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Old 4th July 2007, 03:08 AM   #1
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Angry Precision Power Guru Please Help.

I've got another A600 and I would like to know of an easier way to test the transistors in it? I normally just pull all the components and test them individually. No schematic blues! Are there any typical problems A600's encounter? Are there any test points with corresponding values. I'm a two way radio tech and a lot of the boards I work on have test points with corresponding values. I'm just trying to make things a little easier so I'm not guessing and checking and getting frustrated.

Thanks,

Corradolvr
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Old 4th July 2007, 07:03 AM   #2
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For most problems, you can test the components in the board. The symptoms lead you to the areas where you need to begin checking components.

What is the problem with this amp when you power it up?
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Old 8th July 2007, 05:34 AM   #3
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When I power this one up both the low impedance and power lights come on. I get about a 10 amp draw on the on my power supply. The irfz34 transistors on both sides get a bit warm. The transistors on the side with 5 bdt82's in a row gets hot. I'm thinking I have some shorted transistors but I'm not sure. Whats the best way to test the power supply transistors and channel transistors in circuit? Any ideas on what might be going on? If the transistors are shorted what could have been the cause, besides someone that didn't know what they were doing. What typical in circuit problems could cause this issue?

Thanks,

Corradolvr
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Old 9th July 2007, 02:13 PM   #4
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What are JP1 & JP2 used for? There's no jumpers on them. Test points?
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Old 13th July 2007, 04:52 PM   #5
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Is their anyone out there? I got one channel working but the other one has no output. both of the vertical boards are getting input and have output. Is there a way I can test my transistors in circuit? I did notice one strange thing ohms base to emmiter on one side is 14 Mega and the other side is 1.2 mega. What's up with that?

Thanks

Corradolvr
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Old 14th July 2007, 03:57 AM   #6
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Quote:
Originally posted by corradolvr
What are JP1 & JP2 used for? There's no jumpers on them. Test points?

You jump this to check output biasing I believe it disables temperature control of the bias so you can set the bias without temperature interference. I like to check the voltage across these pins also to make sure the bias voltages are within operating spec's also
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Old 14th July 2007, 05:10 PM   #7
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Cool thanks for the info. The led that's in the middle of the board, what's that thing for. I've never once seen it light up. I'm down to pulling individual components from the board and testing them one by one. I'm almost done with all the vertical transistors. So far I have not found any bad ones. I found 4 strange ones that I can't find documentation on. The c252ac. These test like they have a pn junction and 7 ohms. If it's upsidedown with the flat side facing you I get a pn junction from the center leg to the left leg and a 7 ohm reading from the center leg to the right leg. one of these 4 components I checked had a 10 ohm reading accross the center leg and the right leg. I tested the amp on my service monitor a little more closely the other day. One channel works but I have no output on the other one. both of the vertical boards have input and output w/ some gain. I think what I may have to do is pull all the new transistors and re-test all of them. There were some bad ones that were brand new. Yes I was an idiot for not testing them before I put them in. I also was wondering if anyone knows the lay out of this amp. The components were a little hard to read when I made my map. On the side with the speaker outputs starting at that end I have 4 bdt81's then a c2334 then a a1011 then 4 bdt82's and one bdt81. On the other side I have 4 bdt81's then a c2334 then a a1011, then 5 bdt82's. Is this correct?

Thanks,

Corradolvr
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Old 15th July 2007, 02:49 AM   #8
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The extra BDT transistors are most probably lower voltage regulators for the +&- 16 volt rails. All PPI amp have a balanced set of outputs < i.e. 4 pnp devices and 4 npn devices per channel, or whatever the count as per the amp size.
So those extra outputs are lower voltage regulator pass elements.

as for the strange transistors the numbers don't jog any of my bells but you might be looking a J-fet mute transistors and PPI used a semiconductor mute circuit. And these are normally on without voltage present to the correct lead of the device, and so you will get a 11 ohm or less reading on these. They sound like the mute J-fets < these rarely fail >.

as for the lay out I have successfully repaired every A600 that ever came across my bench so I don't have any carcass's to look at to help you out. I suggest you go to Amp-Guts.com and look up your PPI by model number and right click and save the inside photos of you amp and try and blow them up on your desktop with Windows picture editor software < this works for me in a pinch>

As for in circuit testing a ohm meter will do just fine if your use to the readings you will normally get on a good amp , this takes practice and experience that comes with time. But you can pretty much see bad transistors as shorted devices in this design. If you read the outputs and see 20 ohms or less you can pretty much say something is amiss here. I lift the center leg of any questionable device and then re-read its resistance using the center leg as a common point for the other legs readings. This way if the device is bad it will still read shorted thru the lifted leg and this is a independent reading from all the other devices due to the lifted leg. Does that make sense ?

Do you have the ability to trace a signal from the RCAs thru the circuits to the output??? I ask because we still have no idea where your losing the signal. Maybe I am missing something but I don't read any definitive points where your getting a signal into the channel that silent.
In the old days we used a little signal tracer , nothing more than a battery operated op-amp powered tester that would play the signal into a small test speaker when the tip was used to track the traces.

A simple version would be a nice 100 volt mylar cap feeding a LM-386 power op-amp that drove a little PC grade speaker, and power off of 9 volts or less if possible. this would at least let you know where the signal is dropping out at, so you could concentrated on that area and find your fault.
Today I guess the average tech would use a O-scope to visually track the signal to it drop out point. Either way i think we would both be better able to speed up your repair here.

My experience with a Low impedance lite coming on is pure and simple blown outputs, and a simple ohm meter check should find your shorted outputs. This amp design was fairly well layed out and because of its design the transistors folded over on themselves, and you get that silly yellow led light up as a indicator of low load current draw on the amp, when in fact the sense circuit was sensing the excessive current draw of the shorted output devices.

I would just start ohming them out, I am sure you will see the shorted devices quite clearly with even a junky ohm meter. As they will stand out as the only 1 to 5 ohm resistance you will find, and then when you find them just lift the center leg to find the one or two that usually go bad in these amps.
I recommend you spend the money and replace the entire set of outputs in that channel bank, as it will last longer if you do. just a fair piece of advice from a old timer here that has been there and done that so many times I can't even begin to give you a accurate count number
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Old 15th July 2007, 09:30 PM   #9
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I have the amp at work. I'll be able to trace it more accurately Monday. I use a service monitor for pretty much everything. It's a spectrum analyzer, tone generator, o-scope two way radio device. You can check any radio freq with it, tx and rx. They're really nice but cost a ridiculous amount. Good thing I didn't have to pay for it. Any way thanks for the help. I'll update after some testing on Monday. I was thinking of the led that is surface mounted on the board. the one that you can't see with the case on.

Thanks,

Corradolvr
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Old 7th December 2007, 04:14 PM   #10
ppia600 is offline ppia600  United States
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Not trying to hijack this thread, but I have a friend's 1st generation a600 and apparently he connected the power backwards (generic ebay plug) and fried some of the power supply fets. I bought some IRFZ44's to replace the mtp25n06 to try and improve the reliability of the power supply a little. Will it be ok to use the original gate resistor resistance? Or should I change to a different value? Thanks,

Brian
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