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Sansui 8080 in protection - basic troubleshooting steps?

roger2

roger2

.
Subscriber
8080 with red blinking protector light

I removed all four outputs > still red blinking light
I removed driver board F-2436-1 > still red blinking light

FM signal and tuning meters, and dial lights all work




What is the next most likely place (or board) to look for the problem?

Could the unit be in protection because of the outputs and the driver board being removed?
Hypothetically, could there be a problem on the F-2436-1 board, causing the unit to stay in protection when the board is installed, but then when removed it is in protection because F-2436-1 is missing?

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Step one - replace all of the fuse-resistors on the driver board.
Then test the output and driver transistors.
 
w1jim said:
Step one - replace all of the fuse-resistors on the driver board.
Then test the output and driver transistors.
Click to expand...

I would do that, except I found in digging around through old threads, one case where removing the driver board allowed an 8080 to come out of protection. Not wanting to take that one case as gospel I am asking here. I also came across many cases where the fuse resistors caused the amp to stay in protection, so I know that they are highly suspect. But with the driver board removed, the fuse resistors are also removed from the system.

I am not planning on restoring this unit as I have too many other projects in the queue. This unit also has other damage. My goal at this point is to identify the likely cause of the protection issue, and if it is an easy fix I will do it.

I am hoping to avoid ordering/replacing those fusible resistors if possible. This needs to be done no doubt, by whoever ultimately restores this beautiful Sansui. But if there are other likely culprits I would prefer to focus my time and effort there. Just want to follow the shortest path to either get it working or to identify a likely cause.
 
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It should come out of protection with the driver board removed. If it doesn't, you need to look at the power supply board and the protection board itself. Look for burned parts or circuit traces. Just to be clear, is it an 8080 or 8080DB?

- Pete
 
It could also be a problem with the output circuit between the driver board and output transistors.

- Pete
 
@LBPete it is an 8080. I just submitted the SN to your database thread

Yes, I was expecting it to come out of protection with the driver board removed, but no luck.

I will do a visual inspection of the PS and protection boards.

What circuitry exists between the driver board and the outputs? I assumed, and it appears to me on the schematic, that the driver board directly connects to the outputs. And with both the driver board and the outputs removed, could any circuitry that might be between them have any effect on protection?
 
---there is a lot of thermal paste inside the holes of the TO-3 sockets where the output transistor pins connect. Could this be doing something, is it conductive and/or could the protection circuits be sensitive to this in some way?

---power supply board F-2546: fuse F03 (1A) fuse holder had spread apart and not making good contact with the fuse. Fixed that, powered up no change. F06 (7A) possibly not original ("LittleFuse" brand, unlike all others). All fuses continuity test good, values are correct, and appear original exc. as noted above. Nothing else to report, no signs of heat or other issues visible on either side of this board.

---protection board F-2547: capacitors C04, C06 and C12, all 1uF/50V, appear to have shrunken sleeves. No (other) signs of heat or other issues visible on either side of this board.
 
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Aside from being a big mess, the thermal grease can't cause this problem.

Pins 37 and 38 on the power supply board connect to pins 3 and 4 on the protection board. Measure the DC voltage between each of those wires and chassis ground with the driver board out and the unit turned on.

- Pete
 
Power supply board
Pin37 0 VDC to gnd
Pin38 0 VDC to gnd

Protection board
Pin03 0 VDC to gnd
Pin04 0 VDC to gnd


<to verify DMM and probes were functioning, and I was contacting a good ground, I touched the hot terminal on one of the main filter caps and got ~ 53 VDC>
 
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That points to a fault on the protection board. Take a good close look at the relay contacts. They may be welded to ground. Also check to see if you have 24v dc on either side of the zener diode D02.

- Pete
 
Protection board

Diode D02
+28.6 VDC on the cathode
0 VDC on the anode (relay side)


At the moment I can only see the relay contacts via a small mirror and flashlight. I have a small work bench with a different project amp (disassembled) taking up that space. The 8080 is on a table out in the unheated garage. I don't know how far I will be able to dig into, or disassemble, the 8080. I might be able to replace some components on the protection board with the board in place.

I didn't see any obvious welding, or charring, of the contacts. The relay has stationary contacts for both the open and closed positions. The moving contacts are resting against one set of stationary contacts and appear (given my currently limited ability to view) to be clean. But without removing the protection board I am not able to clearly see all four contacts.
 
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Each side of that diode connects to a side of the relay coil. If you have zero volts on the anode, the relay should switch on. You may have a bad relay or an open on the ground side of the relay coil. Measure the voltage across that diode. That will tell you if TR01 is turned on. Also, if you can get to the foil side of the board, see if you have 24v on either side of the relay coil. That will tell you if the relay coil is open.

- Pete
 
Reading across diode D02 is 30.4 VDC.... which makes no sense to me, as one end is 28.6V to ground and the other is 0V to ground. But I won't worry about that for now as I think the problem may have been found....

Looks like a lifted pad where one of the relay coil pins connects. I scratched through the green coating on the track in the location shown below. I found 28.6V to ground on the track, and 0V to ground on the pad right next to it. Must be a hairline crack because I can not see it.

This has got to be it, right? Would broken continuity in that location explain the other readings?

Sansui 8080 protection board troubleshooting - marked.jpg
 
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That's your problem. Scrap the green screen off the trace and solder a piece of wire across the break. That should get the protection circuit working. It should come out of protection with the driver board removed, then you still need to determine if it will come out of protection with the driver board installed. If it doesn't, then it's time to replace the fuseable resistors on the driver board provided the outputs are all good.

- Pete
 
Yep. I have fixed broken or lifted traces/pads before. I did a quick jumpering of that joint last night and got the steady green light. Will test the outputs before re-installing. Looking forward to checking this out this 8080 :)

Thank you for your excellent troubleshooting help @LBPete :thumbsup:
 
I repaired the broken pad/trace at the relay coil pin on the Protection board as discussed in posts 14 & 15 above.

Further testing was done as described below, with unexpected results.


with driver board removed; TO-3 outputs removed
---amp came out of protection
---measured 0 VDC at speaker outputs

with driver board installed; TO-3 outputs removed
---amp came out of protection
---measured -49.2 VDC at both L and R speaker outputs

with driver board installed; TO-3 outputs installed
---amp came out of protection
---measured approx 52.4 VDC on both L and R channel speaker outputs (did not make note of whether this voltage was + or - when I took the measurements)
---also measured approx. 52.4 VDC from the each case of the two middle TO-3 outputs to ground, and 0 VDC to ground from each case of the other two TO-3 outputs to ground
---no fuses blew while the amp was powered up under this condition


After testing described above, I removed the TO-3 outputs and all four "diode-tested" good, and all fuses still visually appear good (prior to this round of testing, I had removed and tested all fuses).
 
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It's a pretty far stretch, but closely study the schematic and layouts to be certain that the outputs are/were installed in the correct, corresponding sockets.

Most likely a driver is firing an output(s) on hard, but having an NPN output where PNP should be could result in that behavior.
 
You might think I would have remembered that, having been through a couple of the 9090s. I suspect you are going to find issues around the driver board.

If it (F-2436) hasn't been heavily scrutinized, it might be a candidate.
 
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