I have not posted to the group, but saw some of the new topics and thought I would share some of my experiences with members. I have built, rebuilt or restored hundreds of Dynaco products. I built kits in the 60's and 70's and built many SCA 35's for friends and customers, built so many of them I could do it in a week-end. I have also rebuilt over 100 ST120's(although I knob this a tube forum, but they built some really decent SS equipment also. My suggestions cover all Dynaco equipment, but will limit my discussion to tube equipment.
We have all see the oxidation and "rusting" of the chassis of Dyna stuff. There are of course people who sell new production, it yours is really bad, but I use a great product to clean and polish decent ones. It is called Micro-Diamon Pro-Polish. It is sold in many car stores and is deigned to remove rust, corrosion, and grime in general, it is safe and effective on most metals and paints, and plastics. I use it extensively on cars and mags. Comes in a 16 oz bottle and is a green liquid. (make sure you shake it well and frequently as you use it. While it of obvious that it is great on chassis, it also works very well on Dyna faceplates to remove small scratches, fading and fog. Use it on a soft rag and GENTLY, as to much enthusiasm will remove lettering. Buff the finished product with a soft terry or microfiber cloth. Great stuff!
On the subject of restoration there are some hard and fast rules about what to replace first:
1. ALL electrolytic caps should be replaced before you start anything else. Original caps(especially the cans) have a rated life of 15 years max. Most today are 40-50 years old. If the piece has been used frequently they still may be working, but I have NEVER had a Dynaco tube amp that didn't have leakage(electrical) in the can caps and meet specs. I use the German made cans but have used a lot of the new CE ones, with only one failure. That one was right out of package, so I run ALL can caps on my restorer/tester before I put them in the circuit. The German ones are about twice the cost of CE's but you get what you pay for.
2. I also replace all electrolytic caps and any selenium rectifiers on board.
3. It is always best to use a metered variac(both current and voltage when powering up anything! If yours doesn't have metering there are some great digital meters that are self powered.(so they don't indicate much until about 50 volts AC) They are well less than $20 and you can build them into a plastic box with AC cord and outlet. With his system you will greatly diminish the likely hood of smoke, fire, exploding stuff and damage to the amps.
4. Tubes should all be tested and any off value resistors replaced.
5. I always use grounded line cords, but that is personal choice.
6. I add a HV fuse in the line to the CT of the output transformer. 1 amp FB works well for most and it will protect valuable transformers should a tube fail.
Just an aside, I resolder ALL connections and sockets, but sometime you run into some strange things. I recently had a buddy who wanted to rebuild an ST 70. He wanted me to check his work and fire it up for the first time. He did a superb job and replaced ALL the resistors on the board with metal film 5% and replaced all the coupling caps with new dipped caps. After checking all the wiring I brought it up on the variac and set the bias on his new Mullard outputs. Everything was great except the right channel produce no sound. I used a signal tracer to determine that the signal was stopping after the grid of the 6GH8(he used adapters for the original preamp/phase inverter tube- 7199) A quick voltage check showed the voltage to be way off from the working channel. I swapped tubes, then adapter and the problem remained. Flipping the chassis revealed that the right tube filament was not on.(DAH) Started tracing the 6.3 volts and found it right up to the connections from the EL34 to the circuit board. From ground I had 3.6 volts on one leg and 0 on the other. I checked all the wiring and resoldered all the connections, still nothing. Dynaco used small rivets on many connections the accepted hard wiring and once I added a little solder to the connection trace and rivet point it worked perfectly and sounded quite good. I had seen this in some SS Dyna equipment, where connections had been redone several time, but it is the first time I have see it in tube stuff. Sometimes observatio can save you a bunch of time!!!
I hope these thoughts are helpful and will respond to any other or questions.