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Old 3rd Sep 2017, 11:47 am   #1
Pete_kaye
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Default Toaster FB10 elusive fault

I have recently bought a tired Mk2 version with following valves:- 6BE6, 6BA6,6BW6, 6AT6, and 6X5GT. Originally it worked, albeit quietly. A replacement rectifier and output valve brought the volume back and the voltages all about right. Any remaining wax caps have been replaced . However, there is an intermittent fault. After a few minutes, the volume drops suddenly but still OK to listen to on strong stations. Occasionally, the volume comes back temporarily. Various checks have shown the AGC voltage on V3 changes a little when this drop occurs . From -.04 to +.08. Would this be the likely cause?

My question is which components are likely to fail in this way? It takes about 30 mins before it has cooled down enough to return to the correct volume.
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Old 3rd Sep 2017, 12:07 pm   #2
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Default Re: Toaster FB10 elusive fault

Scope the output of the detector, if this drops with the volume you can discount the audio stages.
You need to try to isolate the stage in which the gain is reducing, no use guessing. The AGC changing could be a clue but it doesn't show which stage is faulty.

Have you checked voltages on the radio with and without the fault?

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Old 3rd Sep 2017, 12:19 pm   #3
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Default Re: Toaster FB10 elusive fault

I don't possess a scope. Just basic tools. I will collect some voltages tonight but from memory , the one described earlier was the only one that changed suddenly. The rectifier and OP voltages on the valves were stable.
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Old 3rd Sep 2017, 12:44 pm   #4
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Default Re: Toaster FB10 elusive fault

Dry/poor soldered joint is favourite in my experience such as it is.
Poke around with a plastic needle with it on. If you have a change that is sufficient to hear, there must be a voltage change somewhere.
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Old 3rd Sep 2017, 6:03 pm   #5
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Default Re: Toaster FB10 elusive fault

The forward voltage on the AGC diode suggests there is significant leakage somewhere - the question is where? Both V1 and V2 grids are connected without any series resistance so no help with diagnosis there.

Clean the valve sockets, valve pins and the wavechange switch which is very exposed in this model. Tapping each valve gently when the fault is present may also help you track it down. Failing that I would add a 10K resistor in series with V1 and V2 grids and measure the voltage across them under fault conditions.
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Old 3rd Sep 2017, 9:08 pm   #6
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Default Re: Toaster FB10 elusive fault

Valve socket forks is my bet, if they are the little ones like tuning forks the brass goes brittle and one half tends to snap off. You should be able to detect this if you just gently wiggle each valve.

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Old 4th Sep 2017, 9:16 pm   #7
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Default Re: Toaster FB10 elusive fault

It's doubtless temperature related - so judicious application of freezer spray may well reveal the culprit. But don't let this stuff get anywhere near a hot valve [or valve bases/sockets .... whether they are 'suspect' or not] ...... the risk is too high.
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Old 5th Sep 2017, 12:50 pm   #8
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Default Re: Toaster FB10 elusive fault

I have tried the gentle tapping method on every component and valve base. No change here . I have noticed that the Anode voltages are now lower than they were last week by ~10% but they are the same under both conditions . I wonder whether one of the original, un-replaced valves {6BE6,6AT6,6BA6} is failing slowly as heated up??
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Old 10th Sep 2017, 2:50 pm   #9
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Default Re: Toaster FB10 elusive fault

Still mystified. I have noticed that switching ON another appliance in my flat , even in another room , will sometimes switch it back to good volume ; but it is never repeatable and usually only lasts a few minutes . What does this suggest?
I have removed speaker and discovered a replacement for the 32 +32 UF cap. It is a 16UF in parallel with what looks like half of a 22+22 can and a 10UF in parallel with the other half . See photo .Is this a good idea? It looks well done and gives stable voltages.
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Old 10th Sep 2017, 4:46 pm   #10
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Default Re: Toaster FB10 elusive fault

You need to measure the grid currents. See post 5.
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Old 10th Sep 2017, 5:23 pm   #11
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Default Re: Toaster FB10 elusive fault

