Ampeg b-100R Output Short?

[Mr_bibbles]

Hey All-


Got a B-100R Ampeg Bass Rocket on the bench. Did my homework, seems like these have problems with their output trannys. Read a short on one of the two TIP142s, replaced it and started it back up on my current limiter. Still shorting.... Here are some measurements. Can you solve the mystery?

Diode Checks-

Q5 TIP142 (new)
Base - Emitter: 0
Base - Collector: .543
Emitter - Collector: .543
Emitter - Base:0
Collector - Emitter: .658
Collector - Base: .658

Q6 TIP142
Base - Emitter: 0
Base - Collector: .542
Emitter - Collector: .543
Emitter - Base:0
Collector - Emitter: .636
Collector - Base: .636

Q7 TIP147
Base - Emitter: 0
Base - Collector: .806
Emitter - Collector: .807
Emitter - Base: .001 / .000 (fluxuates)
Collector - Emitter: .561
Collector - Base: .561

Q8 TIP147
Base - Emitter: 0
Base - Collector: .805
Emitter - Collector: .806
Emitter - Base: 0
Collector - Emitter: .561
Collector - Base: .561

Voltages:
Emitters -.08mv across the board

Base Q5 20.7mv
Base Q6 1.8mv
Base Q7 -3mv
Base Q8 -1.8

Collector Q5/Q6: 4.1mv
Collector Q7/Q8: 6.4mv


On the Schematic, where it says +40v and +20v, I don't even get a single volt.

AND I just found out all emitters have about 1.4ohms resistance to ground Should I be looking for the short to ground and ignore the trannies?

[52 Bill]

AND I just found out all emitters have about 1.4ohms resistance to ground Should I be looking for the short to ground and ignore the trannies?

Disconnect the speaker and see what happens.

[Mr_bibbles]

was already running it without a load, same result with speaker disconnected or connected.

[nosaj]

was already running it without a load, same result with speaker disconnected or connected.

Start at the top with the power transformer check the voltages with it disconnected. Cause if it's not right nothing else will be.
Then move to the power supply once power supply is supplying correct voltages then move down the chain. They have a thing around here about doing you homework. Each amp is like an individual different issues. Kinda of like not all muslims are terrorists even if if says so on the internets.

nosaj

[The Dude]

I'll add, it's a good idea, when you post a new thread, to include a schematic so the rest of us don't have to go hunting for it. Posting diode check readings is great, but without a schematic, we have no idea what you're talking about.

[Jazz P Bass]

http://music-electronics-forum.com/a...00r-b100ra.pdf

[The Dude]

What nosaj said. We need to figure out why no positive rail first. Also, leave the speaker disconnected until you get it fixed and verify that there is no DC on the speaker output.

[Enzo]

If the transformer were bad or unpowered, we'd be missing the negatives as well. Are you? +20 is derived from +40 in the power amp. DO you have the +16 in your preamp for the ICs? If you do, that means the +40 supply is there, but not reaching the power amp. If the preamp is not seeing +16 either, then we suspect the +40. Point being the +40 mioght exist but due to a cracked solder joint or something is not reaching the power amp circuits.

Now when you say you are missing +40, are you also missing -40?

[Mr_bibbles]

Sorry, should have mentioned I did the transformer continuity/short tests and also can power up the amp without drawing excess current while the secondaries are disconnected. (dim lightbulb limiter)


Coming off the secondaries leading to the rectifier:

j5: 23mv and j21: 20mv

Exiting the rectifer is -23 mv and 73mv instead of +40 and -40


At D10 and D14, 40+ reads as about 70mv and 40- reads as about -1 mv

[The Dude]

Measure and post the AC voltage from J5 to J21, please.

[Mr_bibbles]

Thanks Dude-

To Clarify, do you mean measure the flow of AC voltage BETWEEN the secondaries while connected to circuit? Or just the between the secondaries? Or from each Secondary to ground?

[The Dude]

One probe on J5 and one on J21- meter set to read AC volts. It's not really "flow" but rather a voltage potential we are looking for. We need to find out if the rectifier is getting AC. No need to disconnect anything. If we had a short, the fuse would be blown.

[Mr_bibbles]

Thanks for clarifying and explaining!

I have been using a lightbulb limiter this whole time and it's glowing bright as the sun... assuming if I took it off I would blow a fuse immediately.

Voltage between the secondaries measured on the Rectifier is 2v AC, and it starts to decrease. Watched it get as low as 1.8, but it was decreasing very slowly at this point.

[The Dude]

Ah! I wasn't aware we had the bright bulb. With the low voltage readings there and the bright bulb, I suspect we have one or both shorted rails. With power off, measure the resistance of each of the 40V rails to ground. When you replaced the outputs, did you make sure any sil-pads were put back, isolation washers installed, etc. Maybe one of your new transistor's tabs is shorted to the heat sink?

[Jazz P Bass]

"Maybe one of your new transistor's tabs is shorted to the heat sink?"

Or all of them.

All of the Collectors could be sitting on the heatsink.

B-100R Output Section.pdf