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Sansui 990 intermittent noise, squealing, cracking sound on one channel

rschweitzer

rschweitzer

Audio Fuse
Subscriber
Hello audiophiles,

Have a Sansui 990 receiver that I am restoring for a customer. The unit exhibits an intermittent noise, squealing, cracking and popping sound on the left channel. I cannot tell if this was one of the original problems because this unit had serious distortion issues when I got it and it was generally noisy with very dirty controls.

Here is what I have accomplished:
  • This problem is so intermittent that it could be 30 seconds to 30 minutes to an hour before it shows up and it is only the left channel and only lasts for 5 to 10 seconds. My scope shows this as square waves in nature, positive and negative.
  • The distortion problems were due to the fuse resistors on the 2436 driver board; several were marginal in resistance and a few were almost open. Replaced those defective components with metal film resistors. Also, replaced the input/output capacitors with Nichcon/Wima.
  • I did have most of the driver transistors, zeners and diodes for the 2436 driver board. And one by one, swapped those. Reduced the overall amp hiss slightly, but did not change the overall problem I am left with.
  • I eliminated the other circuits such as the tone board, volume, etc., by removing the jumpers on the main in/out to drive the power amp only and I have no signal at the inputs.
  • I also swapped the left and right channel driver transistors and replaced the thermal grease and MICAs while I had them off. This did not solve the problem. Thus, I assume that it is on 2436 driver board.
  • Since I was restoring this beauty, I went forth to replace aging capacitors, components, relay and lights throughout the rest of the receiver. Cleaned all the controls. Everything else is fine.
  • Power supply main caps (6800uf, 71v) and bi-polar used at the fuses (4.7uf, 80V) PS caps are on order. The existing 6800uf caps are fine, but since I am in the unit, they will get replaced.
  • Offset and BIOS are dialed in.
I know I am overlooking something that is probably fundamental, hence need some additional TLC!

V/R,

Rick
 
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Reading your description I am wondering whether this is a repeat of a problem I have seen a few times now in Sansui amplifiers, namely, intermittant internal connections in small signal transistors. Sometimes you can reproduce the fault with careful 'tapping' of the chassis or PCB's with the blunt end of a screwdriver. Other times you need to gently nudge each transistor on the driver board for example, to get a reaction and find the offending transistor. Other times carefully directed squirts of freeze spray helps with tracking down the offenders. I have only found one transistor in each case being responsible for this issue, but once you get the offending transistor out of the board, you can sometimes pull gently at one of the legs and feel a click, as the lead makes and breaks contact. I know it sounds a bit far fetched, but once you have experienced it for yourself it sticks in the memory.

Whatever it is, it does sound like a transistor problem, maybe not exactly as described above, but with the same resolution - by changing transistor(s). Please don't take this as a signal to 'shotgun' all the driver board transistors, sometimes this can plunge you into a world of problems. See if you can get closer to the location of the problem, by some of the techniques above, and any others you can dream up. ;)

Good luck.

BTW: Welcome to AK! and especially to the Exclusively Sansui forum. ;)
 
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The driver transistors with the Uniwatt case are known to be problematic and should all be replaced anytime a Sansui piece has them. They look like this:
MPSU51.jpg

Also use freeze spray. Spray a small amount on each transistor, one at a time. The noisy one will show up.
 
I found the sources of the little "gremlin" running around the left channel of that amp. Turned out to be one of the 10 amp fuses that drive the main B+ voltages (-61V and +61V). The scope cannot tell a lie. I saw the noise prior to the amp. So, problem solved.

Lesson learned. Pay attention to the checklist and do not skip the step of replacing 40 year old fuses. They may look good. But chances are they are not.
 
A fuse is either good or bad. It can't sort of work or cause noise. I think you will find it still makes the noise eventually, unless maybe the fuse or holder had some corrosion on it.
 
Dr Audio,

You are correct it is still acting up and just to make certain I was not loosing my mind, I did a direct in to the amp, removing the front end tone circuit as a potential (removed the main in/out jumpers).

I have been running the amp all morning, about 4 hours and not a peep. Last might night however, it made noise. Cycling power did nothing. Then it went away.

As to the driver transistors, yes replaced those. I checked the every resistor and diode on both channels to make certain they were identical. I also replaced 4 transistors (TO-220) in case they were bad. They were not. In addition, using 10x magnification eyepieces, insured that each solder joint was clean and solid. So, the driver board is clean. Will keep monitoring.
 
There are at least a couple known noisy transistors on the preamp board too, they also tend to. "Screech". Ccant remember offhand what number, but worth a peek!
 
Finally got back this after the holidays, found some cold solder joints around the fuses on the 2436 driver board and it ran for days, nice and quiet.

Well it is back on my bench after being returned and it is squealing and making noise intermittently and on both channels. Every transistor was replaced, bias voltages for each stage are correct as noted on the factory schematic, hot or cold temp does nothing.

The o scope is showing noise at the metal film resistors that replaced the fuse resistors of the same value. As noted earlier, front end is out of the loop, jumpers removed and only the power amp is being driven. However, I need to double check that there isn't an op amp still in the signal path from the EQ board 2541, even though the schematic says there isn't. Will post my results when found.
 
Pre-amp is not the complete issue. The protector board, 2547 left and right channel, R13 and R14, 4.7 ohm resistors are bad. They are discolored and measure at 6.2 and 10.3 ohms. Replaced with metal film 1/2w. R 12 was good, 1.5 K, but cracked in appearance. Could not see this unless viewed under magnification. Mylar caps are fine. I had already replaced the relay and electrolytics that were marginal.

Squealing noise appears to be gone. Think I am still hearing some scratchiness in both channels, not at the same time and very intermittent.

WOW!. I know the amp sounds awesome, but this amps negative feedback design makes it tough to find where an issue starts. Time to go back to the beginning and trace. One thing is certain, the initial distortion on both channels is gone,
 
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Last follow up. Replaced TR 5 and 6 along with 9 and 10, dialed in the bias and now quiet. Turns out the replacements for those transistors that i purchased were no good and the originals were obviously bad to begin with. Replaced with KSA1220A and KSC2690A.

Pre drivers, TR 1-4, although appeared fine were replaced with NOS 2SC1708.

Much thanks to the responses and additional data found elsewhere on AK.
 
Replacements for 5, 6, 9 and 10 were NTE equivalents. Won't mention the outfit I purchased from. The specs were correct. But after reading a few blogs, I found the ON semiconductor devices, better specs, higher current rating and gain. The ON semi's come from Mouser.
 
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