Sansui QRX7001 repair

[par4ken]

What happens if I set the bias lower than the recommended?

You will get a bit of crossover distortion, a raspy sound most noticeable with low volume settings.

The bias is there to partially turn on the output transistors to prevent that condition from happening.

 

[SPXER]

I let the 7001 idle for about an hour and set the bias again. It did not drift much from yesterday.
Here is the "current" condition. The unit will start fine if the volume is at zero. With the volume at listening level, it will go into protection. Also, believe it or not, if the rear treble is turned all the way up, it will trip on overcurrent also.
Starting with the volume at zero and letting it set for a while, it works ok thereafter.

 

[par4ken]

It could be something acting up on "The Protector Board". Check the components, most suspect would be the electrolytic capacitors.

 

[SPXER]

I am ordering caps now. The current draw is real. The DBT and amprobe agree.
So why the hard current at startup? I keep going back to the PS recap. This issue did not exist before.
Let me ask this. What current should an amp of this size pull at startup? Maybe it is not unusual, and you (par4ken) are correct.

 

[par4ken]

I am ordering caps now. The current draw is real. The DBT and amprobe agree.
So why the hard current at startup? I keep going back to the PS recap. This issue did not exist before.
Let me ask this. What current should an amp of this size pull at startup? Maybe it is not unusual, and you (par4ken) are correct.

There will be more of a surge at turn on, not a problem as long as the diodes can handle it. You increased the capacitance by 50% so logically the current should increase by the same amount, a bit more if the old capacitors were below specified value which often happens as they age.

That surge at turn on should not affect the protection circuit. It is monitoring the amplifier outputs.

 

[SPXER]

That surge at turn on should not affect the protection circuit. It is monitoring the amplifier outputs.

That is what I thought. I was thinking the surge was going throughout the amp. It does not surge with the volume pot at zero.
It was clicking out the same way when the driver boards were at fault. (not only at startup)
Order for many caps will be sent by Monday. I will start on the protection board.

 

[SPXER]

I recorded the current at startup at the fuse holders, F1, F2, F5, F6. The same fuse holders used to set bias. The current at F1=1172ma, F2=1008ma, F5=720ma, F6=856ma.

 

[barfle]

I let the 7001 idle for about an hour and set the bias again. It did not drift much from yesterday.
Here is the "current" condition. The unit will start fine if the volume is at zero. With the volume at listening level, it will go into protection. Also, believe it or not, if the rear treble is turned all the way up, it will trip on overcurrent also.
Starting with the volume at zero and letting it set for a while, it works ok thereafter.

Hmmm. Not being familiar with that circuit, it seems like something ahead of the volume control is simgle-ended (probably only a positive power supply), and it’s giving a DC pulse while its coupling capacitors charge up. Check the signal on startup on the volume control to see if it has a bump on startup, then follow it back.

 

[SPXER]

Hmmm. Not being familiar with that circuit, it seems like something ahead of the volume control is simgle-ended (probably only a positive power supply), and it’s giving a DC pulse while its coupling capacitors charge up. Check the signal on startup on the volume control to see if it has a bump on startup, then follow it back.

I follow what you are suggesting, but I don't know exactly where on the circuit to start. At the input of R05-R08 would be a convenient spot. What would a "bump" look like on my meter? Here is the schematic of the F-2425 tone control board. Thanks for the help.

 

[SPXER]

I don't know what happened, but the amp has stopped the overcurrent. I can't say I did anything. I have just tweaked some switches and pots around. Now I can't reproduce the issue. Which is good, but very unnerving as I wonder if it will return.

 

[barfle]

I follow what you are suggesting, but I don't know exactly where on the circuit to start. At the input of R05-R08 would be a convenient spot. What would a "bump" look like on my meter? Here is the schematic of the F-2425 tone control board. Thanks for the help.

View attachment 98518

Well, that's definitely a single-ended quad of circuits. There's a positive power supply for each one (looks like 24VDC?), and when the circuits are powered up, the voltage at, for example the left side of R901, 902, 903, and 904 would start at zero volts, then rise for a while, hopefully just a fraction of a second, then drop to zero. But if you ended up with it working, I'd probably say "stop fixing it."

Edit: You asked about VR05-08. Are those the volume controls? The schematic says it's the tone controls, so I would guess that those ganged pots are tone controls, but then there are very few reactive parts around them, so yeah, they might well be volume pots. Interesting that they're all set to cut the input level, and three wired in series is odd.

But I'd check each of those outputs (R901, etc.) for a turn-on surge, if there's still any issues.

 

[SPXER]

Still good today. I would rather suffer with an issue until I found it, than have it mysteriously disappear and not know what it was and if it may reappear.

 

[MidiMagic]

I don't know what happened, but the amp has stopped the overcurrent. I can't say I did anything. I have just tweaked some switches and pots around. Now I can't reproduce the issue. Which is good, but very unnerving as I wonder if it will return.

Possibly the wiper on a pot had oxidized into an open state (slider not contacting the resistance element). Moving it restored normal operation by removing the oxide.

 

[SPXER]

Possibly the wiper on a pot had oxidized into an open state (slider not contacting the resistance element). Moving it restored normal operation by removing the oxide.

Could be. The pots are remarkably quiet considering its age. I wonder if I should even bother with cleaning them.

 

[Doug G.]

I never clean pots until they need it.

Doug

 

[SPXER]

Edit: You asked about VR05-08. Are those the volume controls? The schematic says it's the tone controls, so I would guess that those ganged pots are tone controls, but then there are very few reactive parts around them, so yeah, they might well be volume pots. Interesting that they're all set to cut the input level, and three wired in series is odd.

But I'd check each of those outputs (R901, etc.) for a turn-on surge, if there's still any issues.

I take the VR01-VR12 to be volume and balance. VR13-VR20 look like tone.

 

[barfle]

I take the VR01-VR12 to be volume and balance. VR13-VR20 look like tone.

Good eye. I agree.

 

[SPXER]

I got a delivery of another box of caps. I replaced the caps in the protection circuit after all the Thanksgivings festivities were over. So, how does one test the protection circuit safely? Or do I just assume it is good?

 

[MidiMagic]

I take the VR01-VR12 to be volume and balance. VR13-VR20 look like tone.

Careful if you are in the decoder area. A number of pots calibrate phase in the psi shifters. If you mess with them, you need an X-Y oscilloscope to get them right again.

 

[mandrix]

I know you want to find what the problem is/was. But sometimes good enough is good enough.
Something will/may eventually fail on these old boards but I think you're ahead of the game at this point.

I don't know how far meters have come these days, I'm still using an old BK Precision 2831A.
Might keep your eye out for a deal on a 'scope.