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Old 23rd May 2015, 02:37 AM   #1
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Default Dynaco Stereo 400 - suggested TLC?

So, I've got one of these old Dynaco Stereo 400 power amps from the 1970s. I've has it all these years. It still performs beautifully.

I'd like some advice on things that would be a good idea to replace to head off any future trouble.

First, here are the things I've already done:
Changed out the 2W carbon 1k and 750 ohm power resistors on the PC-28 boards (replaced with 5w ceramic wirewounds)

Changed out all the electrolytics and non-polars on the PC-28 boards.

Disassembled and cleaned the gain pots (were getting scratchy, but are great and quiet now).

---------------------

I'm wondering about those two big 10,000 uF power supply caps. They show no sign of any bulge or leak, and the amp is dead quiet. They measure within spec with a capacitance meter (I don't have an ESR) - about 10,800 and 11,500 uF.
I ponder replacing them, but then again, they seem like they are in such good shape I fear that it's just a big waste of money. I know the common advice about electrolytics. But my experience has been so far that changing out all the other ones on the driver boards made zero difference to the sound, so I doubt that a single one of them was bad.

Whaddya think? And thank you.
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Old 23rd May 2015, 11:08 AM   #2
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If it works well, leave it alone.
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Old 23rd May 2015, 11:16 AM   #3
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I think you were smart to replace the caps. I would have left the resistors alone unless there were signs of excessive heat. I would replace the power supply caps because of the age. Be careful of polarity on the caps. The mentality of leaving things alone because they work is not sound advice in the case of these older amps in my opinion. I would replace the outputs also and probably the drivers.
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Old 23rd May 2015, 11:25 AM   #4
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If left alone, the components will last another 25years. It is a fools errand and a waste of money, to replace in tollerance capacitors!
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Old 23rd May 2015, 09:51 PM   #5
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Thank you.
I replaced the big carbon resistors because they are supposedly a notorious
failure point, and the replacements are pretty cheap.

The thing about the big power supply caps is, even though they measure the correct capacitance, I'm more keen to know how these particular components in that Dynaco amp held up over time. Caps aren't all made to the same level of durability. These were apparently pretty tough and pretty leak proof since they have lasted all this time. And of course, in that section of the amp, if one of those caps shorts out, it could make for a pretty nasty blast if they explode.

The driver transistors... that's one thing I did not consider replacing. My experience with transistors is that unless they've been damaged, they pretty much work the same forever. It does look to me like the power transistors on the big heat sink may not be original, since some of them have 2S numbers and the Dynaco originals were (for the two banks of 4) 2N6030 or 2N6029 and 2N5629 or 2N5630.

Why would I replace the transistors?
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Old 24th May 2015, 02:51 AM   #6
wg_ski is offline wg_ski  United States
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Modern drivers are faster and moden outputs are more rugged. But unless you're planning on blasting it with sustained clipping with 4 ohms or DJing with it, leave them alone.
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Old 24th May 2015, 04:32 AM   #7
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Thanks, wg_ski. That makes sense.
But you are right, I'm only driving a pair of AR-5's with it. And although they demand good power to sound good, 100 watts is plenty for them, so this amp is loafing. In fact the natty blue panel meters, even on the lowest range setting ("4"), barely move unless you play it pretty loud. It has always been that way. My cousin used this amp before me, and he used to drive a pair of AR-9's with it. Even those 4-ohm loads at high volumes never seemed to make the amp break a sweat. I also used this amp as the PA for my rock band for a few years, driving pairs of 8-ohm cabinets in parallel (so it was driving 4 ohm loads there too), with live electric bass among other things being driven through it.

The amp is not only a pretty darn clean and dynamic amp, but it has always seemed to have a lot of muscle when it needs it. That's why I have kept it all these years.
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Old 24th May 2015, 07:18 AM   #8
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Old electrolytic capacitors basically cannot explode, because they practically do not contain any electrolyte any longer.
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Old 24th May 2015, 09:49 AM   #9
djk is offline djk
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" I would have left the resistors alone unless there were signs of excessive heat"

Those he mentioned do have a heat problem, 5W parts are a good idea here.

"Modern drivers are faster and moden outputs are more rugged"

And require a redesign. The outputs have good SOA for the series connection.

With the series connected outputs there is no real need for different outputs, unless you want to convert it to a 416 to be 2Ω capable.

I might replace the TO-220 drivers, just because they're 150C plastic and won't last as long as the 200C metal outputs.

I would add a pair of 22F/100V bypass caps to the main filter caps, they can get a bit inductive as they dry out.
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