Dynaco Stereo 400 to Stereo 410 Conversion

[Triglav427]

Hey everyone, new member, long time lurker that needs some opinions and direction.

I recently did a light restoration on a Dynaco Stereo 410. Needed a refresh so put in new PC-28 boards and replaced the output transistors (old man had previously redid the two large caps which improved bass significantly when he owned the amp years ago). Long story short, amp sounds fantastic. I really think the 410s are underrated, most people seem to only talk 400s and 416s, but that's just me.

I now pull off the shelf an old Stereo 400 I got for a steal about a decade ago when I was in high school. Life got in the way, so she sat. Now, I have finally dove into it and picked up where my adolescent self left off before college, bars, women, and starting a career got in the way. Looks like someone mucked with the PC-28s, pretty dirty inside, and possibly run for some long hours (heat damage in some areas). I was able to rework the power supply board and get one channel to operate when I plugged it in. After trying a few other small things, I believe the problem to really be in the PC-29 board on the other channel (Note that I tried a good PC-28 board from my 410 and still nothing happened). I attempted to bypass via the input (wire directly into the PC-28 as it is in the 410) but looking at the schematic, I can see there is a lot more going on with the output side.

My questions are as follows: Has anyone bypassed the PC-29 successfully? If so, any direction on how to complete it and was it worth it (ie, better sound quality)? From what I am looking at, I could just remove the PC-29, PC-30, and drop resistors - essentially wire it as a 410, but this would pretty much require gutting the rig. Or was there an easier work around I am missing.

 

[djoffe]

This link has schematics for 400, 410, and 416, so you can compare them:
https://www.updatemydynaco.com/Stereo400.html

Bypassing the front end of a 400 is pretty simple...
1. currently, pin 1 of the PC-28 goes to pin 20 of the PC-29.
2. you would disconnect pin 1 of PC-28, and instead connect it to the center of J301. You could also instead connect pin 1 of PC-28 to pin21 of PC29, and just leave the volume all the way up.

I have no idea what the sound difference is, but the mod should be pretty easy.

 

[GregDunn]

You will also forego all the Dynaguard protection, including DC offset; and the input impedance will be considerably lower (about 20K vs. 50K). Unless you're driving speakers which are able to withstand the 400's power or have their own protection fuse, make sure the output fuses are sized appropriately because you won't get a second chance.

 

[idiophool]

One should think carefully before hooking up a Dynaco Stereo 410 to inefficient, low impedance speakers. I bought two 410's from Stereo Cost Cutters many years ago. I bought the amps to drive a pair of Infinity RS IIIB speakers. RS IIIBs are low impedance and low efficiency speakers. I installed the first amp in my system and put on some rock and roll. That amp lasted less than an hour. I tried to test the second amp less vigorously. The second amp lasted into the night before it too failed. I love most of Dynaco's gear, but the ST400 and the ST410 both had a reputation for becoming unstable when they were pushed hard into low impedance loads. The ST416, with 8 output transistors per channel instead of 4 output transistors was supposed to be stable down to 2 ohms.

 

[pjrengineer]

One should think carefully before hooking up a Dynaco Stereo 410 to inefficient, low impedance speakers. I bought two 410's from Stereo Cost Cutters many years ago. I bought the amps to drive a pair of Infinity RS IIIB speakers. RS IIIBs are low impedance and low efficiency speakers. I installed the first amp in my system and put on some rock and roll. That amp lasted less than an hour. I tried to test the second amp less vigorously. The second amp lasted into the night before it too failed. I love most of Dynaco's gear, but the ST400 and the ST410 both had a reputation for becoming unstable when they were pushed hard into low impedance loads. The ST416, with 8 output transistors per channel instead of 4 output transistors was supposed to be stable down to 2 ohms.

i will try my 400 that’s been restored on infinity kappa 8 and report back. I believe if I put proper fuses in (maybe 3 amp on the b+) and 1 amp on the speaker outputs I could at least test it without doing too much damage.