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).
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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.
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.
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.
If left alone, the components will last another 25years. It is a fools errand and a waste of money, to replace in tollerance capacitors!
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?
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?
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.
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.
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.
Old electrolytic capacitors basically cannot explode, because they practically do not contain any electrolyte any longer.
" 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 150°C plastic and won't last as long as the 200°C metal outputs.
I would add a pair of 22µF/100V bypass caps to the main filter caps, they can get a bit inductive as they dry out.
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 150°C plastic and won't last as long as the 200°C metal outputs.
I would add a pair of 22µF/100V bypass caps to the main filter caps, they can get a bit inductive as they dry out.
Dynaco 400s
Hello Mmagliaro ,
Who said: "Changed out the 2W carbon 1k and 750 ohm power resistors on the PC-28 boards (replaced with 5w ceramic wirewounds."
One of my current ongoing projects is to carry out that modification on my old Dynaco 400 (unfortunately without the cool meters), and it's pal a Dynaco 410.
One of those under- engineered 2w resistors in the 400 let go a couple of years ago, curiously while my wife was out of town😀. I dont think it was a coincidence, as it was being driving pretty hard with the warning lights flashing on peaks. That failure took out a string of parts that needed replacing!
After recently reading about this mod on line, I am resurrecting them for use in a biamp project running some AMT ESS Monitors. When I put them out of service they were working fine, but it is clear that these resistors are running hot, discolored and surface texture irregular, and I want to do this mod to prevent another catastrophic failure.
I thought the 5w wirewounds would be a safe replacement.
I need to review the procedure on line about how to adjust the the variable resistors on the 28 boards to minimized the distortion at crossover. How did you do this adjustment? The procedure on line, link to be added, just used a meter to fine tune the various currents. http://home.indy.net/~gregdunn/dynaco/components/ST400/chnl_fix.html
Does anyone have a procedure using a scope, and would that be better actually looking at the traces while tweaking the pots.
I enjoy these threads, as they jump start me on these projects, that I have been meaning to do! Also wife just happens to be away...
Hello Mmagliaro ,
Who said: "Changed out the 2W carbon 1k and 750 ohm power resistors on the PC-28 boards (replaced with 5w ceramic wirewounds."
One of my current ongoing projects is to carry out that modification on my old Dynaco 400 (unfortunately without the cool meters), and it's pal a Dynaco 410.
One of those under- engineered 2w resistors in the 400 let go a couple of years ago, curiously while my wife was out of town😀. I dont think it was a coincidence, as it was being driving pretty hard with the warning lights flashing on peaks. That failure took out a string of parts that needed replacing!
After recently reading about this mod on line, I am resurrecting them for use in a biamp project running some AMT ESS Monitors. When I put them out of service they were working fine, but it is clear that these resistors are running hot, discolored and surface texture irregular, and I want to do this mod to prevent another catastrophic failure.
I thought the 5w wirewounds would be a safe replacement.
I need to review the procedure on line about how to adjust the the variable resistors on the 28 boards to minimized the distortion at crossover. How did you do this adjustment? The procedure on line, link to be added, just used a meter to fine tune the various currents. http://home.indy.net/~gregdunn/dynaco/components/ST400/chnl_fix.html
Does anyone have a procedure using a scope, and would that be better actually looking at the traces while tweaking the pots.
I enjoy these threads, as they jump start me on these projects, that I have been meaning to do! Also wife just happens to be away...
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