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 HMV A13B Excess sound with volume turned down
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 Return to top of page · Post #: 1 · Written at 5:37:21 AM on 6 May 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Hi all.

I have a problem with my HMV Little Nipper A13B. With the volume at minimum there is still quite a bit of volume evident and it's not going through the volume control but is circumventing it somehow. It also sounds a bit tinny at that point, but OK when turned up. Link to circuit diagram below.

Even with the base of both secondary IF coils momentarily grounded the sound gets through. Lifting the grid cap off the IF amp/demod/AVC/Audio Amp valve (EBF35) it will still get through until I move the grid cap wire well away from the valve.

I have replaced most of the caps except 2 in the oscillator which test OK and I also checked them by substitution with new caps. All resistors are slightly high but within 20%. I have substituted the three valves except the rectifier to no avail.

The voltages are fairly close except the EBF35 plate voltage is 110V instead of 94V.

Two points of note are... the circuit diagram shows the EFB35 shield connected to ground via a 100pF cap but it is soldered directly to ground, installing a cap leads to howling. The volume control has been replaced with a 1 meg pot with no tap at 25k Ohms and the bottom lead is grounded instead of connecting to the output transformer secondary as in the diagram, connecting it per the diagram causes the set to howl.

This is a reflex circuit and I'm not all that familiar with exactly how it works.

Can anyone offer any suggestions on what to look for? Thanks in advance.

Circuit Diagram: https://photos.app.goo.gl/pKE3n7jVm47LmYri7


 
 Return to top of page · Post #: 2 · Written at 7:54:39 AM on 6 May 2019.
Robbbert's avatar
 Location: Sydney, NSW
 Member since 18 September 2015
 Member #: 1801
 Postcount: 1074

The diagram shows the shield connected to ground, not through a capacitor. That cap you refer to is one in IFT2.

Reflex refers to the fact the the IF comes into the grid from IFT1, gets amplified, goes to IFT2 like any radio. But then the output from IFT2 is fed to the diode to be rectified, and to the volume control. The output of the volume control then goes back into the EBF35 for a 2nd time, gets amplified, passes through the primary of IFT2 unaffected, then on its way to the output valve.

Reflex circuits always produce some sound at the minimum setting. For example, audio can make its way into the unbypassed negative power supply, then into the output valve. As you've seen, even the nearness of components is enough for the signal to jump across.

Check that R7 is still 40 ohms.


 
 Return to top of page · Post #: 3 · Written at 9:07:20 AM on 6 May 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Hi Robbert, Yeah thanks, the 100pF refers to the cap in the secondary IF, don't know what I was thinking there.

Thanks for the reflex explanation, that's all I needed, I can see how it works now.

I did assume that I should expect some sound bleed through at minimum volume but this is higher than I imagined would be acceptable for a radio.

It's no big deal, I hoped someone might suggest putting a capacitor from here to here to quieten it down.

R7 is spot on 40 Ohms

Thanks Robbert, David


 
 Return to top of page · Post #: 4 · Written at 10:06:49 AM on 6 May 2019.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 813

Did the wiring as you received it look original?

Tapped volume pots are usually used to provide better bass at low volume, so if it is not there it may account for tinniness at low volume.

Taps from the secondary of the output transformer can be used to provide feedback, again to improve sound quality, but at a slight cost to output power. It looks like feedback in this case because the tap is across the voice coil from the voice coil earth, which would give a potential useful for feedback. If that is supposed to be there the sound might be degraded. If the circuit looks untampered with maybe your circuit was modified at the factory. If it is tampered with eg changed output transformer and/or volume control, maybe someone decided to do without the feedback and better sound at low volume.

Looks like you are on the right track as both modifications involve the volume control.


 
 Return to top of page · Post #: 5 · Written at 11:03:57 AM on 6 May 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Hi SRC830, the radio has been modified in that the connection to the output transformer from the low side of the volume pot is removed and the pot's low side is directly grounded. Apart from that some capacitors had been replaced with mid 50s - 60s yellowy green caps which I have since replaced again in an desperate attempt to shotgun the problem, not my normal approach.

I replaced the volume pot as the original was damaged.

The tinniness is only when the volume is on minimum and the sound can still be heard, when the volume pot is turned away from minimum the sound is quite good.

As I said, if I attempt to connect the low side of the volume pot to the output transformer the set howls so loudly it makes the dogs bark. Maybe I should try a shielded cable to make the connection Perplexed

Cheers, David


 
 Return to top of page · Post #: 6 · Written at 11:09:15 AM on 6 May 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Sounds like the feedback loop is positive not negative phase.
Try reversing the secondary or the primary leads of the output transformer.
That item may have been replaced with the monkey doing it not knowing about phasing the feedback.
Fred.


 
 Return to top of page · Post #: 7 · Written at 12:35:30 PM on 6 May 2019.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1406

I concur with Fred. Also have you replaced wires and are they in exactly the same spot as the original?


 
 Return to top of page · Post #: 8 · Written at 12:37:15 PM on 6 May 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Fred... you are a genius! As soon as I read it I realised that would be it, I had forgotten about phasing of the output transformer.

I reversed the input and of course it works perfectly, even the minimum volume level has dropped to a reasonable level and it sounds even better.

I am the monkey who changed the output transformer on day one as the original had an open primary. It didn't occur to me to check the phase but I have read it somewhere and knew about it. I'll go and check up on it again.

Thanks Fred, STC830 and Robbert for your help, I picked up a lot from your collective input.

Cheers, David


 
 Return to top of page · Post #: 9 · Written at 2:11:49 PM on 6 May 2019.
STC830's Gravatar
 Location: NSW
 Member since 10 June 2010
 Member #: 681
 Postcount: 813

Another win for the Forum.


 
 Return to top of page · Post #: 10 · Written at 4:09:10 PM on 6 May 2019.
Fred Lever's Gravatar
 Location: Toongabbie, NSW
 Member since 19 November 2015
 Member #: 1828
 Postcount: 574

Hi Tippy, ooops, apologies for the monkey crack!
I'll just swing back to my cage and eat a banana!
Fred.


 
 Return to top of page · Post #: 11 · Written at 7:18:28 PM on 6 May 2019.
Tippy's Gravatar
 Location: Mount Cotton, QLD
 Member since 20 February 2018
 Member #: 2214
 Postcount: 125

Ha ha... not a problem Fred Smile I'm always happy to learn, mistakes can be the best way to learn as you rarely forget them.

It's great that we have a resource like this forum to help people like me get in contact with people with experience like you.

Until next time... Cheers, David

Hi Carl Smile


 
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