Jerry's "Gonna Rebuild me PLWO 1000" thread

[Gepetto]

Lee the schematic for the PL0171 shows a pair of chassis mounted back to back 1N4004 diodes that go between chassis ground and signal ground (basically across the 2.7 ohm resistors or from pin 5 to pin 2 on the circuit board)

Like I said in earlier post, this is fairly common static discharge practice to keep one ground no more than a diode drop away from the other ground. If you zap the input jack due to static charge on your body as you are installing cables, etc. the discharge current will route through the resistors to ground and if that discharge current flow produces a voltage that exceeds a diode drop across that resistor, then the diode will conduct in a low impedance fashion to route the rest of the charge to the chassis.

 

[laatsch55]

So were there ever "flyback" diodes on a 700?

 

[Gepetto]

Yes Lee there are on the final output stages. These are 1N4004 and are ref des D14/15 on the PL14 and PL20 versions. Ref des is D12/13 on the PL0171 version. They are usually wired into the point to point in the power transistor area. The ones that Jer photographed were on the RCA input jacks.

 

[laatsch55]

Ok, got it straight now. The 1N4148's or were they 914's ya don't see on all of them at the RCA inputs. Was this a sometimes thing or I just haven't got ahold of that many virgins??

 

[jbeckva]

Ok, back on it here... We're GETTING CLOSE now!

I'm outta gas tho.. tomorrow.. aye.. there may be some Shponglin' going on here tomorrow..

 

[laatsch55]

Heheh... yup

HEY... You know, I"m looking at the DCP wiring diagram right? And am I seeing this right? Basically if the DCP ever tripped, it would shunt both outputs right to ground?? Yipe!!??

Did I see that right? So if that happens, aint we going to toast a few things.. maybe even on the channel that didn't have a problem too??

Edited to add.. no.. that's not right... I'm seeing things now - must be those flux boogers again.

Yep, the DCP just opens the output path, doesn't shunt anywhere.

 

[laatsch55]

Nice wire discipline Jer!! Should have sent you some 1" busbar!!!

 

[Gepetto]

Yes very neat work Jer.

 

[speakerman1]

Yes very nice job. How do you guys get the paint off for your grounds. I know a way that will take a couple seconds.

Jerry did you guys ever tie wires with string? If I had a bad lock on a pin in a canon plug. I would tie it with string to another wire. I could tie it so tight that I knew the wires weren't going any where. The only time I used zip ties in the interior was on CBs to lock a component out. So it could fly. You did what ever it took to get a flight off the ground on time. We had a MEL as to what had to be working to fly. Not every thing was functional all the time. You had so long to fix it and other conditions to fly in. You may not be able to fly ILS Cat 3. Which means the autopilot will land and take off at 0 feet of visibility. You can punch in the gate number at a slow airport and it will taxi to the runway. Pilots have it rough. LOL

Larry

 

[jbeckva]

Hey Larry,

Yeah, I remember the cable shop (69C, matter of fact) used to use that string method quite a bit. Worked pretty good.

ALRIGHT.. now... I want to make sure I have this procedure airtight before I do it. I am going to construct a DBT and put it inline with a variac for "the test". How should I proceed with the variac from power on to full power? Do I start it at a certain voltage level, or ?? First time I needed one.. hehe

 

[speakerman1]

If it were me just take it up slow. You will more than likely see smoke before 120 volts. If you want to rest you can. LOL This is just my opinion. I have been wrong before.

Larry

PS what is the light bulb for? Is it a buffer?

 

[jbeckva]

DBT will be to prevent said smoke heheh.. current limiter

 

[speakerman1]

That is what I thought. Just trying to make sure. Thanks.

Larry

 

[laatsch55]

Start at 0 volts and slowly wind it up. You'll see a lot of DC volts till it gets to 60 or so then the front end takes over and starts taking the DC down.

 

[jbeckva]

Yep, that's what I'm doing. 2 blown fuses later, got it to take full power without anything catastrophic happening.. (long story)

But now seems I may have wired up them 2n5088's a bit.. err.. wrong... no wonder no bias current! Time for a break and a smoke, then I'll get the CBE's straight... hehe

 

[jbeckva]

But I were a thinkin.. the DBT may help to find a dead short, especially on the primary 120 side, but I realized that it's just going to take longer to charge the caps on the other end. It limits it a bit tho... just not as much as I was thinking earlier.

 

[laatsch55]

It gives you an indication something is wrong before you go to far. If it doesn't dim after a few seconds....say after getting to 50 volts don't go further till it does.

 

[jbeckva]

Yep, got it on full power now.. no arc n sparks. Bias is about 350 on either side. Right side offset is kind of high tho... 50mV. Looks like no noise with the inputs shorted, so that's cool.

Looks like I have to get some nuts for the pots.. the ones ya sent have thinner shafts. No biggie... got 'er covered, I think.

 

[laatsch55]

Damn, I had lots of nuts too. Got any TIS-97's?? I could send you a couple matched sets and get the offset down. You have the 470uf in C6 right?? And the back to back 1N4148's on the backside of the board accross C6's leads right??

 

[jbeckva]

Got the C6 and the diodes, both. Yeah, maybe send those TIS's... We will see when I finish here in a few...