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Dynaco Stereo 120 Resistor Smoke

klitzy13

klitzy13

New Member
I am hoping the brain trust can help me out on this one... I have a Stereo 120 that was previously worked on by someone other than myself. The output transistors and R16/R17 are replaced on the PC-14 left channel. The outputs were replaced by 2N3772's. Everything else in the unit appears to be stock. The unit was brought to me because it was still not getting any output on the left channel. I got it on the bench, did some initial checks to make sure nothing was blown again on the left channel, all looked good so I tested it and had output on both channels. I had the cover off and had it running (no load, just a scope hooked up to the speaker terminals to watch for the 1k signal I was inputing into the RCA's) and smelled the dreaded burning electronics smell. I powered the unit off and noticed that R16 on the left PC-14 was letting off a little smoke. Checking the resistor it didn't blow but is now reading about 260 ohms instead of 300 like R17.

I have checked all of the voltage test points in the instruction manual (powering the unit on for only a few seconds at a time, letting things dissapate then moving to the next) and all of the voltages are close and match the right PC-14 board. The only differences I can find between left and right are the voltages at Q1. I checked D1, D2 and D3 on the left PC-14 board and all test fine. The output transistors test fine as well.

I'm stumped as to what to look at next or where to go on this one. Any suggestions?
 
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I thought about that but the unit isn't mine and the owner doesn't want to go down that path. Any suggestions on how to fix the amp without rebuilding it?
 
klitzy13 said:
I am hoping the brain trust can help me out on this one... I have a Stereo 120 that was previously worked on by someone other than myself. The output transistors and R16/R17 are replaced on the PC-14 left channel. The outputs were replaced by 2N3772's. Everything else in the unit appears to be stock. The unit was brought to me because it was still not getting any output on the left channel. I got it on the bench, did some initial checks to make sure nothing was blown again on the left channel, all looked good so I tested it and had output on both channels. I had the cover off and had it running (no load, just a scope hooked up to the speaker terminals to watch for the 1k signal I was inputing into the RCA's) and smelled the dreaded burning electronics smell. I powered the unit off and noticed that R16 on the left PC-14 was letting off a little smoke. Checking the resistor it didn't blow but is now reading about 260 ohms instead of 300 like R17.

I have checked all of the voltage test points in the instruction manual (powering the unit on for only a few seconds at a time, letting things dissapate then moving to the next) and all of the voltages are close and match the right PC-14 board. The only differences I can find between left and right are the voltages at Q1. I checked D1, D2 and D3 on the left PC-14 board and all test fine. The output transistors test fine as well.

I'm stumped as to what to look at next or where to go on this one. Any suggestions?
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I had the same problem. Every semiconductor tested ok on my peak meter so I started with the resistors. I found at least 6-8 out of the 5% limit so I replaced every resistor on the boards. They were were both getting extremely hot still and no output on the same channel. Changed all the new capacitors again on both boards. Now, OK but still no output on one channel. Started signal tracing from input to c1 with a cheap Radio Shack hand amp with homemade probes only to find signal drop off at q1-q2 . Replaceced the new parts with better quality parts and Wa ! La ! Stereo ! But hold on. The cheap Chinese resistors at R6 and R7 discolored from heat on both boards. Replaced with 1wattt same quality. No heat, no discoloration, no problem to date. Even with all the money I spent on transistor test equipment, you never know if the part will work under a load. The new parts were bought as kitted parts for those parts on EBay. They are rarely found to be of good quality. Believe me, I know.
 
ivandezande said:
I would just rebuild it with the boards from UpdateMyDynaco honestly.
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I have a pristine ST120 that I've mildly modded - MOSFET power supply mod and TIP mode using quality parts. It would be interesting to double blind it against a completely rebuild unit as once rebuilt using new amp boards and power supply. I'm not saying you couldn't hear a difference, just thinking what it might sound like. Mine drops 2-3mV of DC at the speaker terminals when run across a 10 ohm resistor after a 10 minute warmup.
 
