First PL700 Check Out

[Kma4444]

Thank You guys for the input. I checked it in circuit using the voltages called out in the service manual, which may or may not have been correct to do. I compared it to the left channel which maybe I shouldn't have. The left channel did show voltages very close to what was called out. The Q10 on the right channel showed .5 volts on the center pin, the one that goes to the red wire on the board and the other pin shows just thousandths of a volt. The reason I was checking around at all was that I had no bias voltage on the right channel. I'm not sure if what I was checking had anything to do with that, I was just using the diagram and checking for stated values at points I could identify. I am beginning to figure out what is where on the boards, not what they all do though. Trying to be careful and learn what is what.

SO I went ahead and ordered more of the silicon transistors and I will replace the one as suggested here. I'm working on digesting what Watts posted and I will get it all, might take a couple reads and looking at diagrams etc. I can't say enough how much I appreciate you guys taking the time to reply here. I'm learning,,,,,,, slowly.

 

[wattsabundant]

Hopefully my attempt at humor in the previous post was understood.

You can't gain much by reading voltages on a direct coupled amplfier which is where the P/L's fall. One stage drives the next and they all appear out of whack. Put a meter on the diode scale and compare readings on each transistor. Check every diode also. Pull the silicon transistors for Q8/9 for the time being as they aren't needed until the last phase of the test procedure.

 

[Kma4444]

Copy that and I appreciate the " Hey dummy, try reading it with a healthy dose of intended sarcasm" Alert. Some times I can be a tad thick. I will press on

 

[mlucitt]

You are getting some first rate advice, Don (watts) knows his stuff.

 

[Kma4444]

I have no doubt that is the case Don and LOTS of other folks here. I'm trying to be a sound study. (Now that's pretty funny)

 

[Kma4444]

Alright, so back at it, been working some which is a nice change. So right channel Q8&9 pulled. Still only .5 volts and thousandths on Q10 as described above. So to check the diodes do I need to take them out of circuit? Checking the voltages through D6, 7 & 8 they are not anything like the ones on the left channel. I'm seeing lots of different voltages than on the other channel, probably fairly obviously. But I'm not sure where to go next. I did remove and check Q10 and it tested fine with a cheap little tester.

Thanks all.

 

[wattsabundant]

with power off,and fuses pulled, check eveything in circuit and compare readings with known good channel. Prefer to use diode scale.

 

[Kma4444]

Copy that, thank you

 

[Kma4444]

Not seeing a lot of differences checking the components side to side with the fuses removed and power off excepting the diodes 6, 7 and 8. On the right channel I am getting about .62 volts one way and 1.3 the other. On the left I am getting the .62 but the other way I am about 1.8. No idea if that's enough to cause any issues or if that should point me in a direction. Those numbers are using the diode range on the DVM. Thanks all.

 

[Kma4444]

Q8 & 9 are still in the left channel, was I supposed to remove them as well? Sorry for the maybe simple questions.

 

[Kma4444]

Double checking and I found C1 on the right shows 1.7 in one direction and 1.8 in the other. It shows infinity on the left channel.

 

[Kma4444]

I went ahead and pulled C1 out and tested it with my little TC1 and it shows 109pf I assume that's fine?

 

[Kma4444]

I pulled one end of R3 out because I still measured the 1.8 across the points for C1 even with it removed. R3 tested very close to the 39k it's spec'd. Still checking.

 

[laatsch55]

Where you have 2, it 3 diodes next to each other one end will have a common tie point or they're in series. Out of circuit on those. But if you dont have a dead short or an open they are probably good...probably..

 

[Kma4444]

Well, not making any appreciable progress. I appreciate the help though I've been unable to put it to much good use. I will run through again and see what I missed.

 

[wattsabundant]

...excepting the diodes 6, 7 and 8. On the right channel I am getting about .62 volts one way and 1.3 the other...

The .6 volts across the diodes is good. Has Q6 been tested? I think it would be best to make a table with columns for both channels and write down the readings one at a time for both channels. For the time being skip Q1-Q5 and associated diodes because those transistors affect offset which is ok.

Looking at previous posts the issue has been around Q10. If it has not yet been changed out it should be changed to the proper one. It's entirely possible it is installed backwards or has failed.

 

[Kma4444]

Gotcha, I will do as you suggest. I did change Q10 to the same that had been installed previously. It tested ok after I removed it but I had purchased some new ones so I changed it anyway. I understand about putting the correct components back in instead of these other ones. I was hoping I could get this one working to get me by for a short while until I could do a full WOPL on one of the others. I picked this one as it seemed, at least initially, to be the closest one to working. Maybe since it's been modified, it was a bad choice.

 

[laatsch55]

And it's good practice prior to assembling and installing a White Oak board. Probe slips, components put in wrong , etc, those are all learning experiences better learned on an old expendable board.
I have all components that board requires if you want to shotgun the whole mess.
Follow Don's recommendations and you should get this board up and running.

 

[WOPL Sniffer]

WOPL it and save yourself headaches and money. It's like replacing a carb on a car you are going to change the motor in.....

 

[MarkWComer]

WOPL it and save yourself headaches and money. It's like replacing a carb on a car you are going to change the motor in.....

I couldn’t agree more, even if it’s a cool thing to have an original operational “antique.”