This is one of the gear I acquired long ago. This is not much fun or expertise needed to restore this, as it simple regulated power supply with Tubes. Thought will share the procedure just in case someone is looking for some guidelines.
Read this CAREFULLY before you proceed.
This is a very dangerous instrument in terms of the voltage levels. It can deliver lethal DC and AC Voltages at substantial current levels to RIP you in one shot. AC voltage at “Secondary” is as high as 300V. DC levels can reach over 450V. There are better ways to die if you are up to that.
DO NOT ATTEMPT TO REPAIR/RESTORE ONE OF THESE UNLESS YOU ARE QUALIFIED AND KNOWS WHAT YOU ARE DOING AND HAVE THE RIGHT TOOLS, PROBES AND FOLLOWS ALL SAFETY PRECAUTIONS.
EVEN IF YOU ARE QUALIFIED, PLEASE DO TAKE GOOD CARE AND ATTENTION WHILE WORKING ON THIS.
This was the condition when I picked it up,
Any vintage instrument like this, first rule is to do a detailed inspection. Never even think of powering it up. Not that it damages itself, which will be irrecoverable, as parts may not be available in some cases.
First Glance at Panel. I notice rust on the screws, output sockets have oxidation.
Lets look at inside, like I mentioned before, there is nothing exciting here to troubleshoot.
Typical vintage gear with leaky/Bulged caps, rusted screws. Notice the PCB is kinda burned because the person who assembled this put the zener way too close to be board. They do get hot and was burning the board. Dust all over inside as expected.
Now, first step is to remove the caps and inspect in detail. Also if you observe there are not protection around terminals. I like to heat-shrink those terminals when I restore things.
Time to clean the chassis, connectors, check for cracked wiring, rust, and I did re-solder few points.
These are not factory made products. Its assembled by someone. Highly advised to check the wiring, with schematic, as there could be mistakes in the whole assembly/wiring.
Time to check the tubes out. There are only three, 6AU6 Pentode and 6L6GC beam Pentode as the main series regulator. For me the 6AU6 was bad, shorted. Decided to change as I had it in stock. Luckily main Pentodes were testing good in the Tube tester.
I didn’t have the caps in stock, so ordered caps. Decided to order the best in the class as I can not take risk at 450V or wanna risk my regulator tubes being burned.
While waiting for caps to arrive, decided to fix rest of the issues. I personally don’t like rusted screws, one POT was rusted too. I cant replace the POT so repainted it, but replaced all screws. Removed the Volt meter selector switch and cleaned it too.
Time for Electrical troubleshooting. As expected, many of the zener diodes were shorted. I assume this be due to overheating.This is a 27V Zener. The design use them in series to get the high voltage reference they need.
Trip to HSC (Halted Supplies ) or Excess solutions got me those diodes ( I am in no way affiliated with neither of these).
Caps arrived too, and time to put things together.
[ You will see some box caps, they are the precision 1% tolerance ones ordered for EICO 950B, check the post on that in case you wonder what that is]
CLEAN THE FLUX AFTER SOLDERING WITH ALCOHOL. VOLTAGE LEVELS ARE SO HIGH THEY CAN ARC THROUGH FLUX AND CATCH FIRE.
Here is the rest of the process in pictures. I replaced the front panel indicator 6.3v Lamps with LEDs. Remember Raise the Zener off the board, as much as you can to provide cooling. They do get hot.
Now, time to power up. Disconnected the HV Transformer and keep only the Filament transformer in circuit and use variac to power up and ensure tubes are glowin.
And the Filament voltage on the front panel is delivering 6.3 and 12.6 (output will be slightly over this as there is no load). The VTVM in the rear is reading the 12.6V output from front panel.
Now time to connect main transformer and monitor main output. Do ramp power slowly via Variac, check regulation, output levels.
If you checked all components and fixed the faulty ones, it should be working like above. You can see the -output and +ve output. The Fluke output shows the +400V output.
The last picture is with a load on the +Ve rail to show the ammeter in action.
CAUTION : -ve rail is for Grid. Its regulated by the resistance of -ve voltage control Potentiometer in front panel. There is no active component involved to deliver high current. Do not connect anything heavy to it. It will burn the Potentiometer.
And here is the left over from the whole restore project.
Here is a quick summary of the unit and schematic. Click on pic to enlarge.
Transformer: Two main transformers, one for heater and other for output voltages, that is – B+ voltage, screen bias of main series pass 6L6 tubes and negative bias voltage
Rectifiers : Half wave rectifiers developing screen bias for the 6L6. Same way a doubler for B+ out and simple half wave again for -ve bias output .
Main Regulator : B+ voltage is regulated by the two 6L6s, who is controlled by the 6AU6, which is used as error amplifier.
Voltage Reference : R27, R26 and the zener stack is the reference source for the whole circuit, basic form of zener regulator.
Regulator Control: The cathode of the 6AU6 is referred to the Zener reference. 6AU6 grid samples output voltage and control the grid voltage to 6L6 main series regulator ro regulate the output voltage.
Bias output Control : The negative Bias output is directly developed off from the zener stack sitting on the bias supply half wave rectifier.
Remember, the output of -ve Bias supply is controlled by a tiny pot R19. Do not load the -ve supply, as it may burn this one out.