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Old 15th February 2007, 12:18 AM   #1
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Default repairing PPI PCX-2200

Hi I have recently aquired a PCX-2200 PPI amp , I have it so it turns on but the little stand off boards start to get hot quick , the right speaker output plays at low volumes with static and does not change in volume much when the gain is turned up, the left channel plays descently and adjusts with the gain but has constant static , I also noticed between the input RCA's and the gain there seems to have been a larger resistor maybe but it is missing and the board is burnt between there , what was there ? also there is a tiny resistor behind it that I cant tell the value of and the tiny resistors underneath the gain are burnt beyond recognition aswell , does anyone know the values of these resistors and maybe what the missing part may have been ? and lastly down by the powersupply when the cover is off to the bottom left of the larger torroid there is an A06 and an A92 that i can tell are factory installed BUT on the oposite side it seems that someone has replaced those using a -A56 and an unreadable 1 should those also be A06 and A92 ? and if so which 1 goes in which spot since I cannot find any schematic nor is this amp on ampguts ?

thank you for your help in advance
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Old 15th February 2007, 07:19 AM   #2
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The large resistors were 10 ohms at 2 watts. I just replace a set of 4 on a PC-4400. They are current limits for the RCA shield. These fail when you have badly blown channel or channels, and or DC voltage on the the RCA ground possibly by a output being shorted to the case.

As for the MPSA06 its match is a MPSA56. I have rarely if ever seen it matched with a MPSA92 which is a 200 volt rated transistor and the A06 / A56 are 80 volt rated device.
If your describing the Gate drivers for each bank of power fets < near the toroid, I am sure its the A06 / A56 combo not A92. A18's are the input pair on the American built amps, maybe the offshore also. The only A92 usage I am aware of might be the bias transistors or predriver sets.

PPI never to my knowledge released schematics so you will be press hard in that area.

Can you post a pic of the smaller burnt resistors ? I am sure about the big ones as listed above. But need a better view of the others your mentioning.

Have you ohm'ed out the output transistors ? When reading across the two outside leads it will read low, about 99 to 100 ohms for 2N6488 / 91 outputs, and even lower if its TIP35 / 36 outputs or MJL Motorola outputs- this is normal. typically 5.0 ohms or less is shorted.
Its the readings between the other leads that should not read below a simple junction value typically. Look also for shorted driver device feeding the outputs, and open emitter resistors on the blown outputs if any. The ceramic resistors open up like fuses and have to be ohm'ed out to make sure they are not open. A visual in not enough to check these they must be metered.

This amp has clamps holding down the power devices correct ? If not then don't try to operated this amp without the outputs being sinked properly, They will fail if you do. Older PC and Art series and Am and M series amps needed the bottom reinstalled before testing at any level above idle no signal. The PCX series were supposed to fix this issue as they were most likely built overseas, not in America.

By your description of the Right channel I would say its blown. How bad needs to be checked and measured with a ohm meter.

On the PC-4400 I just fixed today which has the symptoms your describing, I replaced the entire semiconductor compliment of one channel to restore that amps proper "Like New" operation. That was 11 small signal devices, 4 P & N outputs, and 2 of the large ceramic emitter resistors.
Then I had to replace the large 10 ohm resistors on the input RCA board with new flame proof type devices, and repair burned up traces there also.

A lot of work, Yes but it played like new,and it met original spec's. Good luck
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Old 15th February 2007, 09:56 AM   #3
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ok , without removing the MJL transistors I get .099 ohms across the 2 outside legs and .000 ohms on the large resisitors , sorry for the camera pics they are blurry but maybe you will recognize the layout im hopeing, between the RCA's and gain you will see the burnt area and what looks like a place where a large resistor was, and there are 4 small resisitors beneath the gain that are burnt and unrecognizeable by far , and the pic by the torroid is where they replaced the 2 A06, and A92 parts , on the other side it looks to be factory installed still no tampering and those 2 are A92 and A06. if you meant for me to remove the transistors to get the ohm readings I will rmove them just tested them while still soldered to the board. again sorry if the pics are too bad , I will try to get better.
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Old 15th February 2007, 10:03 AM   #4
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Cant figure out how to ad multiple pics to 1 thread so here is 1 other pic
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Old 15th February 2007, 10:05 AM   #5
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And last the pic by the torroid , sorry the pics are in different posts
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Old 16th February 2007, 06:06 AM   #6
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OK last pic first:

Those are supposed to be A06 and A56 pairs there.

