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Messages - ilyaa

#61
just an update -

I ended up replacing every IC on the preamp - after a fiddly 'biamp crossover' pot that almost sent me back to square one - it was one of those 'everything works!' and then put in back in the chassis and nothing works again! - but luckily just a bad solder contact -

we are back in business!! I replaced the flimsy ribbon cable with individual stranded wires - time consuming but much more sturdy - and had to replace a couple broken send/return jacks on the back -

looks like there was something serious that happened to the +/- 15 rails because all of the op-amps were fried. but I've ran it for quite a while now and we seem to be back in business. I did replace the zeners and regulator transistors for the low voltage rails  - think those were the initial problem.

anyway, thanks guys! kind of a long saga but all's well that ends well
#62
any reason those ones i posted links to wouldn't be suitable replacements?

#64
k ill change Q219 and Q220 - what about Q215-Q218? those measure okay, too, but wondering if it'd be safer to swap them, as well.....

and, just as a note: i have been using a bulb limiter (see the rest of my post above), but i can't put the amp into 'real' operational mode with the limiter inline because all the voltages are about halved by the bulb.
#65
hm.

thought I was being thorough!

replaced the outputs. tested everything very carefully. with no load made sure voltages on the bad channel matched the good channel as close I could. plugged it into an ammeter (no load still) and watched it sit - just the bad channel - quite stable at about 0.64 amps (just like the good channel).

plugged in a load - carefully and slowly turned it up, thinking to just run it at a couple watts and watch the current - and before I knew it BAM. again....fuse and outputs blown....

(I had ran the good channel for a while - totally stable totally linear at up to 100 watts - didn't push it past that yet)

I did not replaced the drivers - but they tested fine in and out of circuit!

what do you think, Enzo, should I just replace all the drivers and pre-drivers (and anything else, transistor-wise) even if they are testing good? I'm pretty baffled at this point....
#66
edit:

lemme run a thorough go through then ill get back to you!
#67
how crucial is having matched output transistors in this amp?
#68
thanks, enzo -

biased the 'good' channel properly and its staying stable!

replaced the transistors in the 'bad' channel intending to try to bias it and BOOM - blew them - this time in a split second.....

hmmmm. i saw that this time they took R239 with them and also R241 - R246.  i noticed while replacing R241, etc, that these are 4.7 ohm resistors in this yamaha. on the schematic, the PNPs have 4.7 ohms in this spot and the NPNs have 47 ohms in this spot.....why does my amp have 4.7 ohms in both??
#69
hm good thought -

im waiting for replacement output transistors for the 'bad' channel, but in the meantime i plugged it in with only the 'good' channel hooked up and measured output mains current - at first it seemed okay but as i let it sit (no load, no input) it slowly started to increase - after a little while it was pulling 1.4 amps or so - and it was hot to the touch! seems like there might be a bias issue on the 'good' channel, as well, so very likely there's one on the 'bad' -

there are two bias trimmers in there, what's the best way to adjust the bias on this thing?
#70
oh boy - everytime i have to pick this thing up im afraid its going to be the last time i pick anything up!

schematic can be downloaded here: https://elektrotanya.com/yamaha_pc4002_pc4002m_sch.pdf/download.html

anyway, here's a brief (i guess not that brief - buckle up) rundown:

1) belonged to a studio - sat around for a long time - inherited by another studio.
2) was going into thermal protection - i traced the issue to the left channel
3) took a look at the board - seemed from sitting around some solder connections looked cracked and not so good - did a general clean/resolder
4) went out of thermal - worked fine - i tested it running at 100 watts for thirty minutes or so - no problems
5) went back to the studio - worked for about 25 minutes then went back into thermal mode
6) got it back on the bench - once again issue in the left channel
7) this time, however, i noticed the fuse powering the left channel was blown
8) transistors measured okay - i plugged it in with a light bulb limiter and it worked fine - no thermal mode, but (because of the light bulb) the voltages were low so it wasnt quite in full operating mode
9) ran some signal through it - no problems
10) took the limiter off and plugged it in - worked fine for about 5 minutes at 10 watts then blew another fuse
11) check and got some bad measurements on output transistors - one PNP and two NPNs had blown
12) i took those out ran it without them and it worked fine at lowish wattage (10-25) for enough time to convince me it wasnt going to blow
13) OKAY i figured it was some bad outputs. ordered some new ones (these are hard to find dammit!), put them in, it worked fine for about 5 minutes then BLAM - blew a fuse and more outputs. these time they were different ones and in different locations.

any thoughts what might be going on here? i measured all the bias and load resistors they are fine - im particularly puzzled as to why its running perfectly fine for a few minutes and THEN blowing up - i dont wanna keep ordering these expensive resistors without a better game plan -

thoughts?
#71
i know, g1, isnt that weird??

but it matches the compressor and there doenst seem to be a problem with the input so i wont look a gift horse in the mouth!

i ordrd a couple NE571Ns - ill let you guys know what happens!
#72
loudthud - i appreciate the link but if you read above in the post the whole reason im in this fiasco is that ISNT right the schematic - that is NOT the chip in this amp -

that schematic is very similar to this amp but, the preamp at least, does not match up. ive got a 16 pin compression chip, as well as some other significant differences....
#73
seems right!!!

pins 8,9 are left open in this design - the VCC/ground pins match - and the input/output pins seems right - 6,11 are open, as well -

it would be really great to have the right schematic - kind of a brain-buster to trace the whole circuit around this thing - but i think you might be right on, J M!

#74
here are some pics!

look familiar?

id say late 70s.....

#75
think ive given up on finding the schematic -

anyone have any luck with asking fender for it?

all i really need, at least to get cracking, is to figure out what this compression IC is - its NOT an ssm2012 or ssm2013 - its some 16 pin thing -

anyone know what the alternate coliseum 300 preamp used for compression IC??