ST-70 owners! Please Read!

organ

I finally decided to check how much the resistor values on the driver board have drifted over the years. I couldn't believe the kind of readings I got! Out of the 24 resistors I measured, only about 4 were still in their tolerance range! Some were slightly off and many were WAY off. Some were off by a few hundred Ohms. These parts are 30-40 years old.

So guys, check your resistor values. As good as our 70's sound, we're not hearing what this amp is capable of right now and the sound will only worsen as the resistors keep aging. If you can't figure out resistance value by color coding, download the driver board restoration module from www.dynaco-doctor.com

I'm going to the electronic parts store this weekend to purchase resistors and should have it done by Sun. I'm going for carbon type. Not going for metal film. Want to keep the amp 'stock'. I also hear carbon have a smoother and warmer sound.

Oh yeah, I performed another mod on the driver board which sounds pretty damn good. It was posted by Joe Rosen at the Asylum. This guy really knows about the ST-70. I'm not going to post the technical info because he could explain it much better and it's very long. I'll post a link later. All you have to do is remove the two 390pF caps, two 82pF caps and the two 18K resistors that run in series with the 82pF caps. One set of caps (can't remember which ones) are there to limit the HF bandwidth. After removing the caps and two resstors, I must say, I'm very impressed with te sound. I love this amp. There's so much you can do to improve the sound.

I'll report back on the sound quality after I replace the whacked out resistors.

Maurice

dcarlson

Thanks for the info. I'm certain mine needs a good reworking. New board and all. I'd like to say I'll get at it in the new year.

madmax

I'm assuming you pulled one side of each resistor loose as you measured so as to get the proper reading?
madmax

organ

****! I didn't think of that. I re-measured the resistors I took out and many were still in their tolerance range. I think the worst one was a pair of 18K resistors. One measured close to 18K while the other was around 16K. The others were off by 5 to 10% of their value.

I bought a whole bunch of each value and installed the ones that are dead on +/- 1%. This was critcal because I needed a lot of matched pairs (one for L channel and the other for right). I replaced 12 resistors. The store didn't carry carbon comp so I went with metal film. Also, I needed some 3W resistors but the store didn't carry them. The guy asked me wat they were going to be used for and I told him I was restoring a vintage tube amp. He said a 2W in place of a 3W should be safe because today's metal film can handle extreme temperatures and using a 2W in place of 3W will just make the resistors run a little hotter. So I raised them slightly off the board to help with heat dissipation.

So was there a difference? YES! The image seems more stable, overall sound is clearer. It was somewhat blurred a bit before. HF extension is much better and bass is more defined. This was clearly heard when I listened to Queen's "Under Pressure". At around 40sec into the song, when the bass really kicked in playing the same repetative riff, but this time each pluck on the string was clearly audible. Before you can hear the bass but it was hard to hear each note and string. It was more sloppy before. Now it sounds way better. Dynamics is better too.

It's hard to say what made the improvement. Was it the "better" quality metal film vs stock carbon comp? Fresh solder joint on the board? Resistors being matched 100% for L&R side? Resistrs that measured dead on for the given value? I don't know. I'm guessing it's a combination of all.

One thing I need to do real soon is replace the crusty old tube sockets. No joke, they're old and crusty. I snapped the pin of one socket in half while wiggling it around. It was on the socket for one of the output tubes. I took the broken piece out and soldered the wire and resistor to whatever was left. I'm going to replace them with gold plated ceramic sockets.

After replacing the sockets and re-wiring the inside, I'll be done with the restoration. Well, I'm still going to order a replacement driver board.

Maurice