in need of some PPI ceramic driver modules

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Perry is correct the jumper limit's channel bias, but if your amp had no bias applied when you first got it then I would say it was it had been tampered with.

Have you verified the bias circuitry including the transistor , It s located directly after the SIP card. If that section checks out then the card may be the issue since it directly feeds the bias circuitry.

If possible could you read the voltages on the active pins on the SIP card as referenced to power supply secondary ground and post them in order of pin number ? There are only 8 pins used.

Pin 1 is + 60 VDC
Pin 15 is - 60 VDC
Pin 7 & 8 are ground connected to a 0.1 cap to the -60 rail.
Pin 3 is + input
Pin 4 is - input
Pin 13 is + output to bias circuitry
Pin 14 is - output to bias circuitry

Pin 13 and 14 are my main interest, but the rails should also be balanced , if not then something is pulling one of them down with current loading. Also what voltage is on the input pins, mine all have 12 to 16 VDC mystery voltage
I have three PPI's on my bench currently and have 12 to 16 volts DC floating on the input pins to the SIP cards...What a mess :whazzat:

I think you might be chasing the same ghost I am on these three amps. All three were treated very badly by current and previous owners.
The A1200 was run in competition with water cooling. It ran so hot it actually melted the solder holding the components on the SIP card till they fell off.
The 21400 again the same issue high heat till the SIP card melted.
The PC 1400.2 had the same melted SIP symptoms with both channels failed and a burned out power supply.

All have floating mystery voltage on the input to the SIP cards that goes away with the mute jfets removed, but returns with replacement Jfets installed. Even with all drivers and outputs removed the mystery voltage still appears just to a lesser degree. All those transistors seem to check out electrically showing only mild signs of heat damage when curve traced.
The SIP rails are unbalanced by 3 to 7 volts and I have verified that there is excessive current draw at the current limit resistors by milli-volt drop method or measurement. One rails reads 94 MVDC drop and the other reads 487 MVDC drop and thats the rail that is down.
So I am looking for a defective passive component at this point perhaps a leaking or shorted Mylar cap perhaps. If I solve the mystery I will post the solution as I think this a common mode failure with abuse to these amps. If not then it sure does happen a lot for no good reason......:smash:
Pull and check the bias transistors. I don't know which ones they are on your amp but the collector and the base will be connected to the bias jumper header.

If the amp does not produce normal audio with the muting transistors removed, check the 100 ohm resistors that go from pins 1 and 15 to the emitters of the large SMD transistors on the driver card. I've seen these open or change value. When they do, the output is severely clipped or clips early on one half of the waveform. It could also be responsible for the DC offset.

The outputs from the driver board are 12 and 13. 14 is unused.

The large SMD driver transistors develop bad connections but it sounds like you've already re-soldered those.
Perry Babin said:

The outputs from the driver board are 12 and 13. 14 is unused.

Whoops My goof it is 12 and 13 Sorry for the mis-information, Thank you Perry :)

Please replace the number 12 for 14 in my previous post...

I am currently testing without the driver boards installed. This leaves the front end open to any influence except the SIP card. It also prevents further damage to the SIP cards. My replacements are repairable but I would rather not be causing them damage by having them in circuit at this time, until I can find the power supply unbalance reason, and the mystery voltage on the input... THX again Perry :)
the bias transistors checked fine. measuring across the emitter and collector of the bias transistors, it is about 0.1V with the jumpers in place, around 1.1V with jumpers removed, bias set to min. 1.3V with bias set to max. with no change in input current from PSU.

the amp doesn't appear to have been repaired previously. I was the first one to touch it with a soldering iron. the owner took great care with this as he used it only in an SQ setup and the case looks new and has very few scratches.

edit: driver supplies are +/- 44V, main supplies are +/-33V with 13.8V input.

a deeper look into the circuit appears that there should be around 4.2V across the bias transistor. is that correct?
I would expect ~2v across the bias transistor. Maybe 1moreamp can confirm if he has a working amp there.

It appears that the voltage across the bias transistor would be determined by the constant current sources. The 560 ohm resistors could be reduced slightly to produce more voltage across the bias transistor. You'd have to be careful. A small change in current in the CCS would likely make a big difference in the voltage across the bias transistor.
I was afraid of changing those resistors as the dissipation in the driver cards would increase. :(

what about the 3k3 resistors? I think I could also try that as in the orig schem, it was about 3k36 ohms. :confused:

also, you think it is a factor that I used BC546B instead (only ones available). I think the orig parts were 546C which is a higher gain batch.
I fixed the thing. I think :D

just so happens the 3K3 resistor on the driver boards have one side soldered on the component side and to unsolder this, I have to remove the rest of the parts on the card. :xeye:

I tried dropping the 560R on the CCS's to 470R but it got too warm for my liking but still didn't make enough voltage increase on the bias servo.

I ended up replacing the 1k2 resistors beside the bias jumpers to 2k7 (to change the range of the bias adjustment pots) and increased it just so that there is a slight increase in input current draw. don't know if that was enough though.

idle current before was exactly 1A. now it is at around 1.2A.

it is now running on the bench idling to see if it would go to thermal runaway but so far idle current seems stable and heatsink remains cool.

I forgot the exact voltage across the bias transistor but I think it was somewhere between 2-3 volts.
I got it to cook to 54degC (external sink temp, before my tone generator drained its batt :rolleyes: ) and idle current dropped to 0.9A.

edit: at 49degC idle current is 1.05A.

although I think bias is *somewhat constant* since if the SMPS is lightly loaded (as before with zero bias at 13.8V) the regulation isn't stable, the supply voltage "pumps" such that you can see the current meter needle swinging back and forth slightly. when hot, idle current drops but the needle is stable meaning that there is continuous load on the SMPS. so I guess bias is ok now. I might be wrong though.
Hello everyone.
Resurrecting this thread.
I bought a ppi pcx 1500 fully working apparently
I got it and it works. However i scoped the output and i got had some very unstable offset/very erratic as in -80-95mv.
So i wired an 8ohm speaker thinking that it might make it stable.. But it had no effect on it.
I hope someone can help me find out what is going on.
Thank you in advance
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