Dynaco Stereo 400 - where to start?

[AdamAnt316]

Update!

Been busy lately, so I haven't had a chance to do much testing to the amp as of late. I did amass a collection of replacement op-amps for the thing, though (several NOS TA75558s, as well as brand spankin' new NTE778As and NTE1171s). Earlier tonight, I finally got around to do some more looking around inside it. First off, some observations:

• PC-29 in this is the earlier version, with Q103 present instead of a series of diodes in its place
• D107 in one channel looks normal, but in the other, it's been replaced with a pair of what look like 1N4148s in series (which is probably what a 'BZ102' was to begin with, methinks; some posters in this thread concur)
• One of the solder pads which one lead of one of the Q3s is soldered to has partially lifted off the board, but seeing as it's a triangular bridge between three parts, I don't think that's the issue
• Spotted a quasi-amusing board error near IC103: the hole for one of its leads was never drilled, so whoever put the board together soldered said lead to the lead of a resistor connected to the bridge pad where the hole's supposed to be!

I have yet to test the voltages going to all of the semiconductors on PC-29, but I did check all of the pins for each IC101 dual op-amp, aka NE5558/NTE778A. Each pin measured in the ballpark of what the manual specifies. However, several pins have an annotation (M) nearby, with the legend for said annotation stating that said pins "will drop close to 0 when DynaGuard is on." Since said pins are measuring more than 0V, this would seem to indicate that DynaGuard is not actually activated, and that the indicator lamps are (surprise, surprise) a false alarm! I have not tried measuring said pins with the DynaGuard switch turned on, but I suspect that the readings would be the same.

Given these revelations, what might this point towards? I'm guessing it's something common to both channels, but I don't know for sure. How likely is it for both examples of Q103 and/or Q104 to have failed simultaneously due to the high negative voltage? Could IC102 and/or IC103 still be at fault? Curiouser and curiouser..........
-Adam

 

[llwhtt]

Most if not all Dynaco PCBs were assembled by the factory, so your "quasi-amusing error" is a factory error. I've seen a few factory errors on several different models.

Q104 does the actual turning on of the lamps so it could be bad or it's getting an erroneous signal on it's gate. Q104 should read as an open on it's anode and cathode with no power applied, also you'll need to remove the lamps other wise you'll be reading thru them and the secondary winding of the power xfmr. There are many diodes in the circuit so try testing each one to find one that's possibly shorted.

 

[AdamAnt316]

Most if not all Dynaco PCBs were assembled by the factory, so your "quasi-amusing error" is a factory error. I've seen a few factory errors on several different models.

Yeah, that's what I figured. Pre-assembled circuit boards was a hallmark of Dynaco stretching back to their earliest tubed products like the Mark II/III amps and the PAM/PAS preamps. As I said, the hole which one lead of IC103 is supposed to go into was never drilled to begin with. I can only wonder why they didn't go back and drill the hole when they found it missing. I've never seen something like this on any of the tube gear of theirs which I've restored. The substitution for one of the D107s is another possibly-factory "mod", methinks.

Q104 does the actual turning on of the lamps so it could be bad or it's getting an erroneous signal on it's gate. Q104 should read as an open on it's anode and cathode with no power applied, also you'll need to remove the lamps other wise you'll be reading thru them and the secondary winding of the power xfmr. There are many diodes in the circuit so try testing each one to find one that's possibly shorted.

Could Q103 be a possible cause? As I said, my Stereo 400 is this version, which is different from the later version which did away with it. The Update My Dynaco page lists a bad Q103 as a possible cause of stuck DynaGuard lamps, so I'll have to check the voltages on both of them, as well as both Q104s (which the manual has listings for, unlike the regular diodes elsewhere in the circuit).
-Adam

 

[Dahlquist]

Hi all,
Just pulled my Stereo 400 off the shelf after years. I broke the front fuse holder immediately! It crumbled in my hand .
I just ordered a new set from E-Bay, so there’s not much I can do until they arrive, but I do have two questions. There were no fuses in the holders. What belongs in them? And Is there a difference between a Stereo 400 and a ST 400?

