Yamaha C-60 Woofer Pumping Problem

kermit_xc

Active Member
... and loose output RCA

god, this preamp sounds amazing (paired with a matching M-60).

But - this weird thing happens where random channels (sometimes both) start this woofer pumping thing. When this starts happening, the volume adjustment makes no difference, but the Loudness knob definitely does, it increases the amplitude of the pumping to where the sound definitely becomes audible. If I don't touch the loudness knob all is fine - but the meters on the amp keep a steady noise floor, without the meters I probably would never notice the thing

So - what would cause this issue and how do I go about troubleshooting it ?

I tried cleaning the volume pots with deoxit (through the 3 holes in plastic housing) but the issue prevails - I cleaned up some glue mess around the voltage transistors (see before shots below).

DC offset: R:2.1mV; L: 1.9mV

IMG_1250.JPG IMG_1251.JPG

cheers!
 
Probably cruddy solder joints in a whole bunch of spots, especially on those pass regulators stuck to the corroded heat sinks.
Remove those and clean up the corrosion. There should be holes where you can use screws to properly mount the sinks. The resistors in front of the sinks are probably corroded as well (47k I think) and should be replaced.

I'd grab a magnifier and see if you can identify any other cracked joints and clean those up as well.
 
Probably cruddy solder joints in a whole bunch of spots, especially on those pass regulators stuck to the corroded heat sinks.
Remove those and clean up the corrosion. There should be holes where you can use screws to properly mount the sinks. The resistors in front of the sinks are probably corroded as well (47k I think) and should be replaced.

I'd grab a magnifier and see if you can identify any other cracked joints and clean those up as well.

I went through the backside and couldn't find any obvious offenders in terms of bad solder joints. One of the resistors is pretty badly charred - it still measures properly though (assuming i did it right).
No shorts on the transistors themselves. Any chances I get lucky with swapping just that one resistor ?
 
Pick up a couple 4.7KΩ 1/4 watt film resistors. They are glue damaged. They sit right in front of those heatsinks.
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I am wondering about the continuous loudness board - I can't easily access it for visual inspection. Would the regulator resistor answer the sysmptom
Pick up a couple 4.7KΩ 1/4 watt film resistors. They are glue damaged. They sit right in front of those heatsinks.
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Resistors replaced, volume control assembly deoxit’d ... issue persists. The odd thing is that no matter the input ... the woofer keeps pumping, and turning the loudness only increases the effect. Changing the volume does nothing ... but the right channel meter on the m-60 stays at 1/4th of the scale (2ohm setting) while listening. Super aggravating and I want this resolved without a expensive trip to the audio shop.

HELP please
 
Do you know your way around a schematic ? We need to verify if your power supply voltages are in the ball park and getting where there suppose to go.
 
Do you know your way around a schematic ? We need to verify if your power supply voltages are in the ball park and getting where there suppose to go.

I'm a fast learner :) talk to me

I have the B1 and B2 readings in the meantime

SM calls for B1 +/- 20V, B2 +/- 16V

-B2 = -15.9V
+B2 = 16.1V
-B1 = -14.7V <=== this is a problem, correct ?
+B1 = 19V
 
Where did you measure these voltages at ? Copper buss bars ?
 
Yes it is.It should be alot closer to the +B1 voltage.

This is what I have done so far:
- replaced R271 and R272 in the T149 and T150
- visually inspected all joints and components
- cleaned out all pots and switches with deoxit
- tried adjusting DC bias on rec out - but the trim pots are impossible to work with

what's next boss ?
- measure all transistors in circuit ?

Screen Shot 2018-04-13 at 10.20.52 PM.png
 
Check the resistance of R276 and compare it to R275. In circuit may suffice. We are looking for 2.2 Ω ballpark. These are 1/4 watt fusible resistors.( flameproof carbon film )
 
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If you measure greater than 2.2Ω remove or isolate from circuit and remeasure it.
 
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