Right channel distortion on vintage fisher rs-2010

[suzook]

I have a vintage fisher rs-2010 that uses the "famous" stk packs. I get sound on both channels, but the rt channel has some distortion. My dc offset on good channel is 7mv, and the distorted channel is 32mv. That shouldn't cause my issues. Any idea where to start? Thanks

 

[Nigel Goodwin]

With a scope and a sinewave generator - see what type of distortion it is.

 

[suzook]

With a scope and a sinewave generator - see what type of distortion it is.

I have a scope, no sinewave gen(can i use a smartphone for that?) Sorry, I'm a newb, that has soldering skills, and such. Guessing 1000khz?

 

[Nigel Goodwin]

I imagine you download an app for a signal generator?.

A quick google shows various ones, such as this:

Frequency Generator - Audio Tools & Ultrasound – Apps on Google Play

Ten free audio tools gathered in one app! Tone generator, sound tests and more!

play.google.com

 

[suzook]

Hopefully this helps! That's directly from the positive output of l, and r channel

 

[Nigel Goodwin]

I take it the top one is the good channel?, as the bottom seems to have some visible 'missing bits' - what range is the scope set to? (V/cm).

 

[suzook]

Yes, that one is clean, the other obviously looks bad. Scope at 2 on 10x probe. No idea where to go from here. Lol. Trace it back?? If so, I don't know where to start

 

[rjenkinsgb]

The amp schematic gives voltages at various points - are they correct on both channels?

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[suzook]

The amp schematic gives voltages at various points - are they correct on both channels?

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So I measured voltages on amp board on all transistors. Most were within 10%. Some though were off more. My issue though is both sides of the board were identically off, so I would expect distortion on both. Maybe my problem is before the amp board?

 

[Nigel Goodwin]

Yes, that one is clean, the other obviously looks bad. Scope at 2 on 10x probe. No idea where to go from here. Lol. Trace it back?? If so, I don't know where to start

That's not very clear - do you mean the scope is set to 2V/cm, then a x10, so it's 20V/cm? - in which case that's 40V p-p, which is about 25W.

Do the same test with the volume turned down, so you've only got 1V or 2 V p-p.

 

[suzook]

That's not very clear - do you mean the scope is set to 2V/cm, then a x10, so it's 20V/cm? - in which case that's 40V p-p, which is about 25W.

Do the same test with the volume turned down, so you've only got 1V or 2 V p-p.

Yes, thats exactly what it was. Will redo test after my beer and swim in the pool! 90 outside right now!
Sorry, still learning how to use a scope. Appreciate all your help.

 

[suzook]

Ok, here is 1v, power at 50%.. Can I check before, and after the speaker relay? I'm thinking bad relay

 

[suzook]

Welp, I got the same signal on the power board. So it's definitely something there. Hmmm

 

[Nigel Goodwin]

Welp, I got the same signal on the power board. So it's definitely something there. Hmmm

What frequency are you putting in?, and it's still unclear what the scope is set to, as you can't see the line on the knob in the picture.

Set the gain control to 50mV, set the probe to x10, and make sure the Red 'Cal' control is fully clockwise (it should almost always be there, and may have a click position), this gives 500mV per division on the scope. Then adjust the volume for 2 squares on the scope, giving 1V p-p.

The chances of a relay causing distortion is extremely low, but you can certainly check both sides of the contacts if you wish.

If you're getting the same signal on the power board?, where on the power board? - as it's only on one channel it can't really be on the power board.

 

[rjenkinsgb]

If you're getting the same signal on the power board?, where on the power board? - as it's only on one channel it can't really be on the power board.

The mute relays are part of the power board.

So I measured voltages on amp board on all transistors. Most were within 10%. Some though were off more. My issue though is both sides of the board were identically off, so I would expect distortion on both. Maybe my problem is before the amp board?

Check the collectors of the upper and lower transistors that drive the STK module with the scope, on both channels?

If the signals there are identical on both channels, the fault can only really be in the STK module, something failed within its bias control presumably..

 

[suzook]

What frequency are you putting in?, and it's still unclear what the scope is set to, as you can't see the line on the knob in the picture.

Set the gain control to 50mV, set the probe to x10, and make sure the Red 'Cal' control is fully clockwise (it should almost always be there, and may have a click position), this gives 500mV per division on the scope. Then adjust the volume for 2 squares on the scope, giving 1V p-p.

The chances of a relay causing distortion is extremely low, but you can certainly check both sides of the contacts if you wish.

If you're getting the same signal on the power board?, where on the power board? - as it's only on one channel it can't really be on the power board.

1000khz signal.

 

[Doug]

Just some thoughts
Is he testing under load with speakers?
Maybe square wave output on scope for testing?

 

[JimB]

What frequency are you putting in?,

1000khz signal.

What?

I would agree with 1000Hz or 1kHz
but 1000kHz No way.

JimB

 

[Lightium]

Why not do a full spectrum sweep?

 

[suzook]

What?

I would agree with 1000Hz or 1kHz
but 1000kHz No way.

JimB

Why not do a full spectrum sweep?

Sorry. 1000hz. And what's a full spectrum sweep? Yes, I'm a newb. I have tools, and some knowledge. This thing is in new condition on outside, would love to get it working properly.