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Sideband 11th Dec 2011 10:15 pm

Re: KB FB10 "Toaster"

Originally Posted by westcliff (Post 487333)
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.


In that case please carry out the suggestion by Station X. If you still have significant DC voltage on the grid then either the valve is faulty (prove by pulling it out), the valveholder is leaky (will have to be replaced), or there is a wiring error. Check the grid leak resistor (R9 470K Trader Sheet) and make sure it's connected correctly.


PJL 12th Dec 2011 12:13 am

Re: KB FB10 "Toaster"

Originally Posted by westcliff (Post 487296)
Control grid: varies between 7 and 13 volts.
Anode: 236 volts
Screen: 206 volts
Cathode: 9.26 volts
Control grid/cathode: -8.92 volts

Hi Gus,

I asked you to measure control grid to cathode as well in case the grid resistor is o/c in which case the DMM input resistance would pull the grid to wherever you were measuring and both grid to chassis and grid to cathode would show near zero.

There is some inconsistency here. Are you saying only the grid to chassis was fluctuating and all the other voltages were stable?

- The grid to cathode looks correct.
- The 9.26V on the cathode equates to 38.6mA which sounds about right.
- The HT is a little high which suggests the set is running at a slightly lower current than intended

My guess is you made poor contact when you were taking the reading from grid to chassis...try disconnecting the audio coupling capacitor as suggested and see if the hum goes.

If the hum remains then:
- We are back to the HT or output transformer

If the hum dissapears then reconnect the capacitor and:
- Does the hum vary with the volume control position?
- Try removing V3, does the hum go?

westcliff 12th Dec 2011 1:39 am

Re: KB FB10 "Toaster"
I have done what Station X (and just about everyone else), suggested. I disconnected C15 from the output valve and measured the grid voltage. The reading was 0.008 volts DC. There was a very faint hum, which I would consider acceptable for a set tuned between stations and there was also a quiet hiss when I touched the pin with the DMM probe.

I then measured the voltage at the free end of C15 (still connected at the other end to V3), and got a fluctuating reading in the low mV range (say -5 to +30), which occasionally showed 200 or 300 mV, just for an instant. This was difficult to read as it changed very quickly.

Then I checked V4 pin 5 again and the reading was 11 mV. I checked it once more and got 16 mV.

The last thing I did tonight was to remove V3. There was no hum.

The grid leak resistor is another brand new component and measures 469.8K ohms.

The hum does not vary when I operate the volume control.


PJL 12th Dec 2011 9:28 am

Re: KB FB10 "Toaster"
As I suspected, there is nothing wrong with the output stage. We are getting very close. Don't forget to put C15 back.

The hum is around V3, I suggest you provide us with some voltage readings on V3 (grid and anode) but whilst you are there check R6 (10M) and R8 (220K). Very high value resistors tend to drift and the anode load on the AF amp often fails due to the high voltage across them.

Sideband 12th Dec 2011 9:51 am

Re: KB FB10 "Toaster"
Something is very strange here. In one post you say that you have a varying control grid voltage of between 7 and 13 volts which is very wrong and confirm that you are testing between control grid and chassis and then in post 103 you now say that you got .008 volts (80mV) which is absolutely fine! The fluctuating voltage at the free end of C15 may well have been audio which will vary with whatever the set is tuned to. Was there any hum with C15 disconnected? We now know that there is no hum with V3 removed which clears the power supply and output stage. Where was the 7 - 13 volts coming from?


Station X 12th Dec 2011 10:05 am

Re: KB FB10 "Toaster"

Me too. We need confirmation that with C15 reconnected there is no voltage on pin 5, the grid. Has C15 been changed?

PJL 12th Dec 2011 10:52 am

Re: KB FB10 "Toaster"
Yes folks but look at all the other evidence:
- The grid measured from the cathode is very close to the cathode voltage
- The cathode voltage is correct
- The HT is correct
- The cathode current is correct.

If there was leakage in C15 then:
- The HT would be low
- The cathode voltage would be high
- The cathode to grid would be slightly positive or at least less than the cathode voltage.

The bias conditions for the valve are evidently correct so the error must be in the grid measurement. A bad connection or maybe the meter lead set the whole thing oscillating?

Station X 12th Dec 2011 11:23 am

Re: KB FB10 "Toaster"

The bias conditions for the valve are evidently correct so the error must be in the grid measurement. A bad connection or maybe the meter lead set the whole thing oscillating?
We'll just have to see what happens when C15 is reconnected. 7 to 13V is a lot of Volts.

PJL 12th Dec 2011 1:20 pm

Re: KB FB10 "Toaster"
I am not a gambling man so have to agree there remains a possibility there is a problem with C15 with the conflicting information we have been given!

- Connect C15 up and measure V4 grid voltage from chassis again! It should read less than 1V.
- Then pull V3 and measure V4 grid again. This is to remove any AC on the grid that might be coming from V3. V4 grid would go more positive if there is a fault with C15.

westcliff 13th Dec 2011 1:27 am

Re: KB FB10 "Toaster"
This evening I repeated one of the voltage measurements I did yesterday. It seems that I did something wrong the last time, probably mis-reading the DMM display. Apologies for that, the last thing I want to do is waste people's time. Thank you all for your comments.

C15 is new.
There is no hum with C15 disconnected from V4.
There is no hum with V3 removed.

All measurements were taken with the volume set low (about 1/4).

V4 control grid: fluctuating, 2mV to 12mV.

