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Old 25th June 2013, 11:50 PM   #1
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Default Dynacord Powermate 1600

Hi, I have a dead powermate 1600 and was wondering if anyone could help. It started to blow fuses instantly. I open it up and found two shorted output transistors which I replaced. I was taking some measurements before I powered it back up and noticed I had a short between +vcc and -vcc. Does anyone know if this is normal? I have a schematic but I'm more used to simple valve guitar circuits and cant follow it very well. Any help would be appreciated.

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Old 28th June 2013, 11:01 AM   #2
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This diagram won't expand enough that I can read it.
Often when output transistors go, drivers and parts further back go. often the driver transistors and passive parts aren't rated for the rail voltage being applied at that point.
An ohmmeter across the power supply won't tell you much with more than 1000 uf capacitance. You can check the rail voltages to see if they are proper.
One tests an amp with a light bulb box in series with the AC power input. Use a circuit breaker and a grounded metal box around the lightbulb to keep AC wires from flying around. If the bulb lights up, you have a large short in the amp. If the bulb doesn't light up and you have less than 50 mv DC on the output, you are making real progress to trying out with a trash speaker. Your rail voltage will be a little low with a light bulb. My 1.3 kw amp had a 10 v drop on each power rail with a room heater element in series with the AC (instead of a light bulb). But after I replaced enough parts, it would work. I removed the base of all transistors and one end of all diodes to get a good measurement. Once rail voltage got to them, passing the DVM double diode test on transistors (b-e, b-c) did not prove them good. That test is done at 2v with my meter. I had to do a b-e shorted current leakage test at higher collector DC voltage to sort out damaged transistors. I used 17 VDC, the ones that leak more current than new parts are the bad ones.
I wouldn't worry about matching part numbers much, just matching parallel replacment transistors. Just make sure Vceo is higher than your rail voltages. Something like MJL4281-4302 for output transistors and MJE15032-33 for drivers, for example. Or 2SC5200-2SA1943 for outputs.
Before buttoning up checking idle current on the output transistors is necessary. something like 20-50 ma each is normal. You measure on the emitter resistors. If wrong, sometimes designs have no potentiometer to adjust, you have to fiddle with offset resistor values or something. I used cheaper modern output transistors than went in my amp in 1994, and have to put in different biasing resistors than the print.
Edit: eserviceinfo.com has this schematic diagram. there is a bias adjustment pot.
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Old 28th June 2013, 11:41 AM   #3
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Hey, thanks very much for the detailed reply. I have since found that one of the new transistors was shorted (e to b). I'm going to work on it again this afternoon and see what progress I make. I ment I put up a schematic of the amp section , sorry about that. I'll report back with my progress!
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Old 29th June 2013, 11:30 AM   #4
Tarzan is offline Tarzan  Belgium
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Hi,
The schematic is not of the poweramp.
Here it is. (attached)
Compare it with the details given on the pcb.
There are a few versions...

You have to work slow and very carefull.
The tracks are very thin so you can tear them very easy.

So you found two bad transistors.
The others are probaly stressed to and I advise to replace all of them.
But it is not a MUST...

Check the resistors, the white parts with 3 legs.
These are two resistors of 0.22Ohm.
Very hard to get.

It is good to make an extra copy of your schematic and indicate all the defective parts in red or orange.
Check all semicons and resistors around the defective ones.
When you reach a good part, mark it in green.

When all nodes on your schematic are terminated with green parts, you found all defective parts.
And I can tell you; you will be surprised.

If I have the time, I'll check my notes on this type of amp.
Usually it's allways the same parts that go.

ps: wich transistors have been detectd bad initially?
Q327 and Q329 or others?

Let us know.
Good luck,
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Old 29th June 2013, 12:44 PM   #5
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Hey, I have been going over the surrounding components over the last couple of days. So far, the only faulty parts have been the transistors (MJ15023 and MJ15022). I have a whole new set of output transistors.

I have been going through the schematic (I managed to find a service manual last week) like you have said. Tomorrow I'll finish checking rest of the amp. I have all output transistors removed and I cleaned the heatsinks.

I need to get some thermal compound on Monday then hopefully I can test it initially on Monday or Tuesday. I have a lamp limiter and a variac (for my valve amplifier builds) so I can closely monitor current draw.

What should the idle current draw be like on a mixer like this?

Thanks again for the reply, it has been very helpful.

Peter
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Old 4th July 2013, 06:41 PM   #6
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So, all components checked out ok. Replaced all transistors, new thermal compund, used IPA to clean the board.
I used my variac and a dim bulb tester to ramp up the voltage gradually. I got up to 220v without a hitch. I checked some voltages and they all seemed within spec (considering the dim bulb tester). No DC on the speaker terminals.
I unhooked the dim bulb, went to power on and it blew a fuse after about 20 seconds...... Transistors blown again.
I double checked every component again and couldnt find a fault. I reflowed all solder joints. I bought a couple of extra transistors, put them in and the same thing happened.

I thought there must be something up with these transistors. I cut open one of the blown transistors and one of the originals. The original looked good. It had a nice big die (6mm) I think.
I did the same with one of the new transistors and wasn't surprised to see a 2mm die!!!!
I got counterfeit transistors. I got these off ebay but they weren't cheap!
Anyway, I got a new set from RS and all looks good with the mixer. All functions/voltages and bias are correct.

You live and learn Thanks for the help guys...
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Old 4th July 2013, 07:40 PM   #7
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Congratulations. Your mixer works.
Another test of e-bay parts, failed. I think that has been covered in the parts forum pretty thoroughly. My Transistors, original or copy? The cost of a silk screen mask for relettering $10, the cost of new ON semi transistors, $5, a no-brainer to the sleazy re-label artist.
The 17v current leakage test (b-e shorted) might have shown the fake transistors before you went to the trouble of installing them. A guy in Brazil claimed a 250 V leakage test will show fakes up every time, but I don't recommend monkeying around with 250 V with your hands for reasons of attempted longevity.
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Old 7th July 2013, 05:21 PM   #8
Tarzan is offline Tarzan  Belgium
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Hi fastpete,
Did you do wat I proposed in my previous mail?
I guess you didn't.
Well, You are back at the start and are the proud owner of a pile of bad transistors.
So try the solution I gave you and slowly bud sure, you're amp will work again.
I repaired at least 20 of them and this method is the fastest, easiest and cheapest.
We are here to help you, but you have to listen...
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Old 7th July 2013, 07:07 PM   #9
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Hey Tarzan,

Maybe I didn't make myself clear. I did go through the board like you said and everything other the output transistors tested good. After realising that I bought counterfeit transistors, I went to RS and got a new set of genuine ones.
I installed these and the mixer now works well. I'm a very happy man as these mixers are not cheap!!
Thanks again for your help,

Peter
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Old 8th July 2013, 06:40 AM   #10
Tarzan is offline Tarzan  Belgium
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Sorry I misssed the last part of post #6.
Good to see that a good power mixer is back and ready for the road.

Again, we have learned one important lesson:
Buy from a reputable source...
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