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Yamaha C-4 Preamp Recap

Jake O Bob

Jake O Bob

New Member
Gone through replacing all caps along with variable resistors on this C-4. Plugged into DBT and the good news is I've got a dim glow. The bad, some smoke came up from the function board near VR123 trimmer resistor. Looks like these 2 resistors are toast (see photo). I think both R257 ( 1.2KΩ ) and R261 ( 150Ω ) is where the smoke is coming from.

My question is when trying to find replacement resistors for these two on Mouser / Digikey, how do I figure out what 'Power Rating' or 'Temp' I need? The service manual only has the Ω rating. Can someone please guide me to what replacements would work best here? I'm still learning here so go easy on me :) Thanks!
 

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Jake O Bob said:
Gone through replacing all caps along with variable resistors on this C-4. Plugged into DBT and the good news is I've got a dim glow. The bad, some smoke came up from the function board near VR123 trimmer resistor. Looks like these 2 resistors are toast (see photo). I think both R257 ( 1.2KΩ ) and R261 ( 150Ω ) is where the smoke is coming from.

My question is when trying to find replacement resistors for these two on Mouser / Digikey, how do I figure out what 'Power Rating' or 'Temp' I need? The service manual only has the Ω rating. Can someone please guide me to what replacements would work best here? I'm still learning here so go easy on me :) Thanks!
Click to expand...
They are most likely carbon film 1/4 watt .. I only see one smoked resistor in the photo.
 
Jake O Bob said:
R257 ( 1.2KΩ )
Click to expand...
Manufacture change after service manual was published. In the drawing R257 position is now actually a VD1212 varistor and it looks like it should. Its the same part as D127.

Now you have to figure out what smoked the resistor.. I would check underneath those components at the solder work. I suspect you have a solder bridge.

The resistor ( R261 ) is a carbon film 1/4 watt 150 Ω
 
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If nothing is specified in SM, it’s usually a 1/4W resistor. The resistor to the left doesn’t look like R257 and the one to the right doesn’t look like a resistor, it looks like a double diode, a VD1212 maybe (discontinued)?. If so and if it’s failed, you can replace it with two 1N4148 diodes back to back (+-+-), don’t get the polarities wrong.
 
Thanks to both of you for the quick reply. I've ordered a new carbon film resistor for R261. I'm looking on the board for solder bridging and can't see anything obvious. I'll be taking some more time with this in the coming days

In the meantime I've turned all the variable resistors all the way down counter-clockwise. Need to take a look at resetting the voltage adj and DC offset adj. Not sure if that would have caused the resistor to smoke before?
 
Have you the right tools to make the re-adjustment to VR121 and 122 ? MC amplifier adjustment.
 
avionic said:
Safety check TR197...
Click to expand...
Testing TR197. See image

Not sure that these are the results I should be getting for the Emitter-Base (2.5 & 2.3)? Should it be open and around 5.4 like the other Base-Collector and Emitter-Collector readings? Does this mean faulty junction?
 

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C-4 MC amp distortion adjustment, workaround

Step 3 of the adjustment procedures needs a distortion analyzer which most of us don't have. After replacing the trimmers with Bourns multiturn trimmers I needed to adjust MC amp distortion according to step 3 in the SM. I tried a software based THD-Analyzer, which did not work. Step 3 is...
audiokarma.org
 
Dr. Ear said:

C-4 MC amp distortion adjustment, workaround

Step 3 of the adjustment procedures needs a distortion analyzer which most of us don't have. After replacing the trimmers with Bourns multiturn trimmers I needed to adjust MC amp distortion according to step 3 in the SM. I tried a software based THD-Analyzer, which did not work. Step 3 is...
audiokarma.org
Click to expand...
Thanks! Good to know there's a work-around for this adjustment
 
Latest update: I've replaced TR197 with new transistor ( 2SC1213A ---> KSC2383YTA ). Also replaced the burnt resistor R261. Powered on via DBT, but NOT even getting a dim glow (no light from bulb). The power lights at the front of the unit flash on for a second, then go out.

I'm stuck. Not sure where to go from here...
 
avionic said:
Power supply crowbar circuit TR632 and 633 is being enabled. Try powering up without the dim bulb.
Click to expand...
Same thing happens without the dim bulb. Power lights flash on for a second, then go out.

Had to google crowbar circuit. Not sure where to check for excess voltage...
 
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