PL700II turn off, delayed thump

Netfly

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#1
So I checked out the new PL700II and all seems great...until I trun it off, then 10 sec. later one loud thump. Very repeatable. Input levels don't matter. No bad noise at shut off, just a delayed thump. Any ideas? Thanks
 

laatsch55

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#2
Normal turn off thump. Only thing you can do is closely match Q1, Q2....if it has the LF 351 .......well......
 

laatsch55

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#5
Regardless of which one it has, if you have turn off thump ...that's the culprit...
 

oldphaser

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#7
Meaning if its a 351 I should replace it anyway?
There was a 700 II service bulletin which was issued:

In the first year production runs of the model 400 and 700 Series Two PL36 driver boards we used a TO-99 packaged Fairchild or Signetics LF356 front end BI-FET op amp. This particular IC package was found to cause an unacceptable turn-off "thump" in some amplifiers. We later discovered and used in production a 9 pin mini-DIP packaged LF356 (National) which reduced this thump.

As of about May of 1979 all production runs of the PL36 driver board began using a National LF351 IC (Phase Linear part number 126-0114) which was found to have a better slew rate, slightly lower distortion, and substantially reduced the delayed turn-off thump. They are also cheaper.

If a model 400 or 700 Series Two amplifier has excessive delayed turn-off noise the front end IC's should be replaced with the LF351. These are a direct retrofit device and require no additional modification to the PC board.


NOTE: If you replace the LF356 with a LF351 you will also need to change out R1 and R2 (2.4K/5W) resistors to 7.5K/5W.

I also install 8 pin sockets with gold contacts.

If you have a handful of LF351's, you may be able to use trial and error replacement until the turn-off thump is reduced to an acceptable level. Changing out the IC's will also change the DC offset. (Factory spec was +/-10mV for DC offset. You should be able to get much better than that.)

Ed
 

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#8
So I checked out the new PL700II and all seems great...until I trun it off, then 10 sec. later one loud thump. Very repeatable. Input levels don't matter. No bad noise at shut off, just a delayed thump. Any ideas? Thanks
Normal for the stock PL amps. If you upgrade that problem will go away.
 

Netfly

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#9
Thanks for the detailed explanation Ed. I'll look tomorrow, May have a few or not. Havn't looked for those in a while.


Joe this may be a candidate, if another 351 IC is not an good fix. I'd like to fix the problem. My other one never did this.
 

orange

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#11
I wouldn't touch a capacitor on that amp with a 3.38 meter pole if it holds a charge THAT LONG. That's nasty.
 

Netfly

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#12
Open the can of worms today. I didn't look too bad, aside from supply caps that look replaced.
IMG_20170930_091623974.jpg IMG_20170930_091609981.jpg IMG_20170930_091458793_HDR.jpg

Checked the rails .Not too bad.
IMG_20170930_094603257_HDR.jpg IMG_20170930_094528997_HDR.jpg IMG_20170930_094438752.jpg IMG_20170930_094433842.jpg



A bit more noise on the outputs at idle than I anticipated.
IMG_20170930_094817360_HDR.jpg IMG_20170930_094954364.jpg




My search for LF351s continue, As usual seem to have everything but...
IMG_20170930_081851188.jpg IMG_20170930_082608199_HDR.jpg IMG_20170930_084940993.jpg IMG_20170930_084959430.jpg IMG_20170930_085006697.jpg IMG_20170930_085037875.jpg IMG_20170930_085108687.jpg IMG_20170930_085015648.jpg

Several places to look. Still haven't read what's in there now. The board will come out later.
IMG_20170930_091134566.jpg IMG_20170930_091145501.jpg IMG_20170930_091232801.jpg IMG_20170930_091241995.jpg
 

Gepetto

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#13
Open the can of worms today. I didn't look too bad, aside from supply caps that look replaced.
View attachment 30477 View attachment 30478 View attachment 30479

Checked the rails .Not too bad.
View attachment 30480 View attachment 30481 View attachment 30482 View attachment 30483



A bit more noise on the outputs at idle than I anticipated.
View attachment 30484 View attachment 30485




My search for LF351s continue, As usual seem to have everything but...
View attachment 30490 View attachment 30489 View attachment 30488 View attachment 30487 View attachment 30486 View attachment 30492 View attachment 30491 View attachment 30493

Several places to look. Still haven't read what's in there now. The board will come out later.
View attachment 30494 View attachment 30495 View attachment 30496 View attachment 30497
What do you have going on at 167 KHz?
 

Netfly

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#16
On the bottom that's just the scale. Here is a better look:

IMG_20170930_134104426.jpg


Just a/c noise. Think that is too much? Maybe the bridge. I was more concerned withh the output noise.
 

orange

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#17
I don't know it that's too much or not but if Gepetto is curious it's probably a good place to look. Uncouple and post that same test if you would, or at least decrease the noise input a good deal and work up from that.

WOPL mods are probably going to help a lot.
 

orange

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#18
By the way, have you checked your preamp, taken it out of the loop to compare as well?
 
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