help repairing crossfire CFA602 amp

[grjr]

Hi, I'm trying to repair a crossfire CFA602 amp that has a few problems. so far I've removed one output transistor Q125 (D718) that was shorted, I replaced the two +/- 15V zeners D106/D107 which shorted, I've removed all the preamp OPAMPs (just in case), I replaced small signal transistors Q131,Q132 (one of which was shorted) and the amp will still not power up.

Pin 4 of the TL494 is being pulled high ~4.8V which seems to be shutting down the PS. every few secs the pin will go low but then go back up again. I'm guessing it has something to due with transistors Q131,Q132 because they seem to be part of some feedback to the PS for protection. I don't have a schematic so this is just a guess from following traces. The thermal resistor is measuring around 4.7k ohms, I'm guessing that's typical?

All my measurements are being done with a DMM, if need be I could probably take this thing to school and use the scopes there. If anyone could give me tips on what to check or has a schematic that would be really helpful, thanks!

http://picasaweb.google.com/grjr2000/Crossfire

[Perry Babin]

Q130 also fails. Sometimes the transistor is only leaking and not completely shorted so you may have to remove it to determine if it's defective.

[grjr]

ok, turns out Q230 (on the other channel) has an open BE junction. Q130 seems to be fine. The part is C3200, I looked in my collection of new and used transistors... and I don't have it. Any other common transistor that I could replace it with just for testing?

[Perry Babin]

If this is the transistor that is connected across the emitter resistors for current sensing, the amp will power up without the transistor. The over-current protection will be defeated so you'll have to watch the current draw carefully.

For a replacement, virtually any NPN transistor with the same pin configuration that's rated to handle the total rail-rail voltage will work.

[grjr]

Hi, Perry... thanks for the help, don't think I would have found the open transistor if you hadn't pointed me in that direction. I think I am going to wait on trying to power up the amp until I can get replacement transistors. Don't want to fry anything else on the board. I checked the specs for the C3200 bjt and it's rated for 120V VCE. Anyone know of a good source for KEC parts?

[Perry Babin]

http://www.mcminone.com/

KEC doesn't show a KEC3200 on their site so it's probably not a KEC part. Search for a 2SC3200 on the mcm site.

[grjr]

I've been doing a little bit of searching and I've found some transistors and OPAMPs that I believe I can replace the originals with.

replace:
KEC KTD718 with Fairchild FJA4310
KEC KTC3200 with Fairchild MPSL01
KEC KIA4559 with NJR NJM4559

I would upload the datasheets but the network I am on seems to be blocking FTP. Here's the sites where the datasheets are available:

http://www.keccorp.com/
http://www.fairchildsemi.com/
http://www.njr.com/

I believe replacing OPAMPs and small signal transistors with these other parts won't pose any problems. The power output transistor might be a problem? The reason I sought out these replacements is because mouser carries these parts so they are easy to get. Any opinions?

[Perry Babin]

Unless the op-amps are defective, you should not change them.

If Fairchild or Mouser crosses the parts to the numbers you posted, there's a very good chance that they will work properly.

[grjr]

The transistors I listed cross on the Fairchild website to the original parts. The NJR OPAMP I chose because I think they make good OPAMPs. The Fairchild cross is KA4558 for the original OPAMPs.

Isn't OPAMP selection not very critical? I mean as long as they have the same pinout, voltage specs and such should it matter? They just provide gain control and filtering correct? I destroyed the original OPAMPs when I removed them from the board (I was lazy and cut the legs )

[Perry Babin]

Generally, if you replace the op-amp with exactly the same part number, there won't be any problems. If you use a different part number (as some do when 'upgrading' their amps), there can be problems.