Peavey Backstage 30 - Loud Constant Hum

[Jrm]

Greetings to the group, this is my first post and I've spent a fair amount of time scouring the boards for this specific amp and haven't really found much. It's an older model Peavey Backstage 30 that hums loudly as soon as it's turned on and without the speaker moving at all. I checked the switching jacks and cleaned them, replaced the large electrolytic capacitors, and the hum is still there. I'm reading 114mv DC and and .5v AC across the speaker leads. Do I move on to the rectifier bridge diodes next? I've got the schematic and a few years experience in electronics, but am fairly new to amps. I've got a meter and an (old) scope, but got weary of Googling and decided to check the forums.

I can't find a year stamped on the board or amp itself, but there are no tubes and no ICs. Just a bunch of transistors, resistors, electro caps, etc. and some very strange-looking diodes with what look like SMD caps stuffed inside.

Thanks in advance for any and all help/advice. Just wondering what my next move should be. Even though it's an old little amp, I've got the time to put into it if you guys think it's worth it.

Cheers,
Jrm

[52 Bill]

Welcome to the place.

With what you described, I would have guessed a bad filter cap. Seeing that you have replaced them, then it's time to move on to other things.

If you post the schematic here, everybody will be able to chime in. In the mean time, have you tested all of the power supply voltages? Do any of the controls change the hum in any way?

[Jrm]

I just turned it on again to feel around for any transistors heating up and found that the heatsink that the RCA1C13 transistors were bolted to was quickly getting hot. Should I swap out that whole section (2xRCA1C13/2xRCA1C12) for 2xTIP30/2xTIP29 transistors? And no, moving any of the knobs doesn't affect the hum at all. Backstage-30.pdf

[52 Bill]

I'd meter test the three diodes CR1, CR2 and CR3 first. Then meter test the transistors in the power amp before replacing anything. Remove the speaker load from the amp until the problems have been worked out.

The schematic lists TIP30C and TIP31C as replacements for the outputs.

Have you taken any voltage readings in the power amp yet?

[Jrm]

Thanks, Bill.

Should I take the diodes off the board and test them or do you mean check the voltage between them?
Thanks for the correction (that I edited incorrectly) on the TIPs.
I haven't taken voltage readings, but will do so now with the speaker removed and comparing with the data sheet.
I will report back. Thanks again for the advice/help.

[Jrm]

I went ahead and swapped out all of the old caps for new ones and connected another amplifier via the pre-amp out and did not get the same loud hum. A little with it cranked up, but nothing out of the ordinary. Then I re-soldered all the pots started checking voltages with the speaker unplugged. I got roughly 40v at pin 1 of both transistor pairs in the power section and the schematic shows them at 20.1v and 19.2v, respectively. I also measured the positive ends of both C21 and C22 and got 40.6v and 41v, respectively.

Hope this gives some insight.

[52 Bill]

I went ahead and swapped out all of the old caps for new ones and connected another amplifier via the pre-amp out and did not get the same loud hum. A little with it cranked up, but nothing out of the ordinary. Then I re-soldered all the pots started checking voltages with the speaker unplugged. I got roughly 40v at pin 1 of both transistor pairs in the power section and the schematic shows them at 20.1v and 19.2v, respectively. I also measured the positive ends of both C21 and C22 and got 40.6v and 41v, respectively.

Hope this gives some insight.

Your voltage readings are so far off, you need to start testing the transistors for shorted terminals. This amp uses a single polarity power supply and the output transistor pairs are connected in series across the +40 volts. If the amp is working correctly the voltage should divide evenly across the top and bottom pairs. So you should have +40 volts on the collectors of Q9, Q10 and Q11 and half that +20 volts on the emitters of Q12, Q13 and Q14. Because there normally is +20 volts at the midpoint where the speaker connects to the circuit C21 is added to the circuit to keep the voltage from getting to the speaker.

Replacing all of the caps and resoldering the pots is a good thing, but you should try to locate and fix the problem at hand first before changing parts in hopes of getting lucky and finding the cure. Doing random things can add to the problems and can make finding the original problem even harder.

Does your meter have a diode test function? If it does, test all of the transistors in the power amp for shorts and test the diodes also. Test everything in circuit and only remove something if it doesn't seem to test correctly.

Reviewing the schematic, it looks like the speaker itself is part of the grounding circuit of Q8, so reconnect it for the time being.

[J M Fahey]

My oh my, old memories.

