Phase Linear 700 series II restoration

[P.L.F.]

Originally Posted by llwhtt

Recheck all of the wiring you replaced, make sure all of NPNs and PNPs are in the correct locations, diodes in correctly. On a 400 (not sure on the 700) the DC fuses supply only the output stages so if those are removed and the Variac can be turned up without any problems you'll know the problem is in the output stage. It'll be something stupid, it always is. Oh did I do that? If I remember correctly the 700 has separate DC fuses for each channel.

Craig

Hi Craig,
Many thanks for dropping first thoughts. Some remarks to your suggestions:

• I'll do the recheck again, but for 100% NPN/PNP outputs are installed properly.
• There were no flyback diodes at all the model restored - see the previous posts.
• Very good point on isolating the output stage with fuses taken out - didn't think about it :-)
• Yes, there are four separate DC fuses, two per channel - have a look pls at post #6 here with schematics attached.

I will confirm the situation bringing up the amp with no DC fuses in place.

Regards, Przem

[P.L.F.]

Just checked - with no DC fuses, no current drain.

One more info to share is that before initializing the tests all outputs were checked in line with step 6-1.3 of service manual and no probs found. Also isolation of TO-3 sockets from chassis was OK. Maybe something wrong is with Vbe multiplier. I see from previous repairs that right channel uses BD139 while the left one 2N3404. Still measuring them with ohmmeter no warning readings observed.

I will now try to put in DC fuses channel by channel to see the outcome...

Any other directions to pursue?

[P.L.F.]

Reason found and eliminated: broken leads at both bias transistors - again difficult to spot. Things back to normal. Thanks to the light bulb current limiter I saved newly installed outputs - a good lesson to remember!
Tomorrow final stage of bringing up the amp to normal operation, if no more surprises :-)...

[llwhtt]

What a coincidence that was the last problem I had to find and repair. After a first initial run and adjustment everything looked good, screwed everything back together and it was drawing current. Leads broken on the bias transistors, that's when I decided to replace all of the PCB interconnect wires with something more flexible. I even went thru the trouble of drilling holes thru the solder pads and inserted the wires from the other side of the board. Tracked down some those once common transistors and everything was okie dokie again. Good to see you tracked it down, hope all goes well for the second initial firing.

Craig

[P.L.F.]

OK, need some help and guidance here. Didn't go as expected... :-(

I have way too much DC measured on outputs (ca. 40VDC). Cannot find the reason. Testing configuration:
- variac 0-240 VAC,
- light bulb limiter (60W/230V) in series between the vaiac and the amp,
- no inputs,
- no load,
- the amp doesn't draw current,
- outputs are good (re-checked)
- drivers, too.

What I might do wrong? Where to start diagnosis?

Tx a lot for all valuable advice!

/Przem

[Netlist]

Perhaps the variac is not turned up high enough to get the circuit stable?
If the amp doesn't draw any current, omit the bulb but do replace the fuse(s) with smaller ones, at least half the nominal value and see what you get.

/Hugo

[P.L.F.]

Originally Posted by Netlist

Perhaps the variac is not turned up high enough to get the circuit stable?

Hi Netlist,
Thank you for the advice. The VDC measured on big PS caps is slightly over 80VDC when variac is turned up to the max level (230VAC). I thought a good level for testing. However while measuring across each zener diodes (D1, D2) the reading is 0.7V B+ pole and 2.4V on B-. Maybe the reason for unbalanced voltage is that I hooked up speakers relay circuits to the B+ pole (takes ca. 80mA of current)? I will disconnect these two units and if that was my problem - another supply source needs to be designed.
Your view?

[llwhtt]

Did you stay with the LF351? That's the only thing connected to the LVPS (zeners), do you have good voltages into the dropping resistors? IC in backwards? Both channels acting the same? Until you have proper voltages to the IC you'll have DC offset. If you need more current from the LVPS for the relay circuit you'll need to adjust the dropping resistors, less Ohms.

Craig

[P.L.F.]

Originally Posted by llwhtt

Until you have proper voltages to the IC you'll have DC offset.
Craig

Yes Craig, I agree. Let me come back with measurements to share. Tx!

[P.L.F.]

Here is the next 'report':

Still light bulb limiter in the AC supply circuit - I am looking for 100W traditional bulb, but in EU that value is now not allowed (green policy...). Need to search a bit.

step 1:
- replaced D1 and D2
- replaced Q103 and Q203
-> result: proper offset level on right channel, the left one still too much DC (27VDC at 80VDC on PS caps), across each Zener diode similar voltage :-)

step 2:
- temporarily replaced (only for re-check purpose) Z101 and Z201
-> result: no change, left channel gets DC

What else to check/replace?