PL 400 Rebuild

[Magic Smoke]

..so I joined to beg questions and directions from fellow PL owners.

So, I was given amp two years ago, condition unknown with no meter lights. I found a very hard to read service manual PDF, read through the troubleshooting to see if safe to use variac with no output load. Fuses appeared original and tested good, Outputs also appear original with blue, delco?, ink. Both power supply caps read 6200 uf using Fluke meter. However, DC was only reading 77 +/- volts respectively.

Watching ac clamp current I brought up to full voltage without any drama. DC offset less than 32 mv on both channels. I let it idle for an hour with no signs of heating. I did not measure offset as work called and stuff packed up. Returning home, I used amp to drive old fused AR stereo sub..for two years! Anyway, yesterday, I now get a quick meter spike with pop in both channels during power up. This is intermittent but also noticed faint popping noise out of right channel, also intermittent.

Not being smart I'm guessing driver board so would love to use the PL14 20 Control Board Kit for upgrade. Trying to keep cost down so thought I would get opinions and what else I need to check. I'm learning this stuff as I go so don't hold back. I have a scope with leads in the mail.

Thanks for any advice, Mike

 

[laatsch55]

Q1, Q2 are probably getting a little wonky. 34 mv of offset is a good clue. Replace the old electrolytics on that board. They are a ticking timebomb.

 

[laatsch55]

BTW , welcome to Phoenix....

 

[nadude98]

I second that welcome to Phoenix.. good guys here!

 

[Fishoz]

Glad to have you here Smoke. Welcome. Pretty sure you'll learn what you need here.

 

[Northwinds]

Welcome!!

 

[Magic Smoke]

"Thank you, thank you very much" for the welcomes.

@ laatsch55

What bothers me; both channels make the short turn on "pop" with both meters partially or fully pegged. LOL, dog came off the floor, cat still stuck on ceiling. I have the common turn off "thump", are the start up pops normal? I ask because it's been mentioned in long lost threads. The right channel has the intermittent noise.

I'm missing something but the PS is the thing common to both channels? Also, the 77 VDC readings across main caps, are these near spec or do you feel supply is being loaded down a bit?

Thanks in advance!

Mike

 

[Northwinds]

cat still stuck on ceiling

I LOL'd, my cat sleeps on the CV D9's at full crank. Nothing bothers her

 

[laatsch55]

77 volts is a little low. What is your bias voltage ?

 

[Gepetto]

"Thank you, thank you very much" for the welcomes.

@ laatsch55

What bothers me; both channels make the short turn on "pop" with both meters partially or fully pegged. LOL, dog came off the floor, cat still stuck on ceiling. I have the common turn off "thump", are the start up pops normal? I ask because it's been mentioned in long lost threads. The right channel has the intermittent noise.

I'm missing something but the PS is the thing common to both channels? Also, the 77 VDC readings across main caps, are these near spec or do you feel supply is being loaded down a bit?

Thanks in advance!

Mike

Mike
Your bulk caps are 45 years old, not much good anymore and they will come up very fast. That is part of your pop problem. Putting fresh caps in will slow down the initial rise and allow the control board to do its job more effectively on startup.

The other part of your pop problem is that the front end on the stock PL400 does not come into control until the bulk caps reach approx. +15/-15V, it is that way by design. The pop you hear is the amp putting energy into the speakers before the control board brings the output back into control. Does it do it with no input plugged in from your preamp?

Turn on pop and meter bump in the stock amp is not uncommon in stock form (as is the turn off thump, which is again the control board losing control before all the energy has left the bulk caps).

 

[Gepetto]

77 volts is a little low. What is your bias voltage ?

And what is your line voltage. This voltage is a function of the line voltage and the quality of the bulk caps.

 

[oblivion321]

Welcome, Smoke! I'm a new member here myself and these guys have been walking me through my rebuild as well. Great bunch here. You've come to the right place.

 

[Magic Smoke]

77 volts is a little low. What is your bias voltage ?

Opps, sorry, enjoy your vacation if I read the posts right! I'll log everything after I build DBT and clear up bench.

Mike
Your bulk caps are 45 years old, not much good anymore and they will come up very fast. That is part of your pop problem. Putting fresh caps in will slow down the initial rise and allow the control board to do its job more effectively on startup.

The other part of your pop problem is that the front end on the stock PL400 does not come into control until the bulk caps reach approx. +15/-15V, it is that way by design. The pop you hear is the amp putting energy into the speakers before the control board brings the output back into control. Does it do it with no input plugged in from your preamp?

Turn on pop and meter bump in the stock amp is not uncommon in stock form (as is the turn off thump, which is again the control board losing control before all the energy has left the bulk caps).

And what is your line voltage. This voltage is a function of the line voltage and the quality of the bulk caps.

Thanks for the info!
Yes, I'm saving slush funds to get the W.O. audio control board (kit). I found out about the kits last week so checking if old output drivers can last a while longer after control board upgrade, then, speaker protection relay board. Instead of me chasing a lot of outdated parts the upgrade makes a lot of sense to me and is priced right.

I'm using an old 10 amp variac to keep line voltage right on 117VAC for no load checks. I plan to cobble up a DBT tonight and use the variac to adjust for any loss thru the bulb if I'm doing this right. Please correct if wrong.

Clarification please, "bulk caps"? The two main power caps or the control board caps? The two 85VDC supply caps measured 6200 uF each using Fluke 187 but yep, they old! I plan to dump them when funds allow.

Sorry for ramble and appreciate you guys time!

Mike

 

[Magic Smoke]

Welcome, Smoke! I'm a new member here myself and these guys have been walking me through my rebuild as well. Great bunch here. You've come to the right place.

Hey, thanks. Been reading thru yours and other threads as time allows. Looks like a PL bonanza out here!

Cheers

 

[laatsch55]

Can your Fluke supply 80 vdc to the caps while measuring?? Yes, bulk caps are the power supply caps...no longer on vacation...

 

[Magic Smoke]

Can your Fluke supply 80 vdc to the caps while measuring?? Yes, bulk caps are the power supply caps...no longer on vacation...

OK, I set plug voltage to 117 VAC then waited 20 minutes to measure bias. Left channel was 391 mv and right channel 353 mv. Left channel DC offest 13.7mv and right channel 31.2 mv.
Voltage sitting on bulk caps was down to 73.6 volts today. Nothing on amp heating up and no turn on surge (today) noted on panel meters.


The left channel bulk cap read 9 MOhm and right channel read 20 MOhm with meter. Each cap bench charged to 30 vdc and read 28 volts after 30 minutes, for grins.
Would isolating Left then Right channel to check B+ worth it? What do you suggest I check next or order you would mod this amp?


Thx, Mike

 

[laatsch55]

Mike, it sounds like that is as good as its gonna get without replacing the big caps.

 

[Magic Smoke]

Mike, it sounds like that is as good as its gonna get without replacing the big caps.

Yes, just hard headed old me. Ok, will order caps along with snubber and control board.

Thx

 

[Northwinds]

Good man, you ears will appreciate it

 

[Magic Smoke]

Good man, you ears will appreciate it

Lol, i hear ya!

I just learned i have the older 2 fin model with caps mounted on left side so Joe sent me a pic where there are two single 12000 uf vs stacked caps for the newer models. More mods as funds allow. I did my own LED mod last year so now looking hard at adding power switch if I can get it to look right. I had been using one of the "smart" power strips but it drives my class D HSU amp crazy with turn off transients.

Mike