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.
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.