Peavey Special 130 hum

[guitardog]

I want to start by saying thanks to all the folks that help on this forum...I have read many posts, and the assistance provided is outstanding. Now onto my problem..I have a Special 130 that I have had for many years. The amp worked great, but just recently developed a load hum. The hum is present with no input connected and there is a loud "pop" when the unit is switched off. Very similar issues have been raised in other posts here, and it seems that I likely have bad output transistors. I have identified and located 4 of them, all of which are Motorola SJ6392. I have not removed them from the board, so I am not sure if they can be tested effectively. What should I look for before taking the time to take them out?

I have found some transistors that are readily available (I can get them today), but it doesn't appear that they meet the specs exactly. They are 2N3055, and while the specs are not exact, I have found information indicating that they are pretty durable. It seems that they do run pretty hot when used in demanding applications. Any suggestions, recommendations, etc are welcome.

Thanks again!

[tekhed7]

I want to start by saying thanks to all the folks that help on this forum...I have read many posts, and the assistance provided is outstanding. Now onto my problem..I have a Special 130 that I have had for many years. The amp worked great, but just recently developed a load hum. The hum is present with no input connected and there is a loud "pop" when the unit is switched off. Very similar issues have been raised in other posts here, and it seems that I likely have bad output transistors. I have identified and located 4 of them, all of which are Motorola SJ6392. I have not removed them from the board, so I am not sure if they can be tested effectively. What should I look for before taking the time to take them out?

I have found some transistors that are readily available (I can get them today), but it doesn't appear that they meet the specs exactly. They are 2N3055, and while the specs are not exact, I have found information indicating that they are pretty durable. It seems that they do run pretty hot when used in demanding applications. Any suggestions, recommendations, etc are welcome.

Thanks again!

hi guitardog, New to this forum though I've been fixin' these things for years...
First, remove any speaker load to avoid damage if the amp has "DC'ed" & check to
see what it's putting out. If it's near +40VDC then you Might have a shorted output, namely Q7 or Q8, but it's quite likely that just one of those 1-Watt power resistors in the driver stage has gone open. Check R81- a 1.5K ohm resistor (brn-grn-red) & esp. R82- a 2.7K ohm resistor (red-viol-red). If at -40V Q11&/or Q12 might be shorted.
Also, it's common to find that 1 of the driver Qs has gone bad; either the MJE5331 (Q6) or 5332 (Q10).
2N3055s aren't really up for the task here, either power or construction-wise. Much better to use a "diffused" power Q like an MJ15001 or MJ15003 or a replacement series # like ECG-60 or NTE-60.

[guitardog]

Thanks for the quick reply! When referring to what its putting out, do you mean voltage at the speaker outputs? If so, the only time I show DC voltage at either output channel is when the unit is switched on or off. The voltage is low....less that 1 volt. Do I need to disconnect the resistors to measure correctly??

Thanks

[Enzo]

Nope, that answers the question. There are two real sources of "loud hum" on a solid state amp. One is that it puts out large DC voltage onto the speaker. The other is it has lost filtration on one of the power supplies. if you don;t have 40v sitting there on the speaker leads, the amp is not going DC on you. Turn-on pulses are not considered DC on the output.

I would look to power supply issues in that case.

If your 6392s don;t measure shorted, they are most probably perfectly fine.

MJ15015 would be the modern equivalent for that. You could use 2N30355 if you get the 100v ones.

Scope or measure the two power rails for the power amp. Are both present, free of ripple, and about the same voltage? Just measure across the two large filter caps near the fuses. In fact, wiggle those and see if the hum abates. Cracked solder to one or both of them would not be rare.

And we can;t forget the low voltage supplies. There are the two +/-15v supplies for the preamp. They are set by zeners CR36,CR37. Those are located right behind the Reverb and Presence controls. or you could pick an op amp and look at pins 4 and 8. I'd measure across each zener. You get 15v? Clean and free of AC?

