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Toshiba SA-420 Help!!

F

forsgren

AK Subscriber
Subscriber
Okay, I have to admit that I don't like asking for help. But, I am far from perfect, I do not think I know everything, and I feel that I am humble enough to ask those far wiser than I. Which means all of you in AK Land.

So, I picked up the above mentioned unit, very nice, extremely well built receiver. I checked it out with DB Tester, looked good, fired it up, sounded good at first.

The right channel sounded like crap, hooked it to the scope and saw what I was hearing. Checked with headphones, sounded great in both channels. I unplugged the input from the pre-amp and jumped L to R and R to L, the right channel remained like crap. I know the problem is in the right channel on the main amplifier board PA-41B. I looked at the idle adjustment. The left side was at 9.2 mV (Spec is 10mB +/- 2mV), the right read 0? Huh? I thought perhaps dirty pot? I removed it and checked it, works perfectly. Put it back in and still read 0.

I pulled TR613 (2SD525) and TR615 (2SB595) and tested them, at first TR613 was reading as 2 diodes, I changed the leads on my Atlas DCA55 and it read fine. I thought I would swap the right and left pairs which I did. I did these one at a time, cleaned the old grease off the mica insulators and installed them. When I powered the unit up, both 5A fuses on the power supply board blew. I see nothing no reason why this happened, no solder bridges, nothing crossed? I checked the KBL02 bridge rectifier and it is good. Could it be the main cap (this uses a single cap with two caps inside)?

This does not make sense? All of the transistors checked good as I went about replacing them?
 
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forsgren said:
...cleaned the old grease off the mica insulators and installed them. When I powered the unit up, both 5A fuses on the power supply board blew. I see nothing no reason why this happened, no solder bridges, nothing crossed?

This does not make sense? All of the transistors checked good as I went about replacing them?
Click to expand...

What type of grease did you re-apply? CPU-type is not recommended.

--Joe
 
I once worked on an amp (KA-8100) that was not able to achieve proper bias because of faulty connections between the T-03 main output sockets and the PCB. Perhaps a bad solder joint (or two) somewhere could be the cause of the bias issue in your amp?
 
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Thanks all! I am baffled! I mentioned that one of the NPN output devices was flaky when I tested it, this was on the right channel. I double checked all driver transistors and they all checked good. I replaced the 2SD525 and 2SB595 output transistors with MJE15028G and MJE15029G with silpads. Fired it up and it now sounds great! This is actually quite a nice lower powered unit!

Everybody take care and be safe!
 
I have been busy and travelling. I thought about this and decided to pull it out. Although it sounds great I wanted to check the idle current. I could never get the left channel set correctly. The service manual recommends 10mV +/-2mV. The highest I can get is a tad over 4mV. Previously I recapped the entire unit, replaed all 4 output transistors, replaced all 4 driver transistors TR601 - 604 with KSA1815. I replaced TR609 C1382 (it tested flaky) with a KSC2690. Still not adjusting properly. I pulled the 500 ohm VR601 and it tested fine, I replaced it as well, still nothing. The right channel has always been easy to set. I should point out that this has always been the case since I purchased this.

Any suggestions, I have tried to measure voltages on the right channel and then the left, so far, everything measures the same. Perhaps it is just the slight OCD I may have, it isn't right and should be able to be set properly at 10mV.

Any help would be greatly appreciated!
 
Avionic, thanks for the response. I did check both emitter resistors, they are spot on. I checked D601, it was good, I did replace it anyway with a n4148. I will check TR607 and get back later today.
 
I did check TR607, it tested fine, I did have a BC337 transistor which was close to the 2SC509. No change at all. Like I said, it does work and sound fine. However, the amplifier was designed to operate at 10mV idle current.
 
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