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Old 14th Aug 2022, 8:11 am   #1
agardiner
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Default HMV car radio with type B amplifier

Hi,

I have been restoring an HMV car radio for the last few months, which includes a type B amplifier. This is a slightly more unusual amplifier, with a push pull output. Lots has been done, including full recap, a replacement DH77 in the detector position, (original was low emission) new solid state vibrator etc.

The radio does work, and tunes to stations across the whole band. However the audio is very low and the volume control doesn't really do anything at all. The control itself is fine, as are the various resistors in the circuit of V102. The volume control for this amp is connected to socket pins 6, 7 & 8; no ground connection, (other than the screening of the cable).

Would I be correct in thinking that the volume is effectively controlling negative feedback rather than signal level? Any suggestions as to what to test to establish that is happening? I have tried injecting a tone into the volume control with no effect.

Thanks,

Adrian.
Schematic of amp attached.
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Old 14th Aug 2022, 8:13 am   #2
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Default Re: HMV car radio with type B amplifier

Oh, should have said; voltage measurements on V101, V102 & V103 are pretty much as indicated on the schematic.
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Old 14th Aug 2022, 9:31 am   #3
Peter.N.
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Default Re: HMV car radio with type B amplifier

I can't see a volume control, I would think it's on the other end of the screened cable that goes to the grid of the DH77.

Peter
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Old 14th Aug 2022, 9:41 am   #4
barrymagrec
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Default Re: HMV car radio with type B amplifier

Is it the type with the pair of N78s? I repaired one where replacing them made a staggering difference to the volume.
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Old 14th Aug 2022, 11:13 am   #5
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Default Re: HMV car radio with type B amplifier

When working properly, there is no negative feedback involved in the volume control circuit. The clue to this fault is that there volume control doesn't make much difference. The earthy end of the volume control connects to chassis via C112, so the first check should be of that and the connections to it. I would also be suspicious of C114, as the gain will be reduced if that is open circuit.

You could also try injecting a signal into the grid of V103 (via a capacitor) to see if the problem is before or after that stage.

Paula
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Old 14th Aug 2022, 12:23 pm   #6
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Default Re: HMV car radio with type B amplifier

Peter, thanks - the volume control is in the radio part which is connected to the multiplug.

Barrymagrec, thanks for that. I don't have a spare but perhaps will source and check. Both N78s check the same for emission and there is no distortion.

Paula, thanks for your suggestions. Both C112 and C114 have been replaced but I will double check them. I have tried injecting a signal into V103 and didn't really get a response. Maybe I didn't inject enough voltage; what would you suggest, I think I tried upto about 12V. Injecting into the grid of V102 also doesn't really give anything, UNLESS the volume control is turned right DOWN, in which case my tone is audible at the very low level. Any clues from that?

Regards all,

Adrian.
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Old 14th Aug 2022, 12:35 pm   #7
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Default Re: HMV car radio with type B amplifier

Quote:
Originally Posted by agardiner View Post
Oh, should have said; voltage measurements on V101, V102 & V103 are pretty much as indicated on the schematic.
Yes, but have you checked the anodes for voltage on the output valves?

I've found that the output transformers on these old car radios can often be found to be o/c.
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Old 14th Aug 2022, 1:21 pm   #8
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Default Re: HMV car radio with type B amplifier

That would be unlikely give I do have sound, but will check in case.

Thanks,

Adrian.
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Old 14th Aug 2022, 1:22 pm   #9
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Default Re: HMV car radio with type B amplifier

Yes you can still get sound due to stray capacitance etc., so worth checking out.
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Old 14th Aug 2022, 3:34 pm   #10
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Default Re: HMV car radio with type B amplifier

12 volts into the grid of V103 should be enough to get a decent output, so the output stage is most likely the area of trouble, as Techman has suggested.

Paula
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Old 14th Aug 2022, 5:06 pm   #11
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Default Re: HMV car radio with type B amplifier

OK, further tests. The anodes of both output valves have HT. A little low but nothing much and certainly not enough to cause very little output. Unfortunately I found that the grounding on the probe of my signal generator had failed, giving some false results earlier, so corrected this.

Injected a tone at 10V into grid of V103, and I do get an output. Not massive, so difficult to know if it is enough. Tuning to radio Caroline, the sound is clear and the volume is like having a radio on low in the background. Volume control makes no difference.

