Dynaco ST400 came my way!

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A gentleman posted his non functioning Dynaco ST400 on our local town forum. When I called, he offered it free to anyone willing to try and repair it - I was there within minutes :)

This looks like a decent bit of kit, and could be fun to play with. I do have the manual and schematic, and will have a poke about as time permits. At 200W/ch it is quite a beast, but perhaps not quite a match for my Adcom GFA 585.

Anyone have thoughts/ opinions on these amplifiers? I've already found a few web resources dealing with common problems. The gentleman who gave it to me mentioned that it was blowing an internal fuse, and that he'd planned to repair it himself, but never got to it.

Cheers,

Ed
 
Thanks both!

First thing to do is spend a little quality time with the paperwork I have. The donor said he turned his house upside down looking for this, and it I really appreciate it - so much easier to have the tech info readily at hand.

As for spares, there is a very good place nearby that has all manner of semiconductors - old and new- should the on-line sources not prove fruitful. I think this could be a cool project - cosmetically, the unit is in nice shape.

Ed
 
OK, I finally had a chance to open the amp up fully and have a proper look. One channel's PSU fuses are blown, and a tickle around with the DMM revealed some damaged output transistors, which have taken all the drivers with them.

Found a good webpage on repairs and substitute devices, so I'll look into what can be found, and how much it is likely to cost.

Cheers

Ed
 
I ought to post a couple of pics, now the amp is opened up & ready for work. It's quite a beast, sporting a heavy duty chassis, vast heatsink and a big beefy power transformer. Repair parts from Digikey arrived yesterday, so I should be able to get started at the weekend, that is after I've finished tinkering in the garage with the MG...

I wonder sometimes about keeping all this classic stuff running... but find myself compelled to rescue & fix anything and everything that is old but good :)
 
More tinkering this weekend. The faulty driver board was removed, and the damaged TIP42 and TIP41 driver transistors removed. Their replacements were prepared, but I then found that despite my care to keep them together, one of the mica insulating pads had escaped. Not pleased! More are now on the way, but this slowed progress. Focusing on what could be done, I did replace the electrolytic caps, and some high power carbon composite resistors on both driver boards. The resistors have been cited as a reliability issue elsewhere on the net, so nice new metal oxide film parts replace them. If the repair to the bad channel is successful and satisfactory, I plan to renew the outputs and drivers on the "good" channel.

Back to the bench next weekend :)
 
Good luck. I love dynaco transformers. The cases aren't bad either. The last one of these I saw for sale locally, the PAT4 and ST400 were $300 in unknown condition.
The new stuff they sell around here is such ****. The Peavey stuff excepted, but the overlap with the hifi hobby is slim. Then again, $999 for a CS600s amp is nothing to jump into. Has great speaker (1/4 phone plug) shorting protection, though, better than the dynaco.
I've found in this PV-1.3k amp, lots of blown up 1n4148's and zener diodes, and lot of little 50v ceramic caps open. The dip and TO92 packages with the tops blown off were pretty obvious, as were the toasted resistors. The caps are insideous, I don't have a cap checker they just sit there looking original with no action. Hope you don't have anything like this. I lucked out on the ST120 dynaco amp, the O.T. meltdowns didn't even take out the 5.1 V bias zener everybody says needs replaced every time.
You got a MOS surge supressor on the input? Well, not much lightning in Silicon Valley I suppose, but not all good things were invented in 1972. I've got one between line and neutral and between neutral and case on all my amps now. Lost the turn off pop supressor cap and the power switch several times, the transformers just take it and keep producing the current.
 
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Thanks for your comments. I was interested to learn that these amps still have a good following.

I'm still wondering about how to tackle the first powering-up. It would be disappointing to see it go POP immediately. From the circuit diagram, it seem that it would be OK to use a variac (no slow start circuit to burn out like my Adcom beast) If I can get my hands on one. Another option might be to put reduced current fuses in the PSU lines for off load & low power tests.
 
A variac is preferred but if you don't have access to one you can put an incandescent light bulb in series with the 120V feed to the amp. A 40W bulb is a good starting point. In the event of problems in the amp you will see the bulb glowing brightly and it will limit current draw...if things are OK the bulb will be dim once the PS caps are charged.
 
Progress today! Also some pictures that I'll resize and post later.

New drivers and outputs were fitted to the damaged channel, and I decided to test at that point. PSU fuses for that channel were replaced (the other left dead) and the amp run up with a voltmeter on the output. Nothing scary on the Fluke, so I connected a speaker... All quiet, and the only signal I had available was my finger on the input, which produced a healthy bzzzzzz. The other channel tested similarly :). So far so good...


Then, of course, I decided to work on the other channel, again with new outputs and drivers - to keep things consistent. On completion I went to repeat the test above, but when the power was applied and the PSU circuit operated there was a loud POP and a blue flash near the board on which I'd just worked, with one PSU fuse blown. Imagining a total loss, I started to probe. Outputs OK, drivers a bit funny but not shorted, sure enough, I found one of the drivers to have a short circuit to the heatsink, thanks to a metal burr. Miraculously, this was a device whose collector was direct connected to the PSU, so the fuse was the only casualty! A lucky escape indeed! After careful reassembly, the unit powered up fine, and showed operation in both channels. Tomorrow, I'll complete reassembly of the chassis, and try it with music. Quite a day...

Cheers,

Ed
 
Thanks! I may have bought the luck... Humbled am I to say that I recently sought professional helpto get our Adcom monster amp fixed. It had lain unprobed for a couple of years, with no time to investigate it, so I had a local shop look it over and carry out repairs. It was good to get it back in our system and just too flipping good to lose by neglect. This Dynaco project was of a much simpler nature, even including the mishaps, and much easier to work on mechanically. Can't wait to see how it sounds :)

more tomorrow, hopefully I'll find time to sort through the pics

Ed
 
Thanks djk - I actually have the original paperwork explaining the modifiation.

UPDATE: today I was all set to try the amp (speakers, & my phone running Pandora for a quick trial). Alas, during the night, another of the drive transistors had shorted to the heatsink. This time I was not so lucky with the current path - at least one driver and one output has been destroyed.

I'm highly annoyed by this, because I double checked every mounting carefully yesterday after the previous incident, and all were good. A fresh order for new parts has just been completed, including new insulating bushings for the drivers.

Music will have to wait until next weekend :(
 
Not wanting to leave anything to chance, all the drivers were removed again from each board. Two problems are evident

1) The bushings are a tight fit in the new devices, and can crush instead of fitting neatly into the mounting tab

2) There were burrs on the heatsink, though I thought I'd eliminated this AND also on some of the transistors. Evidently just enough in some places for the two unwanted edges to meet...

Now these causes known, I can go about eliminating them properly. I wonder if they were he original source of trouble?
 
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