Phase Linear 700 series II restoration
 

Hello,
I am renovating P/L 700 II with complementary output topology. More historical and current status info is here: Any Phase Linear Experts out there? - Page 12 - AudioKarma.org Home Audio Stereo Discussion Forums.
While following the service manual test step 6-1.3 I came across a problem of shorted/v.low resistance reading - 2-3ohms - between upper pole common collector buss and any of the emitter contacts on TO-3 sockets, unlike it is for the lower pole (the reading is ca. 700+ ohms) and the whole left channel (upper+lower poles) similar measurements.
All output xtrs and drivers are taken out. All xtrs and diodes on PL36 checked 'in circuit' - no shorted one found.
One of suspects is D107, but not sure where it is located as my manual copy is very poor where it comes to parts location.
Any help/advice please?
Many tx!

I just finished a complete rebuild of a full complimentary 400II last month. It was bought off of fleabay complete with counterfeit output transistors and lots of other crappy work. I replaced all electrolytics, semiconductors, bad resistors, and circuit board hook up wiring. This particular amp had NO flyback diodes, the ones you are looking for, and NO .33uf decoupling capacitors and there was no sign of them ever being there. Both are supposed to be soldered to the small three lug terminal strips mounted between the rows of transistors. If you cannot find any diodes in the point to point wiring around the output stage they were probably never installed, the amp will work fine without them in normal situations. So if you cannot find them you have other problems regarding the low resistance readings you are experiencing.

Craig

Check if the mica isolators of the outputs are well seated.
An easy way of fault-finding is to loosen the screws of the TO-3s a bit, one by one, keeping one eye on the ohm meter.

OP indicates that the output transistors are not currently installed. Schematic of output section would be helpful.

Thank you Craig, that's is probably the case. Serial number is 7A12070, there was a slightly different PL36 installed (see pictures). For sure no sign of flyback diodes in the place you said, but the decoupling caps are there. Which all means even more complicated reason to be found... I will measure PL36 resistors and see if any of them is drifted. One more difference to what I saw so far in P/L gears is the bias transistor on the right channel, which is now BD139, mounted with a screw on a silicon isolator where usually was 2N3404. I checked it however if shorted - seems to be OK ... What else you think can cause bad readings?
Przem

Yes Netlist, Kevinkr is right. They are all out for the moment as anyway they will be replaced with MJ2119* (NPN/PNP) and for initializing the gear after repairs I wanted to have only drivers in. Besides, the mess with thermal compound called for careful cleaning of radiators.
Here are the schematics attached. Thank you for advice!

One more drawing with PL36 quasi-comp wiring for comparison. Connections numbering is in line with P/L 700 II service manual schematics.

... forgot to attach the drawing to the last post ...

Are the readings low in both directions regarding how you connect your meter?

Craig

Yes Craig, they are low either way. I was just checking more things and I also have shorted circuit readings between collector and base bus bars... Q201 to be inspected once more - I guess... Maybe I should desolder the whole PL36 to eliminate sockets isolation issue?
Tx for further supporting.