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Old 25th September 2012, 08:52 PM   #1
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Default Dynaco ST400 came my way!

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
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Old 25th September 2012, 10:03 PM   #2
djoffe is offline djoffe  United States
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It's a pretty nice design, really has a lot of modern, desirable characteristics. I've always wanted to have one, but never have been so fortunate as to snag a deal like yours!

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Old 25th September 2012, 10:43 PM   #3
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Built one back in about 1975...indeed a beast and a pretty decent sounding one at that. Free is hard to beat and with the manuals in hand it should be reasonably easy to get working again.
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Old 25th September 2012, 11:36 PM   #4
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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
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Old 22nd October 2012, 12:32 AM   #5
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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
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Old 25th October 2012, 05:09 PM   #6
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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
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Old 30th October 2012, 06:11 PM   #7
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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
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Old 30th October 2012, 07:22 PM   #8
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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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Last edited by indianajo; 30th October 2012 at 07:26 PM.
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Old 31st October 2012, 06:13 PM   #9
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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.
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Old 31st October 2012, 07:03 PM   #10
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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.
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