Phase 400-II Quasi comp to comp conversion

[TimD]

Just wondering if anyone has done this prior *WITHOUT* changing out boards or backplane. It looks pretty trivial - the PL service bulletin seems to indicate that the only change to the PL36 is adding R127 and R128 (56K), and it looks like the backplane change is simply flipping the emitter resistors and a few other components from the emitter/rail side to the output side, changing out the transistors, and all should be good.

(I'll also change the 4 electrolytics on the PL36 and add the 4 diodes to prevent latchup, but those really aren't a part of the conversion per-se.)

This amp is working at this point, so I have very close to zero desire to change driver cards or go to the PCB backplane . . .

Anything I missed?

 

[J!m]

But it’s still an old amp… start working those boards and scope creep due to “while I’m in here “ and “now that trace lifted “ it may make more more sense to call Joe…

 

[TimD]

Competent technique does not lift traces . . . and only 4 caps to change. I work on stuff this ood (and older) routinely with zero issues (and no traces on the backend to damage in the first place), so while I appreciate your response, it's a direction I have zero interest in at this point. These things are just too simple and easy to work on to justify that (unless it had an internal fire).

 

[mlucitt]

You will also need the two PNP columns of ONSEMI MJ21195G Output Transistors for the Fully Complementary operation to be... complementary. Personally, I would change out the existing NPN Output Transistors as well, use ONSEMI MJ21196G.
If your amplifier still has the Silicone grease/mica heatsink insulation, add 8 Sil-Pads (or 16 to get rid of all that Silicone mess).

 

[TimD]

You will also need the two PNP columns of ONSEMI MJ21195G Output Transistors for the Fully Complementary operation to be... complementary. Personally, I would change out the existing NPN Output Transistors as well, use ONSEMI MJ21196G.
If your amplifier still has the Silicone grease/mica heatsink insulation, add 8 Sil-Pads (or 16 to get rid of all that Silicone mess).

Thanks . . . Thought I had covered that under "Changing out the transistors" (this unit, while working, has the SD555 (or close?) parts, and I do plan to swap to the current complimentary pairs and matching drivers). I had just not focused on that here since I was more focused on circuit rework tasks.

When done, if anyone needs any of those I should have a complete working set.

No desire to change to the sil pads. I have all the other supplies, and had actually found a PL service bulletin stating that the pads can aggravate oscillation, and recommending to stay with mica and grease.

 

[wattsabundant]

Last year I drew up the conversion in power point and saved to pdf. I used a lot of Ctrl-c, Ctrl-v. See attached.

As for silpads I've only installed a thousand or so with only a handfull that I over tightened. Use thumb and index finger to lightly grip the nut driver. Turn until it slips in your finger. After performing load share tests, I shoot the transistor cases with infrared gun at about 80 watts which is worst case condition. The absolute value is not as important as equal temps (about +/- 10%) in each column. Temps should not be allowed to exceed about 85C.

For the TO3 hex screws, I use a 1/4" nut driver with the socket ground so it will fit between screws and the case.

 

[TimD]

Thanks. I don't have silpads, but have plenty of grease and mica, so sticking with that . . . Hard to see a reason to change what's not broken, and as noted, the PL docs indicate a possibly increased tendency to oscillate on silpads, and recommend mica . . . Not sure how real that is, but can't really see an upside to the silpads other than more cost. Thanks for the diagram - that should help (along with an already full comp 400-II to compare to). (I recall having a thin enough socket to get to the screws as well - I had to repair my "long term" 400-II back in the early 1990's, and don't recall having any issues finding a suitable tool then.) The heat gun is a good idea, and I have a decent one . . .

 

[wattsabundant]

As for the grease, less is more. All that is needed is a pencil eraser size dot between the two pins on both sides of the mica.