AU-7700 Left Channel - High DC Voltage at output

[Zekeman]

Hello AK!

I have an AU-7700 fully recapped (90%) which will not come out of protect due to high voltage on the left channel.

The pic shows the blue and gray wires connecting the protect section with each channel. I get low current on the R ch but am getting 35V on the Lch...so consequently the relay will not close. If i desolder the gray wire and power up I get relay click and normal play through the R ch.

The schematic on this model has very few voltage specs for the output section so it makes troubleshooting difficult.

The driver board has been recapped the L ch has all new transistors except for one. All the output transistors test OK.

Any advice on troubleshooting this would be greatly appreciated!

 

[Hyperion]

Have you checked R23 & R37 on F-2097 ?

Suggestion - get rid of those old trimmers.

 

[Zekeman]

Yes I can do that but the trimmer is not the cause.

 

[Hyperion]

It could be many things causing this, luckily you have a 'good' channel to compare it to. You are normally very organised with your troubleshooting efforts, if you put together a listing of comparison voltages L & R, perhaps this will lead to a starting point? It is often difficult to find such faults because with DC coupled amplifiers, (as you know), if there's a voltage wrong somewhere it's almost certainly wrong everywhere. Then you have to resort to methodical testing of semiconductors, and resistors, and possible substitution of a capacitor here and there before you find the culprit.

 

[Zekeman]

Yes the comparison of voltages comes next. Yesterday I tested every resistor in the channel and all test OK. All ceramics caps as well.

Stay tuned.

 

[Zekeman]

Some initial voltages at TR17 and 19 on the left channel - a whopping 39V on both the base and emitter of each while the (good) R-ch shows ~1V and 0,4V respectively.

 

[Hyperion]

a whopping 39V on both the base and emitter of each

Have you tested all the transistors? - it looks to me like something that should be pulling current in the earlier stages - isn't - the lack of voltage drop across those resistors should be a big clue - how about the zener? open circuit perhaps?

You should have roughly equal and opposite voltages at C & E of the transistor with C13, 1µF, 50V connected to it (TR07 ?). Something that might prevent this could be TR05 not conducting.

Let us know how you get on.

 

[Zekeman]

No luck tonight...didn't have much time.

TR-05 tests OK. The Zener is OK as well.

 

[sregor]

I'd be checking the voltages around tr05 to get a direction to go. Watch out for gotcha's - C11 positive terminal goes to ground, negative to the - supply coming through r37. If you changed tr05, make sure basing is correct - IIRC the 2sc1124 has base in the center.
That being said, my money is on the input pair, but kind of hard to tell from here. Good luck with it.

 

[Zekeman]

TR-05 is original and checks out OK. C11 is new and installed correctly. I pulled and checked all 4 ceramic caps and they are not shorted.

Incidentally TR-09 was shorted when I initially opened the amp. It was replaced with a BD139G.

I measured more points tonight...

 

[sregor]

Something not right around R37, emitter of tr05. Should be around -39, not 8.5 volts! Most likely r37 is open... should have -40 on the other side. or???

 

[Zekeman]

You're right but still no relay click. R37 had a cold solder joint - note: it was very strange on this unit that the R37 100 ohm resistor was soldered to the bottom of the circuit board! I replaced it with a new resistor installed on the top of the board. However, I still get no relay click and have abnormally odd voltages around TR-05....I'll repost more tomorrow.

 

[Hyperion]

The bias transistor TR07 ? (2SC711) looks like its taken a hit, looks possibly shorted B-E ? but might be due to weird voltages - I still question whether R37 is really OK? - and not entirely happy with the looks of at least one of the input diff pair - but all of this may well be symptomatic of the real fault.

 

[Zekeman]

TR07 was replaced with a new transistor but I’ve been rooting around so much I’ll need to recheck it tonight.

R37 is now a new resistor.

 

[Hyperion]

Ok -

 

[kevzep]

How did you test the Zener? Not saying its the issue, but would still be a candidate.
I would put a new one in anyway, its doing a lot of work and over the years they do become flaky....not something you can test...

 

[Zekeman]

The zener and all other diodes are new.

Tonight I pulled R37 just to confirm its value and installation. Inresoldered it in and powered it up and the relay clicked. Happy but frustrated at not quite knowing what happened here. I adjusted the DC to spec. Still need to adj the bias.

 

[kevzep]

Cracked trace on the circuit board somewhere could be the issue, pulling that resistor may have resolved a connection perhaps?

 

[Zekeman]

This 7700 sounds beautiful. I know the topography is almost identical to the AU-7900 (my favorite Sansui) but this may sound a tad sweeter..?..I must A-B the two.

 

[911s55]

This 7700 sounds beautiful. I know the topography is almost identical to the AU-7900 (my favorite Sansui) but this may sound a tad sweeter..?..I must A-B the two.

Let us know your impressions. I find the AU-7700 presents many instruments and frequencies that my other amps hold back, especially upper mids to lower high frequencies. This is where much of the detail comes from for me without being startling.