Repairing an AR amplifier

[Frank Wissman]

Hi, Gang,

 

     I am trying to repair an AR amp that someone else started, the PS has been taken out already. I have the SM and update notices. I expected the 2 ~16 ga purple wires to be +/- 39V to the drivers & outputs. But when I visually trace the wires, and ohm them out, neither go to the collectors of Q19, Q20, Q23, & Q24. Nothing else looks likely. This should be trivial and I'm stumped - what are the 2 purple leads, please?

 

Thanks,

Frank

[Aadams]

1 hour ago, Frank Wissman said:

 I am trying to repair an AR amp that someone else started, the PS has been taken out already. I have the SM and update notices. I expected the 2 ~16 ga purple wires to be +/- 39V to the drivers & outputs. But when I visually trace the wires, and ohm them out, neither go to the collectors of Q19, Q20, Q23, & Q24. Nothing else looks likely.

First time I have seen a thread like this here at CSP.  This link will take you to the service manual and drawings in the library if that will help http://www.classicspeakerpages.net/library/acoustic_research/special_sections/electronics/ar_electronics_schematicsse/ar_electronics_service_manu/ar_electronics_service_manu_43.html#previous-photo

 

[johnfalc]

+39 goes to the collectors of the 2N3055 TOP outputs

-39 goes to the emitter resistors of the 2N3055 BOTTOM outputs.

Service manual with schematic is here: https://www.hifiengine.com/hfe_downloads/index.php?acoustic_research/ar_amp_tun_rec.pdf

Being an early SS amp, it (with many others of that early era) uses coupling transformers to drive the output stages.

[Frank Wissman]

Hi, John,

     Yes, I agree - I have the SM (service manual), and I expected the purple wires to be +/- 39V, but they do not directly connect to any of the 2N3055 collectors or the emitters through either the 27 or 33 ohm resistors. I mentioned only the +39V in my initial thread, trying to simplify things.

    In the SM, section 1.C.3, +39 Volt Lead Dress - this update refers to a red wire from C50. In this amp and in pictures of others, the purple wires appear to be factory installed and I no longer believe these to be  +/-39V. 

Hi, Anyone who has worked on these - the purple wires go where?

Thanks,

Frank

     

[Frank Wissman]

Oh boy -  I'm starting to think these are ground leads from the driver stage back to the tab in the chassis by the original caps. Anyone know for sure?

[johnfalc]

Frank, 

By your pictures it is pretty clear that the heavy purple leads in the center of your photo are both negative 39 volts.

One of those twisted purple leads connects to the low side of the 0.33 ohm emitter resistors R92 and R98 (one for the left channel and one for the right), the other goes to the cold side of the bias diodes for the lower output transistors - nominally the same electrically but this kept the collector current separate from the feed to the bias diodes.

You can see the bias diodes glued to those emitter resistors and can see that R81 and R82 are selected to be 27 and 33 ohms (your amp either came without the bias pots (which were a disaster) or was revised per the service directive.

The picture is not so clear but it looks like the yellowish leads at the other ends of the emitter resistors mentioned above (the ones closest to the center of your photo) convey connect R92 and R98 to their respective 2N3055 emitters.

The blue leads that are soldered to the driver sockets (i.e., the collectors of the 2N3054s) should have +39 volts.  Your photo doesn't include the output transistors but I'm expecting you will find blue leads connecting the driver collectors to the output collectors for the upper (positive) 2N3054+2N3055 pairs.

What do you measure, relative to the ground (i.e., the junction of C49 and C50) at the purple wires first mentioned here?  And what do you measure, relative to ground, at the blue wires connected to the driver and output (upper) collectors/cases?  The answer should, I believe, be the -39 and +39 volts respectively.

I repaired many of these when they were fairly new but that was about 50 years ago.  Better illuminated photos would help.

[Frank Wissman]

Hi, John,

   I agree, purple is -39V, thank you. And thank you for the detailed, patient walk-through.

   I don't have any voltages to measure, all caps were removed and I've made a board to replace them with fresh caps. I think what threw me is the different wire size, +39 is about 20AWG and -39 is almost 16AWG.

 

Cheers,

Frank

[johnfalc]

Good luck on getting this vintage piece of history going!  John