Need help fixing an odd Phase Linear 400

[tiefbassuebertr]

The service works by this power amp was done around some years ago, but I recall that the amplifier was already very much deviation compare to the original circuit. output transistors comes by original condition from NEC in a TO-3 outline. In the later condition were instead of NEC types such from MOT in the TO-220 housing, which are too weak for the output stage (MJE15030 and MJE15031) I replace it through the BD751 TO-3 housing and halved the power supply voltage - and then I optimized the whole circuit first by "circuitmaker" and second in real life. Unfortunately I don't had the complement, the BD750 and for this reason I perform a quasi complementary output stage. The models comes from "CircuitMaker" and I use types from there. I don't have an original schematic of PL-200
The peak is to disappear with additional capacitor in parallel to the NFB resistor. value: 82 pF.
The reason for the plot-peak before the roll off is a not right compensation (please note the redesign - the peak go away with the mentioned 82 pF).
Now the power output is only one quarter (-6db) compared to previously but in the future it is easier to get cheap replacement output power devices (NEC TO-3 are hard to find).
This work may not, of course, be made if the user great value to the original state sets. In my case it was required primarily a good sonic results and not creation back to the original circuit.
this links are of interest:
http://www.phaselinearhistory.stereomanuals.com/200seriesamp.htm
http://cgi.ebay.ch/ws/eBayISAPI.dll?ViewItem&item=260425937561
http://www.diyaudio.com/forums/showthread.php?threadid=123439
Service Manual Item number: 280378571884
http://cgi.ebay.com/PHASE-LINEAR-PL-200-SERIES-I-SERVICE-MANUAL--NEW!!_W0QQitemZ280378571884QQcmdZViewItemQQimsxZ20090729?IMSfp=TL090729168005r16530

 

[blue lander]

Got the rest of my parts last night, so I hooked up the correct protection transistor. Then I removed all the output transistors since I needed to put new mica and grease on them. I re-inserted only Q107 and Q108 in both channels.

I connected my PC to the "good" left channel and the output looked fine (just like before). However, upon connecting the right channel, both LEDs in the front of the PL400 start jumping around even though the left channel was disconnected. This is what I saw on the scope with just the right input connected to a 70hz sine wave...


This is the right channel:

An externally hosted image should be here but it was not working when we last tested it.

This is the left channel:

An externally hosted image should be here but it was not working when we last tested it.

The scope was set to 5 miliseconds/cm and .1 volts/cm

I'm going to get readings from all the resistors you asked for last time tonight, and I'll also check for any soldering problems.

 

[RocketScientist]

Yeah, check the DC operating points and make sure the channels match. I assume the bias adjusted OK? With output transistor installed, you want to measure across the emitter resistors of each output transistor when setting bias (Q107/108). For now, it should only be a few millivolts but there needs to be some bias to close the loop.

 

[blue lander]

Those are the 10 ohm resistors? I'll check again tonight, but the last time I did that test (before I replaced the protection transistor), I could never get any voltage across the resistors in that channel no matter how much I turned that pot.

 

[RocketScientist]

Those are the 10 ohm resistors? I'll check again tonight, but the last time I did that test (before I replaced the protection transistor), I could never get any voltage across the resistors in that channel no matter how much I turned that pot.

No. I don't have the schematic in front of me. The 10 ohm resistors are the emitter resistors for the drivers. The output emitter resistors should be around 0.1 ohm to 0.5 ohms each. If you have any output transistors installed, set the bias for a few milivolts across those not the driver resistors as before.

 

[blue lander]

I don't have any output transistors in right now, but I can put them in now that I have enough mica.

 

[wg_ski]

Those are the 10 ohm resistors? I'll check again tonight, but the last time I did that test (before I replaced the protection transistor), I could never get any voltage across the resistors in that channel no matter how much I turned that pot.

Sounds to me like you're still fighting an open (or shorted)predriver. There are also a couple of 1N4148 diodes in and around the area - make sure they're not toast.

 

[blue lander]

I replaced Q101 (40327) in the faulty channel with a 2N3440, could that be causing the problem? I'll double check to make sure I put that one in right.

 

[wg_ski]

A 2N3440 should work. Beta is a smidge lower than The RCA part, but that won't amount to anything with modern drivers/outputs. Make sure the 3440 and the PNP have the proper 0.6 volts vbe on them. If these aren't biased right, nothing further down the chain will be.

I had a 40327 shorted base to emitter and the 4003 open. That's after blowing all 6 PL909's. Or was it before/the cause? Can't tell with these things sometimes.

 

[blue lander]

I started comparing readings from the left and right of the board since the problem seems to be with just one channel. R101, a large 7.5k resistor by one of the sandbox resistors, was giving me all sorts of weird readings. I removed the resistor, and it's reading about 200k. So I'm gonna replace the thing and see if that helps...

 

[wg_ski]

That could very well be doing it. 200k collector load in the VAS would reduce the current to a fraction of a milliamp - and that's a recipe for oscillation big time. And if there is insufficient current in the Vbe multiplier bias string it won't turn on properly and you get low bias in the outputs/drivers.

And the resistor isn't obviously stressed/discolored?

 

[blue lander]

Nope, there was no discoloration on the resistor or the board. The PCB underneath its brother on the other channel, however, is cooked to a toasty brown.

So I replaced the 2 watt 7.5k carbon composite resistor with four "flame proof" 7.5k 1w metal film (two in series, two in parallel). I then adjusted the bias, and the channel started working without distortion or other weirdness.

Next I hooked up a small pair of insignia sattelite speakers and fired the system up. Both channels worked at lower volumes, but started to distort at louder volumes. I assumed this was because of sagging DC voltage because of the light bulb limiter.

I was feeling adventurous, so I plugged the PL400 directly into the mains and powered it up. Music played fine at all volumes, and I verified with the scope that the outputs were clipping evenly.

Tempting death and dismemberment, I plugged the PL400 into my double Advents and powered it up. Everything worked! The power on and off thumps are terrifying, but other than that it's behaved itself through at least 10 hours of listening. The warmest I've seen the heat sink get is slightly warmer than body temperature.

So congratulations, you guys fixed my PL400! Are the metal film resistors I used "acceptable", or do I need to find carbon composite ones? If they're okay, I'm going to replace the carbon composite 7.5k resistor in the other channel since it reads closer to 10k than 7k, and it looks like it's cooked the PCB like a poptart.

I'm also going to replace all the caps in the unit, do you have any suggestions for "audio" grade caps? Is there anything else I should upgrade while I'm at it? What about the power cord?