Dynaco st-120: I got a little screwed

[Dennis5587]

Hi everyone...

Ok so I won a Dynaco ST120 that was supposed to be "Nice clean Dynaco Stereo 120 Amplifier", that "has no rust at all and is in great working order".

Well... this guy messed up two for two. It has some oxidation, nothing too bad for the age of the amp, and wouldn't really annoy me too much had it not been for the fact that the right channel doesn't work....

I tried to hook it up to my stereo, and what do you know... All I get out of the right channel is a constant loud hum. I look inside past the grill and see that the top of one of a large capacitor there is a nice big black spot right on top with the wire melted into it, and the top which is supposed to be a dome is caved in. On the upside... the left channel works fine. :/

I searched around on here, and found a site that has the schematic of the amp http://home.indy.net/~gregdunn/dynaco/components/ST120/ and it is the "C7" capacitor.

So basically what is your opinion, should I try to argue with ebay? Because I went back to the auction and sure enough, the black spot is in the picture on the auction as well. Or, should I just try and replace this component, and go from there. From a visual inspection the rest of the amp seems to be fine, as in there aren't any more burn spots...

Any help would be appreciated.

[jaycee]

Unfortunately the best you can do is leave him negative feedback. Ebay probably wont do anything!

The C7 capacitor is a coupling capacitor that connects the speaker to the output of the amplifier. Because the amp is a single-rail design, this capacitor is needed in order to remove the mid-point DC offset. The wire wound around it forms the coil L1.

The other capacitor shouldnt be bulging either. I'd say that they're both past it, and it just so happens that the one with the big brown spot has let go first. Replace them both.

To be honest, with an amp of this age, all of the electrolytic capacitors are suspect. You should at least replace both coupling capacitors, and the other "big cans" which will include the power supply capacitors.

[taj]

I wouldn't bother leaving negative feedback, he'll just retaliate and you'll both lose. It's the single biggest complaint against eBay, who couldn't give a rat's butt about liars, as long as they get their fees.

For the Dynaco, just replace all the electrolytic caps. It's long overdue. They aren't too expensive (unless you get some audiophile blessed varieties). The physical size and type doesn't really matter, and the value should be kept similar (in other words: 4.7uF = 5uF or whatever).

The polarity is important. Don't get the new ones backwards!

The wire wrapped around is important, make sure it remains after the cap-ectomy.

..Todd

[Dennis5587]

Hi,

Thank you for your fast response. I was thinking the same thing, just kind of wondering what anyone else thought. The coupling capacitors are 3300uF, 50VDC. What do you think I should replace them with? I know that I've read somewhere that bigger capacitors should help with bass response and sound quality.

In case you couldn't figure it out yet... I'm pretty new, so any suggestions on capacitance, voltage, or brand would mean a lot to me.

Thank you in advance for your time,
Dennis

EDIT: Hi Taj, I was writing my reply at the same time you posted yours, thank you for the info, and about ebay. This is the second time I used them. The first time I won a Tube amp that had no shipping cost listed, and the person tried to charge me $400 for shipping. I don't have very good luck it seems

[john curl]

I hate to say this, but the 120 is a 'dog' and really not worth anything. It would be better to get something else. Even something that doesn't cost much. For example, would you rebuild an old Yugo? Same thing.

[EchoWars]

I suppose the ST-120 has its place in audio history (and happens to the the first real high-fidelity amp I ever had a chance to listen to). Thing is, as soon as I got into the hi-fi deal myself, I discovered that you can do much better than the 120 without too much effort.

My advice is, if you're willing and wanting to exercise your troubleshooting skills, make the thing work. Only thing to do with a lemon is make lemonade.

[sandstorm33]

Fix it and put it back on ebay. Make sure you include the choke winding on the cap or the thing might oscillate. If you decide to keep it give it more capacitance in the power supply . A couple of 10,000 50volt cans with low ESR might make things better on the low end. Bypass them with some smaller polyprops.

The choke winding isnt critical ...Just put a few turns of 20ga stranded wire around the cap and solder the ends to the cap leads .

[taj]

Originally posted by Dennis5587
Hi,

The coupling capacitors are 3300uF, 50VDC. What do you think I should replace them with? I know that I've read somewhere that bigger capacitors should help with bass response and sound quality.

Bigger is better (to a degree) for power supply filter capacitors -- the ones near the power transformer (10,000 uF is fine for this amp), but not necessarily true for the output coupling capacitor. If its role is to block DC by acting as a 1st order Butterworth high-pass filter, then 5000uF would be rolling off the bass -3db at 8 Hz assuming a 4 ohm load (4 Hz for 8 ohm speakers). So 5000uF would provide more than adequate bandwidth for this amplifier. Without looking at a graph, I'm pretty confident that you're within -1db at 20 Hz so you wouldn't gain anything by going bigger.

..Todd

[myhrrhleine]

Originally posted by Dennis5587
[B]Hi everyone...

Ok so I won a Dynaco ST120 that was supposed to be "Nice clean Dynaco Stereo 120 Amplifier", that "has no rust at all and is in great working order".

If I bought a ST-120 (and I have) it'd be because I'm collecting a 'classic' amp. Not for it's sound quality.
That being the case, i'd want to restore it as close to the original as possible.

Otherwise, save what you can, strip out the guts and make a new amp.
IMO, a basic chip-amp would sound better.

[Dennis5587]

Thanks again guys, I ordered some capacitors to replace both coupling capacitors. I did get the amp because it is a classic amp that I would just enjoy having, why I wanted it to be as stock as possible. I have read that it is certainly not some of Dynaco's best work, in part because it was one of the first solid state amps.

I'd like to keep it as much the way it is/was so I am not planning on stripping it. I'd love to get an ST-70, as well as other tube amps. Unfortunately I'm a college kid on a very tight budget. S o I get what I can, when I can.

I'm going to start off with replacing just the coupling capacitors for right now, and if that works out, probably the rest of them as well.

To go a little off topic, do you think that this amp would work OK with a passive pre-amp?

Once again, thank you for all your responses and help.