[guitardog]

Enzo, Can I test the 6392's in circuit? Voltage across C59 and C60 is right at 41v. The voltage across CR37 is 14v, and CR38 is at 15v. Voltage at the low voltage connector is around 42v and the high voltage at 52v. I looked the board over for lose solder joints, and re-soldered a few questionable connections. There was only one for sure and the was the power LED. I wiggled all of the caps, and the all of the wiring, but no change. I do not have an oscilliscope to do any detailed testing. Any suggestions?

[Enzo]

Generally if they are shorted, nothing around them can mask that. However things around them can fool you. yes, I always check them in circuit first. I pull them if I have to.

Tek mentioned the drivers and predrivers, check them, and check for open resistors. Not just the large ones.

[guitardog]

Ok, so when testing the 6392's in circuit, what exactly am I looking for?

[Gtr_tech]

Thats the easy part. Just find your v rails for the output section and check for shorts to gnd and shorts rail to rail. Complimentary pair outputs are usually the easiest since the collectors (and rail supplies) are right there at the case of the device. Quasi complimentary designs use the same parts for both halves but with different connections on the half of the circuit that takes the place of the compliment so you won't have the rail v on the case.

Now leaky outputs/drivers can cause problems too. They can test fine using a meter with diode junction test, and then fail in the circuit. I use a dedicated transistor checker to check both junction and leakage....saves alot of time and headache.

But....it really sounds like you have cracked solder on a main filter cap. Or one of the caps may have excessive leakage current (failed). While yer in there, check the v-rails to the op amps and make sure they're present and make sense. They should be +/-15vdc or somewhere around there. If one is way lower or missing either an op amp has gone supernova or a 78** regulator or zener regulator has failed. Pwr pins on most every dual op amp are the corners at pins 4 and 8.

[guitardog]

hanks Gtr. I will check voltages when I get home, and post my findings.

[guitardog]

Ok...here are the voltages on the op amps. U1-13.5v, U2-13.5, U3-13.0, U4-13.5. All of the voltages seem to be in line with the specs....scratching my head!!! There is a small capacitor that is tucked up under part of the chassis near the fuses. Is this part of the pre-amp circuit? Moving things around in that area does seem to cause a change in the hum.

[Gtr_tech]

There are 2 supplies to each op amp. You should have both pos 13.5 and neg 13.5. Is that what you found? Supply rails are at opposite corners so where you find a +13 you should see a-13 at the opposite corner.
http://tangentsoft.net/elec/bitmaps/...inout-dual.png

[guitardog]

So there should be -13v at pins 1 and 5? Or would that be 4 and 5?

[Gtr_tech]

4 and 8. Look at the pinout I linked. Pin 1 will be marked by either a dot by it or a notch out of the plastic indicating the top of the IC.

[guitardog]

Got ya....all voltages are where they should be, so I am leaning towards a failed or leaking capacitor(s). I want to replace the main filter caps, and I can get them in a few days. I believe these are the four caps behind the reverb and presence pots, and it looks to me like they have been changed before. The schemo calls for 2 each 4700uF/50v (at C55 and C56) and 2 each 1000uF/35v (at C59 and C60). In this amp, C55 is larger than C56 and is a 4700uF 63v device, so it appears to be non original. There are also two mica or film capacitors that are riveted to a bracket near the power switch. I would change those as well, but I can't identify the values, or if they are in fact capacitors. The print reads "ITW 103+0 20% 125ac DVL". Can any one identify these???

Thanks again!

[tekhed7]

Hey dog,
To bad U don't have ascope & R familiar w/ using it, as looking at all the supply rails would probably illuminate whats happening here.
I would now be willing to guess that it's the old open-negative-filter-cap syndrome that many Peavers from this era develop. That's the main can-shaped cap that has it's POSITIVE side connected to center ground. C56 on the schematic. You could try jumpering in a capacitior you might have laying around that's rated at atleast 50 volts & maybe 1000 uF or greater. Tack solder it across that suspected cap (careful to discharge those caps prior to connecting to 'em... Amp unplugged, of course) and power it back up to see if the hum is greatly diminished. If so, best to change 'em both out w/ new ones.
I'm curious... does the amp pass audio (test tones or music comes thru) along with the hum? If it does , & it will be distorted, those filter caps are the prime suspects.