Asymmetrical amplifier output on sine wave - LR-1500TA

[tabpat]

First of all, still learning things. That said, I have a Lafayette LR-1500TA that works - for the most part. I've recapped it and deox'd it. Gone through bias current adjustment and balanced the voltages. It sounds fine, up to mid-level. At some point as I raise the split-volume control, the right channel walks away from the left. The left begins sounding, well... gritty.
I hooked up my signal generator and get a clean waveform on the right, but the left goes asymmetrical - but not like when there is clipping from one of the two outputs. I'll attach an image.
Where do I begin? Is this usually a weak transistor or a power supply issue? Resistor? I'm new enough that I've never had to deal with this yet.

 

[sregor]

Could be lots of things. First question is load or no load?
My first guess would be that amp is cap coupled, and problem is offset/center voltage related. Semiconductor problem is likely, but could be bad resistor. Power supply is unlikely, as it is only one channel. I'd start with voltages in the amplifier section - need schematic and voltages, or some circuit reading and calculations. Good luck with it.

 

[tabpat]

Under load and this is not at clipping. All voltages are within the specs that were given. It's circa 1970, so it's got silicon transistors, but half of the resistors are those really old style carbon style. I guess I'll start poking around more as time permits.

 

[sregor]

One thing may help - usually when there is clipping, signal tracing will find a point before the problem where the signal will go high (or low) because of feed back trying to compensate, and after the problem, it will be like the output side.
Also, if they have current limiting (look for small transistor connected to emitter resistor going back to driver) could be causing problem, but I don't think they were common when this model was current. My 2 cents

 

[llwhtt]

What does it look like unloaded? If it looks OK unloaded the problem should be in the output stage after the drivers.

Craig

 

[Rolf Zetterberg]

This Lafayette has an unusual form of current limiting the output transistors.
Presets RV507 and RV508 finetunes a voltage that affects Q514 and Q513.to clip.
If they are wrongly adjusted,the power stage might clip unsymmetrically.

 

[dshoaf]

You can also asymmetrical clipping if DC Offset is not centered. In that case, one side of the wave form will flatten before the other. Post a schematic so some of the arm-chair bench techs here can gander a bit.

Cheers,

David

 

[sregor]

Schematic is available at hoifiengine.com (free but must be registered) https://www.hifiengine.com/manual_library/lafayette/lr-1500ta.shtml

 

[petehall347]

interesting circuit . only ever seen squelch on a cb radio . 2 balances . one appears variable feedback .the other setting the clipping .or approx half rail voltage . plus more settings . minimum 2 multi-meters if i was doing it . i might start with dc balance and bias .

 

[gadget73]

Similar problems on my Sherwood of the same era were centering / dc balance related. In my case it was off value parts causing the trimmer to not allow centering.

Also not impossible the emitter resistor on one side has drifted and you aren't getting full current out of the positive side output transistor. Unloading it would confirm if its voltage or current related. Voltage problem likely will not fix itself with no load, current problem will.

 

[tabpat]

I've set the ma setting and adjusted the voltage via scope for symmetrical clipping under load. I have not tested it without load, though. If I have time, I'll try that tonight. Thanks for all the input!

 

[tabpat]

OK, an update (of sorts). I replaced the outputs on that channel and it cleared up *most* of the issue. Now I'm fighting something else, but I thought it was part of the original problem. The right channel is much stronger than the left. I tested voltages and everything was fine. I injected a 1Khz tone and checked the voltage going into the amp board audio-in pin. They're way off! One would be at 60mv and the other would be in the 20mv range. I ohm'd out the volume pots and they are near perfectly matched. If I'm reading this right, all signals enter the preamp board, then go into the volume pots, then up to a bass pot/board and then a treble pot/board. From there, the signal goes straight to the amp. They've been recapped, but there is one JFET 2SK30 on each board. I'm wondering if one of them has gotten weak. It seems like the low end is affected more than the rest. These look like nightmares to pull since they are physically joined together using some kind of flimsy rail setup. Ugh...