Injecting 0.5V tone into the grid of V102, the volume of the tone is actually good when the volume of the set is set to max. As you turn the volume DOWN, the level of tone increases and becomes more and more distorted. So I am convinced the problem remains with the signal around V102. Connecting the fixed signal generator to the wiper of the pot gives similar results to connecting it direct to the grid, getting louder as the control is turned down. Connecting to the 'feed' end of the pot results in the same level as the wiper, REGARDLESS of where the control is set. Reconnecting it to the other end results in no tone when full volume, and increasing level of tone as you turn the set down.

All very strange but hopefully enough to shed some further light? One more thing.... When you disconnect power to the set, the volume briefly increases as the HT dies. Any clues?

Thanks to everyone for their help so far.
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Old 14th Aug 2022, 6:03 pm   #12
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Default Re: HMV car radio with type B amplifier

What's the resistance measurements between the slider of the volume control (at the normal maximum and minimum volume settings) and the cathode of V102 ?

Lawrence.
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Old 14th Aug 2022, 6:13 pm   #13
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Default Re: HMV car radio with type B amplifier

I think if the wiper of the volume control is directly connected to the grid of V102, the grid bias of the triode provided by the grid leak connected to the tap on the cathode resistors will be progressively shorted out to ground as the volume control is turned to reduce the volume. This will cause the distortion you mention. Could you try isolating the control wiper from the grid bias by adding a 0.1uF capacitor in series with the grid and see if that makes any difference.

Equally, you did not say whether you used a DC blocking capacitor in series with the generator when you were applying it direct to the grid of the triode. This is also essential to avoid shorting the grid bias.

Ron

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Old 14th Aug 2022, 6:23 pm   #14
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Default Re: HMV car radio with type B amplifier

Lawrence - resistance between each connection of the volume control and their respective connections in the amp is zero ohms.

Ron; OK, very interesting. Inserting a cap has no effect. However, what is interesting is I tried running the set with the grid totally disconnected from the wiper of the pot, which means not connected to anything. The volume was the same. Also, injecting tone directly onto the grid, now gives bags of volume at only 0.2V. The signal generator does have a series cap.

So, I think this proves that the output stage is fine, and the issue is to do with V102. Very odd; all caps and resistors in the circuit are fine and I have tried another valve. Not sure what's going on.
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Old 14th Aug 2022, 6:27 pm   #15
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Default Re: HMV car radio with type B amplifier

Quote:
Originally Posted by agardiner View Post
Lawrence - resistance between each connection of the volume control and their respective connections in the amp is zero ohms.
Yes but that doesn't answer the question I asked.

Lawrence.
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Old 14th Aug 2022, 6:57 pm   #16
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Default Re: HMV car radio with type B amplifier

Did your restoration include checking out or replacing that rather horrid screened multicore cable between the head end and the amplifier unit? Breaks in this cable are common and, because of the slightly curious circuit positions of the interconnections, can result in some surprising fault symptoms.

Martin
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Old 14th Aug 2022, 7:14 pm   #17
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Default Re: HMV car radio with type B amplifier

Sorry Lawrence; misread what you were asking. I have tried to measure now, but my meter doesn't have enough resolution for the very high reading. (My other meter is broken and I test high value resistors with an accurate component tester out of circuit). However, I have previously tested R107 & R108 which are fine. Just removed the volume control and measured it - slightly high at 618K, (should be 500K) Tested wiper to each end; tests fine, clearly a log pot.

Martin - Yes. One plug was missing and was kindly supplied by another forum member. While fitting it, I tested fully the cable for shorts and continuity. I have also tested continuity between appropriate parts in the radio and amp, while the cable is connected.
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Old 14th Aug 2022, 9:31 pm   #18
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Default Re: HMV car radio with type B amplifier

Have you checked/replaced the cathode bypass capacitor for V102?

If it is open circuit it would reduce the gain.
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Old 14th Aug 2022, 10:05 pm   #19
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Default Re: HMV car radio with type B amplifier

I have indeed. Replaced twice just in case.
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Old 15th Aug 2022, 9:24 am   #20
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Default Re: HMV car radio with type B amplifier

These sets are very prone to faults due to corrosion on the valveholder "forks" giving odd symptoms.
I suggest you use a signal tracer. Try feeding the signal at the volume control wiper/DH77 grid to an external amplifier and then compare it with the signal at the anode of the first DH77 which should of course be much greater, then move on to the phase splitter DH77 and so on. This will tell you where the problem lies.
The reason for the momentarily increase in volume when power is disconnected is the latter removes the bias from the N78's which is derived from the negative "live" power input ...you are still using the set in postitve earth modeI trust.!!?
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