HK A50K one side not playing in stereo

[oldman55]

Hello all
Problems with one channel only playing in mono mode (balance works) but no sound in stereo when I balance to that channel.

Have tried troubleshooting with tubes and sockets/inputs/voltages/wiring/all the usual suspects check out. Output tubes all drawing about 30mA. PS voltages all to spec.

Anyone run across this issue that may be able to shed some light on it for me? Its wearing me out.
From public domain:
http://shermanr.web.prw.net/A5001.jpg
http://shermanr.web.prw.net/A5002.jpg

Oldman

Edit: I seem to recall about a year ago when I put it on the shelf (if it was this unit) that I had thought it to be in the "stereo norm/rev" switch. Unable to figure out how to jumper a bypass to see if that was the issue. But switch does work as far as changing signal when playing the one good channel in stereo so very possibly not the issue.

 

[gadget73]

so throwing the reverse switch moves which side sound comes out of? There is no circuitry between the line level input jacks and that switch, just wiring from the jacks to the rotary selector, and then between the rotary and the reverse. Then from the reverse to the tape monitor. If its not a switch, its the wiring to/from it.

If you want to bypass it, try feeding signal in to the tape monitor jack. If you get signal from both channels its between the input and the monitor switch and all of the electronics are working.

To see if its anything to do with the rotary, feed signal in at the "tape out". That bypasses the rotary but leaves the reverse and tape monitor switch in circuit.

 

[oldman55]

so throwing the reverse switch moves which side sound comes out of?

Throwing the reverse switch does not move which side the sound comes out but does as it is intended and plays the other stereo signal on the working side.
Using the monitor inputs, it reacts the same as it does in aux.
Using tape out inputs, it also reacts the same.

So in stereo, one channel doesnt play in any configuration and symptoms are the same in all input scenarios. In mono, both sides play in all scenarios.

Oldman

 

[gadget73]

ok so the problem is somewhere after the switches then.

try jumping the two center terminals of the balance pot together and see if you get signal from both speakers. That would confirm if the power amp is good

 

[oldman55]

ok so the problem is somewhere after the switches then.

try jumping the two center terminals of the balance pot together and see if you get signal from both speakers. That would confirm if the power amp is good

When connecting the wipers of the balance pot, it plays both channels when the balance is near center position, nothing on either channel when balanced toward one speaker with the aux input.

With tape out input, same.

With monitor input, same.

Hope this sheds some more of that light on this.

old

 

[gadget73]

ok so it sounds like the problem is in the preamp section. Thats a single tube, V3. Half handles the right channel, half handles the left. If swapping V3 for another known good tube does nothing, check the voltages on pins 1,2,3 and compare to 6,7,8. They should be pretty equal and at least somewhere close to what the schematic calls for.

 

[oldman55]

ok so it sounds like the problem is in the preamp section. Thats a single tube, V3. Half handles the right channel, half handles the left. If swapping V3 for another known good tube does nothing, check the voltages on pins 1,2,3 and compare to 6,7,8. They should be pretty equal and at least somewhere close to what the schematic calls for.

Several known good tubes have been put in that position.

Pin 1 and 6 is about 150vdc per spec
Pin 2 and 7 is 0 per spec
Pin 3 and 8 is 1.2v

old

 

[gadget73]

so its conducting and about even, that sorts out power supply and tube function. Does it do anything if you jump the two center terminals of the loudness pot together?

 

[oldman55]

Jumping them does nothing to change it. It did stop playing for a bit when the tension of the jumper/cable pulling on the wiper caused it to loose the internal connection.

 

[oldman55]

jump the two center terminals of the loudness pot together?

One pot does not make a bit of sound (unlike the working one) when you touch it with cables. All its terminals are pretty much dead to the touch.

Edit: Since the Stereo nor/rev switch is getting two channels of signal, but I lose it at the loudness pot, could it be that it is the switch not sending that signal any further and therefore being the switch? Or do any of the other indicators negate that reasoning? Though it looks simple on paper, I am unable to figure out how to jumper that switch out of the circuit.

 

[gadget73]

thats what I was thinking with the loudness pot jumper. If jumping those connections brought it back, the problem was either no signal from the switch or the loud pot itself was bad. A cracked resistance board could do this. Since that doesn't do anything the problem is not upstream of the loudness pot and that rules out all of the switches and the loudness pot.

another thought, does it do anything if you jumper pins 2 and 7 together? That would rule out any issues in the lo cut circuit and the connections between the loudness pot, the switch, and the tube.

since the tube appears to be working it has to be either the tube gets no input, or there is some issue between the tube plate and the balance pot Mostly thats all wiring to/from the tone controls and filter switches. Might be as simple as a lead on one of those is bent and shorted to ground. These kinds of things are usually simple problems that are just really annoying to actually find.

 

[oldman55]

Since that doesn't do anything the problem is not upstream of the loudness pot and that rules out all of the switches and the loudness pot.

Are you saying that the stereo norm/rev switch is ruled out? Or the loudness pot?
When I jump pins 2 and 7 on V3, the "bad" channel plays along with the other.
old

 

[gadget73]

both. If you have working signal in one channel that means its getting to the loudness pot at least on that one channel. Jumping that signal to the wiper on the dead channel bypasses the stereo reverse switch, the tape monitor switch, and the loudness pot. Since you still get no output, that means the fault is somewhere after the pot.

if jumping it at the grid (2 and 7) gives you speaker output, that means everything from the grid of the tube to the speaker is good. The problem is somewhere between the center wiper of the loudness pot and the grid of the tube. That cuts down the possibilities quite a bit. Have a close look at the 68k resistor from loud pot to the low cut switch, and the connection from the low cut switch to the tube grid. Could be a solid core wire broken inside the insulation, bad solder, something like that. There just aren't that many parts involved. There is the low cut PEC, but if you have the low cut switch set to "out" it isn't in the circuit. The A50K should be the kit version of the A500, so you may just be dealing with someone's bad solder work from a long time ago.