Was chiming in to add the same last detail: here you need to connect the speaker or an equivalent load to DC reference R32 to ground.

Before replacing parts, please remeasure and post voltages at:
C21 top , Q7 base and emitter, Q8 collector , anode of Cr1 to cathode of Cr3 (meaning voltage across the entire diode string) , voltage across R29/30/31/33/34.

[Jrm]

Thanks for all the help, guys. I've spent the better part of the day identifying components and following traces. It's not been easy as some parts on the board don't match the parts in the schematic, so it's been a lot of removing the board to follow signal paths and Googling datasheets. Here's what I've got so far, but I'll need some advice before completing the checklist.

With the speaker attached and one lead of my meter to ground, here are the voltages I recorded at the following locations:
C21 Top - 35.2v
Q7 (which is actually a 2N3503) Base - 25.03v
Emitter - 34.2
Q8 (couldn't find much info on it or its cross-referenced part so I measured all pins) All pins read 36.2v
Cr1 Anode - 35.83v
Cr2 Anode - 34.8 and slowly falling
Cr3 Anode - 33 and slowly falling

And as far as reading the voltage across R29/30/31/34, should I keep my one lead grounded and check the voltages at the emitters of Q10-14? Sorry if this is a newb-ish question, but I'm starting to get a little loopy. I'll step away from this project for a couple hours and wait for further input.

Thanks again for all the help.
Jrm

[52 Bill]

All of your voltages are off value, the +40 is probably low due to the extra current draw.

You never answered the question regarding whether your meter can test diode junctions or not and whether or not you have tested any of the transistors or diodes in the power amp circuit.

Juan was asking for the voltage reading across each of the resistors, meaning one lead on one end of the resistor the other lead on the other end of the resistor. You can read the voltage to ground at each end of the resistors and extrapolate the value, but reading directly across the part is so much faster.

My best guess is that you have one or more shorted transistors here and will need to figure out which ones are bad so that you can replace them.

[Jrm]

I do have a meter with diode and transistor multi-function socket as well as a diode setting on the dial. It's a Mastech MS8268.
And I wasn't sure about checking the resistors that way, so thank you for clearing that up. As far as checking the diodes and transistors in the power circuit, should I check each individual diode with the power off and a lead at each end? Again, this is a bit of a different animal for me, but I do recall reading that diodes are best tested out of circuit. If that's not the case in this scenario, just let me know. And in regards to the power transistor pairs, what's the best way to test those in-circuit? I'll try to do some more reading in the meantime, but for the time being all I know is that the heatsink on the RCA1C13 pair gets hot quickly and starts to smell funny.

Just tested the voltage across R29/30/31/34 individually and each read around 190mv and climbing.

Can't thank you all enough for the help, again, amplifiers are a new animal for me. I very much appreciate your patience as I learn my way around this.

[52 Bill]

Okay, here's a quick lesson in transistor and diode testing. Set your meter to the diode test position. The amp should be off, unplugged and dis-charged. Test everything in-circuit until you get a reading that needs you test something out of circuit.

Let's start with the diodes. Take the leads and touch them across the diode and note the meter reading. Now reverse the leads across the diode and note the reading. If the diode is good you should get a low reading with the leads in one direction and a higher reading with the leads reversed. Most meters read the conduction voltage of the diode junction so the low reading for a silicon diode will be around 0.6-0.7 volts. The reverse reading should be the same as with the leads wide open, not connected to anything.

Testing the transistors is similar to diode testing. Start by holding one lead to the Base of the transistor. Now touch the other meter lead to the Emitter and note the reading. Keeping the connection to the Base, touch the second lead to the Collector lead and note the reading. Now reverse the meter leads and note the readings. Just like the diode test, in one direction there should be a low reading and a high reading in the other. The NPN transistors will test the opposite of the PNP transistors, meaning that the red lead connected to the Base will give you low readings for one type of transistor and the black lead connected to the Base will give low readings with the other type of transistor.

Finally touch one lead to the Collector and the other lead to the Emitter and note the reading. Now reverse the two leads and note the reading. When testing the C-E connections, you should get high readings in both directions with normal bipolar transistors.

If you have any questions just ask. Let us know what you find out.

[TomCarlos]

JRM - I bought an Peak Atlas DCA - Semiconductor Analyser - Model DCA55 - very cool, easy to use. The more expensive version has a software interface that does curve tracer like displays on your computer.