Thanks in advance.

JD

 

[djoffe]

For typical listening, Dynaco recommended 1 or 2 amp speaker fuses. For testing the capability of the amp, they recommended 5 amp speaker fuses. They shipped the amp with 5 amp fuses, which were used during final test, but always recommended going to a lower current for safety.

So far as I know, ST 400 and Stereo 400 were two ways that people referred to the same amp. There were other variations...the Stereo 410, Stereo 416...you can see more detail here:
https://www.updatemydynaco.com/Stereo400.html

 

[Dahlquist]

Thank you for the fast reply.

I’ll know more when the fuse holders arrive on whether it’s a keeper or a boat anchor.

I’ll be happy to see the blue glow of the meters again.

JD

 

[dbrucesm]

Hi Everyone!
This may be a little off topic but here we are Aug 2018 . I decided to recondition a Dynaco 400 using the kits from Ed as listed in the thread. I distinctively remember the Dynaco 400 having worked in a nightclub using them in the 70s ,was always amazed re the quality of the sound they produced. Anyway nearly 40 years on here we are with a restored one so impressed being around that sound again ,had the opportunity to put the Dynaco back into a working nightclub as one of the 1000watt amps had failed . Thinking the ol Dynaco 400 will probably blow up its 50 plus years old, but so what lets see what happens ,I even installed a fan on the heat sink to keep it from going into meltdown and for a bit of extra security. Well ......Holy Heck ! The old Dynaco literally blew the supposedly 1000watt amps well away and the sound quality was in a field in its own ,there was so much more available volume I couldn't,t use it.no distortion and Very ,noticeably clean ,tight and clear sound , I had so much forgotten that sound ,but Wow ,so different to the rubbish that's in the market today . The dynaguard protection never even engaged and it was set at its lowest 20 watts ,but the sound was so clean and clear and loud there was no need to turn it up any more ,it was kicking some serious bass . At the end of the night to my surprise the amp was only luke warm ,it hadn,t even broken a burn ,my memory of them in the 70,s was you couldn't touch the heat sink at the end as they were supposed and designed to get very hot. Well here I thought just lets see hat happens ,and I was prepared to let it blow up ,I thought it wouldn't,t stand up to modern equipment ,I was proved so wrong ....big time . For anyone considering some quality gear don,tbunderestimate what these old Dynaco 400s can do ,prepared to be surprised. For the posts that say these are boat anchors ,please send or give them to me ,all as I can say is you don,t know what you have!

 

[maj53]

I just picked up a Dynaco 400 at a used stereo store that has one channel working. I found three of the 4 BJT's in one of the channels either completely shorted or just not working. I've replaced the two NPN's and now waiting for the new PNP's. The problem I'm having is the bad channel will blow the fuse, or if I bring the amp up with a 75 watt light bulb the bulb burns brightly. I'm wondering how to start diagnostics on the amp with a short somewhere. I also replaced one of the 100 ohm resistors on the audio board, but the one I took out was just out of range. I'm seeing about 80v on the power supply fuses, and I've replaced the speaker fuse holders and fuses. Any ideas on how to get rid of the short so I can do some debugging?

 

[djoffe]

This is a divide and conquer problem. Disconnect all the output transistors. Q301, 302, 303, 304. That gives you a chance to limit the expensive damage. The amp will work, e.g. have voltage gain, but just won't drive speakers in that condition. It will also allow you to check the various voltages to see if they are reasonable before you hook up the big, expensive output devices. Be careful...lots of voltage and current...easy to let the magic smoke out.

 

[maj53]

Thanks. I decided to bring up the amp on a variac. I got to about 70v with all fuses in-place. I had full audio on the working channel, and very faint audio on the bad channel. Then all hell broke loose and the bad channel amp card started to smoke. R223 was completely burned up. R23 (from the schematic) looks like it is connect to the input pin 2 of IC2. I'll pull the output transistors and start again.