SX-1980 is in protection when first switched on

[samc777]

A couple of years ago I picked up a SX-1980 in rather rough shape. To be brief the Power Supply Board AWR-154 was almost unrepairable for a rookie like me so it was placed in the back closet and left for another day.

Then txturbo came along with his new board so I took the easy way out and purchased one. The best and easiest decision I've made in a long time. The installation was flawless and in a few hours my SX-1980 was alive.

Alas, although alive it didn't play music.

At first I thought the new board was bad or I had made an installation error but No, that was not the issue. Besides, txturbo tests all boards and guarantees that they function.

Anyway I soon discovered that the Left Power Amp board GWH-109 was causing the receiver into protection. By removing the Lt Amp power plug that contains pins 1, 2, 3, 4, 5. I was able to hear music through the Rt Amp GWH-110. Everything works OK in that configuration.

So, obviously to me at least, I have a problem with the GWH-109 board.

So far I haven't been able to find a fault on this board and I'm finding as I get older that peering through a magnifying glass is a hard way to troubleshoot. I'm looking for suggestions as to where I should use my time to find this fault. I enjoy doing this work but the aging process makes it frustrating when ones eyes have trouble reading the schematics.

Because the receiver falls into protection it removes power from the board and one cannot chase down the problem in the normal way with a multimeter. I've read many previous SX-1980 posts but cannot find one that specifically discusses this protection issue.

Thanks
Dave

 

[rcs16]

It might or might not be on the pcb, it might be one or more of the output transistors(bjt) are toasted, usually a shorted.
Read up on threads regarding the repair of the o/p stage. Learn how to test transistors using a DMM in diode mode. This means to test with the power off (un-plugged from AC) and the main rails have fully discharged by measuring the voltage across the big ecaps.
I assume after you remove GWH-109 the power supply voltages measure fine?

Good luck

 

[zebulon1]

And:
I was wondering if you checked the amp settings after you installed the new power supply.
The amp is powered when the set is in protection. It's just not letting the bad amps - bum output to the speakers.
What are the DC Balance and Idles settings?
Any fuses blown?

 

[samc777]

What are the DC Balance and Idles settings?

Bingo!

Thanks Zeb, the reason I couldn't find anything wrong with the Power Amp board was because there was nothing wrong with the board... A few adjustments to DC balance and idle cured the fault. How it was so far out is a mystery but the previous owner sold it because it no longer worked. This forum is awesome.
DeOxit time coming up as "gently flipping" (Thanks MTF) the numerous levers has uncovered other audio issues.
Thanks guys!

 

[markthefixer]

the +/- 80V voltages were ok when 1-5 plug was connected? pin 1 is the protect signal for problems on the card.

when you change the power supply you also shift the dc offset.

Plug it back in and measure / set the dc offset before proceeding ANY further.
UNLESS

Because the receiver falls into protection it removes power from the board and one cannot chase down the problem in the normal way with a multimeter. I've read many previous SX-1980 posts but cannot find one that specifically discusses this protection issue.

The power is NOT removed from the board in protection. If the +/- 80v goes away, there is a major fault, and the FIRST things to go are the 15 ohm 1/4 watt R105, R107.
Then nothing will work on the gwh-109 board. It gets the MAIN power from the big caps through the transistor side connectors.

edit: grrr (sigh)

 

[samc777]

Plug it back in and measure / set the dc offset before proceeding ANY further.

Yes, that suggestion fixed the protection issue after adjusting to the manual specs, thanks.
DC Balance and Idle current set per the manual on Page 33. How stable and exact should these settings be? Both boards are floating ~+/- 10mV, sometimes more and do not maintain a steady reading at all. I've got no problems hitting the zero and 30mV settings but they wander up and down the scale. I've seen this floating issue on other Pioneer receivers but I have also seen some that hold exactly to the spec. I allowed over an hour of warm up time but the wandering settings continue. Any thoughts?

The power is NOT removed from the board in protection

Upon checking I found that You are correct, I was wrong. Not sure how I came to that bad conclusion.

As it stands now the issue appears to be in Tape Monitor Switch 2. I'm losing the audio signal for the Lt Amp in that switch. Flipping the switch sometimes allows the amp to start working again. Is DeOxit called for here or is their a better solution?

 

[zebulon1]

Sometimes the pre-amp will influence the amps settings.
You can separate the two at the jumpers and see if the readings stabilize.

Your right. If the Deoxit treatment hasn't been done by you, then it probably needs it.
The signal goes through all the pot and switches.
Remember to lube them with "Fader 5" after a day of drying.
The Deoxit might fix the fluctuating amp settings.

How about some images of this crusty beast?

 

[markthefixer]

Settings for DC offset at the power amp output are usually pretty stable - but the idle current is a moving target.
Temperatures of the driver boards, output transistor heat sinks, environment etc all make it fluctuate.

 

[samc777]

How about some images of this crusty beast?

Err, pictures of the Fluke meters showing 46.48VAC with the watt meters at 270 or the face shots showing the multiple buttons and levers?
Yes, pictures will eventually show up when its back together in one piece. The Lt and Rt amps track almost identical from zero to max. I am of course driving into a large resistor bank. Very pleased at this point.

After the DeOx job on the switches I have discovered the Rt Amp Idle will not adjust. VR4 is a new Bourne unit and reads the full 100 ohms adjustment through its single turn. More fun and games.

Dave

 

[rcs16]

You are a crazy nut, running it full open with a undisclosed load and then saying that you are unable to set Rt amp ops idle I correctly. The VAS is a constant current source, calculate the current by measuring the voltage drops across the resistors and devices in the path.