QuoteBUT  Ouch!! it's a PNP power transistor  and if that be the case then no wonder the amp does not work as the circuit uses all NPN power devices

Ahhhh!!!!!!!!!  I ordered these from Peavey and they sent me the wrong ones. I know I quoted them the original part and I even told them the cross-reference part as well. I didn't think to check before inserting. Okay, there's hope!! Thanks Phil for noticing! Awesome!! I'm going to order from a local supplier and get the right ones.

Also, Phil, thanks for the post on continuity buzzer. I never use it and now will always use it when looking for shorts. So, yeah, I'm finding shorts on the output transistors  from collector to heat sink, and it comes down to the screw making contact. However, Peavey did not supply any kind of isolating washer. The screws go right down through the holes in the heatsink and the holes are tight to begin with. Is there something I can use/purchase to stop the screw from making contact with the hole?

So when the screws go through the output transistor hole they insert through the insulator but the thread makes contact with the case collector and that is cool. Right? I should test to make sure the collector isn't grounded out to the heat sink......or that the base and emitters aren't grounded out.....thanks!

I used R.G. Keen's transformer test. Since I used a 9V battery I reduced the resistance to 10K. The bulb didn't flash. But when I removed the resistance the bulb did flash. Not sure if I've used the correct ratio of resistance to voltage (my battery reads 7V so I reduced the resistance down to 10K instead of 100K). Hmmm....I then tried using a 100K and again the bulb didn't flash but did without the resistance.

Could a small internal short cause the voltage to drop when connected to the power amp as is happening to my amp?
Can putting a load on a transformer with an internal short create an Inductive Kickback in the transformer so that current tries to flow in the transformer to compensate for the short and therefore not send current out into the power amp?

Thanks Phil. My DMM zeros out completely when touching the leads. I'm still wondering about the power transformer as you suggested. I'll do some resistance readings on the 42V windings. From red-to-red I get .5ohms. Red to center tap-.3ohms, other side .2 ohms.....seems low!!

Readings:
I didn't tests the new transistors but might just in case. Here are test results from 4 of the six original output transistors in amp. All transistors were removed for all readings.

TR 7 Showed Good on DCA55 Tester. Resistance Readings trans. out of circuit.  E/C-C/E-0L/0L,  B/E-E/B 75M/0L, C/B-B/C- .71M/0L

TR10 Showed Good on Tester.  Res. readings trans. out of circuit. E/C-C/E-0L/0L, B/E-E/B 5.7M/0L, CB-B/C .6M//0L

TR11 Showed Good on Tester. Res. readings trans. out of circuit E/C-C/E-0L/0L, B/E-E/B 6.8M/0L, C/B-B/C .703M/0L

TR6 Showed Good  on Tester. Res. readings trans. out of circuit E/C-C/E-14.8M/0L, B/E-E/B 7.17M/0L, C/B-B/C .7M/0L.

Thank alot Enzo, after I fix this amp I'll be going over your notes and check out the  math and relationship between components in the circuit.

I feel at this point I've come full circle. I already pulled out the driver and output transistors and found them to be good but one--I used my Atlas DCA55 tester and it showed a short on T13.

Once I replaced the output transistors, plus the driver transistors which I had to replace due to destroying them when removing, I have only tested the amp using the bulb limiter. Is it possible to blow them when using the limiter? Can't see how as the current is so low.


I've retested the  .33 emitter transistors and my meter vacillates between .3 and .4 so hopefully that will not have an adverse effect. Should I replace them just to be sure?

I've included a pic of the output transistors. I was told by Peavey that I needed to use a matched set so I replaced 12 and 13. I used the same grey rubber insulators that came with the old transistors.

I'm going to re-test the output transistors.

Okay, have gone through all the resistors and diodes in power amp. Here are the readings that are suspect:


CR25 Diode Test .656/.515 . Shorted, I'd say
R92, 27K instead of 33K, as should be
R99 .824K not 1 K
R105 31.9K not 100 ohm
R107 54 Ohm not 100 Ohm
R108 54 Ohm not 100 Ohm
CR32 Diode Test .032/.032 shorted I'd say 1.
CR33 .457/.457 shorted
R118 26.5 ohms instead of 22
CR26 We know that this is already shorted.
CR23 Resistance Test 4.78M  one way, 365.5 Kohms the other (but the meter reading is still creeping up as we speak and has been doing for a while, why does that happen? Is it charging C56?) I switched to Diode Test and it reads  .709/0L so it seems good.

CR28 is also shorted .

Haven't taken out each leg of the small capacitors to measure but wonder if I should wait after replacing the bad resistor/diodes. Any thoughts? Thanks!!!!

Replacement of CR26, dual-diode.... What effect would this have?

Is the shorted CR26 pulling too much current through the amp before a signal is applied and turning on the drivers in advance, therefore drawing too much current through the Output Transistors and therefore popping the fuse?

Basically, what is it's function and could replacing this give good odds of bringing the amp back to life? 

