Yamaha C-6 preamp power supply problems

leesonic

Captured on security camera...
Subscriber
I have a Yamaha C-6 preamp in my possession that I offered to look at for a friend. I feel bad that I haven't done much to it, so I thought I'd start a thread up and see if I can get things moving a little more quickly.

When I first got it, I noticed a couple of burnt out resistors on one of the control boards, R218 and R220, which are (were) 56 ohm resistors wrapped in some kind of flame-proof tubing. I pulled the corresponding transistors off, and they both tested bad. I replaced the 2SA999 (TR142) with an MPS4250G and the 2SC2320 (TR140) with an MPSA18RLRMG, replacing the 56 ohm resistors as well. I also re-soldered the two power supply transistors on heatsinks (TR421 and TR424) which had obvious dry joints.

Well, the preamp powered up, and "worked" to a certain extent. There was however a fairly noticeable buzzing noise through the headphones I was using. It seemed to be more on the right channel, the mono button just making it, well, mono. Armed with the service manual, I started testing, and it failed at the first step. The positive and negative supply rails should be plus or minus 25v within 0.5v. The positive measured 25.2v, but the negative measured -38.5v which is way too high.

I did some more testing tonight, and have attached a picture from the service manual, you can see the voltages I measured in green. Keen eyed observers might find some different voltages for two transistors connected together. This was due to the voltages drifting a little between me testing them. It looks like the problem is starting all the way back at the base of TR428 where the voltage should be zero but measures at -6.4v. The emitter connections between TR428 and TR427 don't look to great at -5.6v when they should be -0.6v.

Lee.

YamahaC6PSU.gif
 
It looks like the problem is starting all the way back at the base of TR428 where the voltage should be zero but measures at -6.4v

The -6.4 is due to the imbalance ie. +25 and -38 across R520 and 521.

The emitter connections between TR428 and TR427 don't look to great at -5.6v when they should be -0.6v.
Probably enough bias current on the base of TR428 to switch it on.
 
Last edited:
Tested today :

TR424 tested OK
TR423 was shorted across B-C, measured 77 across B-E, 69 across E-C
TR427 tested OK
TR428 tested OK

Where B = Base, C = collector, E = emitter. My meter shows healthy diode and transistor junctions anywhere from "600" to "800" on the display.

I went ahead and ordered TR421 to TR428 (eight in total) yesterday afternoon just because they only added up to a couple of bucks. Since I have them, I guess it would make sense to replace them all anyway? They're all the originals, and if one has gone bad, how long before the others do?

This preamp has several hot running transistors, all with TO92 plastic packages. I wonder why they didn't use TO126 or TO220s, something you could get a heatsink on? I know over on the Pioneer forum, Mark suggests using TO126 package devices in the phono preamps of certain Pioneer integrated amps, such as the SA-7800.

Lee.
 
Here are the components (circled in yellow) that I replaced on the control board. The bent transistor legs are due to the replacements having a different pin-out to the original.

YamahaC6-01.jpg




Here are the transistors I replaced in the power supply, all of those in the negative side, and I also replaced the big one on the heatsink on the positive side as well. Both of these were almost spot-welded onto the heatsinks from years of running hot.

YamahaC6-02.jpg




I only took out one screw, and it fell to pieces, honest. Actually, it took more than that to gain access to the pots and switches for cleaning with DeOxit. One of the source selector paddle knobs had been crazy-glued on, I had to cut it off with a Dremel.

YamahaC6-03.jpg




Back together again, sort of. Since there was already a paddle missing, and I had to cut another off, I found some old knobs from Radio Shack that fitted. They make them with blue and red inserts, of course I only had the red ones.

I replaced the power and muting lamps with blue LEDs, I tried using green, but the ones I ordered weren't bright enough. One thing is strange, and I don't know if it's peculiar to this preamp, or others as well. With the muting on, as in the output level reduced, the button illumination goes out. With the muting off (normal output level), the light is on. I didn't do anything to change it. Strange... everything else works fine.

Lee.

YamahaC6-04.jpg
 
Back
Top Bottom