Yamaha pc4002M inside pics and a meter problem

[Craig405]

Hi all,

I have a pair of Yamaha PC4002M amplifiers, theyre pretty chunky things and I think the same era as the MX1000 commercial amplifiers (which come up in a few threads here) so I thought some pictures of the guts might be of interest to some people.

I also have a problem, the VU meter on the right channel of one amp seems to be stuck at around 0.1 watts when the amplifier is on or off, repeated flicking of the meter will align it almost to zero again.
The odd thing is when the power exceeds this 0.1 watts the meter jumps into life and works perfectly. Any suggestions on fixing these things?

Cheers!

P.S. if anyone has a service manual id be very interested!

[Craig405]

and another pic of insides

[Leolabs]

PCB bad solder joint/contact?

[eyoung]

hello again,

I have Yamaha B2 middle 70,s amp with a meter issue. this amp has more transistors and circuitry for the meters than most of the amps we discuss and drool over in these forums. Quite a daunting task for most of us even with some skills. That being said be brave and start poking around with all the usual caveats
leathal voltages and such, I use bamboo skewers and nylon pot adj tools for discrete poking when looking for errant solder joints.
good luck with your meter and keep us posted.

Regards, Elwood

[Craig405]

Thanks for the replies.

It could be a bad joint, although when the unit is off the meter remains stuck and will eventually settle back don to zero again with enough flicks on the glass. So im thinking its a mechanical issue, i have no idea how servicable these things are though.

I dont know how complicated the meter driving circuit is but if it is an electronics fault i hope its not too involving.

[Craig405]

Some more pics, ive just pulled the amp modules out of the chassis for cleaning and am impressed by the quality of the PCB's.
They seem to have copper bus bars running around them! I guess its some kind of shielding / special grounding scheme, either way its a sign of quality!

If anyone has the service manual for these id love to have it.

Capacitance seems low with only 18000uFx2 per amp (so 4 caps in total as its completely Dual mono).

Specs are:
430W ito 8R
700W into 4R
1.4KW bridged into 8R
THD 20hz-20khz < 0.05%
IMD 20hz - 20khz less than 0.01%
slew rate +/- 60v/us stereo, +/- 100 mono
response = 10hz - 50khz +/- 1dB

[Craig405]

Some of the copper rails.

[Craig405]

another PCB pic

[ostripper]

I see it all with that last shot, a very typical topology.
those tall to-126 trannies in middle (VAS) , a very well compensated input (lots of poly caps) the two small
to-220 drivers (on heatsink) and those dual rails (20A short
term currents) Those meters sticking could be mechanical.
nice amp...
OS

[Bonsai]

Its really nic e to see a well engineered produc t like this. Not cheap, but I bet if you clean it up, do a bit of re-capping and sort the meter problemout, you will get another 20 years out of it.

Good luck with your project.