Pioneer SX-1980 Odd Short

[ivandezande]

I restored a beautiful SX-1980 for a friend a while ago, and unfortunately he shorted a channel by crossing speaker wires while on.
I made some boards to replace the unique output transistors, and they are now set, but what’s odd is that the DBT is still a bit brighter than it should be!
And there’s a very large hum/white noise, equal in both channels.

I removed the +/- 35V wires from the power supply, and the DBT was now fine, the filament just barely lighting up. Hum/white noise is gone. Amp section sounds great.

Those go to the tone, flat amp, and phono boards. I can’t for the life of me know why they would be damaged if the speaker wire was shorted.

Other than replace all the transistors on those boards again, what could be going on?

(Photos are just because I’m proud of the adaptor boards)

 

[mbz]

Reads a bit like a failed voltage regulator/transistor. Will dig up the sm and add more details later tonight

edit Yeah bit difficult to see the connection between shorted spkr wire and hum/noise in 35V rail. Suggest troubleshoot the power supply (AWR-155). Check voltages and waveforms at,
pin 17, 80V
pin 9, 34V
pin 11, -34V
pin 18, -80V

When you see the noise/hum then check corresponding regulators, eg
+34V rail, Q214, Q216

Maybe problem with D212,3 or Q212,3 or...

 

[rcs16]

The brightness of the DBT depends on the AC line current being drawn and the wattage of the lamp. Normally The DBT lights up/gets brighter as you increase the amplifier output stage bias current which you have not provided any info about if you were able to set it properly to spec, which means you have to be off the DBT to do so.
I assume that to start with you have the amps set for minimum ops bias current which you should be monitoring at first power up.
You have not provided any voltage readings so it is difficult to determine exactly what’s going on.
It quite possible that the regulators are not working correctly, thus the hum/noise because the input-output differential voltage is not sufficient enough for proper regulation due to the use of the DBT.
You can determine the current being drawn from the +/-35v regs by measuring the voltage drops across the external voltage drop chassis mounted resistors and dividing by the resistor values, can’t remember exactly what they are 470,560? to determine the currents being drawn.
You can also measure the voltage regulators for mvAC hum and noise but this should be done while off the DBT.
My suggestion is to power up one amp at a time. Check/monitor the ops bias current while on the DBT, see what the minimum is. If it too high there is an issue so do not proceed. If it’s really low then you can short out the lamp on the DBT ( full AC) while monitoring the ops bias current, what is the minimum? Are you able to set it to spec? If it’s over spec, while off the the DBT then shutdown immediately.

 

[ivandezande]

Reads a bit like a failed voltage regulator/transistor. Will dig up the sm and add more details later tonight

edit Yeah bit difficult to see the connection between shorted spkr wire and hum/noise in 35V rail. Suggest troubleshoot the power supply (AWR-155). Check voltages and waveforms at,
pin 17, 80V
pin 9, 34V
pin 11, -34V
pin 18, -80V

When you see the noise/hum then check corresponding regulators, eg
+34V rail, Q214, Q216

Maybe problem with D212,3 or Q212,3 or...

The brightness of the DBT depends on the AC line current being drawn and the wattage of the lamp. Normally The DBT lights up/gets brighter as you increase the amplifier output stage bias current which you have not provided any info about if you were able to set it properly to spec, which means you have to be off the DBT to do so.
I assume that to start with you have the amps set for minimum ops bias current which you should be monitoring at first power up.
You have not provided any voltage readings so it is difficult to determine exactly what’s going on.
It quite possible that the regulators are not working correctly, thus the hum/noise because the input-output differential voltage is not sufficient enough for proper regulation due to the use of the DBT.
You can determine the current being drawn from the +/-35v regs by measuring the voltage drops across the external voltage drop chassis mounted resistors and dividing by the resistor values, can’t remember exactly what they are 470,560? to determine the currents being drawn.
You can also measure the voltage regulators for mvAC hum and noise but this should be done while off the DBT.
My suggestion is to power up one amp at a time. Check/monitor the ops bias current while on the DBT, see what the minimum is. If it too high there is an issue so do not proceed. If it’s really low then you can short out the lamp on the DBT ( full AC) while monitoring the ops bias current, what is the minimum? Are you able to set it to spec? If it’s over spec, while off the the DBT then shutdown immediately.

