Pioneer SA-500 Restoration Help Bad CH2

[misterc57]

Purchased a dirty PIoneer SA-500 to restore. Thought this would be a fun project. My SS debugging skills always need help.

Cleaned it inside and out, sprayed pots and switches. Brought up slow on a variac.

I noticed that the current meter kept randomly pulsing while bringing it up to full AC voltage. At full AC the current would swing between 0.5 and 0.6 A (seems too high a current?), with sometimes a wider swings on the low side.

The output transistors on CH2 were hotter than CH1. About 145 degrees vs 100 degrees. Reading 42 VDC at B+ of the main board (W15-055) which is at spec. Eventually the current dropped to zero and I later found that the CH2 1A fuse had developed a very high resistance.

It seems that CH1 is good so I hope to be able to compare readings from each side. I am not convinced that the final output transistors are at fault as I seem to have identical resistance readings.

Schematic attached hfe_pioneer_sa-500_schematics.pdf

Greatly appreciate comments, suggestions and advice!

Thank you, MC

[misterc57]

and now I find that there is DC on both outputs. I can dial them down to 2 VDC

[g1]

The amps use single ended supply, so the outputs sit at half the supply voltage and there are caps that couple to the speakers (C15 and C16).
With no load connected you might see some DC there, but it should discharge through some high resistance used instead of a load (try 10K or something).
Or were you seeing that 2V with load connected? If so, those caps must be bad.
Not sure how you changed the DC on output, but you should set it back. There is probably an offset adjustment that is used to set the output at half the supply voltage.

The current surging and hotter output transistors might be from a dirty bias adjust pot, and bias should be set to correct level.
There is probably a better service manual that includes the adjustment procedures.

[Helmholtz]

Outputs are AC coupled as always with unipolar supply.
Without a load you will see the emitter voltage of the input transistor at the output.
Connect a dummy load and check again.

It is not a good idea to use the trimpots to lower output DCV.

Edit: Missed post above.

[misterc57]

Yes I had mistakenly checked for DC output without a load. I reset the outer trim pots back to where they were, about middle.

After adding a 10 ohm dummy load to each channel there is no longer enough DC output to be concerned about. Note that the current is still randomly pulsing.

I read the following in another thread...The trim pot with the long shaft is the one for the Bias adjustment. Since the heat sinks are very small, I would adjust these trim pots to have no more than 5mV across the white resistors close to the fuses.

CH 1 is compliant across the resistor (which I think is R39 10 ohms 2 watts), CH 2 is at about 0.8 VDC across that resistor (which I think is R40 10 ohms 2 watts). Those Bias trim pots (with the white body and longer shafts) which I think are VR3 and VR4, neither of them will budge.

I could use a better service manual if anyone has one.

[g1]

Yes, VR3 and VR4 are the bias trimmers. The schematics in attached manual are much clearer.
The trimmers may be locked with glue or sealant, and if the current was fluctuating, they may need a shot of cleaner.

[Helmholtz]

I read the following in another thread...The trim pot with the long shaft is the one for the Bias adjustment. Since the heat sinks are very small, I would adjust these trim pots to have no more than 5mV across the white resistors close to the fuses.

CH 1 is compliant across the resistor (which I think is R39 10 ohms 2 watts), CH 2 is at about 0.8 VDC across that resistor (which I think is R40 10 ohms 2 watts). Those Bias trim pots (with the white body and longer shafts) which I think are VR3 and VR4, neither of them will budge.

You can't have any DC across R39/40. These are part of the Zobels and have nothing to do with bias.

Bias is typically adjusted for some voltage across the emitter resistors of the outputs (R31,32,33,34).
5mV across 0.5R would mean an idle current of 10mA.

[misterc57]

Yes, VR3 and VR4 are the bias trimmers. The schematics in attached manual are much clearer.
The trimmers may be locked with glue or sealant, and if the current was fluctuating, they may need a shot of cleaner.

Thank you for the better schematic. So do I force VR3 and VR4 with a screwdriver?

[misterc57]

You can't have any DC across R39/40. These are part of the Zobels and have nothing to do with bias.

Bias is typically adjusted for some voltage across the emitter resistors of the outputs (R31,32,33,34).
5mV across 0.5R would mean an idle current of 10mA.

Now that I have a better schematic, I meant R33 and R34
CH 1 is compliant across the resistor (which I think is R33 0.5 ohms 2 watts), CH 2 is at about 0.8 VDC across that resistor (which I think is R34 0.5 ohms 2 watts). Those Bias trim pots (with the white body and longer shafts) which I think are VR3 and VR4, neither of them will budge.

[Helmholtz]

I would check/replace the small ecaps.
A bad cap might cause oscillation.

BTW, 0.8V across the 0.5R emitter resistor means an insanely high bias current of 1.6A.
Measure R34.

What DCV do you measure across R32?

[g1]

The board layout on pg.15 of the pdf I posted shows which resistor is which.
If you can see any glue or fixative on the bias trimmers, try to break it away or crack it first. Otherwise, you will just have to try to force them.

[The Dude]

Before you just crank on them, it might be a good idea to spray some Deoxit or similar in them first to help them break loose.

[misterc57]

I have sprayed VR3 and VR4 and see no obvious glue.

VR3 and VR4 are 100 ohm VR

Here are some interesting readings.

VR4 resistance = 760 ohms
across R32 1.0 V drop
across R34 1.0 V drop

VR3 resistance = 68 ohm
across R33 just a few mV drop
across R31 just a few mV drop

I am thinking I should remove and inspect VR4 next step?

[g1]

Yes. If it is shot you could replace with resistor around 68 ohm and see how it comes out.

[misterc57]

Yes. If it is shot you could replace with resistor around 68 ohm and see how it comes out.

Good progress. Replaced VR4 with the closest I had (56 ohms, 2W). Amp is working now with stable and very low overall current draw.

The V across R32, R34, R31, R33 are all close and under 5 mV now. All 4 output transistors are running cool (around 76 degrees F).

What wattage rating can I use for a replacement resistor for VR4? I may have a 68 ohm 1/2W.

And what is the final fix? Replace both VR3 and VR4 with a fixed and equal R or does it need to be a variable R?

Any final adjustments needed?

Thank you all!