QuoteWith no circuit connected the DCV read 37 Volt rails but the moment you connected a load the DCV dropped like brick, close to zero DCV.
This was over 20 years back so I can't recall exact details but I don't think the transformer even hummed.

Thanks for that Phatt. I'll do a transformer test when I get home from work.I believe I used this method before on this transformer but that was a while back. I'll re-test for sure. http://www.premierguitar.com/articles/the-super-secret-transformer-tester-1

QuoteWith such low rails there has to be a major issue somewhere else. I may have missed it but, Have the main filter caps been checked? My old Laney combo came to me dead, instantly blowing fuses.
The main filter caps where the cause. Laney was likely similar age.

Tested with an ESR meter and they are good.

Thanks again guys.

Quote
The power amp works without the transformer, but it allows full power at both impedances.

So, by two do you mean with the transformer connected (one impedance) and not connected (another impedance, or lacking impedance because it's not connected)? You say the power transformer works without the transformer, but you mean not properly, correct, otherwise we wouldn't have a problem?

Tonight, I'll comb through every component on that power amp. Easy to want to give up at this point but I'll keep at it...................

QuoteAnd remember, we are using NO speaker or load.  If the output transformer is unhooked, NOW does the bulb act the same?

Yes, acts the same.

QuoteSo with the red wire connector pulled off, measure AC volts from the center pin to either end, and you ought to get roughly 30v.  Yes, a little less with the bulb in place.   I suspect it is OK.

27VAC

QuoteThe rectifiers are now new?  so that ought to be good.  Assuming they are in the right way.  If neither rail is shorted to ground by C66, C67, then the power amp has the problem.  A shorted filter cap there is pretty rare.

Yes, rectifiers new


Dr. Gonz:Edit:

QuoteNote that we still need to know if that C69 is shorted. Measuring the voltage at the bridge, the way you did, is also a correct way to go about this too. I am concerned about that C69 still. If the secondary leads are connected you will not get an accurate reading and you will have to lift one leg of C69 to do an ohms test.

Removed one leg and got an infinite reading. Checked other good caps out of circuit and got an infinite reading so I guess is infinite is normal for a good cap.

QuoteBut did you also check all the 0.33 ohm 5 watt resistors associated with them?  They will either be open or they will be OK

They test OK.

QuoteThose two transistors, Q5, Q9 are your drivers.  They are often damaged when outputs blow.  Check them, I usually replace them when I have to replace outputs

I replaced them as I buggered them up when I removed for testing. Brand new. Only used with the light bulb limiter.

QuoteAlso the three 47 ohm resistors asociated with the 5331 and 5332, are they open?

They all show 47 ohms.

QuoteA little left of those are CR26, CR27 diodes.  Make sure neither is open.  CR26 is a dual one - two diodes in one part.  So on your meter, it will look more like 1v on diode test instead of 0.5v.  CR27 is a regular diode.

CR26 .015 V on diode test, both directions. CR27 .515 one direction, 0L the other.


Thanks again.

Thanks! Up early for work. Will post tomorrow :tu:

Sorry about that will try to be  clearer. 4.9VAC, 2.45VAC per side, on the secondary side of the transformer, measured from one side of the red wire connector to the other with the connector plugged in. 1.4VDC  rectified. And this same 1.4VDC shows up on the rails.

What's also strange is how, when I remove the red transformer wires from the board and test them, it has 53.6 AC volts from red to red, the light barely lit, but, again, when I connect it and measure, on the pins, I get 4.9VAC and the light goes very bright!

Silly question but if the cap is short should I get an infinite reading? The .1uf is too low a value for the ESR meter to read it.  I have a multi-meter. Also if this cap is shorted then will it be pulling the voltage to ground?Thanks.

QuoteSo what resistance readings are you getting across the +/- ends of C66 and C67? Are either of those caps shorted?

Thanks for this Dr. Gonz. I used my ESR meter and both caps checked out good.

So is  this a case of a shorted supply that could have something to do with the Output Transistors shorted out and pulling tons of current? I tested them with my Peak Atlas DC55, out of circuit, and they all showed good but one. I ordered a matched pair from Peavey and installed them. I haven't fired up the amp without a bulb limiter.

I installed them the same direction as the original transistors--serial number/ brand--unless for some wierd reason these new ouput transistors have different pinouts than the originals.

With the variac at 100V, .69Amps and the bulb limiter very bright, I have 4.9 VAC coming in for the rails and 1.4V DC leaving to feed the rails. So with the voltage drop of the rectifiers this DC voltage doesn't seem too far off.

Again, I've measured the resistors in the power amps  and they appear good. Is there anything else in the power amp circuit that could be hauling current through the amp? "Shorts between transistor case and heatsink, due to a punctured mica washer"? This is my toughest amp yet :grr

Hey Roly, thanks for all your help. I really appreciate you trying to educate me with your responses and they were thorough and totally worthy of referring back to as I try to figure this stuff out. People like yourself give of your time and that's sometimes a rare thing in this day. May your todays and tomorrows be peaceful!

Thanks Roly!

Hawk