Thanks for the response John, basically to fix your issue you had to rebuild the boards? Or for the capacitors were you referring to the large caps between the boards? I've tested them all disconnected and they test fine but like you said they could do different things under load.
 
Bill S said:
I have a pristine ST120 that I've mildly modded - MOSFET power supply mod and TIP mode using quality parts. It would be interesting to double blind it against a completely rebuild unit as once rebuilt using new amp boards and power supply. I'm not saying you couldn't hear a difference, just thinking what it might sound like. Mine drops 2-3mV of DC at the speaker terminals when run across a 10 ohm resistor after a 10 minute warmup.
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Hi Bill,
I just purchased an ST120 that needs some work. When you did your mods, did you use the updatemydynaco power supply kit? Also, where did you get the components and specific instructions for the TIP mod?

Thanks

Jim
 
Bill S said:
I did the mods maybe 8 years ago and then put the unit on a shelf. The information for the MOSFET power supply mod and TIP mods are here. It also has links to an article written by Frank Van Alstine.
https://davidreaton.com/dynaco/stereo-120/
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I've seen that post and will probably do the TIP mods detailed there. I've gone ahead and ordered the power module kit and "large capacitors" from Updatemydynaco. I'm hoping the amp boards themselves are ok. Gonna pull it apart next week, clean it up and rebuild.
 
Are the large caps bad? Mine were fine. The MOSFET PS is a good mod. I sold my amp and the buyer accidently shorted the speaker outputs. The only damage was a blown fuse. The amp runs nice and quiet and makes good power for its size. I think you'll enjoy it.
 
One of the C7's has been replaced (very poorly) with who knows what, C9&C12 are bulged big time, and C11 was replaced (again rather poorly) with who knows what. My thought is to at least start with a clean slate and stable power supply and see what I have. Hopefully parts will start to show up soon.
 
I have 2 of these that I got cheaply. Replaced the amplifier modules , put on a better cord, redid the grounding. Haven't touched the power yet, but I listen to one of them every day. The only thing that bothers me is the thump from the speakers when powering up. I just wait to turn on my pre until I power up the amp.
 
Spacey37308 said:
I have 2 of these that I got cheaply. Replaced the amplifier modules , put on a better cord, redid the grounding. Haven't touched the power yet, but I listen to one of them every day. The only thing that bothers me is the thump from the speakers when powering up. I just wait to turn on my pre until I power up the amp.
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The turn-on thump is normal for an output capacitor-coupled amplifier and there's no way around it. It's annoying, yes, but it doesn't hurt anything. The caps have to charge up...
 
Good looking St-120, nice chrome. It is hard to find one with non rusty metal work. I have a flea market find St-120 from Florida's forgotten coast, ended up having to use bondo and Krylon metallic grey on the gone chrome. Updatemydynaco will bring many 40-53 year old pieces into the modern age, and at a decent price.
 
It turned out pretty good. the chrome is probably 90% intact, just gave it a good polishing inside and out with Blue Magic. I really like the updatemydynaco stuff, but I wanted to stay as close to original as possible. I did use their power supply because it appears to be more stable than the original, but the amp boards are all original (except for upgrading the 300 ohm resistors to 10 watt). I pretty much use it every day, but I am on the hunt for a PAT4 to feed it. Right now I'm using a Rotel preamp to drive it, but I'd like to have a Dynaco "set" in the rotation.
 
I had a PAT-4 and installed the Van Alstine boards, two long PCBs with TL071 op amps. Had a little op amp edge but with tremendous bass response. Used a Zenier regulator for the PS. Updatemydynaco has a PAT-4 rebuild. A PAT-4 would look great with the St-120 real retro 1970. A PAS 3 or 3X would be a nice match too as SS and tube can compliment each other. Get a retro Dual 1214 and some AR or Advents and really go back in time.
 
I have my eye on a couple of PAT4's, so hopefully something will come together sooner than later. Speakerwise, I just built a pair of Altec 19's (not quite complete but functional) and they're great with the Dynaco (and the Rotel power amp as well).
 
Wow! That will be awesome. They don't need much power with those horns and a big paper cone woofer.
 
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