Yamaha C-85 Preamp Advice

While waiting for parts for the M-45 I made some progress on the C-80 (not C-85 as originally stated).

-New 4700uf filter caps are in, took a little creativity to make them fit; the caps that I chose were a little too wide to fit back to back in the main power supply section.


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Finished replacing the rest of the caps, cleaned and re-greased the transistors/heatsinks. 2 resistors were corroded from the glue used for the the heatsinks so I replaced those as well out of an abundance of caution. The heatsinks were secured with M3 screws and nylon washers; the holes already exist in the bottom of the board. I can't figure out why Yamaha went with glue instead of screws here.

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It looks like someone dropped this poor preamp and dented the rear corner near the power cord. Luckily, the circuit board with the extra outlets didn't break. With some gentle pounding with a hammer and block of wood I was able to straighten the panel out. Notice the voltage selector, was this a common feature? I'm guessing it might have been sold at an overseas US military exchange back in the day?

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I cleaned the flux off of the boards and touched up a few rough looking solder joints; other than that everything looks to be in excellent condition.

The preamp passed the DBT, I dialed in the DC offset for the MC phono preamp and hooked it up to my CR-2020. So far it sounds great!

The signal cuts in an out on the left channel when tone bypass is selected. I'm going to try taking the switch apart since the Deoxit didn't seem to work.
 
Ok, new parts are in on the M-45. Passed the DBT, (no class A), and powers up no problem plugged directly in, idle current looks OK, however the relay doesn't engage. Didn't have much time to troubleshoot, but did notice that the collector on Q150 is reading around -0.4v...which isn't right.

I'll start testing, but any advice would be appreciated.
 
I took Q150 out of circuit and it tested fine. Just in case, I replaced it with a KSA1013 that I had lying around and same exact result, still -.4v on the collector of Q150, the base and emitter both are around 60-61v. I checked diode 130 and it is fine too.M-45 Relay.PNG
 
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Did you replace R221 or 222?

R221 ----> 2.2 M Ω
R222 ----> 10 K Ω
 
Measure the resistance to chassis ground of R224. One end should be near zero ohms. The other end should be around 150 Ω.
 
Ok, making progress. I did not replace R221 or R222, took them out of circuit and both test OK. I checked voltages around Q149 and they were off too but the transistor tested OK, started following the rabbit trail and Q147 and Q148, same deal, voltages off but testing OK. Finally, tested Q143 which was bad as were Q144, Q145, and Q146 in the muting and over current detection sections. Q139 and Q141 tested OK. Replaced the bad transistors and fired it up on the DBT and success, relay clicked!

I'm a little concerned about firing it up on full power and even more to engage class A...anything else I should check before taking the plunge?
 
zaskarx said:
I'm a little concerned about firing it up on full power and even more to engage class A...anything else I should check before taking the plunge?
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No guts,no glory..:D Adjust your bias to zero volts with the DBT, before going full mains voltage.Do not hook up a load. Then readjust your bias with full mains.
 
Seems happy and stable in class A and AB! Idle current is right around 40mv on both sides in class A, DC offset around 15mv on both sides. No magic smoke, sparks, etc.

Still need to test for irregularities on the scope and dial in the meter levels but looking good so far. Thanks again Avionic.
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