Jer's PLWO to be... (Jer vs the 700, Round 2... it's "Personal" this time)

[jbeckva]

First off, gotta give the props to the master sniper and overall GREAT guy here, Lee. I turned him on to this one and said I'd do another attempt at a PLWO if he sniped 'er, so here it is...

The ad said it didn't work, which it doesn't. The seller obtained it with the power cord cut, so basically it was a good "as-is" listing. Got it in today. I haven't done much with it, but I did check the fuses figuring ok.. at least that would give me a clue on what to expect on the inside.

Fuses? Every one of them is perfectly FINE. So there's a mystery... hmmmm... HMMMMM!! I will shortly replace the power cord and put it on the variac to see what's what, but I really am wondering if I may be in for a big surprise.

Then I popped the top and front panel, and ... WOW... this thing looks like a VIRGIN (Hey you ole PL guys.. what do you think by looking at the pics here??). The only thing (and look carefully, but you should see it too)... is it would appear that one of the TO3 sockets on the right side right at the top is unseated and not properly installed. I wonder... if I fix that and the cord up, what will the variac test reveal???

In any case, I don't plan on leaving it "as is". At the very least, a DCP is in order for upgrading (Lee.. who do I ask?). Then I'll be looking for a full PLWO overhaul to work on. I'm going to complete this one, dagnabit... it will not kick my butt.

Anywhoo.. take a look at the pics! More a comin' as the new project takes flight!

 

[laatsch55]

DUDE!!!!! they don't get any cheerier than that!! In fact I've never seen a series I in that good of shape!!

 

[laatsch55]

Jer, PLEASE --leave that board intact. If your troubleshooting requires some board work, I would really like to have that board intact. Ed has asked that if I ever came accross a cherry PL 0171 if he could have it. that looks like the candidate right there.

 

[laatsch55]

wattsabundant (Don Imlay) on the DCP Jer.

 

[jbeckva]

Chances are I'll be switching it out for a rev C WO anyway, sure.. of course it'll be intact.

How are these caps here... I think if memory serves these were the ones ya had in my SN1?

http://www.ebay.com/itm/2-ADCOM-100V-22000uF-Computer-Grade-Low-ESR-Capacitors-/200834525297

(just planning ahead.. like I said, it's going to be interesting to see what happens when I get some power to it... )

 

[laatsch55]

Date code says 45th week of 2001. they have some life left. Good price , good seller too. Nice guy.

 

[kevin]

0 hell yea... Another good thread to follow.. lol.. Didn't need or want this thing but I watched it on e-BAY..

 

[jbeckva]

Yep, it's been my "bane" here. Before I got out in 98, I was one mofo of an avionics tech and it was kinda embarrassin to say "uncle" to the one Lee's got back. But that was also learning "what not to do", as working on these PL's is more challenging *mechanically* than what I was used to which was circuit boards (easy to get to and probe out). Didn't have to muck with any wires either... heheh.

Anyway, so I have the cord replaced and that one socket reseated and buttoned down. I'm going to go burn one then hook the variac up, and we'll see what's what here..

 

[jbeckva]

Ok, so it's up on the variac. I haven't gone full power yet, because I have seen that the right channel is swung completely in the positive direction, which isn't good. Or is it?

Joe/Lee - How far up do I need to bring it before I can say that with a minimum of "x" number of volts on the rails, if I still have a large DC output then I definitely have a problem? (Yeah I'm a "chicken" to go higher right now... ) I pulled the fuses looking for any of the outputs having a bad insulator but that looks good. Is that the "latch up" issue that I've heard you speak of in the past?

 

[Gepetto]

usually kicks over at ~40 V Jer. If it still keeps going after that, something is wrong.

 

[jbeckva]

Ok, yep.. even at +/-50 the right channel is at 38 volts on the output. Left is down to 500mV so I'll concentrate on the right first. That also happens to be where that one socket was misaligned.

Gotta consult the parts placement and schematic .. get familiar with where everything is...

Hmmm.. would swapping the drivers (the SJ's on the bottom row) between right and left be a good idea to isolate? Or should I just think now about what to order for the "makeover"? I'd like to troubleshoot a lil bit first to get familiar with the circuit...

 

[laatsch55]

When I've had lrge offsets if it wasn't "latching up" 90% of the time it was a Q1-Q4 problem. If it goes rail voltage without blowing fuses it's almost always in the low level section.

 

[jbeckva]

Aha! Found it!

Well Lee - Upon closer inspection, this board definitely HAS been worked on. The problem was with Q17. It's emitter lead was barely BARELY long enough when it was replaced, and it eventually (or just "did") lost contact with the trace. So basically Q17's emitter was going "nowhere".

I had to take off the plastic spacer on the bottom to gain just enough lead to get it to the pad on the backside. Soldered that up, tested.... Went for power up... 50 mV offset on the right.. about half a volt on the left but I haven't tried adjusting anything yet.

There's a good stopping point for the evening! Slow and easy... Tomorrow should be hooking up the loads and seeing what it can do with a signal...

Well.. maybe not.. I don't think I'm getting any bias current on the other channel.. right is "fine" at around 360 mV

hmmmm... welp, that'll be tomorrow night's puzzle.

 

[laatsch55]

A PL 0171 board has 4 trimpots . do you have the manual?? You adjust one with the volume down, then turn the pot up then adjust the other, then do it again, and again, and again until it won't go any lower. A half a volt offset is completely in the realm of possibility with a PL0171 board, it's also possible to trim it to zero from there IF the resolution is going in the right way, if not, Q1 thru Q4 will need replaced.

 

[laatsch55]

On the bias....... There is no Q6 on the wall. They monitored output temp with a tranny on the board. You will be putting a Q6 on the backwall if you put a Joe board in. Don't do any loads until you get that offset down.

 

[laatsch55]

BTW the 0171 stands for the first Phase Linear board for 1971.

 

[jbeckva]

Ok, so far I've done the DC offsets on both... not bad, but wow are those pots sensitive! Makes you wonder how they'd stay adjusted..

But still.. no bias on the left channel... No bias means no quiescent current (did I say that right?). I've gone somewhat down the line of the bias circuit.. Q6 collector/D5 anode is reading .7 versus about 1.6.. Difference to me seems to be a PN junction "away".. hence possibly a shorted diode or one of the trannies south of Q6.

Still prodding n probing... haven't found it yet. Hints welcome, as always...

 

[Gepetto]

Most likely D5, 6 or 7 is defective (shorted) or Q6 is defective (shorted). Are you getting 0.7V on one of the diodes and near zero on the other 2? That points to shorted diodes. If your 0.7V is fairly evenly divided amongst D5,6 and 7 that more points to a bad Q6

 

[jbeckva]

Ok, looks like Q17 is borked (ey Lee.. there is a Q17 on der PL071 board for the series I.. I swear I sees it on the diagram..)

Going to pull it to be sure.. but looks opened up to me...

 

[laatsch55]

Yes, I edited that out last night after perusing the schematic.