Thank you guys. Sorry for missing some information.

- Power supply is the brand new remake.
- All voltages coming out of the power supply are correct (or expected on DBT) including the +/-35V.
- With amp sections completely disconnected, the DBT is still too bright.
- Hand in hand with this, again, with just the 35V rails disconnected and the amp boards fully connected, the DBT is at the correct level, just barely lighting up the filament.
- DBT is a 75W bulb.
- Amp sections are set a zero bias, but does bias correctly.

Give me some time this morning to figure out current draw on those rails, but I'm sure it will be too much as the DBT is telling me.

I know the SX-1980 does normally light up the DBT just a bit at idle, but this is more than that.

 

[HPMguy]

Shorted outputs will often fry the driver transistors and sometimes current mirror transistors as well, and the damage can go up as far as the differrential pairs. And, I've seen blown main filter caps! No reason to think the voltage regulator wasn't cooked, or had solder joints soften and crack.

I'd check the ripple on the filter caps for the 35V supply.

Is the hum on all inputs?

That adapter board is quite nice by the way!

 

[ivandezande]

Shorted outputs will often fry the driver transistors and sometimes current mirror transistors as well, and the damage can go up as far as the differrential pairs. And, I've seen blown main filter caps! No reason to think the voltage regulator wasn't cooked, or had solder joints soften and crack.

I'd check the ripple on the filter caps for the 35V supply.

Is the hum on all inputs?

That adapter board is quite nice by the way!

I agree but again, the amp sections are not the issue and have been checked, and the power supply board is a brand new remake from an AK member.

There is zero AC ripple on the 35V rails.

Yes, hum/white noise all inputs all settings all volume.

Thank you, made life a lot easier!

 

[zebulon1]

So the amp(s) sound great but when using the pre it's drawing current and sounds like crap?
Both channels?

 

[mrk229]

Is it possible that under load the + and or - 35 supply is oscillating? I had it happen on an SX-950 using an MJE15034 as a pass transistor, switched out to a "slower" transistor and all was well.

I suppose you would have seen the oscillation while looking at the ripple though.

Nice adapter boards!

 

[zebulon1]

Is it possible that under load the + and or - 35 supply is oscillating? I had it happen on an SX-950 using an MJE15034 as a pass transistor, switched out to a "slower" transistor and all was well.

I suppose you would have seen the oscillation while looking at the ripple though.

Nice adapter boards!

I remember that. And I was wondering about the PS. At least it was the same situation with the oscillation issue in a power supply..
The EQ and Flat amps use a Long Tail pair and could draw some current in an abnormal condition. Enough to see on a DBT.
Most of the time a shorted transistor in the EQ, Flat and Tone amp get masked buy the lamp. I notice them when testing the receiver for function. Speaker pops is the most common trouble before finding the short or backwards component.
If the very large hum/white noise is equal in both channels. Then the PS is the place to start.
Rick's second paragraph in post #3 is spot on.

 

[zebulon1]

If you don’t have a scope, buy one of those ZOYI handheld scopes. I love the freedom and it works well for this older equipment. I find myself using it all the time now. It’s slow but it will reveal most of what you’re looking for. If your new to scopes its perfect for learning on.

 

[ivandezande]

Is it possible that under load the + and or - 35 supply is oscillating? I had it happen on an SX-950 using an MJE15034 as a pass transistor, switched out to a "slower" transistor and all was well.

I suppose you would have seen the oscillation while looking at the ripple though.

Nice adapter boards!

I think you're spot on. I had a 2330B a while ago that same issue, preamp was drawing too much current, it was because I replaced those dang 2SA798's.
Had to increase the ceramic cap next to it.

Guess I'll try!

100pF between each base and emitter. I'll try 220pF.