Peavey Special 130 blowing fuses

[Steve Casas]

Hi all. I have been reading the posts by Enzo and Tekhed7 and Gtr_tech about another members Special 130... and I have one with a similar problem...
I bought this from a friend about 12 years ago. He said it was "blown." The speaker cone was frozen so I took it apart and saw it was half burned, indicating high DC in the output (right?). I had it reconed. I then, simplistically, assumed bad output transistors and replaced them with 70483100 (with a circled "on" logo) transistors that I was told at the time were a good replacement. I powered it up and got a crackle through the speaker and the fuse blows after about one second. Sooooo, I got mad and let it sit for 12 years.

After finding you all, I drug it out, took it apart, cleaned it up, checked all my solder joints, visually checked all the electrolytics, looked for burned components, replaced the fuse, and fired it up... same thing.

Any Ideas? i am a carpenter that is also a very capable tech...I have two years in the Army fixing radios and a CC degree in electronics. I just don't do this often and don't have the time to re-learn everything.

I know one of you guys could probably say, " check r67 for a short" or "probably c32 is leaky" or some such thing that would save me weeks (if not another 12 years!) of head-scratching.

Thanks in advance!

Steve

Any help would be VERY appreciated.

[Steve L]

Peavey Special 130

Steve,

Here is a schematic for the amp.
Besides the output transistors which you already replaced (actually MJ15015 or 2N3055A), some other parts that typically short in that amp are CR28-31, Q-6 and Q-10.
However, as I am sure you already know, not every case will be the same.

Steve

[Enzo]

Disconnect your speaker. Watch the output with a volt meter. Got DC there? Or does it just still blow fuses?

[Steve Casas]

Yes, it is still blowing fuses.

I just ordered the two driver IC's and two 4700 uF caps ( tubesandmore.com didn't have the 1000 uF @35V ones... I'll keep looking ) I'll swap them out when they get here (I'll test 'em before I solder in new ones) and let y'all know what happens. I'm also gonna rig up an 8 ohm dummy load. I was using an old 12" 8ohm speaker that I don't really care about... but it might fry before I am done troubleshooting...

I am wondering about the 70483100 (with a circled "ON" logo) transistors I put in there 12 years ago... does anyone recognize that brand? Are they compatible?

Thanks Enzo and Steve L for the quick replies.

[Enzo]

Those transistors used to be made by Motorola. then Moto spun off the semiconductor division to concentrate on consumer electronics. The spin-off company is called On Semiconductor, that is their logo you are seeing. These are the Motorola parts of old, just new name.

70483100 is a Peavey house part number. it is an MJ15015 under the skin. SJ6392 was the earlier Peavey part number, they changed the number system a while back. SJ6392 and the 704 number are the same part.

DO NOT connect a load to this amp until it not only stops blowing fuses, but also does not produce DC on its output. Once the amp is stable and centers on zero volts, THEN connect a speaker. Aside from senselessly burning up your load, it can cause further damage in the amp.

Driver ICs? There are no driver ICs, the power amp is all transistor.

Check all the rectifier diodes for shorted.

When you replace the power transistors, did you remember to include the insulating mica washer between the transistor and the heat sink? In other words, make sure the case of each power transistor is not shorted to the heat sink.

[Steve Casas]

Thanks for the info, Enzo...
I wasn't about to hook up a good speaker to this amp until I knew there was no DC in the output, but I need a load before I turn it on, right? What's wrong with using a crappy old speaker? Isn't it going to cause damage if I turn it on with no load?

I meant driver transistors, not IC's... I just didn't want to type "transistor" heheh

No shorted rectifiers... maybe open...I didn't pull them out.

I did remember the mica insulators... I checked for shorts...none.

Okay... so I replaced the two 4700 uF filters... the old ones tested good (one was out of spec, maybe, at 5300 uF... over 10%)

I also replaced the two drivers, one pnp and one npn, verified which was which. The old ones tested ok, I think, with my trusty Fluke 179.. no shorts except when forward biased base to emitter, or emitter to base... no shorts base to collector.

I haven't plugged it in yet... I will wait to hear from Enzo re: speaker loads... But I'm guessing it will blow the fuse again... any bets?

