(another) 9090 protection light

M

mntnbkr

New Member
I have a 9090 that I use for sound in my garage. In the winter months, (when it's cold) the unit won't come out of protection mode. It's been like that for a few years now and I'm just getting the motivation to troubleshoot and try to repair it. Last time this happened, I determined it was a fault on the driver board. There happened to be one listed on ebay at the time, so I picked it up for like $30 and I was back on my way. Currently there aren't any available, so I'm wanting to fix the one i have, but so far I'm just making it worse. I'm here to seek help form the pros.

Here's what I've done:

I brought the unit inside the house and removed the driver board. The protection light goes green after turning the power on as expected. After reading through the forums, I went through the board with my DMM and checked the resistors. They all tested within spec. I put the board back in the unit just to check it, and the unit wouldn't come out of protection mode (even inside the warm house). Apparently I somehow made it worse, but I have how.

I then removed fuses 7 & 9 from the power supply board and reinserted the driver board and the protection light goes green when powered on. So at this point I believe I've narrowed the problem down to the right channel.

Being that the problem is (was) temperature dependent, I figured it could be a bad solder joint somewhere, so I re-flowed all of the solder joints on the right channel of the driver board. I put the fuses back into the power supply board and reinserted the driver board. Upon pressing the power button, both fuses 7 & 9 promptly lit up and then popped.

I've looked over the board several times with a magnifying glass trying to identify any place that I might have accidentally bridged some contacts with the solder, but I can't find anything. I rechecked all of the resistors and they all seem good. I'm going back and forth between the left channel (known good) and the right channel checking for matching resistance at different points on the board, but I'm really kind of flying blind (I'm more of a 12V mobile audio kind of guy when it comes to fixing things). Anyway, I did find one particularly odd reading. Resistor 35 reads at about 436 ohms while Resistor 36 reads at 1.5K ohms as it should. When I pull R35 out of the board, it measures 1.5K ohms. Also, maybe of note, when I reverse the DMM probes on R36, the reading changes to 813 ohms while the reading on R35 stays the same regardless.

I'm guessing I put too much heat into one of the components when I was re-flowing the solder and cooked it, but I don't know how to find which one it might be. I pulled out TR03, TR11, & TR13 and checked them with the diode setting, and they appear to be fine. As I was putting them back in, one of the traces started to separate from the board, so I didn't want to do anymore without getting some expert advice first. I certainly don't want to make this any worse than I already have.

For reference, I've attached what I believe are the pertinent pages of the service manual.

Sorry for the long post, and thanks in advance for any help you can give.
 

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bad capacitors can start behaving after applying heat .. so can transistors ..
best start would be checking bias and dc offset pre relay ..
 
Does your driver board have glass fuses in vinyl tubing? Most do and if yours is like most, Sansui modified it during production. The schematic you posted will be different from the actual board.

Put the driver board back in. Turn the unit on it's side, transformer down. Remove the bottom cover. There is a blue and a gray wire coming of the bottom of the power supply board. Measure the DC voltage between each of these wires and chassis ground. It should be close to zero. If it's higher than a couple hundred milivolts but less than 1 volt, you may be able to adjust it within range using the trimmers on the driver board. Be very careful to identify the trimmers correctly. Only turn the ones for DC offset. At this point don't mess with the bias current trimmers.

- Pete
 
My driver board does not have the glass fuses. It's an F-2436-1. I think the -1 is the board rev but I'm not positive. As I understand it, the earlier revisions did not have the glass fuses. I do however know of at least one difference between my schematic and the board that I have. The R901 18K ohm resistor between pins 2 and 5 doesn't seem to exist on my board.

I forgot to mention earlier that I had checked the DC offset toward the beginning of this adventure (before reflowing the solder on the right channel) and the left side was somewhere in the neighborhood of 50mV, while the right side was jumping around wildly. I adjusted VR01 up and down at the time to see if I could get it "in spec" but it didn't really make much difference (if any).

Per your suggestions I put the driver board in the unit and went to check the DC Offset again. When I powered up the unit, I got magic smoke from R30 (390 ohm on the left channel). WTF? I've been very careful not to mess with the left channel for the purpose of keeping it as a reference. Ugh. Now what? Obviously I have to replace the resistor, but why would it have burned up in the first place? (I wonder if I accidentally bridged pins 17 & 18 with my probe when I was checking for the offset).

Seems every step I take makes the problem a bit worse. I'm starting to think this may end up being a job for Radio X Tuners.

Thanks,
Adam
 
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If both channels were under 50mv, it shouldn't be in protection. Something has changed. The erratic reading was probably a function of the trimmer dirty or failing. But now you have a new problem. Resistors don't just burn up. Something was causing too much current to pass through it. Bridging pins could certainly cause it. That resistor is one of the fuse resistors on that board so hopefully it lost it's life doing it's job and you haven't damaged anything else.

It's much safer to measure the DC offset using the method I posted above. Regardless of what it was when you started, we still need to know what it is now. If your overall karma is good, you still may get away with just an adjustment or by replacing a handful of fuse resistors.

- Pete
 
The right channel was jumping into the hundreds of mV when I checked it originally.

I was using the method that you posted, however my work area is very poorly lit and I'm afraid I may have accidentally touched the probe to the pin adjacent to the blue/black wire on the back of the power supply board while trying to get my reading. I don't know for sure if that's what happened, but that's my best guess since I didn't change anything else before the resistor blew.

Currently, the Right channel shows 90.6mV and the right channel shows 53.5V.
 
Both are too high. You might be able to adjust the Right channel into range but the other right channel with 53 volts probably has blown output transistors. If so, you need to check the driver transistors on the driver board too.

- Pete
 
LBPete, The output transistors; what would cause those to blow? Are those the transistors under the heat sink on the back plate?

Anyway, I've changed the path that I'm taking on this repair. I found a driver board for sale and bought it. I don't have it yet, but when I get it I'm going to be hesitant to just pop it in and fire it up, even though I have a feeling the new driver board will resolve the problems. What can I do to check and make sure this there is not something else wrong with the unit that will cause damage to the new driver board? I have a variac that I can use when I power up the unit if that may be of some use.

Thanks for your help with this.

Adam
 
You can damage a good driver board if your output transistors are shorted. Do not plug the board in before checking them. They are on the back behind the heat sink. Mark them so you can put them back in the same location.

- Pete
 
Also, be sure your new driver board is configured the same as the old board. Sansui modified that board at least two time during production. I don't think you can swap an early production board into a later production unit or a late production board into an early production unit. If your board doesn't have glass fuses insulated in vinyl tubes, you can't replace with a board that does have the fuses.

- Pete
 
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