SWR workingman’s 10 combo - blowing fuse- repair matrix request

[chrisdski]

I just acquired a dead swr workingman’s 10 combo amp. The fuse blows on power up. What would be a good flow chart/matrix of component order to test to figure the fault out? I have attached the appropriate schematic. Power transformer comes in on w3/4. SWR Workingmans 10 schem Rev 02_.pdf

[The Dude]

Firstly, which fuse is blowing? Is it a mains fuse not shown on the schematic, or F1 speaker protection fuse? If it's a mains fuse, since you can't check voltages with it popping the fuse right away, the first thing I'd do is check resistance from each rail to ground and see if you can detect a shorted rail. You have +&-15V and +&- 41.6V. If it's the speaker protection fuse, it's likely a shorted output IC. Do not hook up a speaker (or unhook the speaker) until you are sure there is not DC voltage on the output.

Edit: Removing F1 disconnects the speaker if that is easier than unhooking the speaker. I know that it's a PITA to disconnect speakers in bass combos sometimes.

[chrisdski]

Thanks for the reply. Yes, it is the main power fuse that blows. The internal fuse F1 is OK. I checked + side C8 and - side C3. Neither shorts to ground. I checked pin 3 of U3 and pin 3 of U2. Neither shorts to ground. FYI both W4 and W3 short to ground, which are the inputs from the transformer.

[The Dude]

The schematic doesn't show transformer wiring, but I assume the transformer has a grounded center tap, so the low resistance you read on the outer windings is just reading to the center tap. You can unhook W3 and W4 and power up the unit to see if it holds a fuse. That will tell you if the transformer is shorted or if it's something external to the transformer. If you have a light bulb limiter use it, or consider building one. It will save you from constantly replacing fuses. Or use a variac and monitor current.

[chrisdski]

Thanks for the reply. I desoldered W3/4 from the board. Fuse does not blow on power up, and both wires read 31.5v. I have a variac and just ordered a plug in amp meter for it.

[Jon Snell]

In that case I would check BR1 for short ciruit diodes, then C3/C8 for bulges then look for a short between pins 13 & 15 of the TDA. If leaky, replace the TDA. With care!

[chrisdski]

Thanks for the reply. The bridge rectifier checked ok pulled out of circuit. The caps look fine. In circuit there is a short between pins 13/15 of the TDA. Any tips to pulling such a circuit? I have chipquick. Is it easier to snip the legs first?

[chrisdski]

Successful repair. I snipped the legs on the TDA, leaving them long in the board. Then using chip quick flux and solder I pulled them one at a time using needle nose pliers. Then used solder wick and finally a paper clip to clear the holes. Fitted the new chip and soldered. Slowly powered up with a variac. A bit of a scare as no sound Until I realized a switch was set for headphones only. Flipped it to speaker and it worked great. Question- when you zoned in on pin13/15 on the TDA, was that because that’s the power +/- input of the chip?

[The Dude]

......Question- when you zoned in on pin13/15 on the TDA, was that because that’s the power +/- input of the chip?

Yes. The amp was popping the mains fuse, which means high current demand on the supply from somewhere. By unsoldering the power rails from the output IC supply pins or checking for a short between them, we find out whether the IC is the source of that extra current draw.