Peavey Power Amp PV-260

[tmagic650]

This amp trips the RESET switch on the front panel. The 3 LED indicators flash randomly. I have checked the output transistors (4ea 70473100, 4ea 70483100) and they check ok... The power supply is good

[Enzo]

How can you tell the power supply is good if the breaker trips when you power it up?

Measure resistance from output to ground - across the output jack. Is it shorted?

Are you testing with a load? Does it behave the same without a load? Work without a load until it will hold a breaker and NOT put DC on the output.

If it will hold the breaker for a second, check for DC on the output. is there some? Which channel? Or both?

[tmagic650]

I've checked the bridge rectifiers, output transistors, filter caps, power resistors and other transistors and diodes passively using a Fluke 79 III True RMS Multimeter. I'll replace the output transistors and reassemble the amplifier. I have tested it with a load and without a load, with no change in symptoms...

Regarding the thermal sensor, this one is measuring almost short, very low resistance of 0.4 ohms. Does it open when hot?

[Enzo]

Thermal breaker in the mains circuit? Opens when hot. CLose when hot sensors would control a fan maybe.

I asked something specific: what resistance is across the output jack? PV uses a protectiv crowbar triac across the output, It senses DC ther and shorts the output. This prevents speakers catching fire, but usually kills the triac. ALWAYS check for a shorted triac when servicing blown PV power amps.

The finest meter in the world doesn;t put working voltages and currents on components, it uses a volt or less. Parts that seem OK there may fail under operating voltages.

Without me looking it up, do the two channels have independent power supplies? If so: Disconnct the power transformer wires from each. WIll the transformer all by itself hold the breaker? Now connect one channel and retest, then disconnect that and connect the other channel. Will either channel alone hold the breaker?

[tmagic650]

Thank you so much Enzo,
I understand that a fine Multimeter like the Fluke 79III won't power a divice up to it operating voltages or currents, but it is useful in finding shorted or open devices passively. I got the schematic and parts list for this amp in an email from Peavey support. It took all of 10 minutes to get it after my request.

It looks like a single power supply (+ - 15vdc, + - HV) runs both channels and there are 2 triacs on each output. I cannot give you resistance readings until I reassemble the amp this weekend. I will replace the output transistors and reassemble the amp. I am replacing the 70483100 with MJ15015, and 70473100 with MJ15016... Your help has been great!

[Enzo]

Yep, they are great down at PV.

Do you have the 1998 version - I think - or the 2007 version?

The triacs in question are SAC187 in TO220 cases, each is near the wire coil where the output wire connects You can measure them on the board for shorts if you like. The smaller diac I have never seen fail in the 22 years I have been an authorized PV servicer.

Looks like the older version has only one power supply winding, sorry.


I brought up the meter thing only because they CAN point out complete failures, but cannot tell you if a part is really good. Yours is better than my old old original 77.

If you are not aware of it, look up "light bulb limiter" in the search here. If I am not mistaken, RG describes it on his geofex web site. You wire a light bulb in series with the mains - clip it in place of the main breaker or fuse for example.

[tmagic650]

Yes Enzo the schematics are dated July 1995, The power transformer has a single winding out. You reminded me about the "light bulb limiter". I have a Sencore Powerite that I use like the "light bulb". It is a vairable AC power supply with an ammeter included. I will drag it out today. The TO220's (4ea) check good, not shorted

[Enzo]

4 of them? Are they SAC187s? Right next to the inductor.

I much prefer the variac and ammeter to the light bulb anyway, great you have that.


And that single winding, unit holds breaker when that is disconnected?

[tmagic650]

My mistake Enzo... Yes the SAC187's (2) are located near the air core inductors with wire connections to the speaker out plugs. These Triacs are not shorted. The SAC187's are not mounted on heatsinks. They are free standing but bent over. The 4ea TO226 transistors on heatsinks were 2ea SJE5331 and 2ea SJE5332

[Enzo]

Look at the layout drawing, doesn;t it include the trace art? Are there any jumper wires carrying the HV rails we could lift to remove rails from one channel or both?

And I forget, did we test the transformer with secondaries disconnected?

