Return of G K 400 RB.

[catstrat]

Here we go again. This is the same 400rb from a couple of months ago. I decided to give it one more go before giving up. I ordered brand spanking new MG 15003 and MJ15004s from Mauser. Got them installad and tested it . The limiter bulb was lit but not nearly as bright as before. I could get the bulb to go out by adjusting the bias pot. I'm at least on the right track. I wanted to check the bias using the scope method and i plugged a speaker into the spaeker out,turned the amp on and the bulb lit very bright just like before and had a bad hum. I have positive 43 volts dc at the O/P. But with speaker unplugged the bulb is off but the voltage is there regardless. What gives? I keep suspecting a shorted diode somewhere.I replaced one flyback diode already according to the schematic,layout i have. Thanks.

[catstrat]

400 RB

The issue is when i plug a speaker in, the d c voltage is shorting to ground via the voice coil. I'm not so sure the schematic/board layout i have is even the right information. They've only had 4 or 5 circuits they used, or at least that's my understanding. I pulled the filter caps and none of them showed a short.The dc at the o/p is probably what caused everything in the first place. You know, i don't drink but this may be a good time to start.lol.

[Jazz P Bass]

Not too sure which amp version you have.
I thought it was an older one.

I have attached the 1982 version.
The same or not, there are a few static Vdc readings labeled.
I would suggest that you see what is off on yours by comparing them.

A quick down & dirty check is to look at the output transistor bases.
More than 1/2 a volt & you are looking at trouble. (hot bias)

[catstrat]

400 rb.

Thanks Jazz. That's the schematic i've been working from. On the back of the amp is a white block with the numbers 3-85 in it. I'm thinking that's the build date,but not sure. The o/p board has a number of 60041 and the serial number is 31322. Enzo checked his archives and said his board layouts skip over the 60041 board number.

[Jazz P Bass]

Still, lots of pertinent voltages listed.

No matter which amp it is.

[catstrat]

400 rb

The layout i have shows some of the componants facing in different directions. I know wrong bias can cause a hum, but will it cause +43 volts at the output. I rebuilt the bias circuit already. I could trace everything down but i'm not very good at doing that with SS circuits. I'm hoping someone on here has had experiance with this problem before. Thanks.

[catstrat]

It's funny i repaired a QSC 1450 power amp a couple of weeks ago and it's working fine. This 400rb isn't nearly as complicated as that QSC and it's the one giving me fits.

[J M Fahey]

Well, you only started repairing it, still more to go.

1) you replaced all 4 output transistors?
2) you could make the limiter bulb go out by lowering bias? (with amp unloaded)
Can you still ? (if not, 1 or more transistors got damaged while testing )
If you can start with bias to minimum (series bulb very weak or off) .
3) measure DC voltage at the speaker out, still without speaker: how much?

Post results.

[Jazz P Bass]

You do not have to 'trace' anything 'down'.
A few voltage measurements will lead you to the problem area.

Note: solid state Class A/B amplifiers can be tricky repairing, to say the least.
Every section is relying on all of the other sections to balance the output to zero volts.
One little kink, and the output will go high.

To start, we use our senses. Is it hot. Does it smell. Is anything burnt.

After that, you have to use your tools.
DVM, scope, lamp limiter.

[g1]

Not sure if you ever did replace the drivers, Q7 and Q10 ?

[catstrat]

400 rb

Hi J.M., Without a load the bulb is completely out but i have the + 43 volts at o/p. It's when i plug a speaker in, the bulb lights bright with a hum. With a load all i'm doing is dumping that +43 volts to ground. This is a fight to the finish. lol.
Hi Jazz, w/o a load there very little heat. I measured the voltages at the o/p bases and emitters and get a load of this, they're anywhere from + 43 to +45 volts on all 4.
Hi G-one, i replaced all drivers and all of the to 92s on the board. But that doesn't mean one of them hasn't cratered on me. As i look at the schematic,assuming it's the right schematic, i wonder if something could be wrong with the rectifier. I've checked the filter caps out of circuit and none of them showed a short. When i check the O/Ps trans in circuit, they all show a short across B-E,B-C and E-C. Now if i remove them from the circuit and test them ,they test fine. This is the reason on something like this, i use the old shotgun approach. I hate to do it this way but sometimes for me, it's the best way. Besides, i would rather be safe than sorry. Thanks everyone for your input. I'll keep you updated.

[g1]

When i check the O/Ps trans in circuit, they all show a short across B-E,B-C and E-C. Now if i remove them from the circuit and test them ,they test fine.

Do these same measurements at their board connections, but with them removed from the circuit. Any shorts still there?

[catstrat]

400 rb

I just found that if you check the voltage at the o/p while adjusting the bias pot,the voltage starts to go down drastically, but the bulb limiter keeps getting brighter and the o/p transistors start to get hot. So maybe it is something with the bias circuit i've either overlooked or a transistor in that circuit has fried. I'll check again for solder bridges and start removing the trans in that circuit and test them. Could a shorted driver cause this. If a driver is shorted you would be sending possible rail voltage the o/p transistors bases, not good. My question, where should i start my testing? The drivers/pre drivers or the bias circuit? I did notice a couple of resistors that appear to be in the bias circuit showing sign of overheating. That will cause the resistor to go out of specs and that will cause problems. I'll start my search with those. I'm so grateful to you guys for putting up with me and my sometimes ignorance. But, this is how i learn. Shoot, i'm even keeping that flowery language to a minimum,lol.

[Jazz P Bass]

If the bases of the output transistors are on that hard, work your way back.
Q8, Q9 base +.5 Vdc
Q11, Q12 base -.5 Vdc

They are driven by:
Q7 base. +1.1 Vdc
Q10 base -1.1 Vdc

They are driven by the bias string.

[catstrat]

400 rb

I remember checking the voltage before with the o/p trans removed, and as i remember the were normal. If it is another shorted o/p transistor, and these are new, what is causing this? I haven't even had the thing turned on more than maybe a minute total. There is one thing i just remembered, when this first came to me it had two resistors that had overheated to the point ,i couldn't read the color code. So going by the board layout i have, i replaced those resistors. Now they're showing signs of overheating again. But i don't know if i used the right values are not. Seems like one was a 2.3 k and i think the other was 2.3 k as well according to the layout. Again the layout i have is totally different than my p c board layout. Let me look at the info again and i'll get some numbers for you and see if you can make sense out of it. Be right back.