Output pic next :
on the outside pins you should read 99 to 100 ohms to as low as 22 ohms across the outside pins No lower.
Lower means Shorted transistors, are your readings 0.099 K ohms ?? that 99 ohms and OK.

across the big resistor groups, they are connected together in groups. to test these you must lift one end of each and test < i suggest the wire side, not the body side.
In this model those usually burn black or ash colored and darken the board when they fail, so they look Ok . I would check them later.

first pics:
WOW ! your board has been flashed bad. All that burnt looking stuff is carbonized circuit board, and in my opinion it must be removed back to unburnt fiberglass so there will not be any short circuit through the blackened board areas. I believe you have vaporized some traces also.
This is not good, as there will be no board left to mount components back to unless you rebuild the fiberglass board. I have a friend that does this and it is time consuming at best, and requires that you use some fiberglass repair chemicals .

You know this amp may not be worth repairing at this point. Unless i can get a replacement board or repair the existing board to spec, I normally tell my clients to buy a new amp.

You amp was repair by someone that was either guessing or was using whatever he had on hand to try and fix this amp before. I say this because of the wrong transistors in the Fet driver circuitry near the toroids. And the poor soldering I can see even with a blurry picture.
The damage up front is extensive, and requires sharper pics of both sides for me to be able to help more in this area, You also need a pic without the front endplate removed if possible, end on. this is so a judgment call can be made about weather it will be worthwhile to attempt repairing this section of that board.

Ohm out the fets in the power supply section. Nothing should read shorted <below 200 to 300 ohms. everything should read higher.

You are going to have a lot of work tied up in this before it comes back to life. I say this to you so you will not be falsely lead into thinking this is a easy repair, as it is not
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Old 16th February 2007, 07:59 AM   #7
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If you're using this amp to learn, I believe it's salvageable but it may take some work. If you're trying to repair it for profit, now would be a good time to return it to its owner.

The badly burned resistors (some of the later models used 47 ohm resistors) near the RCAs concern me. Assuming that they're the resistors that connect the RCA shields to the secondary center-tap of the transformer, I would think that it would require more than 12 volts to do the damage shown in the photo. It's possible that the transformer is shorted. Measure the resistance from the secondary center tap of the transformer to the primary center tap of the transformer. If you read something near zero ohms, the transformer is likely shorted. Even if there is no short now, it could be intermittent and may not show up until the amp is repaired.

Is there a second burned area that's not visible (under the shaft of the pot)?

Is there a burned 270 ohm resistor near the transformer?

The area that's badly burned is likely conductive and needs to be cut away. If there are no other traces over the burned area, it's no big deal to cut out the charred board. Many high quality amps use point to point wiring. It won't hurt to wire these resistors point to point. You'll simply have to follow the traces back until you get to good copper or a suitable connection point.

It's reasonable to believe that the a92s are the originals. While I don't understand why they used 300 volt transistors in that location, they are generally comparable to the a56s that are more commonly used. Only you can tell if the solder connections are the original flow-soldered connections.

You said that the meter read 0.000 ohms across the emitter resistors in the audio section. It seems like the meter may have been set to diode check or continuity. A meter that can resolve 3 places beyond the decimal point should have read higher than 0.000 ohms.

What meter are you using?

Pull the outputs and check them out of the board.

For the smaller burned resistors, try to get a better photo. Someone here may have one of these amps. If you ask nicely, they may be willing to check the values for you.
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Old 16th February 2007, 09:40 PM   #8
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I verified by looking at my doc's and the other 7 PPI amps awaiting pickup on my shelf the gate drivers are supposed to be MPSA06 and MPSA56.
In your pic it appears that someone has been soldering around in that area. And I would say they threw in what they felt was compatible instead of using all original parts.

Who knows maybe the A92 works reliably in this application. The A92 costs more so it looks like a fast repair I would guess.

I <might> have a spare input board floating around in storage. If you remove it and post a couple more pics I might be able to find you a replacement <Maybe> as PPI does not support these products anymore.
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Old 17th February 2007, 06:23 PM   #9
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I am trying to get my digital camera back form a friend , but I looked at the board and the other side of the toroid , it is A92 and A06 and the solders are deffinately factory , I will get pics of it today in fact , if the A56 will work I will use those instead.
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Old 17th February 2007, 10:14 PM   #10
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Yeah the A56 will work just fine. its used in all the rest I have seen over the past 19 years or so.
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