Peavey XR684 blowing fuses

[52 Bill]

Hi thanks for that,
I cut the track and the bulbs still bright. (100w)

Did you also remove or cut the two jumpers right next to the rectifier bridge?

I'm getting mixed signals here, you stated that the rectifier bridge was already tested with your meter, but now you are asking about reading across the two ac input terminals. What sort of level of electronics are you experienced at? Are you certain that all of the transistors and diodes have been tested correctly? It would help us to know what you know.

[J M Fahey]

Ok, then mark diode bridge pack for later proper reinsertion and unsolder/pull it, then test again: bulb *should* shine dull orange or less; if not I'm afraid we might have a PT problem.

Measure the bridge "outside" , check you have the proper foreward/reverse diode measurements.

Also solder a couple 4k7 1W resistors or 10k 1/2W across each large capacitor, or disconnected (as they are now) may hold charge for a LONG time and surprise you in a bad way.

I guess that besides the track cut you also removed the links connecting caps to positive rail.

As of paid advice, would you consider $ 320, like you paid earlier?
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mmmmhhhhh , I also guessed not

Don't worry, that's not the way we run things here anyway

[guitarman51]

thank you - bridge rectifier faulty
I replaced the bridge rectifier today, reconnected everything (apart from the mixer unit) and i now have 7.6Vdc on the board. Something else dragging it down.
checking the O/P transistors i have +7.6Vdc at the centre pin of the PNP transistors Q106, Q107, Q206, Q207 but 0v at the NPN's 101, 102, 201, 202
now that we have something to see, whats the next step?

[52 Bill]

A couple of silly questions. Did you reconnect the trace that was cut earlier? After replacing the bridge rectifier, did you prove to yourself that both 60 volt supplies were working?

[guitarman51]

Yes.. i did reconnect the cut track - and jumpers. but in my excitement to try it out i didnt check that both 60vac supplies were working under load. (they're ok measured straight off the trnsfmr) will check tmrw.

[guitarman51]

Cut the track again, disconnected the jumpers and have now checked voltages.
bulb limiter does not light up
P2 - P3 42Vac
P3 - P4 42Vac
116.2 VDC
Feels like we are getting somewhere, Should i order a bottle of bubbly now ?

[52 Bill]

P2 - P3 42Vac
P3 - P4 42Vac
116.2 VDC

From this, I will assume that you in fact have working plus and minus 60 volt dc supplies and not a single 120 volt supply.

Your earlier post stated that you had +8 volts on the collectors of the PNP output transistors. If you look at the board and follow the trace, do the collectors in fact connect to the positive power supply line?

[guitarman51]

Hi Bill
i measured the PNP transistors and could only get a voltage on the middle pin in situ. (the base). Nothing from the collector or emmiter pins
looking at the scematic, i think the 'base' connects to the -60vdc so why would i get +8vdc ?
In answer to your earlier question - your right i'm a newbie to electronics. so really appreciate your help
I'm an electro-mechanical engineer and a musician i have always wanted to learn this stuff, thought i'd give it a go.

[52 Bill]

...could only get a voltage on the middle pin in situ. (the base). Nothing from the collector or emmiter pins

Based on the transistor part numbers and on the board layout, the transistors are plastic case TO-3P or whatever the other number is. In either case, the center lead is the collector not the base. Looking at the transistor from the front with the leads pointing downwards, the pins are base/collector/emitter.

With this pinout in mind, please retest the output transistors in situ and see what you find. Test from base to collector and to emitter as well as from collector to emitter. If there are any odd readings, then you may need to remove the transistor from circuit to retest.

[J M Fahey]

Please download the datasheets of all transistors involved and look at the pinouts.

Then draw transistor packages on a sheet of paper, label pins, and staple it to the wall, near the workbench.

Proper part pins identification is essential for any troubleshooting.

EDIT: in general terms, old "TO3P" , TO218 and TO247 are basically the same case/package, with slight variations but all basically compatible, so any of them if found on a datasheet means roughly same size and almost always same pinout.

[guitarman51]

[ATTACH=CONFIG]36041
thanks for that - already done.
I need to establish that the power generation is ok, but
I'm struggling to understand the schematic
Looking at the board layout i see what i assume to be the
positive and negative DC tracks as i've marked on the attached sketch.
But when i look at the schematic i don't see the -60vdc or +60vdc connections onto the board ?
Im obviously missing something, help pls. thanks
Do i need to connect a resistance across the circuit to test it under load ? if so what and where.

[52 Bill]

Looking at the board layout i see what i assume to be the
positive and negative DC tracks as i've marked on the attached sketch.
But when i look at the schematic i don't see the -60vdc or +60vdc connections onto the board ?
Im obviously missing something, help pls. thanks

You have everything correct as drawn on the pc layout. The ac voltage from the transformer will be less than 60 volts. The dc voltage will be about 60 volts (+&-).

The schematic does not show lines connecting the 60 volt power supplies to the power amp circuit. There are points on the schematic that are marked plus and minus 60 volts. You need to connect the dots in your head.

The test of the power supply would simply be to remove the two jumpers next to the bridge rectifier and cut the trace under the board. Now read the dc voltage from the + and the - outputs of the bridge rectifier as referenced to ground. You should get +60 volts and -60 volts dc (black meter lead connected to ground, red to each test point). If the voltages are correct or even a little high, then you can assume that the power supply is working. There is really no need to test it with a load unless there is some reason to suspect that it is failing.

There is something that is drawing too much current in the amp. That is why the fuse was blowing and why the light limiter lights up brightly. You said that the bridge rectifier was shorted and replacing it has gotten the two 60 volts dc supplies working again. When the power supplies are connected to the rest of the circuit, the limiter is still lighting up bright, so there still is something that is drawing too much current.

You stated earlier that you had removed the output transistors and tested them. Did you have the correct pinouts when you tested these transistors? When you replaced them did you use the insulator pads or mica washers to isolate the cases from the heat sink? Are you certain that you have replaced the transistors in the correct locations on the board?

[guitarman51]

Amp now working - The fault was originally the rectifier - i eventually replaced it, after removing the output transistors for bench test. When i put the transistors back i hadn't noticed that i had bridged 2 tracks (see pic) the tracks at this point are very close together and now that i have sperated them both amps are now working albeit a little distorted?, need to investigate further as i suspect i may have inadvertently damaged other components. for info i am now getting 54vdc + & - : Schematic says 60vdc is this ok?

[Enzo]

That is fine, it is within 10%, which sounds OK to me.

[52 Bill]

Is that reading with the light bulb limiter?