Dyna St-120

[pcheb]

Well it's Sunday & my ST-120 finally gave up. I see no great love on here for them ... but I'm cheap!

Replaced P/S caps a couple of years ago after hum. Had some motorboating this week, a whump & no sound out. I had the output coupling caps still around so replaced them last night.

Still no sound, P/S Q9 is running hot, all finals are cold. No scorching/burning. Sounds like the P/S is shutdown. Of course all my test eq is 2000 miles away in storage. Except for the big electrolytics all components are original.

I can get a cheap DVM and troubleshoot, or get a used Yamaha RXV757 for $135 and just use the power section.

Worth the effort to repair?

[djoffe]

they're typically pretty easy to repair...I'd probably try and do that first...

[djoffe]

Enclosed pdf shows some analysis I did on the power supply at the end of last year, beginning of this, when things were a bit slow. Oops...nevermind...it's bigger than the pdf upload limit...if you drop me a line, I can send you an email copy.

[indianajo]

Yes, unload the P.S regulator by disonnecting the output PCb's from the cap, and see if the Q9 lets the big filter cap come back to 72 VDC. You can test the PS regulator PCB with a 10 ohm 225 w adjustable resistor; adjust until reaches 6.5 A out, if it does and stays 72v , the PS regulator is okay. Then one of your output PC15's has a problem. See D Joffe's Updating the Dynaco Stereo 120 for a complete shotgun replacement of the PC15. See Dynaco Stereo 120...can be beautiful for a 5 transistor per channel update that makes the original PC15 sound great. With modern output transistors I believe mine is putting out more than 60W per channel, but haven't proved it yet. (Trying to produce a stable sine wave source for $0). I fixed ST120 with a simpson meter, no scope. Pulling base leads off the output transistors and doing the forward double diode test is step one. Then the 330 ohm resistors, then the 5.1V zener. Check all your electrolytic caps, even the little ones, they are way past design life if not replaced already. Typical high frequency failures. It is a **** of a power transformer. If it didn't burn out your speakers when the power transistors went out, then the "obsolete" capacitor in series with the output design did it's job. Direct connected output transistors on split supply amps will toast the speaker coil if there is not sophisticated protection circuitry. If you stay with PC15, do the TIP mod mentioned in the links, it helps the circuitry not oscillate with modern fast ft transistors. The original 1966 design used really slow low gain output transistors, the cause of some of the distortion the 1966 reviewers complained about in my humble opinion. The low volume notch distortion, Mr Joffe has designed out two different ways.

[pcheb]

Originally Posted by indianajo

See D Joffe's Updating the Dynaco Stereo 120 for a complete shotgun replacement of the PC15. See Dynaco Stereo 120...can be beautiful for a 5 transistor per channel update that makes the original PC15 sound great.

indianajo, thanks for the tips & links ... I had read the 'can be beautiful' thread, and have been slowly copy & pasting the thread into one doc for easy reading offline. I hadn't seen the 'Updating' thread yet.

[pcheb]

Originally Posted by djoffe

they're typically pretty easy to repair...I'd probably try and do that first...

Yes very easy to work on. I guess it's time for a full rebuild though, as I've only replaced the big cans ... depends on what I find.

I'm off to the pawn shops to find something to hold me over while my Dyna is down.