Output Voltage on 701A Sagging

[ETN550]

I've been runniing a 701A and have noticed that after about 10 - 15 minutes of running the output voltage drops off. Starts out around 120 when the knob is cranked almost fully clockwise. Then, as it warms up the voltage drops off until it gets to around 106 or 108 at full clockwise on the dial. turning the dial counterclockwise can get it as low as 90. This seems to repeat once the machine is cooled off and then re-started. Load does not seem to make much difference.

So to get things moving along I swapped the entire A1V1 voltage regulator assembly with another I had in a complete and functioning control box.

Problem solved as it seems to be holding 120 and not at full scale on the dial.

Now I want to fix the bad A1V1 so my spare control box is back up to snuff.

Does this sound like the transistor is bad? And if so is the transister the component on the top with a dome the size of a nickel and screwed down with heat sink tape under it? Can it be replaced separately? Where do you get them? I know complete A1V1 is very pricey..

What else to look for?

Thanks for help.

 

[Isaac-1]

I have a similar problem on my MEP-701a, max output voltage varies with ambient temperature, in the winter it seems to go up to 128V or so, but on hot 100+ degree summer days it will max out around 118 volts. I have changed out the top transistor you mentioned with no change in symptoms. Someone on the Smokstak board suggested the most likely candidate would be one of the capacitors, I plan to change them out next.

Ike

 

[ETN550]

Well the wholesale swap of the entire A1V1 regulator did seem to fix it. However, I will keep running it when I get a chance to make certain.

Are the capacitors you refer to the ones in the regulator's board or others bolted directly inside the control box to the walls of the box?

It looks like that big @ss regulator can be taken apart and the board will slide out with the two end screws removed. The little transformer is riveted in but that would be easy to drill out and re-rivet.

I change some capacitors a while ago on a frequency transducer and it was pretty easy, just took patience and a steady hand with the solder iron.

 

[derf]

Capacitors are the components that usually go out on electronic stuff. Resistors seem to last forever unless overheated. Transistors seem to last, too, unless overdriven or overheated. Capacitors, on the other hand, seem to fail the most, especially when they get some age on them.