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Dynaco amp mods and tweeks


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I have a few mods or tweeks information on Dynaco's ST-150, ST-400, ST-410, ST-416 amps also series 80 amps and ST-120.

If there is sufficient interest I will follow up here in the near future.

If I remember correctly there was a phase 2 Dynaco ST-400, I have no information on why there was this added model designation. It may have been an internal doc.

Please post here if you are interested in following up on these issues.

Remember there is a serious risk of shock if you are not experienced.

There was a TIP mod well known for the ST-120 but there was also a similar TIP mod for the 80 series. 

The mod for the ST150 - ST-416 amps is on Greg Dunn's web site. It was posted there around 2002. You can print out the article.

A set of 8 pages has been deleted there that would have created a 600 watt mono amplifier. No explanation posted. Perhaps liability issue.



I do not have an email address yet.




Edited by dynaco_dan
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Hi Joel

The site referral is a site without an active forum.

The repair was written by Kevin Boales for PC-28 circuit boards.

I can print out the St-400, etc, mod for my use but I do not know how to just save it.

This mod is for usually one dead channel and it covers replacing a common cause of the one dead channel.

I do not feel it is ok for me to print out that file and re-scan it and post it here.

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  • 1 month later...

Hi Joel

Yes the original composition resistors were slightly under-rated and that chap, Kevin, takes us through the steps to get to and replace them.

After completing those replacements we get a helping hand to reset the bias.

Only listed tools, parts and a VOM are needed.

Which caps did you replace and were the new ones 85' or 105'?

Did you strip the amp right down to it's basic components?

Great job for someone here to do, you must be highly skilled.

I picked up a Dyna 400 with meters here because one channel was dead.

It is really ugly as it was tinkered with and likely stored in a damp carport.

Less than $50.00 though, another of my rescues.

I went to the directory here and see over 80 viewers have seen some or all of this topic.

I wish some of those viewers would leave a comment or?

Nice to see this amount of interest in this topic.

Edited by dynaco_dan
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  • 3 weeks later...
On ‎06‎/‎04‎/‎2018 at 8:23 PM, dynaco_dan said:

Hi Joel

Yes the original composition resistors were slightly under-rated and that chap, Kevin, takes us through the steps to get to and replace them.

After completing those replacements we get a helping hand to reset the bias.

To add to this heading I was reading about a Pat-4 preamp that was noisy in only one channel.

The writer replaced only one composition resistor that must be 40 years old.

Noise disappeared.

There is a need for replacing those old composition resistors with metal film or ?


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1 minute ago, dynaco_dan said:

To add to the Dynaco ST-400 amp story.

I have a vague memory of the resistors across the top of the relay plate that was also a choke-coil assembly.

My memory cannot be relied on but I believe the coil wire was originally a red colour and a mod was instructed by Dynaco at that time.

I quote from Audio Magazine May 1975 page 49, a review of the Dyna 400.

This is a direct quote from Dynaco.

"Dynaco informs us that a very recent change in the value of the resistor R305  to 1.8 ohms reducing the Q of the output inductor-resistor combination effects an improvement in high-end sound quality.

This is easily made on existing St-400's by adding a 2.2 ohm resistor (2 watt minimum) in parallel with each output inductor (choke-coil assembly) on top of the relay mounting plate.

This will also noticeably improve the square wave trace into a capacitive load, especially at low drive levels."

This review was 3 years after it's introduction.

If my memory  serves me correctly the colour of later kit coil wires was changed to black.

If you should have black no mods are needed.

The St-410 and 416 would also have black as they were designed later on.



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On ‎28‎/‎04‎/‎2018 at 12:21 AM, dynaco_dan said:


Hi again

Here I am with some more trivia about the Dynaco 400, 410 and 416 amps.

Today I found an article which I will write about another day and perhaps scan and post here.

It is a more detailed story about the coil/resistor which I discussed earlier.

The colours were correct but they used a black plastic sleeve over the coil.

I also came across a copy of the MBI-400 super-charge mod which can increase the mono output to 600 watts

This is one of several links that has disappeared from, home.indy.net/~gregdunn/dynaco/components.

That site does not have a forum, sadly, because Gregg has done a beautiful job setting it up.

Also I came across price lists both retail and wholesale which I need to scan first.

To change the subject here, I see lot's of viewers but only a few get involved with this site.

If everyone visiting could post an answer or a question it would be great, even if no one here can answer it.

Especially me.

Also if everyone could post a photo of an article or ad they have of old Dynaco.


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  • 1 month later...

