Kustom PA 10 Public Address System blowing fuses

[TerryD]

I got a set of speakers from one guy and the Kustom 10 from another, both for free, to use at church.  That was good.  I plugged my guitar (for testing purposes) into the Kustom and the 3 amp slo blo fuse blew after about a minute or so.  I stuck a couple of regular 3 amp fuses in with a known speaker and they both blew right away.  I only had one side of the two speaker outputs plugged in.

Is there anything that I might do as a novice to repair this stupid thing?

 I've followed trouble shooting advice from you guys on a number of tube amp projects that were not completely easy and came out smelling like a rose

Thanks,
Terry

[pompeiisneaks]

I would guess you want to try to replace the power tubes, but it may be more than that.

I would look for anything burnt on the board.

~Phil

[TerryD]

No tubes in this solid state amp.  I checked the power section and no signs of anything foul.

Thanks,
Terry

[shooter]

do some surfing for a schematic, otherwise it's monkeys throwing darts for stock picks

[drgonzonm]

Jack Darr has a procedure for trouble shooting SS electronics.  I found copies of the chapters at tubebooks.org. 

I got my Kustom 300 head for $10. at a garage sale held at a friends electronic shop.  If you can get it working, good snag.

[TerryD]

I can't find schematics for this for love or money.  I looked up the website tubebooks.org and found tons of authors but found no Jack or Darr.  I perused the list twice.  That sounded good.  I have very little test equipment here except a multimeter

Thanks,
Terry

[drgonzonm]

try this site, Please note I have not used it.  https://openlibrary.org/works/OL4985155W/Electric_guitar_amplifier_handbook

[shooter]

here's a link to the kustom 25;
http://www.vintagekustom.com/
You *might* be able to use it as a basic guide.  I'd start by Identifying the power section, the 25 uses a split rail +and- supply.  find where that originates then you can *temporarily* disconnect it from *the load*.  Power up and see if the voltages hold.

[drgonzonm]

http://www.musicparts.com/products.asp?Company=Kustom

above link might have the schematic.  but it will cost you a few bucks. 

[TerryD]

I'm afraid most of this is above my head, plus I'm not finding things.  The Darr book doesn't seem to open on that site.  Not wanting to put money in a schematic.  Was hoping for just a couple of ideas to trouble shoot that anyone could do in the power section, being as careful as with tubers.

[PRR]

Does it blow fuse with no speaker? (A transistor amp does NOT need a load.)

Basic: does it have "B+"? In a tranny amp this is likely two supplies, +30V and -30V from ground.

If it blows fuse no-load, then of course there won't be "B+". Put it on a lamp limiter. Now go through and see where things look a little low, where it gets WAY low.

But 9 of 10, it is a shorted output transistor PLUS half the small transistors (they fail domino fashion). Partial repair attempts get expensive fast.

[pompeiisneaks]

No tubes in this solid state amp.  I checked the power section and no signs of anything foul.

Thanks,
Terry

I think this is the third time I've forgotten to look at the category and said something tubey about SS ones :) I should pay more attention.

~Phil

[drgonzonm]

Please find attached a copy of the 4th chapter of Darrs book.  You will find he recommends that on SS devices, troubleshooting starts at the preamp and works through the power amp. 

[TerryD]

Thanks for all the work.  I'm printing it out now.  Terry

[eleventeen]

I'm afraid PRR is quite right, as he usually is. The big issue with troubleshooting SS audio amps is the most common fault, by far, is a failure of the output devices. And depending upon the exact way they fail but massively more common than anything else, they will take out the (smaller) transistors that drive them in a heartbeat. The driver transistors are not cheapo little 10 cent 3904's. They are a few bucks each. This means that a failure takes out at least 4 transistors and they are the more costly ones. This can happen a lot of times in between shoving in a replacement part, turning the power on, and having 2 or 4 brand new parts cook in an instant. Long ago and far away, when I worked as a tech in an electronics company and could get bags and bags of brand new parts out of the dumpster (I once got a bag of 1000 LM317 voltage regulators) I had loads of transistors which I could burn up. Today, having to go buy these things at present day prices gets expensive and irritating. You cannot just shove new parts into a sick SS amp. You have to bring up voltages very slowly and carefully, monitoring things.


It is true that a bad shorted tube will often kill a resistor, but that's a resistor. No big deal. If you catch it in time, maybe you just heat up the resistor and can reuse it. The transistors that fry in a cascade SS amp failure are gone in an instant. So depending upon how you source your new parts, it can cost you $50 just to try turning the thing on again and then you're smoked. 

[TerryD]

Thanks