Any other Ideas as to other places to look for failed components? I'm thinking of disconnecting all the secondaries on the PT and testing them...

[Enzo]

You have it exactly backwards. It is a solid state amp, so it does not require a load. RUn it all day without one if you like. COnnecting a load to an amp that is not right can cause added damage. If the output is slammed over to one rail - the +50v rail for example - with no load it just sits there with DC on its output, waiting for you to measure it. When you place a load on it, the load draws current. SO now instead of simply putting a DC voltage on the empty output jack, the transistors now have to provide tons of current. Good way to burn out transistors. And all that excess current through the load is often the difference between an amp sitting there with DC on its output and the amp blowing fuses. If both sides of the output turn on at once, then it blows fuses no matter what, but with just DC on the output it only blows fuses when a load is present. No load, no output current.

Open rectifiers don't blow fuses.

If you want to pull the two secondary wires off the board and power up the transformer, go ahead. If it blows fuses that way, then the transformer is probably bad. This would be extremely rare however. But if a bad transformer is unlikey, having the voltage be wrong in some fashion is even far more rare than that.

If all you did to it is replace the not shorted filter caps, then i would agree it likely will still blow fuses. Unless it is DC on the output, and you have removed the load.

I recommend against throwing parts at it. The failure can just as easily be a broken solder connection or a cracked copper trace on the board. Replacing parts won;t cure that.

[Steve Casas]

Ok, so I powered it up, no load, and it didn't blow the fuse. No DC in the output. I thought I had a bad Zener, but apparently my Fluke thinks 22 ohms is a short ( there was a 22ohm resistor in parallel).

But I noticed a rattle on the board. no moving parts, no washers on the pots. no loose bolts, heat-sinks, sockets rivets...NOTHING! Is there anything in a thermal breaker that would rattle? Actually I swear it's coming from one of the power transistors. Mystified.

anyway, it seem like it's time to plug in the old speaker to see it it blows a fuse, eh? Unless someone knows something else I should check first? Perhaps some normal data for some test points?

[R.G.]

anyway, it seem like it's time to plug in the old speaker to see it it blows a fuse, eh? Unless someone knows something else I should check first? Perhaps some normal data for some test points?

I keep a set of four 2-ohm 100W power resistors on a board for issues like this. They cost me about $2 each at some surplus joint or other. Not only are they cheaper than a speaker, they're much more durable.

And oddly enough, I can hear audio coming from them at power levels over 100W...

[Steve Casas]

I have the same thing ( 8-1 ohm 10 watt in series) But the speaker I was referring to is an old piece of junk with torn up cone that's not worth reconing. I have it mounted under the upper cabinet on my workbench and I use it for a load and to keep screws on the magnet so they don't get lost.

Enzo: If I plug in a load and it blows the fuse, could it have caused damage in the half-second before the fuse blew?

[olddawg]

If I plug in a load and it blows the fuse, could it have caused damage in the half-second before the fuse blew?

Can you run faster than an electron? Hell yes! Every time you blow a fuse when troubleshooting you are risking more damage and stressing components. A shorted output will take out drivers,etc.

[Steve Casas]

Well, that's what I'm getting at... So, before I plug in a load and turn it on, I should rig up some sort of current limiter in series with the dummy load first, eh? Say, a 60 watt light bulb? That way I won't get too much current flow through the output transistors, etc?

Also... I found an open resistor, R23, gonna swap that out right now...

[olddawg]

Well, that's what I'm getting at... So, before I plug in a load and turn it on, I should rig up some sort of current limiter in series with the dummy load first, eh? Say, a 60 watt light bulb? That way I won't get too much current flow through the output transistors, etc?

Also... I found an open resistor, R23, gonna swap that out right now...

The standard way you would do it on a shop bench would be to use a bench variac with an amp meter. As you slowly bring the line voltage up you watch the current draw. If it starts to creap up you shut off the amp and look for a problem. Yeah, you can use a 100 watt lightbulb and a jerk cord. That's an old rock and hammer TV tech trick. But you still could fry something. Still it's better than nothing.