How to Fix Sansui Sansui G-6700 G-7500 G-7700 e G-8500 Oscilation Problems Damage Output

[ghazzer]

Should these test signals be applied to the receiver's input terminals, or to the input connections on the amp board?
And are you monitoring the signal at the receiver's output terminals or at the terminals of the speakers?
I'm assuming that both channels should be tested independently.

Thanks - - -

 

[Hyperion]

Should these test signals be applied to the receiver's input terminals, or to the input connections on the amp board?

I think you should be able to use either, ('AUX' or 'Power Amp in' ?) but if you suspect either the pre-amp stage or the power amp stage you should test them seperately. Or at least check the power amp first, make sure it is stable, then connect the pre-amp, that is if the source of instability is unknown.

And are you monitoring the signal at the receiver's output terminals or at the terminals of the speakers?

I guess you would be using a dummy load at this point, so receiver or amplifier speaker terminals is the answer.

I'm assuming that both channels should be tested independently.

Thanks - - -

Yes independently - if you manage to excite one channel into oscillation, you'll want it to be the one you are monitoring.

 

[dr*audio]

I would use poweramp inputs. You don't want the preamp circuitry to affect the signal.

 

[dapbmw]

Counterfeit parts are often a possibility, but I have been careful, and try to always check my parts. OP TRs come from reputable sources. Components are checked with meters and I use a curve tracer for the other transistors.

IMHO, there is definitely a gremlin lurking in the design of the F-2980 board. My G-6700 thread is ~350 posts over nearly five years and my memory is fuzzy (some things you try to forget) but I remember there being three (or more) events. After replacing parts and thinking "I got it this time" I would power it up and watch the meters for a few minutes, and then something would jerk the smile off my face and take out the OP TRs. Once it actually played for a few hours.

Some knowledgeable people here have suggested oscillations as a possible/probable cause, but I have never observed that at the right instance of time. I think you'd have to be very fortunate to time it just right, spend a lot of time watching a scope, or figure out some way to record it.

For now, adding a couple of caps is an easy way to take a precaution, so what the hell.

Same issues here with the G6700 I replaced both channels outputs with the njw0281/0302.
The driver transistors 2sc1845 = ksc1845 and 2sa939 = tt004 the pairs for each are HFE matched, 2sc2071 = tt003. All 4 100ohm VR’s replaced with Bourne multi-turn. For some reason the right channel 2sb536 and the driver transistors tt-003/004, ksc1845, ran hot and the VR for bias is at the lowest setting and I can only manage it to go down to 8mv with a 100 watt dbt, dc is fine at 0mv.

I added the 47pf caps as per Aleksei’s post dropped down to 5mv with the dbt still in circuit. I dare not plug it straight until I can resolve the issue, I’ve checked all the resistors in the area of that board one end lifted to confirm and they are within spec. One 22pf cap was off beside the bias VR, it read as OL as in its open.

It plays ok with my cheap sacrificial 8 ohm speakers, but it runs hot after a few minutes on the dbt. I’m stumped, so any ideas would be appreciated.

 

[kevzep]

Same issues here with the G6700 I replaced both channels outputs with the njw0281/0302.
The driver transistors 2sc1845 = ksc1845 and 2sa939 = tt004 the pairs for each are HFE matched, 2sc2071 = tt003. All 4 100ohm VR’s replaced with Bourne multi-turn. For some reason the right channel 2sb536 and the driver transistors tt-003/004, ksc1845, ran hot and the VR for bias is at the lowest setting and I can only manage it to go down to 8mv with a 100 watt dbt, dc is fine at 0mv.

I added the 47pf caps as per Aleksei’s post dropped down to 5mv with the dbt still in circuit. I dare not plug it straight until I can resolve the issue, I’ve checked all the resistors in the area of that board one end lifted to confirm and they are within spec. One 22pf cap was off beside the bias VR, it read as OL as in its open.

It plays ok with my cheap sacrificial 8 ohm speakers, but it runs hot after a few minutes on the dbt. I’m stumped, so any ideas would be appreciated.

Can you see it oscillating on the scope?
Also do not boas the amp and run it on the DBT. The DBT is a go/no go test, once you have no shorts, get it off the DBT as it severely limits current and voltage which plays havoc especially in an amp which is very voltage sensitive like these.

 

[DVNOBLE]

Same issues here with the G6700 I replaced both channels outputs with the njw0281/0302.
The driver transistors 2sc1845 = ksc1845 and 2sa939 = tt004 the pairs for each are HFE matched, 2sc2071 = tt003. All 4 100ohm VR’s replaced with Bourne multi-turn. For some reason the right channel 2sb536 and the driver transistors tt-003/004, ksc1845, ran hot and the VR for bias is at the lowest setting and I can only manage it to go down to 8mv with a 100 watt dbt, dc is fine at 0mv.

I added the 47pf caps as per Aleksei’s post dropped down to 5mv with the dbt still in circuit. I dare not plug it straight until I can resolve the issue, I’ve checked all the resistors in the area of that board one end lifted to confirm and they are within spec. One 22pf cap was off beside the bias VR, it read as OL as in its open.

It plays ok with my cheap sacrificial 8 ohm speakers, but it runs hot after a few minutes on the dbt. I’m stumped, so any ideas would be appreciated.

Hi! The capacitors 47pf are to contain oscillation, you have to know if you are heating by DC bias current or by high frequency AC oscillation current. Use a series light with fixture while insecure. It would be nice to have an oscilloscope to check. You can put a 1uf capacitor in parallel with the output if the series lamp lights up strong, there is certainly oscillation in the circuit. Hug!!

 

[Hyperion]

One 22pf cap was off beside the bias VR, it read as OL as in its open.

I hope you replaced this one as it could be you whole problem.