Someone put a cap replacement board in the amp and I am not sure one wire in in the correct spot.
On the picture the red with yellow tracer is the wire I am not sure about. The others I have traced back to their spots on the power board. This red/yellow wire comes form the main transformer and the connection on the cap board reads C- and the schematic calls it the Transformer Bias Winding. The is also a photo of the transformer with the schematic on it. This amp has not worked since I bought it, all the tubes test good, I have replaced all the caps. I can trace the sound from in put to output on the amp, but output is really garbled and not as clear as the input. It has K 77 and 7199 tubes in it. I tried to upload the photos by the directions on the forum but all I could get was a little red box. So pics are at link below,,sorry

http://s276.photobucket.com/albums/kk15/oakdust/ST-70/

Well you will have to find out who made the replacement circuit card before you can find out whats wrong




The transformers not stock either , looks like you have quite a mess on your hands

Oakdust,
You got yourself a SDS labs cap board in there.Just do a google search
for SDS labs to get the info on the board wiring instructions.They are sold
by Triode Electronics.The power transformer you have in there is also from
Triode Electronics and is a heavy duty upgrade replacement for the marginal
rated stock Dynaco transformer.It has a taller lamination stack than the
stock Dynaco transformer and runs alot cooler.You can get the specs and
wiring info from Triode Electronics.You got a good deal there,two of the
big problem areas of that amp have already been addressed with those
new upgraded parts.I found the link for SDS labs:
http://www.quadesl.com/refurb/refurb_dynaST70.html

Triode Electronics link:
http://store.triodestore.com/dytr.html
Regards,
Swanson

Thanks guys, Nothing like a challage egh!

Whoever did that soldering should be sternly disciplined. Reminds me of me when I was 8yo and never done it before.

Hi,
That's a very good power supply and that board is a good way to take advantage of
modern capacitors and resistors. The bias supply is a bit tricky to get right on target,
so measure the voltage carefully. There are two pots (one for right channel and one for
left channel) that should be adjusted so that you don't run your output tubes "too hot".
Check the two potentiometers in the bias supply and make certain
they are in good working order.

One word of caution - the power transformer is a noticeable upgrade. I fried the first
filter capacitor after about an hour (it gave up the magic smoke in a big way, and I was
listening to Pink Floyd). Use 630V rating or higher capacitors if you rebuild the
B+ part of the board! The rectifier tube (GZ34/5AR4) will handle
almost anything, but that first capacitor takes the majority of abuse (voltage spikes &
high ripple current). (That's one of the reasons Stokes designed that power supply
board.)

You should be able to find everything you need between the SDS Labs site and the Triode
Electronics site, but if you can't, post a question here.

And, use you favorite meter to check for continuity -
those solder joints could use some attention.

Jimmie

Yeah,
The soldering and wire connections on that cap board
are atrocious.It's obvious it looks so bad I did not think
I had to initially comment on it.For all we know that could
be the cause of most of the problems.I can't imagine
what the underside of that board looks like.When
that amp gets all fixed up it will sound real good.
The ST-70 has always been one of my favorite amps.
Regards,
Swanson

I agree, that may well be true. After verifying the wiring, I'd resolder that entire board, if it were my amp.

Well I found out the red/yellow wire is a ground wire and the red/black is the bias. Switched them, cleaned up some connections and now I have sound from the right but not the left. On werd and up werd

You'll nail it. Last time mine did that it was a driver tube. I have the Triode driver board on mine. Swapped the two tubes around, and the problem moved, so there it was. Back in business with a replacement tube ever since.

Found a bad solder joint on pin 7 of one of the K-77 tubes. Problem solved

Which preamp/driver board is on that ST-70? (What's the tube lineup?)

Jimmie

KT-77, I think

I have built, rebuilt, or modified a large number of ST-70s. Even though it is working now, I strongly urge you to go through the entire thing and resolder everything, or at least somehow verify that the solder joints and wiring is OK and neatly done. Start with the board, then the point-to-point wiring. Cut any that look bad and remake them (like ALL of those shown in the picture, EEK!).

Also, tighten ALL the screws, particularly those holding the sockets to the chassis. It doesn't hurt anything to run ground wires from the socket lugs to the central ground. If the sockets are riveted, it was a factory job, and it becomes even more important to ensure that the sockets are grounded. Unless you can re-set all the rivets firmly enough to get a solid ground to the sockets, you may have a big problem getting a reliable bias setting.

I forget, but I think the SDS board also replaces the bias supply rectifier (selenium) and associated parts. If it doesn't, redo that, too, with a silicon diode and fresh filter caps.

Brett

Adding a UF4007 in series on each secondary lead of the PS trans. to pins 4 & 6 of the 5AR4/GZ34 is a good idea as it is under a heavy load, especially with new production tubes which will fail early. It doesn't hurt sonics, will make all rect. tubes last longer, and will help save tubes in a power failure flash off and on in quick succession etc.

The diodes can be attached to unused adjoining tube pins saving using terminal strips. The banded end goes to the tube socket. As they are in series they could also be enclosed in heat shrink tubing so hiding the mod and insulating the leads.

http://ns1.audiokarma.org/forums/showth ... p?t=405473

Randy

Agreed, this is a nice mod that make the rectifier tubes last MUCH longer, although you can still kill it with the wrong on/off/on sequence.

Brett

Keep the 77's happy and rock on!

Jimmie

How do the diodes extend the life of the tube, it's not obvious to me.

See post #175.

http://www.diyaudio.com/forums/tubes-va ... ve-18.html

Randy

Essentially, the diodes do the rectification, and the tube is more-or-less a soft start circuit. This corrects the major blow-up inducing feature of using an entirely solid-state rectifier. It saves the tube by mostly eliminating the reverse-bias, which keeps it from arcing over.

Was blowing Sovtek rectifier tubes about every 6 months before this mod, since, I have been on the same tube for about 2 years (which is A LOT of hours).

Brett