Let's Restore A Dynaco ST-400!

Jim Marantz

Active Member
This thread was originally posted in "General Audio Discussion" by mistake. Moderators please remove that thread.

OK. I picked up another ST-400 from a local dealer. I paid probably too much but I liked the fact that I could see it in person before paying for it. The amp does work 100% but I could tell it needed a lot of work before I even opened it up. The DC offset was checked before I put speakers to it and it measured under .2mV on both channels, so I assumed it would be safe to listen to.
The sound is very distorted especially on the top end frequencies. The Low end kicks like a mule and the mids are a little dry. Plus, it has a nice hum, seems to be 120Hz so I'm thinking power supply problems.
Here's her pretty (subjective) face. Not perfect, but a good polishing will help a lot.

As a note, I'm approaching this "restoration" as if I'm dealing with an antique, more than a piece of "vintage" electronics. This amp has the possibility to produce 600W @ 8ohm in mono-block configuration. The transformer is 1,000 VA and is not to be toyed with for your own safety and it's complimentary equipment.
My other restored ST-400 puts out over 285 WPC @ 8ohm and 340 WPC @ 4ohm before any clipping can be detected, so it's a worthwhile endeavor.
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Opened up, it's very dirty inside. I don't think any service was ever done to this amp.
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The chokes on the fuse plate are black, meaning this amp was produced after 1975. The resistors in the choke assembly were originally 10 ohms in the first versions, but changed to 1.8ohms in response to some reviewers of the day not liking the 400's top end frequencies. To delineate the change, Dynaco covered the coils in black tubing.
Owners could bring their first generation models up to spec. by piggybacking a 2.2 ohm 2W 5% resistor to each coil.
Also, this unit has first generation output boards PC-28 which is a single sided board. Later boards were double sided to save a lot of jumpering. I would approximate a manufacture date of 1976.

Taking a measurement at the positive fuse holders I get a reading of 93V. I should be getting about 74V. My power supply suspicions are correct so far.


PC-30 BOARD BACK.jpg

Here, the fascia has been removed and the cabinet face folded forward. I've removed the fuse plate with speaker relay and the main filter caps. We can see a little discoloration on the power supply board PC-30. (This is common on the ST-400)

PC-30 BAD COMPONENTS.jpg
Here's the other side of PC-30. One zenner is blown, the 850ohm resistor is cooked, the capacitor is dried out and the bridge rectifier gets flakey at higher currents. All component were checked "out of circuit". I will be changing every component on PC-30 and installing a new bridge rectifier of 35A 800V.

cap test.jpg
I tested the old filter caps (silver) and you can see the results above. The new caps should be a nice upgrade in performance.

As of today, I have some large orders coming in from Newark and Mouser with tons of components to rebuild the power section and also all new components for the (2) PC-28 output boards. I'm wanting to square away the power regulation before I attack the audio section. I'm still under $200.00 in on this restoration, not counting the purchase price. The Dynaguard / "Preamp" board PC-29 still has to be looked at, but I'm trying to not overwhelm myself at this point.
Jim
 
Teardown still continues, but I have managed to get some of the amp back together.


Dude, where's my amp?!!?
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I took the opportunity to strip the old paint off of the chassis and "Z" bracket.
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The power supply board PC-30 is completely rebuilt.
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I rewired the power switch, circuit breaker, tranformer and thermal sensor system. Painted the chassis components with textured paint. Cleaned up and tested the transformer, rewired / rebuilt the fuse plate and speaker relay with all new components, and installed the main power caps.
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I'm currently replacing all components on the output boards and scrubbing everything clean to start on the output/heatsink section of the amp.
I hope you like it. It's been a lot of work thus far.
 
