Polytone 104

[lowell]

Amp has negative DC on one of two power amps speaker out and other power amp has no sound. Need schem please!

[J M Fahey]

Don't have that one, but AFAIK there are 2 or 3 classic versions only. Is it the one with a DIP8 LM391 driving output transistors or the full discrete one?

[lowell]

I haven't pulled the power amps out yet. I can tell you that both power amps have 2 heatsink-mounted TO-3 transistors. Does that clear it up for you or should I pull them?

[vintagekiki]

Polytone power amp mod 378


Polytone power amp mod 378B - Solid state pwr
http://www.murchmusic.com/Polytone%20Info/schem2.JPG

[J M Fahey]

Hi lowell.

both power amps have 2 heatsink-mounted TO-3 transistors. Does that clear it up for you

Not fully, because all known versions do have 2 TO3 transistors per power amp; the difference lies in how you drive them
The one kindly posted by vintagekiki is the LM391 driven one. Only way to know is to pull the chassis out and look at the board.
Anyway you will have to.
There are other two, discrete versions. All three are classic designs, no big trouble there, thanks God.
As usual, a couple pictures will help.
Good luck.

[lowell]

Ok Cool... I'll check 'em out Mondee. thanks!

[lowell]

It's the LM391 version. Thanks guys I'll report back if something else arises.

[lowell]

Ok so one power amp has a bad 2n5882 power transistor in TO-3 package and the 100ohm R18 is also fried. I'm thinking it may be best to replace the drivers and both power transistors. What yall think? FYI also I cannot find any 2n5882, 2n5880, NSDU07, NSDU57. I need a substitute. Any suggestions?

[52 Bill]

Polytone switched to the MJ4502 and MJ802 for the outputs.

[vintagekiki]

IC (LM391N) replaced only with version for 80 or 100V
Output transistors (2n5882, 2n5880) replaced with MJ15003 / MJ15004 or any 150W PNP/ NPN transistor with VCE > 100V
Drive transistors (NSDU07, NSDU57) replaced with BD243C / BD244C
Check pinout transistors
When replacing the any transistor, adjust bias output transistors at 25 mA, using the trimmer potentiometer R15/ PA378 or R14/ PA378B (2500 Ω)

[lowell]

thanks guys!

[lowell]

Ok so how and where do I measure the 25ma bias current? The amp is up and running.

[J M Fahey]

Basically measure voltage across one of the emitterresistors. Example: 30mV across 0.33 ohms= 90mA and so on. Substitute values with those present in your amp.

[vintagekiki]

Voltage drop on R21 (0.15 Ohm) at the emitter current 25-30 mA is 3.75mV. It will be hard to read. Direct measurement of collector current is precisely.
Adjust output transistors collector current at 25-30 mA, using the trimmer potentiometer R15/ PA378 or R14/ PA378B (2500 Ω).

[lowell]

It's taking about 5 minutes for the current to reach 25ma. I had the amp on for quite a while and kept backing off the current because it kept rising over time. Finally after stabalizing at 25ma I shut it off and powered on again only to find it takes 5 minutes to go from 18ma to 25ma. I'm not seeing any AC voltage on the collectors, thought maybe that was the issue. Anyway, is this typical?