Heath/Zenith High Voltage Power Supply SP-2717

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Heath/Zenith High Voltage Power Supply SP-2717

Postby Thermion » Sat Aug 13, 2011 6:10 am

Does anyone have a schematic and/or the manual for a Heath/Zenith regulated high voltage power supply Model SP-2717? Mine is acting up and I need to do a little troubleshooting.

I found a manual for the SP-2717A online, but the circuit was modified between the two models.

This family of power supplies are very useful for doing tube work. They are tube regulated using a pair of 6L6s as the pass tubes. They are rated at 0-400 v and 100 ma. There is also a 0-100 v negative supply available and 6.3 and 12.6 v filament supplies.

JT
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Postby dcgillespie » Sat Aug 13, 2011 2:04 pm

Hi Thermion --

I have the older Heath version of this unit (SP-17A), and it is indeed an excellent unit. I do not have the manual for the unit, but did draw out the schematic for it (somewhere!), so I'm very familiar with it.

My unit was not operating correctly with I got it either (years ago at a ham fest). The output voltage would run up much too high and blow out the output filter cap. In diagnosing the problem, I found an inherent problem with the build as offered by Heath in at least my version:

There is a 1 meg screen resistor for the 6AU6 error amplifier that is only specified and installed as a 1/2 watt device (again, at least in my version), yet it is dissipating over .5 watt in normal operation. When this resistor opens (as mine did), it prevents the tube from drawing any current to limit the voltage output. Simply replacing the resistor with a good 1 watt device repaired my unit some 20 years ago now, and it's still running.

I don't know if this is your problem, but it is a classic problem of this basic design due to the resistor specified being too small in wattage. Even if this isn't your problem, you will at least want to check this to ensure dependable long term operation.

Dave
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Postby Thermion » Sun Aug 14, 2011 6:31 am

Dave, Thanks for the reply. The screen resistor is not my problem. I am suspecting that one of the 0A2 gas regulator tubes is acting up. When a gas regulator tube fails, do you know if it fails as a short or open?

I was using my power supply to power a 1626 headphone amp prototype and was letting everything warm up on the bench before conducting power tests. So, everything looked good and I had a 1k sine wave going into the input at about 25% full power. When I came back to the bench, something smelled like very warm electronics. The heat was coming from the high voltage PT in the power supply. I turned it off and disconnected from the circuit. The next day I powered up the unit to see if it would produce its rated voltage. I only got about 125 v and it sounded like the PT had excessive current flowing through it again.

At that point I shut it down and removed and tested all of the tubes. They seem fine, but I have not been able to test the 0A2s. Actually, I use this power supply to do all of my tube testing now days so I'm really missing it. Now I am putting the tubes back in stages and checking voltages and such. The problem is my schematic is for an updated model that has diodes for the 6X4 rectifier tube and zener stacks for the 0A2s. Now I just have to find time to work my way through the troubleshooting.

JT
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Postby EWBrown » Sun Aug 14, 2011 2:03 pm

First of all, check all of your electrolytic caps in the PSU, they are probably about 45 years old and can be dried out, or otherwise in a failing condition. Look for any bulging or leaking of electrolyte, and carefully, after running the PSU for a while, and then letting the voltage drop bac to zero, very carefully feel if any of teh caps feels unusallywarm or even hot, which would indicate a bad one.

When a gas regulator tube goes bad, it usually has drifted well out of its specs, so the voltage across it will be too high or perhaps too low. They don't generally fail in a short or open mode, though if there is no glow from inside the tube, then the gas has leaked out and it is essentially "open".

For example, an 0A2 or VR150 should measure pretty close to 150VDC across it, but 140-160 VDC is still acceptable, outside of that range means it's gotten weak or otherwise going bad. Also check the series dropping resistor(s) feeding the 0A2(s) if it has drifted out of tolerance, then replace it with a new one of the correct resistance value.

I have two of the Heath Sp17s, one is 100% functional, and has been my tusty lab bench supply for years. The other one, which was $5 at a hamfest, doesn't achieve the full 400VDC, and the DC voltage slowly drifts downwards, after a few minutes' use. I suspect either a flakey 0A2 or more likely, a bad electrolytic cap.

HTH

/ed B
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Postby dcgillespie » Mon Aug 15, 2011 8:21 am

Thermion -- The only things that can draw significant current inside the unit are the two voltage doubler caps, and the output cap (unless of course, a rectifier is shorted). As Ed suggests, check them all first.

I would again emphasize to check the 1 meg screen resistor to make sure it is in spec. If it has failed, the output voltage will rise and short out the output filter cap. The 6L6s will then turn on hard (full cherry red plates), which will also damage the 1000 ohm screen filter resistor. This is exactly the series of events that happened in my unit, producing the exact same symptom as yours: a laboring power transformer with excessive current flow. If it is not this, then it is likely the voltage doubler caps or rectifiers -- which is better than the output cap going and hurting the tubes!

Regarding the OA2s, you can use the bias output terminals to check them for accuracy, as one of the two tubes (in part) provides a stable source of voltage for this feature. Granted, only -100 volts is available at the terminals, as 50 of the volts are lost by the current required to operate the front panel meter through a common dropping resistor that supplies the output terminals and the meter. But basically, try both tubes in each position and see if the maximum bias voltage available is correct. If it is with both tubes in either position, then the tubes should both be ok. If not, the OA2 that is NOT bypassed by a 33K 1Watt resistor is the tube that shunt regulates the bias source. It's a bit of a strange circuit, in that both tubes are required for the HV circuit to operate (they are connected in series), but since both tubes operate below (but ultimately reference) ground, the tube whose plate is grounded is also used to provide the regulated negative source for the bias terminals.

Hope this helps find the problem!

Dave
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Got the Schematic

Postby Thermion » Sun Sep 18, 2011 7:10 am

Dave and Ed,

Thanks for the help and advice in trouble shooting my Heathkit power supply. When I bought the unit a few years ago, the first thing I did was replace all of the electrolytic caps and the the two control pots. I also retubed it with NOS tubes I had on the shelf. The 1 meg resistors still seem to be good but I will go ahead and replace them with 1 watters.

I put the troubleshooting on hold while I tried to find the Heathkit schematic and/or manual which is getting tough to do. Shannon and I went to a hamfest in Peoria yesterday and I picked up a copy of the assembly manual complete with all of the pictorials in good shape. The good news is that it came with a working IP-17 also in very good shape. The guy sold it for $40, so more then Ed's $5 find but I felt lucky all the same. Then later I bought another one in even better shape (sans manual) for $55. So a very good day at the hamfest.

After looking at the schematic I suspect the voltage doubling caps may be my problem. I'll let you know when I find out. In the meantime, I now have a "pair and a spare" on these extremely useful benchtop tools. These things have been selling on Ebay for >$200 lately.

JT
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