Dynaco - ST 400 PC-29 mods/upgrades

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jsmd

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Joined 2015
#1
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?
 
#2
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
 
#3
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
 
#5
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.
 
#6
llwhtt said:
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.
Click to expand...

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
 
#7
FrankW said:
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.
Click to expand...


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
 
#8
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.
 
#10
FrankW - Any chance I could get a copy of that article? I've tried every possible official channel with no luck and my local library has no ETA on an interlibrary loan. I'm about 95% certain it won't have anything new for me ( especially after UpdateMyDynaco released their PC30 ) but I'd still like to look. I'd also be interested if you still have a template for the transistor drilling. I had one from SCC's upgrade kit from decades ago but it is hiding from me now.

Thanks!
-Tom
 
#11
Of course I can give you a copy! And, yes, I have a template available. Send me your email in a PM and we will take care of it. I'm heading out and will return Tuesday now, so all we can until then is exchange information. I should be able to mail out the template Tuesday.
 
#12
Just got a Dynaco 400 with meters up and running from a very terrible shape I’d say catastrophic blowout. I believe the previous tech wire the positive and negative 73v wrong and blew all the output transistors.
I guess he sold the power capacitors as the ones he gave me were all the wrong voltage ratings.

The amp was in pieces and left out till it’s forgotten when he finally sold it in pieces to me. I’m down 500 bucks on parts alone as I recapped everything and changed a bunch of transistors and all output transistors, and both those gigantic 10,000ug / 100v caps.

But it’s up and running so I’m happy.
just got questions though.

Are the Q4 and Q5 supposed to run this hot? As in couldn’t place a finger without 3rd degree burn hot.

Also, are the resistors in PC-30 running these hot as well?

From my limited experience on the PC-30 it gets the 70+ voltage and drops it down to 13V

I don’t think this is ideal and prone to burn damage in the long run.

I know there is a mod that changes this board.
I’m think that if the only job of this board is to step down voltage from 70 to 13 volts, can’t I just add a small transformer in there that does the same job? 120v input the. 13-0-13 secondary voltage?
 
#13
Let's calculate the power dissipation in Q4 and Q5 on the PC-28 driver board. Current source Q3 puts a current of (14-0.6)/4700=2.85 mA into Q1 and Q2. Half of that current appears across R6, and half across R7. That makes the voltage across R6=voltage across R7=2200*1.4mA=3.08 Volts. That puts
(3.08-0.6)/100=24.8 mA total, split amongst Q4 and Q5, each then having 12.4 mA. At idle, both transistors will have about 74 volts from collector to emitter, so each transistor will dissipation 12.4 mA*74 volts =0.92 Watts....call it 1 watt.

Q5 and Q4 are 2N3440. Junction to ambient, thermal resistance is 175C/watt. That says that these transistors must have heatsinks. Thermal resistance from junction to ambient is 35 C, so even with a big old heatsink, these transistors would run at around 25+35=60 C, equal to 140 F as a junction temp.

Assuming my calculations didn't slip a digit...I'd make sure that Q204 and Q205 each had one of the classic star heatsinks on them. Of course, I have two Stereo 400's in the closet, but I don't feel like doing "weight lifting" to check this out!
 
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#15
djoffe said:
Let's calculate the power dissipation in Q4 and Q5 on the PC-28 driver board. Current source Q3 puts a current of (14-0.6)/4700=2.85 mA into Q1 and Q2. Half of that current appears across R6, and half across R7. That makes the voltage across R6=voltage across R7=2200*1.4mA=3.08 Volts. That puts
(3.08-0.6)/100=24.8 mA total, split amongst Q4 and Q5, each then having 12.4 mA. At idle, both transistors will have about 74 volts from collector to emitter, so each transistor will dissipation 12.4 mA*74 volts =0.92 Watts....call it 1 watt.

Q5 and Q4 are 2N3440. Junction to ambient, thermal resistance is 175C/watt. That says that these transistors must have heatsinks. Thermal resistance from junction to ambient is 35 C, so even with a big old heatsink, these transistors would run at around 25+35=60 C, equal to 140 F as a junction temp.

Assuming my calculations didn't slip a digit...I'd make sure that Q204 and Q205 each had one of the classic star heatsinks on them. Of course, I have two Stereo 400's in the closet, but I don't feel like doing "weight lifting" to check this out!
Click to expand...
Thanks for the response, I forgot to mention they each have heatsinks but yes I feel like the star heatsink could be larger.

PC-29 board I could smell the board roasting over under that poor old 10watt resistor.
 
#16
Just found out one of the meter is stuck, opened it and coil is burnt, ebay says nothing is for sale as replacement. I was thinking of replacing just a coil using an old analog meters coil. Anybody ever tried such a method?
 
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