I then measured the voltages at V3 grid and anode.
V3 control grid: rose to 0.9 volts.
V3 anode: 68.6 volts.

I measured four resistors, with the set switched off.
R4 (2.2M ohms), is old and measures 2.608M ohms.
R6 (10M ohms), is old and measures 11.96M ohms.
R7 (220k ohms), is new and measures 219.2k ohms.
R8 (220k ohms), is new and measures 219.4k ohms.


Station X 13th Dec 2011 8:41 am

Re: KB FB10 "Toaster"
PJL wins the bet then!

Resistors are OK, but R4 and R6 are getting close to the 20% limit. It's probably worth changing them in case they go higher.

There should be no DC voltage on V3 control grid. It's possible you're measuring the audio signal. Check the voltage again with the volume control at minimum. Also check it with C14 disconnected.

PJL 13th Dec 2011 9:49 am

Re: KB FB10 "Toaster"
So what we know:
- It's not the audio output V4
- It's not the HT supply (pulling V3 would make it worse)
- V3 bias is not too bad (grid resistor is 10M, 5M with DMM, 1V=0.2uA)
- It is not from before the volume control

That leaves us with:
- AC pick-up on V3, either wiring around the grid or direct to the valve
- Leakage in the valve or socket

As it was intermittent I would be looking at the volume control, wiring to it particularly the earth connection and cover.
- Follow Graham's advice, disconnect C14 and see if the hum goes.
- Might be a good idea to clean V3 valve socket and pins as well

westcliff 14th Dec 2011 2:38 am

Re: KB FB10 "Toaster"
Success at last. ;D

This evening I cleaned V3's socket and replaced V3 with the 6AT6 from my donor set. I also replaced R4, R6 and R3, as it was around 10% high. The result was the usual loud hum.

Following PJL's advice, I then inspected the volume control and compared it with the one on my donor set. There is one major difference in that the rightmost tag on the donor set is connected to the metal cover of the volume control and also to the lead which connects the on-off switch to the chassis.

On my set, this tag was connected only to the volume control's metal case. Several posts ago, I mentioned that the switch/chassis connection comprised two wires joined by a blob of solder, but not actually in physical contact with each other. That was fixed with a nice strong repair, covered with heatshrink tubing, so that it could not accidentally touch anything.

I think that the blob of solder was at one time also connected to the volume control earth tag and my repeated fitting and removing the case eventually broke it off.

I joined the volume control earth tag to the on-off switch chassis connection and switched on, to be greeted with an altogether different kind of hum, much gentler than before. Operating the tuning control produced music; the hum was now only between stations.

So I switched off the computer monitor, which is about a foot from and parallel to the aerial. No hum at all.

It's not yet perfect; there is a high-pitched but fairly silent whistle and there are two Gold stations, fairly close to each other on the dial, so I'll have to fix that. Is a bit of alignment needed?

PJL, I owe you a beer or three. :beer:


Sideband 14th Dec 2011 8:55 am

Re: KB FB10 "Toaster"
So after all that, it was operating with a poorly earthed volume control.....well done for finding it.

Be careful regarding 'Gold'. It is not uncommon for it to 'link up' with other Gold stations during the night and if you are able to receive several of these fairly close together at different strengths then it could be normal. I can receive three at night along with the main one on 194 metres. There appears to be another at around 197 metres somewhat weaker and I suspect this is for the Medway area. Check during the day before you suspect a problem as you may not have a fault. Whatever you do, don't start trying haphazard adjustments to the RF sections. It is possible to carry out reasonable alignment without specialist equipment but it needs care and a certain amount of experience.


bhamham 14th Dec 2011 10:02 am

Re: KB FB10 "Toaster"
Well done, Gus, and to all the experts.

Great thread on troubleshooting the little 'toaster'.

westcliff 14th Dec 2011 1:50 pm

Re: KB FB10 "Toaster"
Good point about the various Gold stations Rich. I'm also thinking that the whistle may be connected with my computer and its wireless system. The PC and wireless transmitter are about two feet from where I was testing the set last night.

I certainly don't want to mess about with the alignment unless it's absolutely necessary. In addition to that, it was working well enough before and I have been careful not to touch any of the alignment adjusters, so it should still be okay.

Tonight I'll test the set in another room.


Sideband 14th Dec 2011 1:59 pm

Re: KB FB10 "Toaster"
Not good with AM reception! You need to keep computers and routers well away from any sets being tested. There is so much rubbish radiated from these items plus power supplies and central heating controllers that it makes AM reception very difficult. Someone I know had a microwave that obliterated AM just by being switched on (just the clock working). Off topic for this thread though so just search 'interference' for some good threads on the subject.


westcliff 14th Dec 2011 7:38 pm

Re: KB FB10 "Toaster"
4 Attachment(s)
The least I can do is post a couple of pictures of the finished set. The new owner, Mrs Westcliff, is delighted with it and it's singing for her as I type.

I am very grateful to everyone who gave me advice and encouragement, sent me parts and even made stuff for me (thanks Mick).


Station X 14th Dec 2011 9:06 pm

Re: KB FB10 "Toaster"
That looks very nice. There's a lot of rejoicing here when a thread reaches a successful conclusion. Especially one as long as this. :)

vinrads 14th Dec 2011 9:13 pm

Re: KB FB10 "Toaster"
Phew it took some time getting there, I only played a small part, but I love the finished article well done, regards Mick.

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