Modular Drive 480V Conversion with Scissio Controls SCR

Greetings all,

Been a long time lurker and finally decided to join to add a potentially useful drop to the ocean of knowledge here that I have selfishly used over the years...

A while back I was searching for a modular drive 10EE for home/hobby use. I ran across one that fit most of my wants except it was a 480V machine. I did a bit of research on the voltage conversion but put it on the back burner because I didnt want to mess with it. One of my obvious concerns with getting a Modular drive was the the possibility of needing to replace the C16J tubes. I stumbled across Tim's SCR thyratron replacement thread one day and got really interested when I read the replacements are less sensitive to the filament voltage. I contacted Tim about the possibly of using his thyratron replacements to convert a machine from 480V to 240V.

To make a long story short I ended up getting the machine and have it running with the original 500V input transformers used for T6 and T7 with the filament voltage running ~1.47V. I dont have much time on the machine yet because I ended up taking the gearbox apart for unrelated issues. I will update after some use but I wanted to post this up before I forget.

Tim is a stand-up guy, you can tell he has a real passion for these machines. We exchanged 20+ emails and he called me a few times over the course of it. I ended up having a malfunctioning C3J that he helped me troubleshoot and source a replacement for.

Additional reference information: The machine is S/N 50635 with stamped manufacture date of March 1971. Along the way I also checked out options for replacing the filament transformers that included Monarch and the OEM (Control Transformer Inc). Monarch had some used ones they quoted over the phone at ~$300 ea. Control transformer is a custom transformer shop that surprisingly quoted me for new replacements but were ~$1500 ea and 8 week lead time. The OEM was a long shot however everything but the price was a pleasant experience. Similar to Monarch they had great customer service and knew everything from the serial number including that they were originally supplied to Monarch.

-Matt

The SCR tube replacements have been done in the past with success depending on use, industrial use not so much.

On earlier module drives, they used two different filament transformers, high voltage and low voltage. Later on, they used low voltage transformers, and wiring them in series when using high voltage, to produce 2.5V AC.
If, you have the higher voltage filament transformers and you want to run on 200v, the other option to replacement, is wiring in a small booster transformer for the T6 and T7.

Greetings donie,

I left some details out of my previous post to keep myself from doing my normal over elaborating. Since you opened the door I will do so

Tim's initial email reply to me was actually that I didnt need the SCR replacements based on the low voltage transformers in series configuration you mentioned. I expected this for my machine because I considered 1971 a "later" machine. Turns out I was wrong and it has high voltage transformers which I didnt realize until I got it home.

I had seen previous posts referencing the addition of another transformer to get T6 and T7 at the proper voltage however I went the route I did for 2 reasons. First, I wasnt real excited about having to splice and/or route wires through the compartments. Second and more importantly, I didnt get to run the machine when I checked it out. The individual I purchased it from got it in a lot of tools and didnt have 480V for me to run it. For various reasons, which includes being impatient, I came to terms with buying it knowing there were risks. One of those risks was not knowing if the original tubes would function. For me it made since to go the route I did because it also removed the potential of a dead tube that I would have to source after I bought and wired in the transformer. Sort of a bird in the hand vs bush thing I guess.

Regarding the SCRs not being well suited to industrial use. Like I mentioned it is for home hobby use so I am not out to dry if they crap out. My other thought was if they do crap out I have a "spare" albeit untested set of tubes that I could potentially fall back on via the transformer option. Knowing my luck they are probably bad but I can pretend it is a safety net.

The trade offs made sense to me for my application. I fully undersatnd that there are multiple ways of doing everything each with their own merits. Currently my only regret is the missing tube glow.

-Matt

For the SCR replacements I do not think the filament transformers are even needed as SCR's do not have Filaments. The high voltage side can even be disconnected on the old ones.