th Oct 2011, 11:05 am   #41
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Default 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.



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Old 31st Oct 2011, 2:14 am   #42
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Default 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.

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.
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Old 31st Oct 2011, 9:10 am   #43
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Default 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. "

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

Pete
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Old 31st Oct 2011, 6:34 pm   #44
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Default Re: KB FB10 "Toaster"

Good luck Westcliff. This set has tried your patience more than enough.
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Old 1st Nov 2011, 1:57 am   #45
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Default 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.
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Old 1st Nov 2011, 8:46 am   #46
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Default 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?


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Old 2nd Nov 2011, 12:58 pm   #47
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Default 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.
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Old 2nd Nov 2011, 2:49 pm   #48
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Default 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.


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Old 9th Nov 2011, 1:49 am   #49
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Default 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.
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Old 9th Nov 2011, 9:21 am   #50
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Default Re: KB FB10 "Toaster"

Quote:
Originally Posted by westcliff View Post
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

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Old 12th Nov 2011, 3:31 am   #51
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Default 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.
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Old 12th Nov 2011, 5:50 pm   #52
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Default Re: KB FB10 "Toaster"

Quote:
Originally Posted by westcliff View Post
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 View Post
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!!


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Old 13th Nov 2011, 2:16 pm   #53
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Default 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.
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Old 13th Nov 2011, 9:36 pm   #54
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Default 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.


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Old 14th Nov 2011, 7:29 pm   #55
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Default 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.
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Old 14th Nov 2011, 8:30 pm   #56
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Default 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.
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Old 14th Nov 2011, 9:43 pm   #57
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Default Re: KB FB10 "Toaster"

Quote:
Originally Posted by westcliff View Post
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......)!!


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Old 15th Nov 2011, 2:18 am   #58
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Default 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.
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Old 24th Nov 2011, 6:40 pm   #59
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Default 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.

Last edited by westcliff; 24th Nov 2011 at 6:41 pm. Reason: Two words had to be separated.
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Old 24th Nov 2011, 7:19 pm   #60
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Default 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.

Last edited by PJL; 24th Nov 2011 at 7:26 pm.
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