Judging by the number of responses I've gotten on this thread, it doesn't seem to be in the interest of most people here. I will continue none the less in the hope that it may help somebody in the future.....
Lots of scrubbing going on as this amp must have been in a bar or something for a long time. Lots of nicotine coating everything. Luckily, it's all coming off easily with some Spray Nine. Tear down is complete and I'm waiting on new matched output transistors, RCA jacks and one capacitor that I missed on each PC-28 board.
Speaking of which, here they are temporarily mounted on the disassembled heat-sink. Every component has been replaced and/or upgraded.
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Here's what it (they) looked like before hand.
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It was more work than I anticipated as some of the traces fell apart during the de-soldering. Luckily I was able to make the needed repairs.

While I'm waiting for more parts, I figured I tackle the top cover. One of my favorite things about Dynaco amps is the large perforated sheet metal covers. The dealer that sold this amp to me quickly painted it a gloss black, but I could tell there was trouble lurking beneath by some rough looking patches. Plus it had a slight warp that I wanted to straighten.
Here it is stripped of paint (using chemical stripper). There's some pretty coarse rusty spots, nothing to be alarmed about.
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Here it is after repairing the warp and buffing out the rust. She's ready for her acid pickle to take care of anything I missed and give the surface a nice etch for painting.
Hopefully next week I'll have more and better pics.20161009_162743.jpg
 
I'm always interested in Dynaco stuff and anything by James Bongiorno. I have a 400 and 416 awaiting the same thing you are doing, they're on the back burner though. I've been following just haven't had anything to add, keep posting!

Craig
 
I read these types of posts like a menu at a steakhouse! I drool all over them! I appreciate the time you take to post them and the skill you have to do them. Maybe by the time I am 80 I can do something like this!
 
I read most of the rebuilding threads simply because I pick up way and methods I otherwise wouldn't consider. I'm currently running some smaller Dynaco stuff, like a SCA80Q and a couple PAT-4 preamps but only one Stereo 120 so far, all rebuilt and upgraded.

Good solid stuff from back then!!
 
Just found this thread and it's very interesting. You are doing a great job of bringing that beast back to life. Keep the updates coming! :thumbsup:
 
Cool!
Really nice work.
I have a ST-150 sitting on the shelf ready to be brought to the amplifier hospital.
It was received in marginally functional condition and will require a complete rebuild also.
The ST-150 was nominally similar to to 400 but scaled back a tad. I'll post my progress too.
Hope mine will come out as nice as yours.

Lou
 
Thanks for all the encouragement everybody. This has been a lot more work than originally anticipated. The deeper I get into it, the more I see that needs fixing. I'm a little upset about the price I paid for the unit at this point, and the project is costing quite a bit more than I was hoping to spend. I do feel good though that when all is said and done, I'll basically have a new amplifier.
I just got the parts from Newark last night to replace all the components in the heatsink/output assembly. My transistor set should be here by the end of the week. Maybe Sunday night I can try starting her up.
Here's a small update:
One thing I absolutely HATE about the 400 is the input RCA jacks. They are recessed in the heatsink so it's impossible to get a hefty cable in. And if you do reach the jack, it's a very cheap piece of junk.
I drilled out the jack holes in the heatsink on the drill press with a 17/32" drill (.531" dia.) to clear the cable ends of the Monster Cables I like to use.
Then I built a new jack assembly with some 1/8" thick G-10 material (otherwise known as FR4) and installed some gold plated connectors. Only cost me $5 to build and is quite an improvement. The G-10 can be had from McMaster-Carr. A 1" wide strip by 24" long piece was like $2.65. If anybody is interested, I can post the blueprint with dimensions for the plate. Very easy to make.

The new assembly is on the left.
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I'll have more to show soon.
 
The holes for the RCA jacks on my 416 have been hogged out very crudely, I'm hoping a Unibit will clean up the mess before running out of aluminum! I usually use the black Garolite from McMaster for my jack plates. Was your chassis painted or bare cadmium? My 400 is bare and the 416 is rusted paint. Thinking of powder coating both as there's a powder coat shop right behind my shop. Keep posting!

