Sansui 551 with dead left channel

frankiebull

Active Member
OK, so I got what I thought was a great deal on a 551. It is very clean inside and out. One of the 2.5 amp fuses was bad but luckily I already had a replacement and did not have to find in such an odd size. The 7 volt bulb on the FM meter is bad but I can get that ordered.

The problem is that the left channel is completely dead. FM, AM, tape, phono, auxiliary, head phones - no left channel on anything. I cleaned all pots and switches. No help. The service manual states what to check for AM, FM AUX and phono. I was wondering if since the left channel is just dead there might be another problem. Any recommendations?
 
All I have is a digital multimeter and a Mercury 2000 tube tester that tests transistors too but I never used it for that function.
 
The service manual states that " the value of each level in block diagram was measured by the following:
Master volume control - maximum
bass, treble balance - center
input PHONO 2.0 mV 1KHz sine wave, AUX 100mV 1KHz sine wave
Output - - 11.4V (16W) 8ohm"

I can set the volume control and bass treble and balance but can I get by without the INPUT and OUTPUT? I don't have the proper equipment.

The Service Manual TROUBLESHOOTING CHART states that if the lamps light (which they do) I need +52V on collector of TR13 and TR14. I do. I also checked the collectors on TR15 (49.3V) and TR16 (25.7). According to the BLOCK DIAGRAM TR16 is close to correct (28V) Does this mean that TR15 (which is on the bad channel) is bad?
 
I really think I could get this thing working if someone experienced with this unit would assist and point me in the right direction. Any help would be greatly appreciated.
 
You need an oscilloscope to be able to troubleshoot this. Can't fix something with no tools.
 
Check TR9, 11, 13 and 15 and D03.

I was able to do some checking today after returning from long weekend. Checking in circuit, I don't see anything unusual using the diode test on my multimeter. On the transistors I checked from base to emitter and base to collector. On the diode I show voltage both directions. again this was in circuit. Do these need to be checked out of circuit?
 
jumper c31 negative to c32 negative . if no sound in bad channel check speaker switch .
before that check collector voltages of outputs ..one of a pair should be rail . the other about half rail .
 
OK so I got 53.6 on collector TR13 and 50.6 on TR15. That's the bad channel. On the good channel I got 53.6 on TR14 and 26.6 on TR16.

Let me hook some speakers up and I'll jump -C31 to -C32. When you say sound in bad channel, I assume you mean an audible click or static?
 
OK so I got 53.6 on collector TR13 and 50.6 on TR15 . need to track down why this is so .
its unlikely to produce any sound like that .. even it it does it will be bad

possibly bad transistor and or resistor/s
 
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Not finding any bad resistors. All 4 0.47 ohm 5W are measuring high but since its all 4 I don't think that would be the main problem.

This question is going to expose my NEWBIEness. Is it a good idea to swap TR15 with TR16 to see if that switches the bad channel to the right? Seems like I read that the heat may just ruin a transistor anyway so I might just end up with 2 bad transistors.
 
you need to test the transistors . start at the outputs and work your way back . do not swap from working side or you could easily end up with both sides not working .
 
I tested them in circuit using the diode function on my multimeter. I compared results from channel to channel. I did not notice any thing that seemed abnormal. Do they need to be removed from circuit to test?
 
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