[tmagic650]

I got the amp put back together with new output transistors. With the secondaries of the transformer disconnected all I see on the 3 wires is about 4 VAC. With the secondaries connected and with the circuit breaker bypassed, the amp hums loudly and the inrush resistor smokes. I found no shorted bridge diodes or anything else shorted for that matter...

[Enzo]

DOn't bypass the breaker - better it pops than a fire. Seriously.

And leave the speaker off. All this does with a speaker is draw even more current from the amp and potentially damage the speaker

You don;t have to have a shorted output, an open diode in the bias string would turn all the outputs on at once and that will certainly blow breakers.

On your variac, does the current ramp up immediately as you advance from zero? or can you get a few volts before it starts up?

I find that low ACV from the transformer by itself suspicious.

[tmagic650]

Ok Enzo,
don't worry about jumping the breaker. This was done on the bench with another 2 amp breaker in the AC line. I call this "the smoke test". There are times when this test will pinpoint the problem area(s).The variac shows some voltage before it goes full scale. Is the biasing circuit single for both channels or dual, one for each channel? I am not sure just where to look. I have checked for shorted components, but as you know, opens are harder to notice unless you know right where to look.

Another goof, I was checking the AC out unloaded from the transformer using the logic ground instead of the chassis ground. There are 3 transformer output wires, 2 red and one red with a yellow stripe. The 3 outputs read 127, 131, and 143VAC... Sincerely Enzo, thank you for your patients and time helping with this

[Enzo]

I really doubt the transformer is bad. DOn't measure AC to chassis with them disconnected. The red wires are the high voltage, and the red with yellow is the center tap. Measure from the center tap to either end. those two should add up to the reading between the two red ones.

OK, in a solid state amp, when you hear "bias" forget anything you know from tube amps. The only thing they have in common is they affect the idle current draw and crossover distortion. Each amp channel has its own bias, but it only takes ONE whacko channel to blow breakers.

I am looking at a drawing where in Channel A Q103,104 are outputs and Q100 is the driver on the positive, and Q113 is the driver on the negative. From the base of Q100 down to the base of Q113 is a string of three diodes - CR107, 110, 114. CR107 has a 10 ohm resistor parallel, and CR114 is a dual diode. Those diodes are your bias string.

With reference to the output bus, the bases of the output transistors (I will abbreviate transistor xstr from now on), will be one semiconductor voltage drop removed. Since the driver xstr sits on those bases, the driver xstr's own base will be two drops away from that refernce point. The negatove side is the same, so ideally the bases of the two driver xstrs should be about 4 drops apart.

Drops: set your meter to diode test, and conect it to a diode. It will result in something like .5 volt. That reading is the voltage drop across the semiconductor junction in the diode. From the base of a xstr to its emiter or collector should be similar.

In reality we'd prefer a little less than 4 whole drops. SO that 10 ohm resistor tweaks it down a little. The dual diode is convenient for designers, and it will be on the heat sink so the bias will thermally track the power xstrs.

If one of those diodes is open or the circuit to it not complete, then Q101 will be free to pull the base of Q100 right up to the + power rail. That turns it on fully. At the same time down below, Q113 would face a similar fate. That would turn on the negative side fully.

The result then would be both positive and negative power supplies being connected together at the output bus - a dead short across the main power supplies. That is why it only takes one bad channel to blow breakers.

You can look at channel B for the equivalent xstr numbers on your own.


Check each of those diodes. The one with resistor will measure low, of course. The middle one will measure normal, and the dual one will measure like two diodes in series - something like 0.8 volts to maybe a whole volt. And because it is the whole circuit we care about, to test CR107, test it right at the part first, but then also check by measuring from the base of Q100 to the top end of CR110, if you follow me. That way we not only test the part, but we are testing the copper traces on the board.

[tmagic650]

I started this electronics tech thing in 1968, at a TV shop in California. As you can imagine, there were many Vacuum Tube TV's and very few transistor hybrids then. I was 18... I learned about both high voltage power transformers (Flybacks) and low power transformers like our AC input transformer here. I forgot about the center tap, and I know the transformer is ok now. I'm sure it is in the biasing circuits. I don't expect both channels to be bad. As you said, it takes only one bad channel to blow breakers. I have repaired many amplifiers, and I always expect blown power transistors or power rectifiers. This Peavey has been a real challenge