SX-3800 Power Board Issues

[ClintB]

Hi Everyone,

I recently bought a "non-working" SX-3800 and I've been attempting to get it into the "working" category. My first power-on of the unit was on a DBT. The bulb on the DBT dimmed right away but the only thing that came on was the three lights that illuminate the analog tuning scale.

Also, the protection circuit relay did not close.

I know this receiver has other issues but my approach to this project so far has been to try and get the power supply board sorted. I feel like that's where it all starts and if that board isn't working right, nothing else will either. I understand that there are different versions of this board and I have the GWR-122B.

This is what I have done so far to this board:
1. Replaced all of the electrolytic capacitors.
2. Replaced all of the diodes.
3. Replaced all of the transistors except for the 2SK34 units that live in Q202 and Q206.

I didn't do anything with any of the resistors. I pulled all of the fuses and checked them for continuity. All checked good.

I put the board back onto the heat sink and reattached all of the various wires. Powered it up again on the DBT and all of the lights (speaker A/B, input selection, digital frequency readout, etc) all worked. Still no "click" of the relay closing though.

I checked voltages at all of the various pins and compared them to what values are on the schematic and what others have posted here in this forum.

All of them seemed to check out ok except for these:

29: 37.6
31: -37.7
35: 37.8
37: -37.8

All of those voltages should be reading 45.5 or something much closer than they are. My questions are these:

1. What could be causing these relatively low voltage readings?
2. Are these low voltages what could be causing the relay to not close?

I have the board back out. My intention was to start testing and replacing resistors as needed but I thought I might try posting here to see if anyone has any thoughts. I have read literally dozens of threads about the SX-3800 and I do intend on giving the solder joints a closer look as I'm working through this.

Thanks in advance.

 

[AE Bob]

I would suspect the slightly low voltages may be due to the fact you are on a DBT. With the lamp in circuit, particularly if it is glowing at all, the input to the power transformer will not be a full 120 volts, therefore the rectified DC voltage at those points will be low. It is unlikely that the low voltages would keep the unit in protect. Check the DC on the amplifier outputs before the protect relay. any substantial DC there will keep the unit in protect.

 

[ClintB]

Bob,

Thanks for the reply. That certainly makes sense. As soon as I get through my OCD-fueled testing of the resistors on the power supply board, I'll get it put back together and look at what you suggested. I have to say, this receiver is one of the more confounding pieces of electronics I've ever dealt with. At the same time I bought it, I had an opportunity to also buy a very nice Yamaha CR-820. I'm beginning to wish I had.

Thanks again.

 

[markthefixer]

The protection unit senses AC voltage to decide if the receiver is on or off.

DBT's bring down the AC voltage, SOMETIMES enough that the protection does NOT "think" the receiver is turned on.

The protection has to "beat" the failure of the turned off power supply voltages to disconnect the speakers before the "POP" is formed.

the race is literally milliseconds.


the failure is:

POP

 

[ClintB]

Thanks Mark.

I have a follow-up question: I can understand the theory about the DBT affecting the voltages I'm seeing at Pins 29, 31, 35 and 37. If the DBT is the cause of those low voltage readings, would it also affect the readings on the pins that attach directly from the transformer? That would be Pins 1-5 and 26-28. This is what I measured at those pins:

1: 43.8
2: 43.8
3: 0
4: 9.62 AC and 10 DC
5: 1.98 AC and 10 DC

26: 29.2
27:28.8
28: 0

The schematic that I have doesn't address what the voltages at those pins should be. The only reference I have is what others have posted in this forum. Based on that, the readings I have seem ok but honestly, I don't know.

Thanks again.