Pioneer SA-6700 Blowing fuses

[redpackman]

Got a "garage sale" SA-6700 today. Fuses blow as soon as the power is turned on. Checked the transformers, as best I could. Doesn't seem to be a dead short in either primary...around 3 ohms resistance. Then I checked the diodes (D21 thru D24) all GP-20D. All of them were open and failed the test. I've ordered replacements from Digi-Key.

With ALL the above mentioned diodes removed from the circuit, I can turn on the power and the fuse doesn't blow. That's progress.

My question: OK, what are the chances I've found "the" problem, or have I just discovered one of the symptoms and as soon as I power up I'm going to get more blown fuses and/or blown GP-20D's, or other surprises?

I will use a dim-bulb tester on power-up to lessen the flames that may result.

Any insights? Any advice?

 

[markthefixer]

The SA-6700 has a crude protect system, it is INTENDED to blow the main fuses

Temporarily remove THRYSTOR D26 to stabilize things for repair. Do NOT connect speakers until everything is working correctly and D26 has been reinstalled.
The amps are probably putting out too much DC, start by checking the power supply voltages - both regulated and unregulated.

 

[redpackman]

The SA-6700 has a crude protect system, it is INTENDED to blow the main fuses

Temporarily remove THRYSTOR D26 to stabilize things for repair. Do NOT connect speakers until everything is working correctly and D26 has been reinstalled.
The amps are probably putting out too much DC, start by checking the power supply voltages - both regulated and unregulated.

So, if I understand you correctly, I should replace and install the four diodes that are now blown (open), and then remove D26. t

The SA-6700 has a crude protect system, it is INTENDED to blow the main fuses

Temporarily remove THRYSTOR D26 to stabilize things for repair. Do NOT connect speakers until everything is working correctly and D26 has been reinstalled.
The amps are probably putting out too much DC, start by checking the power supply voltages - both regulated and unregulated.

First of all, thank you for your response.

So, If I understand you properly, I'm to replace and install the four diodes that are now blown, then remove D26 and then using the instructions on p. 15 of the service manual to adjust the DC voltage as described. Then once the DC voltage is properly set one can reinstall D26 and hopefully things won't blow as they have.

Yes, that is a different set up than I've seen before

One more question: what is the purpose of cutting the jumpers after the DC voltage is set?

Thank you again for responding.

 

[redpackman]

Update. I got the diodes and installed them. I removed D26 and using a dim bulb tester turned it on (8 ohm load on speaker terminal, aux position on selector, volume turned all the way down). The 60 watt bulb glowed slightly and dimmed almost completely. Good!! Nothing blew. I checked the voltages at the test points described p. 15 of the service manual. They seemed to settle in at 15 mv in the left channel and 21 mv in the right channel. I reinstalled the D26 and still connected to the dim bulb tester I flicked the switch on. The bulb burned BRIGHT. I immediately shut it off. Nothing burned out, no blown fuses, but I think that's probably because I only had it on for literally a second.

Suggestions? It seems I must have a dead short. Could the short be D26 itself? Everything seemed to behave quite nicely until it was reintroduced.

 

[redpackman]

Thank you.

 

[markthefixer]

edit - been working on this on and off all evening - your D26 could be shorted too.



The PROTECT CIRCUIT is blowing the fuse with D26 ON PURPOSE because it is NOT HAPPY or it is BROKEN.
Since there is no relay in this model of amp to disconnect the speakers from the amp to prevent fires with dangerous failures,
they stuck in a "last chance" circuit (D26) to blow the main fuse to prevent HOUSE fires.

You are treating the symptoms, NOT the cause. I didn't say to put D26 back in. LEAVE IT OUT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

You have a dangerous partial understanding of the circuitry.

To answer your question:
Cutting the wires is an idle current adjustment. An increase in idle current.

I said to:

start by checking the power supply voltages - both regulated and unregulated.

I said nothing about checking idle currents (P15 ) - It could have IMPLIED checking the amplifier output voltages.

(I am not in here frequently enough for handholding an impatient repair anymore)

Here's some EXPLICIT instructions that will apply whomever assists you:
1. do not reinstall D26, nor use a Dim Bulb tester YET.
2. do NOT NOT NOT connect SPEAKERS
3. measure and post all regulated and unregulated DC voltages - see if someone else has made a list of them and the probe / measuring points
3. it looks like +40 (Q17 e), -40 (Q16 e) regulated and +40, -40 unregulated at the big capacitors
4. measure the DC output voltage AT the speaker terminals.
4. The DC output voltage CANNOT be adjusted
4.The DC output voltage should be less than 0.025 volt positive OR negative on a good unit that is working well
4.if The DC output voltage is greater than 0.025volt, but less than 0.100 volt it is a problem to be investigated
4.if The DC output voltage is greater than 0.100 volt it is both a severe problem and is starting to get dangerous.
5. For anyone else helping:
5. The protect circuit is just a DC sense, with q19(2sa904a > ksa992) and q20 (2sc1914a > ksc1845) and a few electrolytic capacitors driving D26's gate.

get good power supply voltages first. then get near 0.000v at the speaker terminals - only then can we start looking at the protect circuit.

 

[redpackman]

Thank you for your reply. Most of it, I'm already observing. I know, too, that speakers can be destroyed by a rogue amp, so they've not been connected.

I am a prime example of the wisdom of your motto: "Nothing is ever completely fool proof, because fools are so ingenious."

With D26 out of the amp and plugged into AC without the dim bulb tester I got the following voltages.

Pins 10 and 14 are the commons back to the transformer.
Pin 11 31.15 v AC
Pin 12 31.2 v "
Pin 13 42.4 v "

Around the large caps I get readings on one of -41.5v DC (of course) and the other is 41.8 v

At the speakers I get readings (with the amp on, volume at zero, set on AUX and speaker set A on: Right channel about 9.6 mv but it floats between 8.5 and 10.5. Left channel 11.3-11.7 mv with less floating.

One other thing I noticed on the thrystor D 26 (removed from circuit), there is total continuity (O ohms) both ways between legs 1 and 2 of the component. The data sheet on this component tells me that these two legs are the terminal legs of the thyristor. Is that normal? I was assuming that this acted like a "switchable" diode which meant that there would be continuity in one direction at a time, but not both ways at the same time. The data sheet tells me, of course, the third leg is the "gate." And, yes, I am that much of a novice, but I'm learning.