I got a used SP2717A off an auction on ebay for a reasonable price. The seller was not super knowledgeable about the product, so I knew I had my work cut out for me. After completely disassembling it and replacing the front panel pots, switches, and power cord, the power supply only outputs 150V. The adjustment pots on the main circuit board do nothing when adjusted, but checking them with my multimeter, I see a change in resistance. Could the adjustment tube be shot but still outputting a 150V on the B+ supply? The C- supply works perfectly...
Here is a link to a schematic and manual.
HEATHKIT SP2717A 0..400V,0.1A,0..-150V VALVE STABILISED POWER SUPPLY 1982 SM Service Manual free download, schematics, eeprom, repair info for electronics
HEATHKIT SP2717A 0..400V,0.1A,0..-150V VALVE STABILISED POWER SUPPLY 1982 SM Service Manual free download, schematics, eeprom, repair info for electronics
I gave it a look, and I tried the calibration steps, but tweaking the adjustment pots gave no indication that it was doing anything. The pots test ok with my multimeter, I'm worried if there is something wrong with the adjustment tube, and if there's any way to check that...
All measurements were taken after waiting 30 seconds from a cold start. Test lead clips were used to clip measurement points to a Radio Shack multimeter.
On the first tube, on the left hand side looking at the schematic, pin three goes off the scale of my multimeter, which maxes out at 600V, when I switch it off it quickly falls to around 590V before dropping to zero, so I assume its around 600V.
Pin 4 hovers around 243V when the B+ knob is full counterclockwise (minimum voltage), and it goes up to 460V when the knob is all the way on the right. If I leave it alone, the voltage drops to about 420V after a minute.
On the right hand side tube, pin 3 and 4 show identical behavior.
I checked against the manual, it's set for 120V.
I tried fiddling with the adjustment pots, but I don't see any changes in the max output, despite the pots being good (I tested by turning the power supply off and clipping leads to the wiper and one leg and turning the pot back and forth)
Adjustable between 0-150 using the front face knobs.
All three tube heaters are glowing.
Thanks in advance for helping me out with this; I'd love to get this up and running to play with the small collection of tubes I have growing
On the first tube, on the left hand side looking at the schematic, pin three goes off the scale of my multimeter, which maxes out at 600V, when I switch it off it quickly falls to around 590V before dropping to zero, so I assume its around 600V.
Pin 4 hovers around 243V when the B+ knob is full counterclockwise (minimum voltage), and it goes up to 460V when the knob is all the way on the right. If I leave it alone, the voltage drops to about 420V after a minute.
On the right hand side tube, pin 3 and 4 show identical behavior.
I checked against the manual, it's set for 120V.
I tried fiddling with the adjustment pots, but I don't see any changes in the max output, despite the pots being good (I tested by turning the power supply off and clipping leads to the wiper and one leg and turning the pot back and forth)
Adjustable between 0-150 using the front face knobs.
All three tube heaters are glowing.
Thanks in advance for helping me out with this; I'd love to get this up and running to play with the small collection of tubes I have growing
If the minus -300volts (i.e. pin3 = lower leg of the 400v adjust pot) is off spec - say just -225v or so, this would limit the output voltage to around 150volts.
It would also explain why the adj. pots have no effect ...
Check the -300v.
If it is indeed low, check zeners ZD6 to ZD10 ...
It would also explain why the adj. pots have no effect ...
Check the -300v.
If it is indeed low, check zeners ZD6 to ZD10 ...
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R15 measures 25.80k Ohms
R16 measures 25.28k Ohms
R17 measures 24.82k Ohms
R18 measures 24.09k Ohms
R11 measures 198.0k Ohms
R13 measures 351.8k Ohms
R9 measures 329.3k Ohms
The expected -300V lines measures -292.2V (measured from cathode of Z10 to anode of Z1).
Because it was low, I measured Z6-Z10, which gave me a result of -143.7V
Would replacing the resistors and zeners with newer, identically spec'd components be the next step?
going with better spec and higher rated will not hurt just a space thing The zeners are not that far off. The resistors do replace as well as the diodes d1 thru 6 with soft recovery diodes{ her108 or uf1007} the originals are 750 ma 600 volt diodes. check r6, r7,r8,r28, replace if off. while at it check the 3 pots zero, 400volt and b+ . also r1 in the top bridge. so far this is a low cost repair.
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Hey guys, just wanted to share the good news: I finally replaced the resistors and the tube (I just had another 6AU6 tube lying around and thought of giving it a shot) and that did the trick! I now have a fully functional tube power supply for the cost of a few 3W resistors. Thanks for all your help!
R15, R16, R17 and R18, just clipped the leads off and desoldered the stumps, and soldered in new 1% 27k 3W resistors from digikey.
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