[Jrm]

Thanks Bill. Q7 (which reads 3904 but is actually a 2N3053) was positioned in such a way that I couldn't get my probe to all of the legs, so I removed it to measure it out of circuit. The bottom of the 3053 is charred black and I got the following readings with it out of circuit and on a breadboard with my meter in diode testing mode:

Positive lead to base, negative lead to collector: .608v
Positive lead to base, negative lead to emitter: .626v
Negative lead to base, positive lead to collector: OL
Negative lead to base, positive lead to emitter: OL
Positive emitter, negative collector: OL
Negative emitter, positive collector: OL

Diodes CR1, CR2, CR3:

CR1: .574v Leads Reversed: OL
CR2: .567v Leads Reversed: OL
CR3: .574v Leads Reversed: OL

CR4: .496v Leads Reversed: OL
CR5: .505v Leads Reversed: OL
CR6: .496v Leads Reversed: OL
CR7: .505v Leads Reversed: OL

Other transistors tested in-circuit in diode testing mode:

Q12 (which is a 3644 and not 3638 as listed in the schematic):
Positive lead to base and negative to collector: OL
Positive lead to base and negative to emitter: OL
Negative lead to base and positive to collector: .670v
Negative lead to base and positive to emitter: .674v
Positive emitter, negative collector: OL
Negative emitter, positive collector: .504v

Q9 (3642): Positive base, negative collector: .001v
Positive base, negative emitter: .000v
Negative base, positive collector: .001v
Negative base, positive emitter: .000v
Positive emitter, negative collector: .001
Negative emitter, positive collector: .001

Q8 (690): Positive base, negative collector: OL
Positive base, negative emitter: OL
Negative base, positive collector: .690v
Negative base, positive emitter: .690v
Positive emitter, negative collector: .001
Negative emitter, positive collector: .001

For the power transistors (Q10, Q11, Q13, Q14) I couldn't find much data on them, but with a little research made an educated guess that the pinout is 1(base) 2 (collector) 3 (emitter) and based on that data I tested the power transistors with the diode function to get the following results:

Q10: Positive base, negative collector: voltage rose till it read "OL"
Positive base, negative emitter: voltage rose and stalled out around .910v but seemed like it could slowly keep ticking upward
Negative base, positive collector: .504v
Negative base, positive emitter: .518v
Positive emitter, negative collector: .550v
Negative emitter, positive collector: OL

Q11: Positive base, negative collector: OL
Positive base, negative emitter: .897v
Negative base, positive collector: .504v
Negative base, positive emitter: .518v
Positive emitter, negative collector: .550v
Negative emitter, positive collector: OL

Q13: Positive base, negative collector: .001v
Positive base, negative emitter: .457v
Negative base, positive collector: .001v
Negative base, positive emitter: .818v
Positive emitter, negative collector: .818v
Negative emitter, positive collector: .455v

Q14: Positive base, negative collector: .001v
Positive base, negative emitter: .457v
Negative base, positive collector: .001v
Negative base, positive emitter: .818v
Positive emitter, negative collector: .818v
Negative emitter, positive collector: .455v

Hope this data provides some insight. Again, I did all of those tests with my meter in the diode setting, so hopefully I did all of that correctly.
Thanks again for everyone being so patient and helpful. Really curious about the charred underside of that RCA 2N3053

Cheers,
Jrm

[52 Bill]

Okay, you're making some progress now. All of the diodes have tested fine and Q7 Tests okay as well. Are you sure that the bottom is charred or is it oxidized? I've seen some cases turn black from oxidation before.

If you look at the reading of the other transistors, you should notice some problems like Q9, which tests as shorted on all leads. This is one that you definitely need to remove to retest out of circuit. And once you have removed Q9, I'll guess that the 0.001 C-E reading on Q8 will go away.

Q13 and Q14 have shorted terminals as well, but because they are connected to each other in parallel, one may be shorted and the other one may be okay. Pick one to remove and retest and see what you find. For the time being, if you have some sort of solder sucker or removal braid, you can just unsolder the center lead of one of the two and see if one or both transistor is shorted.

In any case, if you remove the transistor from the heatsink be careful not to damage the insulator that isolates the transistor case from the heatsink. You will need to reuse or replace it when you install the new transistors.

Let us know what you find.

Edit: Are you sure that it's Q13 and Q14 that test shorted from B-C and not Q10 and Q11? When you remove Q9 retest the outputs before you remove anything else. Q9 is also connected in parallel to Q10 and Q11, so the zero reading could come from the driver or from the outputs.