Hi there

Just recently I came across more old paperwork.

The TIP mod for the ST-120 amp is an 8 pages instruction and parts list.

Gregg Dunn site has that TIP mod with a short list of parts with no instructions, which is all that he may have been given.

My comment is not negative for Gregg's site which is very beautifully done, but without a forum.

Also another 8 page mod to increase the ST-400 to a ST-416 watt amplifier output, referred to as "The Double Dyna", substantially doubling a ST-400's wattage output.

Stereo Cost Cutters (SCC) Dec 1983 catalog of most of 13 pages of 24 pages 8 1/2 x 11 paper size remaining Dynaco inventory they had purchased on Dyna's closing, 40,000 pounds worth. The parts list of only 5 pages was done in such a small font, it may not even be scanable.

US and Canadian Dynaco wholesale and retail price lists.

The MBI-400 mod changes around parts, wires, etc to turn a 400, 410 and 416 into a 600 watt mono amps and this is to be considered non-reversible and very prone to hurt the user (hernia). This project is extensive and the possibility of expensive damage to the amp, if an error is made or serious personal injury, also 8 pages long, not for the rookie.

The danger is so great that maybe that is why it was pulled from Greg Dunn's site.

I will scan and post some as soon as I can.

Errata: There was 60,000 pounds of parts and inventory.

There is also 2 follow-up SCC hot sheets which mentions that they had a flyer, 20 pages, of just Dynaco. I'm only mentioning that fact, I don't have it, date unknown.


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  • 11 months later...

Hi folks,

New to this site but Ive attached a few articles you may find helpful.

Did a pretty major upgrade to my SCC 1986  ST416 over the winter and included the PC-28 resistor mods discussed within.

My 28 boards were the updated - doublesided version and I also replaced all caps - obtained from quo-co (Ed Fantasia) on eBay.

Ed used to work for Dyanco / Hafler and is a wealth of knowledge - never a problem with his parts and he responds quickly to inquiries.

My latest item I'm correcting is I had my right channel go out due to a shorted output transistor (Q5)

I ordered a new set of transistors from Mouser - ON brand - 863-MJ21194G (8) and 863-MJ21193G (8) as old ones aren't available new and a couple other transistors exhibited some flakey readings (they are over 30 years old).  new transistors have better ratings and heat dissipation characteristics.

PC-28 boards do not appear damaged from checks performed. Revealed by right channel being out and one of the 4 relay board fuses being blown.


Some pics from SCC catalog forthcoming.  

To date I've modified my unit (410 to 416 conversion dynakit as follows) Circa 1986:


  • Replaced speaker relay with enclosed relay (on line kit) - Akitika.com  RY301 (replace R142 with 1500ohm, 1W resistor to lower relay coil voltage)
  • Replaced output (large) caps under relay board Akitika  22KUF80V
  • Changed PC28 Caps and 6 resistors (ea) on PC-28 boards (R-28,29,30,31,32,33 to wire wound) (Mouser), new 1K trim pots (2 per board - PIHER 1K812M)  (RP011 190103) "qua-co" on eBay (reliable) "Fantasia Audio"
  • Installed led meters and select switch  NOS
  • Installed 2 speed fan thermo switch (KSD301)
  • Rebuilt PC-30 board (all components) - lots of heat degradation (qua-co) eBay
  • Installed short blue LEDs for power and hi-temp lights - 2152A6  NOS
  • Lastly replacing all 16 output transistors as one failed (shorted) with MJ21193G, MJ21194G ON Semiconductor (8 ea)
  • RCA input jacks with gold plated jacks

Hope this helps others! 


Dynaco Repairs For PC-28 Amplifier Boards copy.pdf


Dynaco Stereo 400 Power Amplifier - Dead Channel Fix - Dynaco Repairs For PC-28 Amplifier Boards ~ Dons Deals.pdf

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  • 2 years later...

I have a Dynaco ST-416 that I originally purchased as a kit in the late '70s. I had it professionally assembled and used it for many years as the low end of a bi-amped system. It blew the protection fuses and I just put it on a shelf and forgot about it. Anyway I am going to sell it and thought that I would start on this site. I am asking $400, a restoration kit is about $200 so you could have a great amp for around $600. I also have a Hafler 220 power amp and a Dynaco pre-amp that I am willing to part with. 

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Suggest you list the items for sale in the For Sale section. It's free. Be sure to include some pictures, resized to about 100KB or so. Also give your location. That 416 is over 50 pounds so local pickup is desirable. Good luck


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