Hi guys,

Just thought I’d canvas a few new ideas for this issue - I keep losing the right channel intermittently but only when playing records.
The channel works fine on radio & when it cuts out on records I quickly flick it back to radio then to records and it works again.

I’ve swapped cartridges but same issue.

I’ve also changed the 12ax7 but no difference.

I’ve replaced all electros & paper caps….checked the mute switch on the Gerard but that looks ok.

I’ve measured the resistors around the 12ax7 & they seem to check out….caps I’m not as sure I’d but I think they wouldn’t make a channel cut out would they?

My guess is the pre amp circuit resistors or the main switch unit…but being intermittent I just can not work it out!

And it’s driving me mad!

I could swap the r & l channels on the phono input to definitely isolate any garrets turntable issue although I just don’t think it could be if it works upon flicking to radio then back to gram?

I’d really love any ideas….!!

https://www.kevinchant.com/uploads/7/1/0/8/7108231/11-97.pdf

Thanks!! Andy

You may have a hairline crack in a resistor that opens when hot. I have seen that before. When it happens again check your resistors whilst hot.
Or dirty contacts in the switch.

Quick checks:

Phono/radio switch dirty contacts -- clean with isopropyl alcohol or contact cleaner (never WD40) and work it back and forth a number of times

Intermittent connection between cartridge and pre-amp -- try swapping the channels on back of cartridge

Dry/cold/loose joints on connections, and/or corrosion on plugs and sockets

Else:

When the fault occurs, trace the input signal on each channel from phono input to point where it stops. Work back from there to find the culprit.

Very much inclined to think a flex break in the cable, bad joint or switching. Looks like the chassis is back biased the cables to the PU cannot be grounded.

Marc

Thanks for the ideas guys!

I have tried cleaning the switch internals....basically from what I can see everything is turned 'on' but for the radio so to speak when the off switch is up....and it all works on radio....so I'm just not sure it could be the switch.

The resistor crack is a good suggestion....looking at the circuit it seems a bad resistor that goes open could be it...but hard to know as it's a bit of work to disassemble things to test them and being intermittent by the time I wait for the fault then disassemble it would probably be working again....

I was wondering about a flex break in the cable.....been thinking of swapping the R & L channels where they plug into the chassis to test this....but they aren't easily swappable....although I could make some connectors up and use another old spare plug I have to swap them....

I tried measuring the input from phono with the multi but I couldn't get any reading.....must be a tiny tiny voltage? That or i'm not doing it right....

I tried measuring the input from phono with the multi but I couldn't get any reading.....must be a tiny tiny voltage? That or i'm not doing it right.

It is typically a few millivolts. Need a very sensitive meter, or a scope, or a signal tracer.

Just a reminder to be extra careful doing this. Dangerous voltages present under that chassis.

The crystal cartridge tends to put out less voltage than an old fashioned magnetic one. The specs for crystal ones are published.

I would make a point of checking all solder joints between and including the switch and pickup: That also goes for plug and socket. Wire breaks a most common on plugs. Sometimes you can apply tension to the wire at the plug & if its broken that bit may pull out of its sheath. The jaws or whatever in the socket, may be sprung?

It might be an idea to use IC clips & and ohm meter to power the cable while moving it.

Marc

Yeah it just be too low a reading for my multi…I have checked an awful lot of joints for resistance but all seem ok. I haven’t checked the plug & socket resistance though - another idea to pop on the list to try!

I didn’t get time today to tinker but tomorrow I’ll check the resistance of plug & socket connection.

Then I’ll dummy up a connection to swap r&l channel input….just to rule out issue with turntable or wiring.

That will one & for all (hopefully) indicate whether issue lies with pre amp stage….if so I may just have to try replacing resistors one at a time…there only about 6 around the 12ax7 so it shouldn’t be a big deal although there’s a few styro’s there too which I’ve read heat can ruin so I’ll have to be careful…..

One of the points I fear is being missed it the fact of it being stereo. When it is on AM the two are combined. That basically means that anything after that switching in, cannot be implicated if the fault does not manifest when its running monaural.

It may be an idea to print a copy of the circuit and colour the rails involved. Being back biased the ground of the cartridge may float, albeit the crystal is not resistive. A bad connection at the cartridge is not to be overlooked. I would not trust those Phone jacks either. Seen a few of them loose, damaged, & open.

Marc

Hi Marc,

The fact it is stereo is why I’m looking in the pre - amp stage as that seems most likely given it’s ok on radio. (Although I do have another issue to sort there too - only 1 of the push pull outputs works on R channel…..I’ve swapped those output valves around but no difference - it means the R channel has less bass than L. I’ll pull apart & measure resistors there but it may well be output trans).

I do need to look at the cables / connections more at the cartridge end….I swapped the headshell over for another with a seperate cartridge but no difference.

I have some spare cables I’ll try today for the gram to chassis….process of elimination!

Hmmm well I’ve attacked the push pull issue measuring resistors surrounding valve 4 & 5 but can’t see any that measure up differently from R to L channel. There’s no paper caps left - I had already replaced the .0047 caps there too. Remaining were 2 .033 mustard per channel that I thought I’ll replace on the R channel just in case it could help but to no avail.

I am not really sure how to measure the output transformers but I checked the primary resistance between push tap to chassis & pull tap to chassis they are same for both on both channels.

Is there a better way to measure this?

Is there a way to measure output of each transformer when it is on to see if they are providing equal? Ie. could narrow it down to speaker issue or something? I didn’t measure secondary resistance (forgot!).

Didn’t get time left to change phono cable - baby woke up so it was tools down!

Thanks!

Andy

As I understand it, in radio mode there is no problem, so the output stage which includes the OP transformer and is common to all modes, ought not be the issue.

From the symptoms you describe, the problem lies somewhere in the phono path. Eliminate each part of that, one at a time.

Thanks - there’s actually 2 problems - the R channel cuts out intermittently on gram but it also is - on all modes - less “bassy” - and removing valve 5 doesn’t make any difference to R channel volume on radio or gram so I think there’s an issue there too…..

There could be a problem with a potentiometer. There really should not be any difference in two circuits the same unless there is a changing of parts by either error or, failure. Plate bypass caps are quite vulnerable and are KV rated to avoid being punched through by the audio transients. In the old sets The highest probability of failure in non polarised caps was the audio plate bypass & the buffer on a vibrator unit.

Removing V5 should upset the apple cart; that is a push pull circuit and taking that out will remove one phase. In most cases that can / will cause serious audio distortion. I have seen some serious damage done by a 6GW8 going short.

The oldies method of radio alignment with an AC voltmeter & 0.1mfd in series applied to the pentode plates of 6GW8 is still valid. However, the audio has to be a very clean sine wave, even if its derived from the heaters (with a small value series cap 0.00x)

My preference with audio amps, after sorting out voltages by comparing them not only to the circuit, but between them, if its stereo. The voltage across the cathode resistors will give an indication as to the tubes drawing similar current.

I will resort to one of the oscilloscopes, with the unit set mono and an audio signal bein injected, It will show the amplitude of the signal and its form. By that if its was a pure sine wave, then the shape of the signal at the reproducer should be the same at the top & bottom. Any sagging & distortion will be obvious and the oscilloscope can be used to ping where the signal is going awry.

Marc

Thanks for so much info - most well beyond my expertise though!

I think those caps are the .0047s that I’ve replaced already.

I think measuring voltages will have to be the next step!

So strange that v5 makes no difference…do you know which tube acts at the phase inverter for this setup?

Just so I can try & track the circuit.

These things have so many parts it’s hard to believe they work at all lol!