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Yamaha PC2002M Observations

SoCal Sam

SoCal Sam

Lunatic Member
I recently picked up four PC2002M's locally. Mech986 and I believe they came from a local (and famous) amusement park so they are likely high hour units. Mech986 just bought another PC2002M which had similar markings to mine.

I downloaded the service manual here at AK and set the power supply voltages and the bias. However, the offset which is controlled by a DC servo was too high on two of the amps. One channel in one of the amps was dumping over 20 Volts to the speakers! The offset would slowly come down until around 2.4 Volts which is when the speaker relay triggers.

I pulled out the pre-driver and driver boards and removed the dreaded Sony bond glue. This helped reduce the offset to around 300 mV which is low enough to trigger the relay normally. The offset then continues to decline to around 20 mV, which is above spec but acceptable. The glue or some other gunk looks to have discolored the PCB which makes my think the PCB itself has become conductive. Interestingly, with power off, I still measure well above 150 mV of offset.
 
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I have one here that takes around 15-20 min. to get down below 10mv. I've replaced the diff pair 2SK146's (thanks to Spectrum77:thmbsp:)and the TL072's(DC servo opamps) in both channels.It usually starts out from a cold start around 50mv on both channels.
 
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Can empathise with your DC frustration- similar to the troubles I'm having here with a PC2002M. Any chance there is a photo of the '... dreaded Sony bond glue'? Good luck with yours.
 
Does anyone know if the Yamaha 101M also had that "dreaded Sony bond glue" in it? How long do the amps that have it typically survive, before showing signs of DSBG disease?

Just wondering, since I'll be bringing home a nice 101M next month. It's from Japan and very clean, but the glue stems from the factory and only causes trouble after a long time period, so it's unlikely that any previous owner would have dealt with the issue (unless the unit already failed). I'll be opening it up and examining the insides, of course, but I'd like to know what to expect, and whether I should plan on a preventive repair, before putting it into service.
 
avionic said:
I have one here that takes around 15-20 min. to get down below 10mv. I've replaced the diff pair 2SK146's (thanks to Spectrum77:thmbsp:)and the TL072's(DC servo opamps) in both channels.It usually starts out from a cold start around 50mv on both channels.
Click to expand...

Thanks Avionic. I found the 2SK146's easily enough, they are TR101 and TR201. Still looking for the TL072's.:scratch2:

Edit: Ooph, TL072 is an IC chip.
 
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Arkay said:
Does anyone know if the Yamaha 101M also had that "dreaded Sony bond glue" in it? How long do the amps that have it typically survive, before showing signs of DSBG disease?

Just wondering, since I'll be bringing home a nice 101M next month. It's from Japan and very clean, but the glue stems from the factory and only causes trouble after a long time period, so it's unlikely that any previous owner would have dealt with the issue (unless the unit already failed). I'll be opening it up and examining the insides, of course, but I'd like to know what to expect, and whether I should plan on a preventive repair, before putting it into service.
Click to expand...

The only possible location might be on the secondary DC boards located on the sides of the amp. There are a couple medium-sized electrolytics that lay down on the board and may be attached with that glue. There won't be glue by the through-holes though.
 
I took the offender over to my tech today. He concurred with Avionic on the two possible culprits. He removed a resistor to isolate the two negative feedback paths and determined it was the TL072. New chip went in and problem solved.

BTW, be nice to your tech and buy him lunch. I did!
 
BTW, be nice to your tech and buy him lunch. I did![/QUOTE]

Great news Sam!
What does he like. Ramon or roast?:scratch2:
 
Thanks for the link to the pics, brutal.
BTW- have replaced TR101 and TL072- thanks to Avionic's advice.
Now, let's see about this gloo...
 
Arkay said:
.. How long do the amps that have it typically survive, before showing signs of DSBG disease?
Click to expand...

I think it depends on condtions, humidity, perhaps.

Generally speaking, I'd say by 25 years you're to stage 4, if not terminal.
 
I have had my pc2002m sitting around collecting dust for a long time now. Tried it last july and right channel are -10dB compared to left and have a weak sound. When "pushed" a bit it sounded like the sound were coming trough but I didnt want to fry my ribbon tweet´s so powered it down and put it in storage. As I recall the +/- 80V were spot on but the bias moved up and down. Could it be the TL072 and 2SK146 messing up? Carefully sanded the relay today to eliminate that as a fault. I think its a dry cap somewhere, wouldnt that give problems like this?
It triggers the fuses on the wall outlet, even if they are 16A, 230V troughout the apartment..
 
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IsakAlexande said:
I have had my pc2002m sitting around collecting dust for a long time now. Tried it last july and right channel are -10dB compared to left and have a weak sound. When "pushed" a bit it sounded like the sound were coming trough but I didnt want to fry my ribbon tweet´s so powered it down and put it in storage. As I recall the +/- 80V were spot on but the bias moved up and down. Could it be the TL072 and 2SK146 messing up? Carefully sanded the relay today to eliminate that as a fault. I think its a dry cap somewhere, wouldnt that give problems like this?
It triggers the fuses on the wall outlet, even if they are 16A, 230V troughout the apartment..
Click to expand...

The opamp(TL072) is a DC offsett servo and dual Jfet differential pair (2SK146) is the amplifiers front end differential amplifier which must be pretty a closely matched pair to maintain the amplifiers balance between the positve portion of the sine wave and the negative portion-- directly related to the DC offset..Check your DC offset/DC balance . the closer to zero volts the better( let the amp warm up for 15 min.- test with no input and no load..Try cleaning your two input attenuators controls with deoxit first.:thmbsp:
 
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avionic said:
The opamp(TL072) is a DC offsett servo and dual Jfet differential pair (2SK146) is the amplifiers front end differential amplifier which must be pretty a closely matched pair to maintain the amplifiers balance between the positve portion of the sine wave and the negative portion-- directly related to the DC offset..Check your DC offset/DC balance . the closer to zero volts the better( let the amp warm up for 15 min.- test with no input and no load..Try cleaning your two input attenuators controls with deoxit first.:thmbsp:
Click to expand...

IIRC, the TL072 is readily available but the 2SK146 is not. Anyone have a source or a sub for the 2SK146? I have several PC2002M's that won't go below 200 mV offset. They were run in bridged mode so as a result, the left driver boards have servo problems.
 
Can you speak Japanese? Have you got a friend in Japan?



I tried for a while to get stuff from Interfet directly. I have no idea how to get anything from them.
 
question for you owners.... what's the width of the case of these guys? SM shows the width including the face with the rack mount. I need the width behind that.

Thanks...
 
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