I had exactly this effect on my FB10 a couple of years ago. After much prodding, poking, and measurement (no voltages changed significantly when the fault was present), it turned out to be a faulty 6BE6, which I replaced, without any modifications, with an N.O.S. EH90 I happened to have to hand. Still happily plays away as my much used kitchen radio. Tony.
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Old 10th Sep 2017, 10:50 pm   #12
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Default Re: Toaster FB10 elusive fault

I had the same thing in a thread on here from a year or so ago, on an Ekco festival. That turned out to be tin whiskers inside the volume control. I think the fragile contact inside was acting like some sort of detector that was affected by appliance switching such as the bench light or soldering iron.
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Old 13th Sep 2017, 1:05 pm   #13
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Default Re: Toaster FB10 elusive fault

I have measured voltages at the volume control which has been replaced at some time. It is nominally 470K but mine measures 400K cold. I gave it some Servisol.
When working well , the voltage on the top of the volume control is -2.1v but then drops to -.8v. The same thing happens at the wiper (-.100 to -.050). Switched off , the pot measures 400k again after several seconds as it rises on the DMM. I presume this is due to caps discharging ??
I am looking out for another 6EB6/EK90.
McMurdo did you try to repair the volume control or just replace it?
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Old 13th Sep 2017, 2:02 pm   #14
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Default Re: Toaster FB10 elusive fault

6BE6 or EH90 is what Boxdoctor suggested.

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Old 13th Sep 2017, 5:56 pm   #15
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Default Re: Toaster FB10 elusive fault

I think you are chasing a consequence of the fault. I doubt it is the volume control.

You previously said the AFC line was going positive? No circuit as I am travelling but it sounds like a problem with th ddt valve.
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Old 13th Sep 2017, 6:37 pm   #16
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Default Re: Toaster FB10 elusive fault

As Peter says above, the clue is the AGC line going slightly positive and it could be V3.

Try this:

Wait for the fault to appear and the AGC line to go positive. Tune the set off station so there is little or no signal. The AGC line should stay positive or go even more positive. Gently pull V3 (6AT6) out of its socket and if the ddt valve is responsible the positive voltage will go.

If the positive voltage remains, V1 (6BE6) or V2 (6BJ6) could be responsible.

If the positive voltage doesn't go when you pull V3, switch off and refit V3 in its socket. Turn the set on again and wait for the fault to reappear and repeat the test pulling V2 from its socket. Finally, if necessary, do the same with V1. The positive voltage will go when you pull the faulty valve.

BTW this test is safe for the radio because the valve heaters are parallel fed from a transformer. It is not a good idea to unplug a valve from a powered up set if it uses a series heater chain.
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Old 13th Sep 2017, 6:41 pm   #17
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Default Re: Toaster FB10 elusive fault

Hi Gents, I've just fixed a poorly performing FB10. the set had been wired incorrectly from new. the IF valve had its cathode returned to ground and the suppressor grid connected to the cathode resistor. It is running quite a lot better now.

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Old 13th Sep 2017, 9:35 pm   #18
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Default Re: Toaster FB10 elusive fault

Quote:
Originally Posted by Ed_Dinning View Post
It is running quite a lot better now.
No surprise there then Ed!
Surprise is that it ever worked and that it has remained like that for so long, well caught.
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Old 4th Dec 2017, 2:59 pm   #19
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Default Re: Toaster FB10 elusive fault

Little progress but I didn't say before that there has always been a crackle whenever and wherever it is tapped with light blow with a plastic probe. I have never been able to locate where this is coming from .It doesn't cure the loss of volume in any way.
Anyone else recognise this symptom?
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Old 15th Dec 2017, 1:33 pm   #20
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Default Re: Toaster FB10 elusive fault

I have shown this set to an experienced repairer and he thinks the fault is with the frame aerial and pointed out burn marks on it . He recommended converting to Ferrite rod as was done in the later version.

Has anyone got this later set and could post photos and any relevant details , particularly coils' values ?
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