GK 800RB Mystery

[booj]

I've fixed several of these popular amps by just replacing all the T0-3 transistors. This one came to me with cardboard home made insulators made out of a cereal box, the power switch alligator clipped into the circuit and the fuse holder missing and short circuited. Also when he put the outputs in, he switched one of the NPN's and PNP's.
I was able to just pull the transistors out off the board because he didn't solder the transistors back in, even though the solder "mounds" were still there around the leads.
I diode checked all the outputs, drivers, diodes, and small transistors. All the power resistors measure good.
I did replace one of the T0-3 transistors with a new one. The rest are the originals. I know, I know, not a good idea, but I was just curious. Anyway.
I can only bring up AC input up so far before the amp starts to draw about 2 amps.
AT that point, both the 60 volt and the 80 volt supply are producing + / - 15 volts each.
So, here's the question. If I can get + / - 15 volts from both supplies, what is that indicating? Thanks for your time!

[booj]

800RB_82_91.pdf

Here's the schematic

[g1]

Doesn't really indicate anything to me other than there is no dead short across any of the supplies. Try disconnecting one of the supplies, then the other and see which is causing the current. Sometimes you also just get current spikes when bringing something up with a variac. In these cases you bring it up a bit more and things straighten out. However 2 amps at +/-15 seems a bit extreme for such a scenario.

[booj]

Of course. I love this forum. I forgot to isolate which supply was being drained. That would be a big help. Thanks G-one!

[Enzo]

Sounds like both sides are conducting at once, which leads me to think you have an open resistor.

[J M Fahey]

This one came to me with cardboard home made insulators made out of a cereal box

Well, not bad at all for a 6 year old technician. Ooops, let's correct that to 5 YO.
Back to the amp, don't "lift one supply to see which one is shorted" or whatever, because that's not the answer.
We don't have "two supplies" there, but functionally a single one, or , if you want, two equal ones in series..
We have a center point simply to avoid an output coupling capacitor, and ground our speaker there for convenience, but the amp itself goes rail to rail, with output transistors in series.
If the faulty amp draws, say, 2A from the + rail, the - rail will sink those same 2A.
The only way you can have different currents, is when there is DC voltage on the speaker, but that is the first thing you must disconnect for troubleshooting. Did you?
Good luck.

[52 Bill]

JM:
I don't think that he is talking about one half of a bipolar supply, but about one of the two separate power amp supplies.

The 800RB has a 300 watt amp (+ & - 80vdc) and a separate 100 watt amp (+ & - 60vdc) with two different power supplies.

[booj]

Thanks so much for your time!
I disconnected the positive +80 volt supply (+85v on the scat) by chance, first. The amp did not draw any appreciable current. At least much less than one amp. That's with 120vac input.
I then started replacing all the power transistors connected to that rail, including Q12. No change.
My thought is to go from there changing out Q11 and Q15, then Q13 and Q14. But before I do that I will look again at the resistors and measure them both ways to see if there is an open one.
Thanks for your replys. It aleviates that sinking feeling. Will get back to ya!

[J M Fahey]

Oh, Bill, thanks !!!
Hey !! Where's the Emoticon/Smilie where I hit my own head with a hammer ?
Yes, if there are two power amps with individual supplies it makes all the sense in the world to test them separately, of course.

[g1]

Yes, I meant disconnecting both sides (+/-) of one supply at a time, to isolate which amp is defective. However, it sounds like what JM was afraid of did occur.
Booj, running the amp with one side of a supply not really get you anywhere. I can't even guarantee that the reduced current draw with +85 disconnected means the problem is on the positive side of the amp. What I'm trying to say is not to ignore the bottom side. With no load the center point (output buss as Enzo calls it) is floating so a problem on the bottom half can be hidden when +85 is disconnected.
The other problem is you have to totally second guess the previous repair "attempt". Are there other incorrect parts, parts in backwards etc? Has the bias pot been cranked the wrong way?
Try disconnecting both sides of the 85V supply. Are the preamp & 100W amp both good? If so you have a very close working version of the amp to compare to.
Just a few extra output devices & higher voltages in the 300W.

[booj]

The amp is working now. Turns out one of the MJ15023's was leaky. Something I don't usually see. I wound up replacing all of the transistors on the heatsink and the board. The bias pots did need adjusting. I got a reading of zero for the 100 watt amp. After thourohly checking out the circuit, it turned out to be the bias adjust pot not turned enough. Attached is a picture of the "previous repair fix"

[J M Fahey]

ROFL squared !!!
At least the Pokemon label is readable, as well as the box code.
Check it, you might have won a prize !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Some day, we *deserve* to know the history of that amp.
The owner obviously must know it.

[Enzo]

Digimon output transistors. Sell the thing and tell them it is "Digimon-ready"


That is a hoot.