Zapco Z400C4-SL... Not Again!

[1moreamp]

0.1 mv will work and not cause any undue heating of the semi's after that you can raise it as you like I am thinking...

[AKHeathen]

i generally set them using the current meter. perry has a little interactive simulation. back them off to zero, then slowly raise to the point that current starts to barely draw, and repeat.

[90scaraudio]

Got it... So since it a 4 channel amp, I thinking turn all 4 pots down, start with one and bring it up until the current draw is 1/2 amp, then do this to the remaining 3 channels so I get a total of 2 amps idle?

Originally Posted by Perry Babin

As was suggested, you need to bias the transistors. If you have a power supply with an amp meter, set it as is shown in the following demo.
http://www.bcae1.com/temp/ausettingbias.swf

[AKHeathen]

i pay less attention to how much it is, as long as it is in a good range at idle. it could be 1.5-3a idle typically, but anyways, it goes like the demo x4. back all off, like you mentioned, and raise them until the current just starts to pick-up. sometimes i do it a few times to be sure. then move to the next, and repeat until all are done.

[90scaraudio]

Think I'm finished...
On channel 3 which is rear left, I replaced the MPSA06 near the finals, replaced a TIP102 and 107 Which are part of the finals. Pulled a TIP35C but it was good.

I really don't think the problem was in the final area as everything I pulled was good. So on to the next...

I replaced the 2N6715 and 10uf 50v, part of the PWM, don't think they were bad.

Pulled the 4700uf 50v caps, they are good.

Around the NAND Gates, I replaced two MPSA06 and one MPSA56, again, don't think they were bad.

Replaced the two J109 on channel three, they weren't bad but I could smell capacitor liquid. Cleaned it the best I could.

Replaced the two NAND Gates

Replaced 12 OP-Amps, No more static noise now.

OK, When I pulled the CENU-07 and CENU-57 from channels one and three, I did not use enough heat and pulled small traces off the board. Spent about 4 hours repairing them

Idle current is back to 2Amps.

Adjusted bias so now Idle is about 2.2 amps.

Not sure what fixed it, maybe cleaning the rest of the leaky cap off the board?

Now just have to put it all back together

[90scaraudio]

These are the traces that pulled and ripped off the board.
Also when cleaning this transistor and resistor came off and the pads were very corroded. NOT FUN!!!

Also the semi and cap I changed by the PWM

And the Semis plus NAND Gates.

[90scaraudio]

OK, For testing, I pulled two of the four 4700uf caps out. When I reinstalled them, it went back to protect mode. Every thing I've read and most Soundstreams use 1000uf caps for filtering. Is it ok to use 1000uf caps on this Zapco?

[90scaraudio]

Well I put in 4 new 1000uf 50v caps and it turns on with no faults and no big surge of current. Now I have to push it to almost clip to see if I made any mistakes, so far I've run it to the limit of one of my power supplies and the wife yelled at me...OOOPPPPs. Its a 12 amp supply.

Thanks all about the bias adjust info, really works...

[AKHeathen]

not sure about pushing it ti clip yet. you may encounter a little noise there, since you now have less that a quarter of the capacitance at the rail, and some of the lower frequencies may come through, like the a/c ripple (alt whine) which might come through the power supply. not sure, but it is a possibility. are you planning on going back to larger caps? did you find a bad one?

[90scaraudio]

They are old and they ohm out ok but with them in circuit the amp struggles to start. Plus these boards are so fricken delicate, when I was pulling two of the caps, two of the circular feed through that sits in the hole came out. The positive rail has the main trace in one of the inner layers and that sucks to fix. The ripple with the old caps was about 30 mv, now its down to 2 or 3 mv. I may get larger value caps at a latter time.

How come the old Soundstream reference 1000s only has two 1000uf 50v caps for its rail? See pic...