Heath/Schlumberger SP2717 power supply

[chris_leyson]

Heath/Schlumberger SP2717 power supply arrived the other day, thanks to forum member Kalin  Took the top cover off and was surprised to find more tubes than expected, it turns out that it's actually a Heathkit IP-17. It survived it's journey across the pond apart from bent front and back panels. First job is to replace the 40 year old electrolytic caps dated '78. 15mm spaced radial caps will fit for the 70uF and 40uF parts and all the 20uF parts were replaced with axial 22uF caps. Re-wire the transformer primaries for 240AC, more on that later. Check all of the resistors and they turn out to be within better than 5%. Remove all of the front panel furnature, no soldering required, bend front panel back into shape, re-fit all of the pots and switches and re-fit the front panel. So far so good.
Power it up, tweak the 0V and 400V reference pots and all is good. The load regulation, tested with 30W load, isn't as good as the spec says, it's 1V instead of 500mV and that could be down to tired 6BH6 which may have been replaced at some point because it's a GE branded tube and not EI-MANCO like the rest of the tubes. Regulation is good enough I'm not complaining.

My only complaint is that the heater voltage is 6.95V on load, it's 10% too high and that is on all of the heater windings !! grrr. The HV transformer is spot on however. Next job is to fit a small toroid 115-0-115 to 12-0-12 to step down the primary voltage on the heater transformer. 7VAC heater voltage is way out of spec and I'm running the transformers at 236VAC Grrr

[TERRA Operative]

Tube based bench PSU? Oooh pics please!

[chris_leyson]

Pictures attached, sorry for the delay. Bent the front panel back into shape, a quick clean with a little bit of detergent and it's as good as new. Fitted all the front panel furniture and then I noticed the B+ pot, damn it, the flat is on the wrong side of the shaft so I will have to get a pair of collet knobs to finish the front panel, no worries.

Wasn't happy with 7VAC heater voltage at the front panel terminals, 7.5VAC off load, so I've been doing a bit of tweaking.  My line voltage is 236VAC so I have to drop it by 15V or so to get close to 220VAC. It looks like the transformer primaries are 110VAC and not 115VAC because all of the primary voltages are somewhere between 5% and 7% high. Wired in a small 15VAC 7VA toroid onto the neutral to drop the volts and now I get 6.5VAC on the front panel terminals and 6.3VAC on the internal heaters. On the HV side the AC volts are a few percent down and it turns out that the little 15VAC toroid is actually running at 19VAC so my line volts is 216VAC to 217VAC, close enough. Will have to do a full load test and make a small mounting bracket for the toroid.

[chris_leyson]

I couldn't be bothered to make a mounting bracket to fit the small 7VA 15V toroid securely but there is enough room to drop the heater transformer down by 1/2 inch or so and you can mount the small toroid vertically. See picture.

To drill the panels you've got to take the power supply apart and quite a few screws were siezed so I had to be a little creative in getting them out. One of the transformer mounting screws was so badly siezed I had to drill it out and when it went round and round grind it off those slotted pan head screws really are cheap crap, grrr. To be fair they're built to a price and mounting screws are only supposed to go in once and once only. I don't have any stock of UNC screws so I have to save every screw.
Put it all back together with the odd chewed screw head, power it up and heater voltages are now where they should be and not 10% over, so not bad for a 40 year old power supply still using the original tubes. She'll be good for another 40 years.
I could have spent the same amount of money on a Chinese switched mode HV supply that would deliver more power, but would it stand up to transients and surges and can I fix it when it's broken, I don't think so and I want a bench supply that filters out common mode noise not one that generates it !!!