Peavy XR-600B started buzzing. Video Included.

[Max Rambone]

Hello, everyone!

I've had this old PA for a while now and it's been fantastic. I love vintage music/audio gear.

I was running it one day and it just started buzzing as you can see in the video. If I remember correctly, it always made that noise when you first turned it on, but just for about half a second.

As you can see, it still somewhat lets sound come through, but the buzzing seriously overpowers it. I'm pretty good at soldering and working on stuff like this, but not very experienced with diagnosing them.

If it's something obvious like a swollen/leaky cap or crispy bit, I'm good, but after having a look at the inside, I don't see anything that jumps out at me.

I'm hoping that one of you amp masters will know exactly where I should start looking.

Also, does anyone have any good budget/bang for the buck alternatives to pair with the speakers in case I can't get this thing working again?

If I had to buy a new one, I kinda want it to be stereo, I think. I usually just hook a laptop up to it to "DJ" at family weddings or other events. I really don't need all the different channels because I figure I can just hook up my Yamaha MG10XU to pipe everything in.

Thanks!!!

[Enzo]

Either your output is making DC or you have lost a main filter cap. "Lost" could mean the part is bad or it could simply mean the solder to it has broken. What power amp board is in your amp?

[Max Rambone]

EEyyy, Enzo!! From my man Magic Johnson's hometown! I'm originally from the Detroit area...Troy and then Madison Heights.

I'm not sure what power amp board. Where would I find this? Attached are some pictures of it.

I have an ESR meter and I thought about just replacing the caps anyway, but I thought I would try to get some input first.

Thank you, sir, for your time and reply.

Apparently, I'm over my upload quota, so here's discord links:

https://cdn.discordapp.com/attachmen...113_165511.jpg
https://cdn.discordapp.com/attachmen...113_165517.jpg

[Enzo]

On the rear panel below the power cord and next to the heat sink, does it not have 400BH printed there? Your photos look like a 400BH.

Instead of ESR, the two big blue ones, flip the board over and see if the solder is cracked on one. In fact my scope is always on, and so is my meter. Check the two 50v rails for ripple. Instead of 50v, I bet one of them measures more like 30v with a ton of AC on top. SOme of those blue caps last forever. With the boards still in place, You can measure for rail voltage on the output transistor board. Q3,4,5,6 will have +50 on their cases. The negative side ones Q13,14,15,16 should have zero volts on their cases, but also -50 on the 0.33 ohm 10 watt resistors nearby. Either of those 50v supplies wrong?

You do not need a speaker for testing, but do measure for DC voltage on the speaker out jack.

[Max Rambone]

Yes, it is a 400BH, sorry. I just never paid attention to that before.

So, I didn't check the solder joints on the caps yet, but I did take some measurements. Not sure if I did it right, but the photo shows what I came up with.

The circled numbers are what I measured. I did indeed get +33V on the Q13,14,15,16 and the correct +50v on the Q3,4,5,6 assuming I'm measuring correctly.

The numbers below that is the A/C voltage present.

For the -50v, am I just supposed to measure across the resistor? I got zero volts on some and some were just a few mV.

On the speaker out, I measured 3.3mV. Oddly, when I unplugged the amp, it shot up to 1.1V and then slowly went down.

Thanks again, Enzo, for your help.

https://cdn.discordapp.com/attachmen...76802011_n.jpg

BTW, the schematic is upside down because that's how I'm looking at the board.

[Enzo]

I was asking for voltage readings to ground, not across the resistor. On th case/body of Q3,4,5,6 I want to see +50v. The body is the collector of the transistor. Look at the schematic. I mean the schematic page, not the layout drawing. Note the collector of those four connects right to +52. Now look down to the -52v side, it does not connect to the collectors, it connects to the emitters through those large resistors. SO I cannot measure the -50 on the bodies, but I can on those resistors.

If I interpret right you have zero volts DC where the -50 should be? Or do you have the -50 there but you have +33v on the cases of Q13...

[Max Rambone]

Yes, 0-3.3mV on the resistors the side you said should be -51 and +33V on the case of the transistor.

I did a rough paint edit on the schematic to show how I hooked the leads of my meter up.

https://cdn.discordapp.com/attachmen...76802011_n.jpg

The +50v side seemed good. The side you said should be 0V was +33V and the resistors on that side were the ones that should measure -50, so I'm guessing that if that side is NOT 0V like you said it should be, I won't get the -50 on the resistors, right?

Upon closer inspection, there is alot of corrosion around the transistors on the side that should read 0V, but is actually giving me +33V

[Enzo]

Um, how did you decide to put your black probe THERE? None of the terminals on that power transistor board is ground. We need to have voltage readings with respect to ground unless some other point is specified.

I generally doubt dirt on the parts is the issue.

Put the black probe to chassis. By ground I mean the circuit common, not necessarily the earth ground. Circuit common is usually connected to the earth ground, but if the boards are loose, might not be.

[Max Rambone]

Sorry. I looked at it and it seemed like the thick piece of aluminum that the transistors and board are screwed into were connected to the chassis, but it must be insulated from it somehow.

Like I said, I'm no expert at this stuff...That's why I'm here. Thanks for being patient with me. Here are the new measurements:

https://cdn.discordapp.com/attachmen...04/PA_test.jpg

[Enzo]

The metal heat sink is indeed grounded to chassis, but none of the parts on the board are connected to the heat sink. They are just mounted on it but insulated from it. In your earlier drawing it looked like you put the black probe on the area of the board where the large resistors connect. If you had it on the chassis/heatsink, that's OK.

OK, now it looks clearer. Note you have -50 on the one side resistors and about zero on the cases, and likewise about zero on the other side resistors, and instead of +50, you only have +34. I'd wager if you flipped your meter to AC volts and remeasured that +34v, you'd see quite a lot of AC voltage. That is consistent with losing the +50v main filter cap.

Those big blue 5000uf 55v were custom, they are not made. The nearest standard value would be 4700uf. Of if you like you could up it to 6800uf or 8200uf. 55v won't happen, but 63v is a standard. Or higher.

The blue cans have different lead spacing, so a new part won't line up with the holes. I am comfortable making new holes, but if this is more than you want to do, you ca mount new caps off board and run wires to the board.

[Max Rambone]

Thanks a ton, Enzo. I'll get some ordered ASAP.

[Enzo]

But it seems to me we have not yet pulled the board to resolder the big cans? If so, do that first.

[Max Rambone]

Okay, so I reflowed the connections on the caps and it's still buzzing.

These caps have 6 connection points, but I assume the outer 4 are just for stability/mounting securely to the board?

Thanks again!

[Enzo]

Yes, only one of the outer ones NEEDS to be connected, and the center one is positive.

[Max Rambone]

Success!!!!

https://cdn.discordapp.com/attachmen...1647213538.mp4 (link seems to automatically download video on my browser and it's just a bit over the limit to attach here.)

Thanks, Enzo! You are a legend. If you don't mind me asking, how did you get started doing this? Did you go to school or get training? Self-taught?

Thanks, again, good sir!!