Craig
 
The holes for the RCA jacks on my 416 have been hogged out very crudely, I'm hoping a Unibit will clean up the mess before running out of aluminum! I usually use the black Garolite from McMaster for my jack plates. Was your chassis painted or bare cadmium? My 400 is bare and the 416 is rusted paint. Thinking of powder coating both as there's a powder coat shop right behind my shop. Keep posting!

Craig
My chassis was painted and flaking from some light rust that had developed under the paint. The guy I bought it from gave it a Sharpie paint job, which I spotted as soon as I saw it. Sharpie markers are actually a very deep purple, not really black. Anyway, I painted my chassis parts with Rustoleum Texture paint and baked it for an hour @ 175 degrees F. It's as hard as a rock. I love enamels, but the problem with them is the long curing times. If you have a dedicated oven it's very easy to overcome.
I was tempted to throw the heat sink on the milling machine and mill a nice slot for the RCA jacks. The drill was faster and I have a family that needs tending to, so I made a compromise. It did come out nice. I will post more pictures this weekend. I was up till midnight last night and I got half the heatsink assembly wired. I figure by Saturday afternoon I should be ready to start making cursory resistance checks to make sure she won't blow when I turn up the Vari. Wish me luck!

Teaser pic: Measuring the faceplate with the portable CMM into a Solidworks model. I plan on having a new face cut on the waterjet, and I'll silkscreen it myself.
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Updates have been delayed because of work and family obligations, but also because I'm sometimes not very bright. I've not been able to get the amp to bias up to the output transistors Q-point, and had a very hard time figuring out why. Thankfully I put down the schematic and picked up the calculator and found my error. The main biasing resistors are supposed to be .18 ohm 10w. I installed 1.8 ohm 10w. Those darn decimals! I jerry rigged down to .18 just for testing and everything sorted itself out. The proper resistors are not easy to find and I had to order them factory direct from NTE. Hopefully they'll be here by the upcoming week end.
Here's a couple pics to show the progress.
Heatsink assembly complete (with incorrect bias resistors) waiting to be installed. Matched output resistors are in as well as ultra-fast recovery diodes. All wires pulled and replaced, new resistors throughout etc. I even replaced all fasteners with stainless steel as the originals were well oxidized and pretty ugly. My fancy new RCA jacks are also in. What an improvement!
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Here's two weeks ago.... Thankfully I didn't need to use the dead-blow hammer :)
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And here's an overall view.
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While I'm waiting for the bias resistors I'll get to work putting some fancy led lamps in the meters. It's been a lot of work to get to this point, but so far I'm very pleased.
Jim
 
Ive always liked the looks of these amps. Change the knobs and it would look just like most of my early 70's Marantz a stuff. Very nice work so far. I noticed you ditched the ceramic caps while you were in there. Nice touch. :thumbsup:
 
Ive always liked the looks of these amps. Change the knobs and it would look just like most of my early 70's Marantz a stuff. Very nice work so far. I noticed you ditched the ceramic caps while you were in there. Nice touch. :thumbsup:
I also think they look very nice. The sound is in a class by itself to my ears. I'm also a big fan of Marantz, hence my screen name. The Marantz units have much better build quality. The buttons feel nicer when pressed, the knobs smoother when turned, etc. The Dynaco sound is my favorite, and the ST-400 is my favorite Dynaco by far. Very punchy and tight. Doesn't seem the speaker could distort even if it wanted to, even at ear bleeding volumes. I do warn that this unit is not a "beginner" project unless you have some good mechanical ability. The chassis assembly can be a bear to work with if you're not thinking 5 or 6 steps ahead.
 
@Jim Marantz Ive rebuilt a couple stacks of Marantz equipment and the build quality can get crazy. The model 16 mono blocks for one, lots of fun. My main system now though is a Dynaco ST-70 and a very modded PAS preamp. I had problems with a few traces on my 16. One sided and traces held down with little more than spit. Nice work on the 400 it should last a good long time.
 
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