The Dynaco Dynakit ST70 Stereo Amplifier Restoration Project Part 2

Now in Part 2 of the Dynakit ST70 restoration project, we will begin with the choke as the first component for installation, though I thought it interesting to compare both on the Genrad 1650A impedance bridge to see that the impedance and inductance is.

SPEC
62ohms 1.75h
OLD CHOKE
63ohms 1.50h
NEW CHOKE
64ohms 1.35h

All within acceptable range, the new choke is installed into the chassis, though, the old one is definitely a keeper for other projects. All work is done with the intent on maintaining a scratch free project and with extreme care. The tightening of all screws in this project has the slot of the screws pointing in the same direction.

The two switches followed the choke and interestingly, they were not threaded like the original ones. This was helpful though and allowed the screw heads to maintain a parallel appearance. Following the switches was the assembly and installation of the gold plated input jack board.

Returning to the old chassis, the selenium rectifier, power cable, fuse and stand-off board was removed for transfer. It became apparent that a custom screw for the rectifier would need to be fabricated. As these would be put into the new chassis it was also a good time to introduce the bias pots as well. Tightening in such a manner that the flats would be parallel to the font of the chassis.

The rear jack kit was assembled at this point. This kit did not include the 16 ohm option, instead an internal standoff for 16ohm serving as a tie-off for the feedback connection. These are much better than the screws in the original kit. Upon assembly, these too are mounted. along with the end stand-offs.

Returning from Lowes with a 6/32 screw the fuse socket was installed and the screw was cut as a rod in support of the selenium rectifier, using an acorn nut as the top cap. The standoff board was installed and then I'd assembled some hardware to test the rectifier install. It looked good and was done so that the flat again was parallel to the chassis.

The tube sockets were a little tricky because they are a bit smaller than the hole. So I had to shim it with two turns of paper to center the socket whilst tightening. It seemed to work good though. Mundane is having to repeat this several times. Moving to the power transformer was welcome.

Its challenging though, guiding this heavy thing into the chassis without scratching anything with all of these wires everywhere. Somehow I managed to pull it off. Dampeners and all installed. Immediately I install for regular nuts on the bottom to hold the transformer in for safety. One benefit of the transformer is the ability to flip it to the service position.

Power transformer wiring begins with the EL34 filaments, BROWN and GREEN. This included their center taps to the double capacitor to ground. Following this will be the connections to the rectifier. Then the primary black wires off of the power transformer to the fuse and switch. Following this, is the single cable to the selenium rectifier.

At this point I go back to the old chassis to remove the star ground hardware and install it in the new chassis. The first connection to it would be the ground off of the power transformer. At this point a 3 prong power cable is introduced and wired in. The ground of this cable also meets the star ground.

With the power testing is conducted with the Heathkit IM-11 VTVM using a fraction of the voltage. Switches are tested and then the ratio's determine that the windings are ok. The center-taps to heaters were measured by resistance across the coil.

Having completed this testing part 2, has come to an end. #dynaco #st70 #tubeamplifier #antique #restoration

[julesl]

Wonderful video, you are the man. These amps and the technology behind them are being appreciated today, this technology simply hasn't been improved upon by solid state electronics in the context of high fidelity amplification, nothing beats this stuff. Awesome video thank you!

[johnsampson]

Jordan, at least you didn't "choke" while testing that Great detailed videos, and I'm sure your military training in electronics instilled your thoroughness. Twenty years ago, I was at a yard sale and found an ST70 with EL34s Mullard brand output tubes. The cost? 12 bucks! It was the find of the century. I also scored a Fender Bassman 10 at that time also for a very reasonable price. Happy

[dougmcartin]

I would love to see how your kitchen cabinets are organized. They must be a thing of beauty.

[otekrec]

Your technical knowledge and versatility is matched only by your astonishing attention to detail. Amazing work, Sir.

[sebokaz]

Because I know that they're there! What a beauty

[HipocratesAG]

Man, your attention to details is inspiring, well donr

[thermionic]

Nice, subtle touch that you did by not using that Lowe’s Phillips head!

[midmodaudio]

Excellent video and as others have stated, your attention to detail is impeccable and inspiring. Seeing your avatar, I'm guessing you learned those habits in Navy boot camp.

[edvaioli]

Nice job Retro! One thing I don't like, however, in the original or the redo, is the AC mains wiring. Neutral (white) wire should go directly to transformer, hot (black) should go to rear fuse terminal (as you have corrected) out the side fuse terminal into the SPST power switch and out the switch to the transformer.

[dhpbear]

11:35 - The 'lower nut should be used on topside of the chassis (the control mounted flush with the bottom of the chassis) .The top nut is tightened to lock the wiper in place ounce the bias is adjusted.

[larryshaver]

I use 6-32 screws to replace the ones when i rebuild motors

[matthewenderby]

Great work @Retro Tech & Electronics I think your work is great and attention to detail, but I would not have done this the same as you. Not a criticism but I would have cleaned up the chassis and kept old choke since it was in spec, replaced the selenium rectifier with silicon rectifier. Each to his own!

[Waltkat]

Nice build, although this is not so much a restoration as building a new amp while reusing the transformers from the old ST-70. I may be doing the same thing soon, that is if I can talk my buddy into selling me one of his old ST-70's. Lol. I have some ceramic tube sockets with gold plated contacts waiting for a new home...

[SuperFredAZ]

❤just a minor point 6-32, is pronounced 6-thirty-two. It is a number 6 screw, 32 threads per inch, nothing to do with 32 and of an inch

[thermionic]

Where’d you get the replacement chassis? Looks like one of those incredibly-expensive South Korean replacements.

[larryshaver]

that wire may be a little too thin too[black]

[oldricky]

The ST-70 was compliant to the 1970 electrical code but not today's. I have yet to see anyone rebuilding these amps being compliant to code that's been in place since the late 70's early 80's. If you are rebuilding these amps for others not being compliant to code and someone is electrocuted, you may be liable in both a criminal and civil context. For current safety / electrical code reasons You CANNOT switch the neutral (white) alone and the 120 volt line (Black) MUST be fused - NOT the white neutral. Breaking the line neutral side with either an open switch or open fuse leaves the system as a live electrocution hazard waiting for someone well-grounded to earth to touch it. Remember, milliamps can kill without the fuse blowing. The fact that the green earth ground exists does not guarantee that every receptacle an amp is going to be plugged into has integrity in its earth ground. As well, if others following your advice, fall into the same situation, many a lawyer will say you share the liability. There are precedents.

I'm not trying to be a jerk. I come out of the service industry myself and I own and have rebuilt ST-70 amps and am one that has had a "hot" chassis. One another when I attached an earth ground to an ST-70 that was working perfectly, the caps on the board went off like a bomb which I have yet to trouble shoot. Otherwise, great videos. Keep up the good work at preserving these gems.