 

[oldman55]

both. If you have working signal in one channel that means its getting to the loudness pot at least on that one channel. Jumping that signal to the wiper on the dead channel bypasses the stereo reverse switch, the tape monitor switch, and the loudness pot. Since you still get no output, that means the fault is somewhere after the pot.

if jumping it at the grid (2 and 7) gives you speaker output, that means everything from the grid of the tube to the speaker is good. The problem is somewhere between the center wiper of the loudness pot and the grid of the tube. That cuts down the possibilities quite a bit. Have a close look at the 68k resistor from loud pot to the low cut switch, and the connection from the low cut switch to the tube grid. Could be a solid core wire broken inside the insulation, bad solder, something like that. There just aren't that many parts involved. There is the low cut PEC, but if you have the low cut switch set to "out" it isn't in the circuit. The A50K should be the kit version of the A500, so you may just be dealing with someone's bad solder work from a long time ago.

I apparently posted the wrong schematic. Here is an excerpt from the loudness section which differs:


Edit: in testing resistors, noticed this on both channels, apparently it doesnt matter as one channel plays.

 

[gadget73]

don't really think the different schematic affects it much. If jumping the center wiper on the loudness pot doesn't give 2 channel output but jumping the grids right at the tube does, its still somewhere between the pot and the tube grid. Ohm checking from the grid of V3 to the wiper of the loudness pot should give right about 100k.

where it measures to ground is the phono preamp. That won't even be in circuit if the aux input doesn't work properly.

 

[oldman55]

don't really think the different schematic affects it much. If jumping the center wiper on the loudness pot doesn't give 2 channel output but jumping the grids right at the tube does, its still somewhere between the pot and the tube grid. Ohm checking from the grid of V3 to the wiper of the loudness pot should give right about 100k.

where it measures to ground is the phono preamp. That won't even be in circuit if the aux input doesn't work properly.

Checking in circuit, I get from 350k-500k depending on where the balance pot is. This is from wiper of loudness to the grid of V3 on both channels.
Note that when I jump the loudness wipers, I get sound in both channels only when the balance is in the center position and no sound when the balance is set one way or the other.

 

[gadget73]

ok I think we've gone around a circle now and I'm lost.

The balance pot should not have any effect on anything if the wiper of the loudness pot is jumped. The Blend pot will have an effect on that, so make sure we're talking about the same control here. Balance pot is the one just ahead of the power amp section at the output of the tone control circuits after V3. Blend control is just ahead of the tone circuits. I don't know what is considered normal for the blend pot, I'd guess fully left but I really don't know. Owners manual should explain this. @grindfx has one of these too, he might know. Center the balance pot just for good measure.

Reading from V3 to loudness pot wiper should be 100k per the schematic. If you can confirm what value resistor comes off the center wiper of the loudness control that would help. Having never been in one of these I'm assuming the resistor goes from pot wiper to lo cut switch, the lo cut PEC hangs on the back of the switch, and a wire runs from switch to tube grid but I could be entirely wrong on that.

just to make sure we're on the same page here. All tests with balance centered and blend to whatever is normal for full stereo operation. Lo and hi cut switched out.

selector set to stereo, any input, output on one speaker only.
using the stereo reverse switch changes what signal comes out of the speaker, but its still the same speaker
using the tape monitor jack or tape out jack has no effect on this
jumping the center terminal (wiper) on the loudness pot gives output from the same speaker
jumping V3 pin 2 to V3 pin 7 gives output from both speakers

yes?

 

[oldman55]

Edit to check data.

 

[oldman55]

Confusion here on my end as we may be referencing different valves. There are two schematics that I posted and I am going off of the second one as it more closely resembles this unit. The other schematic have the tubes labeled different and V7 is eliminated.

Below is the one I was using and is relevant to my answers below and came with the manual.

Having difficulties uploading hi res;

The balance pot should not have any effect on anything if the wiper of the loudness pot is jumped.

The grids of V3 go directly to the balance pots with V2 being upstream with a lot of circuitry in between them to get back to the loudness pot. The loudness pot goes to V2 with a RC and PEC between. Refer schematic below which is different than the original schematic posted.

If you can confirm what value resistor comes off the center wiper of the loudness control that would help

It is this schematic at loudness:

View attachment 1900793

using the stereo reverse switch changes what signal comes out of the speaker, but its still the same speaker

using the tape monitor jack or tape out jack has no effect on this

jumping the center terminal (wiper) on the loudness pot gives output from the same speaker[/QUOTE]

correct. but it bypasses the balance and gives full volume to that speaker (jumping the two pot wipers together)

jumping V3 pin 2 to V3 pin 7 gives output from both speakers

It gives lower volume output (and a lot of connection noise to good channel speaker) to both speakers but the balance pot has to be in the center position.

Apologies on the confusion and lost time it caused but I appreciate you not baling out. Hopefully my answers make more sense

Oldman

 

[gadget73]

diff tube numbering is probably the confusion here. One of them calls out V3 as being the tube between the loudness control and the balance pot, the one with all the tone control stuff wired to it. If thats V2 on the schematic you're working from then I see the confusion. So lets agree to call the tube with all the tone bits V2 from here on.


What does it do if you jump pin 2 to pin 7 on V2, the tone amp tube?