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Old 31st May 2009, 11:03 AM   #1
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Default Sansui BA-5000 Oscillation

Working on my brother in law's new (used) Sansui BA-5000 amp. Bought it knowing there were a few issues, but I told him I've fixed a number of these amps, so if the price was right to go for it.

The main issue was simply some worn-out caps in the input board (the power supply for the small reed relays and temp monitoring circuitry & such). Once that was done, it would engage the soft-start properly, and the small reed relays would fire giving you the 'green for go' LED. The transformer was also making an ugly noise, but recapping the small +/-65V regulated supply (and installing new transistors & zener diodes) cured that.

The problem now is that it is oscillating at the output (both channels!!)...about 10V peak-to-peak, at about 500KHz. I figured there were damaged semiconductor junctions with a few of the outputs, so all 16 of them, and all four drivers, were replaced with new MJ21193 & MJ21194 devices (all emitter resistors and base resistors were checked for proper values). That made no difference.

I also swapped out every transistor & semiconductor on the driver boards (both boards). Depending on the device chosen, the output amplitide and frequency would change (a little, as I got the frequency down to about 400KHz once and amplitude to about 8V P-P), but the oscillation never goes away.

At this point the amp is fully recapped except for the four large power supply caps. One of the driver boards has mostly new ceramics in an attempt to kill the oscillation.

Like I told my brother-in-law, I've fixed a bunch of these amps, but I've never seen anything like this. I'm at a loss as to what to try next. I've reinstalled many of the original devices on the driver boards just to make sure that I wasn't imagining things with the original oscillation problem and perhaps caused it by installing some of my replacement devices (same devices that I have used in the past for several BA-5000's). Problem is still there. The oscillation from the output goes everywhere...back into the regulated supply and from there back to the front end of the amp. It's like a contamination that gets everywhere once unleashed.

Internal wiring looks good, except that a previous tech had done away with a couple of connectors and soldered wires directly to some pins rather than rely on the stock friction-fit connector (I desoldered and crimped on small friction-fit female pins so the boards could be removed without desoldering, as it is supposed to be). Grounds look good.

What's the possibility of the large PS caps or somehow a damaged tranformer playing a role here? (yes, I'm grasping at straws) Anyone think of something else I might try?

PDF Schematic (good till 6/8/09 and 100 downloads...if this expires and someone wants to look at it, let me know and I'll upload it again).

http://www.yousendit.com/download/Mn...ckl6RTkzZUE9PQ
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Old 31st May 2009, 12:32 PM   #2
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I would replace the VAS capacitor.
That usually controls osciliation in a power amp.
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Old 31st May 2009, 12:55 PM   #3
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Talking about C05? It has been replaced. So has C03.

Ceramics are pretty reliable, and since both channels are oscillating (when each board is installed by itself, that channel oscillates) I keep looking back at the power supply, but I've never seen this kind of thing...
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Old 31st May 2009, 03:20 PM   #4
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You could try some 100nf's across the power supply electrolytics.
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Old 31st May 2009, 05:46 PM   #5
djoffe is offline djoffe  United States
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Default more possibilities

on the F-2518 boards, check C15 (0.047 uF) and the 10 Ohm resistor in series with it...R31 I think, but it's a bit hard for me to read the schematic. If R31 (are there actually 2 in parallel?) has gone open, the Zobel network loses its effect, and oscillation can result.

If that doesn't do it, then there are some diagnostic tests we can do with the feedback loop that might help track this down.
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Old 31st May 2009, 11:27 PM   #6
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For Sansui, the '2' in parentheses means 2 watts, so that's a 10 ohm 2 watt resistor. This resistor measures OK. The stock C15 was a cheap little greenie Mylar, which I replaced with a nice 0.047f 400V polypropylene. Made no difference.

When considering an attack plan, remember that both channels oscillate, with either or both of the driver boards installed, so it seems to me that a focus on the individual components of the driver boards (F-2518) might be self-defeating.

But, if you have some ideas my friend, I'm all ears.
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Old 31st May 2009, 11:48 PM   #7
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Quote:
Originally posted by EchoWars


When considering an attack plan, remember that both channels oscillate, with either or both of the driver boards installed, so it seems to me that a focus on the individual components of the driver boards (F-2518) might be self-defeating.
So its either the power supply or a change you have made to both boards.

Could it be you have changed a component to a wrong value on both boards ?
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Old 1st June 2009, 12:22 AM   #8
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Right now, both driver boards have all new electrolytic caps (Panasonic FC and Elna Silmic for the input DC blocker), and a new 2SA798 input differential transistor. One board has all the rest of the semiconductors as original, the other is using Fairchild KSC2682 for TR02 & TR03, and KSA1142 for TR04. The bias servo transistor (TR05) is stock (and was temporarily replaced as a test with no change), and the drivers are On-Semi MJE15032 and 15033. The rest is all stock except for some ceramic cap changes to one of the boards as a test.
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Old 1st June 2009, 11:36 AM   #9
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None of the vet's want to take a swing at this? I'm not asking ya to come over 'n fix it. Just an idea...something I can try that makes sense.
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Old 1st June 2009, 11:52 AM   #10
djoffe is offline djoffe  United States
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Default more ideas...

have you separated the output board from the input preamp? What if the input preamp (F-2520) were oscillating, or passing through oscillations from the LV power supply (F-2521)?

If you can separate the two and the output oscillation goes away...you've narrowed your search considerably...
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