Protect Relay issues - Sansui 881

MisterBill2

Joined Jan 23, 2018
14,232
If probing some point with the meter causes the relay to operate then I would suspect an open connection someplace. A few years back I was given an amplifier that "just quit working one day", with no smoke or noise or anything. Although it had a Kenwood logo and a Kenwood part number, Kenwood denied ever maaking it, so no circuit information was available at all. Then I examined it very closely and saw some solder joints did not look good. So I resoldered every joint on the PCB, which took almost 20 minutes, because there wer a lot of them. After that the stereo amplifier worked perfectly, and still works perfectly. On some occasions, resoldering all of the connections is a cheap and easy fix, if one is good at soldering. Certainly there are some who can do mare damage with a soldering tip than other could do with a big hammer. THEY should avoid this advice.

My whole point being that a failed solder connection can cause all kinds of problems.
 

Thread Starter

rfjohnsto

Joined Dec 31, 2020
15
If probing some point with the meter causes the relay to operate then I would suspect an open connection someplace. A few years back I was given an amplifier that "just quit working one day", with no smoke or noise or anything. Although it had a Kenwood logo and a Kenwood part number, Kenwood denied ever maaking it, so no circuit information was available at all. Then I examined it very closely and saw some solder joints did not look good. So I resoldered every joint on the PCB, which took almost 20 minutes, because there wer a lot of them. After that the stereo amplifier worked perfectly, and still works perfectly. On some occasions, resoldering all of the connections is a cheap and easy fix, if one is good at soldering. Certainly there are some who can do mare damage with a soldering tip than other could do with a big hammer. THEY should avoid this advice.

My whole point being that a failed solder connection can cause all kinds of problems.

Yes, good point!
 
These were something like 34.5Mv R and 13.5Mv L or similar on all three.
Those numbers are fine. You start to worry at 750mV. My amp is below 5mv which is very unusual.

VR1 adjusts the offset and Vr3 adjusts the idling current. Most amps don;t have a way to adjust the offset voltage.

There probably is a low value resistor between O and P and Q and P. This is where you measure the idle current. With no signal, the idle current should not run away.

The voltage across Q7e and Q7c matters. It will change based on VR3 and heatsink temperature. You can force it to be zero, but it should not be zero or you'll get crossover distortion. When troubleshooting, I sometimes force this to be zero (short).
All of the low valued resistors matter. You have to get all of the bad parts replaced.

This is where a variac helps. Monitoring the difference between Q7e and Q7c with a signal is the best turn on possibilty. I;ll take that back. With Q7e - Q7c = zero and no load is the best initial turn on possibility
 

MisterBill2

Joined Jan 23, 2018
14,232
If that amplifier is direct coupled then the excess offset voltage might be coming from an earlier stage. That was common on the direct coupled amps: if the $1 transistor of the input stage goes leaky than the $45 output module gets fried. And that was a 1985 price for the module.
 

Thread Starter

rfjohnsto

Joined Dec 31, 2020
15
If that amplifier is direct coupled then the excess offset voltage might be coming from an earlier stage. That was common on the direct coupled amps: if the $1 transistor of the input stage goes leaky than the $45 output module gets fried. And that was a 1985 price for the module.
I think this one resembles that design... Ouch ..... finger crossed !!
 

sagor

Joined Mar 10, 2019
797
Looking at TR04 and 05, should the collector of TR05 also be close to 29,6V? With all 3 pins at 0V on TR05, it seems suspicious. It suggests TR05 is conducting, but there should be a small voltage difference between Collector and Emitter.
 
I built the leach amp http://leachlegacy.ece.gatech.edu/lowtim/ and they specified 100 uF/50v in about the same location as yours. The rails were +-50V. One explosively popped.

I had really big filter caps too and a slow turn-on circuit. My slow turn-on circuit died because of large spikes across the power supply. I had to put ZNR's across the main power supply caps. 50V is to close to the power rail in my opinion.

I worked on a surround sound receiver that had come back to a shop 2x. I was moonlighting there.
The Amp IC's were operating too close to the power supply rails and they would eventually die.
 

sghioto

Joined Dec 31, 2017
3,940
Looking at TR04 and 05, should the collector of TR05 also be close to 29,6V?
No. The collector of TR05 is connected to the base of TR04. On a working unit the voltage there is probably only about 1 volt depending on the current drawn by the relay coil.
The problem was a shorted capacitor C12 which explained all the zero readings.
 

sagor

Joined Mar 10, 2019
797
No. The collector of TR05 is connected to the base of TR04. On a working unit the voltage there is probably only about 1 volt depending on the current drawn by the relay coil.
The problem was a shorted capacitor C12 which explained all the zero readings.
My error on that 29V, I did not notice the resistor divider network between the 100k and the 22k resistors that the TR05 collector is connected to. When conducting, it should be a very low voltage. When not conducting, it should be around 5V. TR05 is just a driver for TR04. However, my point was that there should not be 0V at the collector (maybe 1V, maybe 5V - does not matter directly), and a shorted C12 does explain it.
 

sghioto

Joined Dec 31, 2017
3,940
When not conducting, it should be around 5V. TR05 is just a driver for TR04.
TR04 is normally always conducting so the voltage is never above appx 1.5 volts. TR05 is not a driver. TR05 shuts off TR04
when the offset dc is too high by shorting the emitter to base junction. This is the only time TR04 is not conducting. When that happens the voltage on the base of TR04 is practically zero. So you will never see 5 volts on the base of TR04.
 

rjonst

Joined Nov 4, 2020
1
Hello again,
Thanks for all the replies. The set is working again, thanks to your input, I made a set of test point sheets and constantly measured voltages after replacing a few transistors. The one rail was still low on voltage. Led me to the Driver board, where there were 8 tiny diodes were bad. Four on each side (-)(+). After that, system took off like it was new.
 
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