Gk 800rb

[sonic_tooth]

I am having some problems figuring out what is causing my 800RB to blow fuses. Transformer is good. I checked out the supply and it seems fine, nothing is shorting. The problem seems to be somewhere in the power amp. I had 2 shorted power transistors that I replaced, but the amp is still shorting when I power it up. Suggestions anyone?

[Enzo]

Sure, you didn't finish fixing the power amp.

Furst off, are you using a speaker or load? DOn't. If the amp is putting out DC, it is the LOAD that draws all the current. Try it without a load and see if DC is sitting on the output jack.

Now it may still blow fuses, or not, but quit blowing fuses. Either bring it up slowly on a variac watching the current draw, and back off if it starts up rapidly. Or lacking the variac, remove the mains fuse and use a limiter light. An old trick - wire up a regular light socket with a couple wires with clips on the end. Put a 100 watt bulb in the socket. Now clip this light in place of the fuse. Power up the amp. If it is going to blow fuses, the light will glow brightly and use up most of the current, rather than the amp. If the amp is relatively OK, the lamp will only glow dim.

In any case, the output transistors should be checked, and then the driver transistors - the ones that control the outputs. ALSO IMPORTANT is to check all the resistors in the area. The "emitter resistors" for the output transistors, are any OPEN? Then the drivers have resistors too, check them. Then the drivers will ultimately be connected together through the bias circuit - the thing that adjusts idle current. Make sure those parts are OK.

Fuses blow because either the output has swing over to one power rail or the other and the load is drawing current off that like crazy, OR both sides of the output are trying to turn on at the same time.

[Mars Amp Repair]

[QUOTE=Enzo;12219]Sure, you didn't finish fixing the power amp.

Furst off, are you using a speaker or load? DOn't. If the amp is putting out DC, it is the LOAD that draws all the current. Try it without a load and see if DC is sitting on the output jack.

Now it may still blow fuses, or not, but quit blowing fuses. Either bring it up slowly on a variac watching the current draw, and back off if it starts up rapidly. Or lacking the variac, remove the mains fuse and use a limiter light. An old trick - wire up a regular light socket with a couple wires with clips on the end. Put a 100 watt bulb in the socket.

Another 'trick' if your variac doesn't have an ammeter or has one that can't resolve low amperages is to connect up the mains fuse with clip leads (or like I use an old fuse block) to where you can see the fuse wire in the glass...bring up the variac & watch to see if the fuse element is bending. Nice easy quick visual indication of excessive current draw. You won't have to wait long to see if the unit is drawing too much current.
I agree it sounds like you've missed one or more of the drive transistors...don't forget to meter those bias diodes too. glen

[sonic_tooth]

Thanks for the help guys... It took me a little while to get to this project again (lots of OT at work). Anyway there were quite a few bad transistors that I had to replace. Diodes seem to be fine, but I am still getting a short somewhere.

[drewl]

I know Enzo's the master at these, but I still insist there's something inherently wrong with the design. If you send them (and 400RB's) back to GK they retofit the power section with a much simpler design utilizing TO3 power transistors.

[sonic_tooth]

Well upon further inspection, I think I might be the cause of some of the problems I am having. There is a good chance I might have accidentally swapped some of the output transistors in the process of replacing bad ones. Anyway, the schematic that I have seems to have a couple different types of transistors listed for Q7, Q8, Q10. Mine has only MJ15022 and MJ15023 types in it.

[Enzo]

The MJ150xx are TO3 types. Whatever the schematc says, those should work too. Those are the ultimate heavy duty power transistors. The 15022 is the NPN and the 15023 is the PNP. My schemo calls for Q8 as MJ15001, put the 15022 there, and MJ15002 as Q10, so put the 15023 there.

I know GK gave up on the switching power supply, and they will rebuild those into a linear supply. Is that what you refer to?

Make sure the insulating mica washer is in place. If Q8 is shorted, replace Q7 also. (MJE15030) And if Q10 is shorted, replace Q9 too. (MJE15031)

There are only ten xstrs in the channel, just test them all for junction drops with your meter diode test. And verify every resistor to the right of Q1 on the drawing. If Q7 was bad, then Q2 was sressed, which means R11 might be burnt.

[sonic_tooth]

Getting frustrated but I am determined to figure this out. Checked all transistors and Diodes and all seem OK. The schematic that I have is a little hard to read and I am having problems reading resitor values, making it difficult to check everything to the right of Q1 Went through and checked resistors out. No shorts.... R 11 is OK. Wouldn't I be able to see if any of those resistors were burned out and open? Thanks again for the help..

[Mars Amp Repair]

The notorious TL604CP discontinued IC

Hey guys,
FYI, Sonic brought this amp to me & it turns out that there is a bad tl604cp in the preamp section that is shorted & caused the +-15volt regulators to blow.
I've heard talk of these discontinued tl604's & how Peavey has a replacement in the form of a small pcb for like $20. I've done some searching on the web for this IC & have found a few sources, but would like to know if anyone has had any reliable luck with any particular supplier.
I found one (vancebaldwinl.com) that sells them for around $6 for the original TL604CP, but they have no means on their site to contact them. That makes me suspicious. Can't

Thanx, glen

[Enzo]

And when you called PV parts for it they said...?