Sansui BA-5000 Oscillation

[djoffe]

C714 & R736...did you change that on both channels?

are both channels now oscillation free?

[EchoWars]

Originally posted by djoffe
C714 & R736...did you change that on both channels?

are both channels now oscillation free?

Originally posted by EchoWars
I'm still waiting on the remainder of the parts to confirm that I have the issue licked, but confidence is high.

Won't know till tomorrow night.

[BeatleFred]

Hi EW:

Its nice to be in touch with you again as well. I still have both of my BA-5K's, virtually in as-new condition, atleast cosmetically they are. I imagine some components inside could benefit from replacement due to age if I were to examine the amp internally.
But for now, the amps seem fine, and I'm glad I was able to obtain both of them several years ago from sellers who maintained them. I did have a third one for a while but sold it to an AK member, not sure if he ever had you work on it after he got it. The third one I used to own was a bit scratched-up, but played fine.

I'm not sure why the fan circuit seems to get so tampered with. I do know that both of mine can get quite warm to the touch after its playing music for a while, but its possible the amp just gets a bit hot naturally due to its inherent design. I suppose it might be worth a try to use an infrared thermometer on it to see how hot it gets (such as a Tenma 72-820). I do know that Sansui originally designed the fan to only come on after a specific temperature was exceeded, not to come on automatically as soon as the amp is turned on.

In case you're interested: I was reading a review of the BA-5000 in a 1977 issue of 'Audio' mag where the amp was praised highly, but the writer did make some comments about Oscillation:

"The amp is not well stabilized- when you drive it into clipping, it oscillates. I believe this is due to the fact that the second differential input amplifier's & pre-drivers f t's occur at the same frequency which produces a phase shift in the open loop response of the amp. However it is not bad and doesnt seem to have any noticeable effect on the sound. Another possible problem is the circuit uses 100% dc fdbk. This is because there is an auto-transformer in the output and any small dc offset may cause excessive current to flow through the auto-transformer because it has very low resistance at DC and saturates its core. The 100% feedback requires the use of a blocking cap in the shunt leg of the fdbk loop. When you impress a non-symmetrical unidirectional signal across the blocking cap, it develops charge. When this happens, it pulls the input of the negative side of that differential amp to ground, causing the outpu of the amp to swing positive. Every time this happens, the feedback tries to catch it, and this causes a pulse inside the amplifier. The bias points are no longer fixed and wander around. Only totally direct-coupled amps are free from this, but thats pretty esoteric."

Basically, the writer feels the BA-5K is a top notch amp, but its a little more sensitive to grounds because of the way theyre stabilized.

Anyhow.., yeah... be careful not to get a hernia, I certainly know how heavy they are.

Regards,

B/F

[EchoWars]

Yes, nice to see you too.

As far as your post, I've never had stability problems with a BA-5000, and the reveiwer's comments about the second diff stage and the pre-drivers is a little off, since the pre-driver stage is properly compensated and the unity-gain frequency is then mostly irrelevent.

Originally posted by BeatleFred
I'm not sure why the fan circuit seems to get so tampered with. I do know that both of mine can get quite warm to the touch after its playing music for a while, but its possible the amp just gets a bit hot naturally due to its inherent design. I suppose it might be worth a try to use an infrared thermometer on it to see how hot it gets (such as a Tenma 72-820). I do know that Sansui originally designed the fan to only come on after a specific temperature was exceeded, not to come on automatically as soon as the amp is turned on.

The stock thermal switch is set to close at 70°C, which is damn hot. I have not been able to buy these 70°C devices, but have been using 60°C devices instead. Even at 60°C, you'd swear the amp is gonna melt down before that fan finally kicks in (I run the amp at about 50W output for about 25 minutes solid to get the fan to come on). So suffice it to say that unless you are really pushing the amp hard for an extended period, the chances of seeing the fan come on is pretty low.

Take care my friend.

[stocktrader200]

I would ground the input to the amp (at 2sa798) and see if it is still oscillating. The schematic shows several gain stages before the output amp. they could be the source of the problem.
I replaced (both) the TA7136P in my pioneer Sx3700
with a single op275. works perfectly.

[EchoWars]

I have grounded the input at the diff pair, and the oscillation continued. The TA7136P is used for the power meter amplifier, so an upgrade here is meaningless.

The rest of the parts for the Zobel filter came in last night, and were installed into the second channel. No oscillation whatsoever. Looks like this problem is licked.

[noveltone]

EW,

Nice work. I know at some point you said you returned it to original condition. Will you go back and reinstall all the new caps, transistors and zeners?

Collin

[BFNY]

Originally posted by EchoWars

The rest of the parts for the Zobel filter came in last night, and were installed into the second channel. No oscillation whatsoever. Looks like this problem is licked.

Did testing the removed parts reveal anything? Hard to believe resistors went bad on both channels. How about the caps? They were electrolytic, right? Can/did you measure the ESR? I would put my bets there. On old caps the ESR goes way up as they go bad.
Bob

[EchoWars]

Didn't see this post till now. The resistors measure OK, but were replaced anyway. The caps are cheap Mylar jobs, and the value of both is way off...supposed to be 0.1µf, and are reading about 700pf.