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In Reply to: RE: Pas 2 balance bypass posted by Cleet Torres on March 22, 2014 at 12:14:34
Thanks for your responses. Much appreciated
Joe/Pater
I'm planning to do some tweaking for a friend who managed to get a Pas-2 and Dynaco 70 from (I believe) an estate sale.
I have built and tweaked a number of electronic components before (and currently busy) but never ever Dynaco stuff. Is there an "idiots guide" as to what mods work well and which don't.
As I mentioned, it's for a friend so I want to keep cost reasonable and I cannot spend so much time, so some basic guide as to what works and what doesn't would be very helpful.
Regards
Cleet
Follow Ups:
Almost everything you need is here, at the top of the Dynaco Doctor page. I would recommend starting with reading the ST-70 Restoration Manual to verify proper operation of the amp. It may or may not need a new quad filter cap. Next, read the Bias Expansion Mod, which is usually necessary if you are going to replace the selenium rectifier in the bias supply. Beyond that, a stock ST-70 in proper order sounds quite good.
For the PAS, I would do the following (again, all linked at the top of the page): Restoring The Filament Supply (wouldn't hurt to increase capacitance to double or more the original value), Correcting RIAA Response, Removing The PAS Tone Controls. Will all due respect to the author, I would recommend one change in the tone control removal: instead of jumper wires from eyelets 6 to 7 and 12 to 13, I would instead install 1.0 uF/100V film caps. Otherwise, there will be a small amount of DC at the output after modification, and the ST-70 does not have a blocking cap on the input.
As for resistors and coupling caps, there is no need to change resistors in either unit unless they have drifted badly and your voltage measurements are way off. The PAS already has 1% metal film resistors where they're needed.
I'm not a big fan of high-dollar coupling caps, especially when they're inside a feedback loop like they are here. CDE, Matsushita/Panasonic, Illinois, and Sprague orange drops all do fine for not a lot of money. All readily available from Mouser, DigiKey, Antique Electronic Supply, and others.
Almost forgot, assembly manuals are also available here, and have schematics with the voltage charts you will need.
Hope this helps, and that all goes well with your foray into uncharted Dynaco waters!
Mike
Thanks Mike. Much appreciated.
Regards
Cleet.
If you don’t mind my saying, in my opinion not being familiar with the Dynacos and not having much time to work on them is not an optimal combination. If you intend to work on them yourself, I would say that a good starting point would be to obtain copies of the original Dynaco manuals, including the various pictorial diagrams, and then study them thoroughly to get your bearings - but that's just me.Are both units currently up and running? If not, you could then try bringing them up slowly on a variac over several hours. In either case, you can check the internal voltages against the reference voltages in the manuals. This will help you to gauge their general health and then decide what if anything you want to do.
Of course you could always just send them in to Joe to get checked-out.
Edits: 03/27/14 03/29/14
Hi Peter
I'm not particularly interested in getting into much detail on Dynacos.
I'm am planning to swap out a couple of caps and resistors in my friend's amps to bump up the sound quality, no more.
I'm wanting to find out which give the best bang for buck, what manufacturer (Reliable, Mundorf, etc.) and if a value change helps f.e additional capacitance in the power supply as Joe alluded to out in a post on this thread.
Regards
Cleet.
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