Dynaco - ST 400 PC-29 mods/upgrades

[jsmd]

I am restoring a ST-400 with meters that I recently found locally.

The PC-29 has been modified:

• Traces to the filter switches have been cleanly cut/severed
• The pins in the filter switches had been removed (I bought a NOS double switch assembly from Vintage for replacing them)
• Two jumpers have been installed in the back
• Several wires have been soldered to a kind of post on 22
• 7 and 8 are bridged by solder.
• R19 missing/removed
• Two D5 have been disconnected on one side
• Two D6 have been disconnected on one side

Could it be a modification for by-passing the Dynaguard with or without keeping the audio?

Are they any papers/articles on modifications/upgrades on the PC-29?

I have seen a post from audiobard (May 10, 2010) Originally Posted by audiobard: ... There is an article in TAA (The Audio Amateur) 3/83 by Bill Rollins about an update of the stereo 400. I have a copy on paper, but not yet in electronic form.I'll try to scan it as soon as I can and mail it to you if you are interested...

Can someone send it to me?

I am also replacing few parts for PC-28 amplifier circuit boards as per Kevin Baoles' article (Dynaco Repairs For PC-28 Amplifier Boards)

Any articles on upgrading the PC-28 and PC-30?

 

[llwhtt]

I have a 400 (20 years) and a 416 (3 years) and have been unable to find anybody doing anything in the way of mods. Both of mine have been stripped to the bare bones and ready for reassembly after some painting or powder coating. Bypassing the Dynaguard seems to be a good idea if the amp is used strictly for stereo. I'm going to dig up the AA article when I get home, I'm sure I've read it. There's a lot of the original Ampzilla in these amps.

Craig

 

[djoffe]

Jacques,

I misplaced your email, but saw your post, and I finally have a few minutes:

Looking at the schematic, it's likely that they have bypassed the hi-filter/lo filter switches...or perhaps wired them in permanently! It's hard to say without a close inspection of your amp.

The post on 22 is the main signal ground, so it's kind of sensible that there might be lots of wires tagged onto it.

7 and 8 solder bridged is quite sensible...the schematic shows that.

D5 and D6 and the diodes that provide the Dynaguard limiting action, so...yes, with those pulled, the Dynaguard is disabled.

Here's a link to one of the schematic variations that's out there:

http://www.updatemydynaco.com/documents/schematic_paper_manual.pdf

Here's the other variation:

http://home.comcast.net/~g.e.dunn/ST400/schematic.jpg

Good luck wrestling with the beast.

All the best...

Dan

 

[jsmd]

Dan,

Thanks.

I also put your relay kit.

The meter lights are not working and the meter needles are not moving.

Are the meters not functional after bypassing the Dynaguard system?

All the best,

Jacques

 

[FrankW]

I have Audio Amateur Three-1983 with the Bill Rollins article. Yes, he deletes DynaGuard in it's entirety, and he does an ambitious upgrade to PC-30, as well as gives a good overview on converting to a 416. Let me get my scanner working and I can forward copies to people who PM me. BTW, I made a batch of drilling templates for adding the other 8 outputs, again, PM me.

 

[rayma]

Bypassing the Dynaguard seems to be a good idea if the amp is used strictly for stereo.
I'm going to dig up the AA article when I get home, I'm sure I've read it. There's a lot of
the original Ampzilla in these amps.

Bongiorno worked on the ST416 for a time. The Dynaco ST410 version sounds much better though.
More supply capacitance is also good (like the C-100 module). A lot of info is here:
Stereo 400, 416, and 410

 

[Glasgow Grip]

I have Audio Amateur Three-1983 with the Bill Rollins article. Yes, he deletes DynaGuard in it's entirety, and he does an ambitious upgrade to PC-30, as well as gives a good overview on converting to a 416. Let me get my scanner working and I can forward copies to people who PM me.

PM sent, I would love to read that article. I am rebuilding an ST-400 and I'm down to the PC-29 board, which I am recapping at the moment. But the more I look this board the more I am leaning toward disabling Dynaguard. Seems like an awful lot of complexity for questionable value. Thanks in advance! - Steve

 

[Carl Jacobs]

I know this thread has been inactive for 2 years, but I just foolishly/gloriously obtained an ST400 that I'm going to rebuild from the component level up and convert to a 416. I've built a dozen Dynaco kits over the years so I'm excited to go into the breach once more. FrankW, are you still willing to accept a PM request for the AA Bill Rollins article? The heat sink drilling template you mentioned would be handy too. I was going to create one myself using the 416 pictorial layout as a reference but it would be good to have another persons template as a double check on my work. While I've been a longtime member this is my first post so I may not be able respond quickly because of moderation rules.

 

[FrankW]

Carl - PM sent.Yes to both requests, of course.