Monday, November 9, 2015

Repairing the TUNE capacitor on the Heathkit HL2200 (SB-220) amplifier

Figure 1:
The front panel of the HL-2200 amplifier - which is really just a slightly
modernized version of the SB-220.
Click on the image for a larger version.
Earlier this year I picked up a Heathkit HL2200 amplifier (the newer, "brown" version of the SB-220) at a local swap meet for a reasonable price.  What made it particularly attractive was that it not only had a pair of new, graphic 3-500Z tubes in (Chinese-made, but RF-Parts Inc. tested/branded) but it also had a Peter Dahl "Hypersil" tm power transformer rather than the "just-adequate" original Heathkit transformer and an already-installed circuit that allowed the milliamp, low-voltage keying rather than the 100-ish volts of the original.

Obligatory Warning:
The amplifier/repair described on this page presents lethal voltages in its circuits during normal operation.  Be absolutely certain to take any and all precautions before working on this or any piece of equipment that contains dangerous voltages!  This amplifier was unplugged and the built-in high-voltage safety shorting bar operated by removing the top cover was verified to be doing its job.

Problems with the amplifier:

While it was servicable as-is, it did have a few known issues, namely a not-quite-working "10 meter" modification (the parts are all there, apparently having been pulled from an old SB-220 or from the previous owner having obtained a "10 meter" kit) but my interest at this time was the tendency of the "Tune" capacitor of the output network to arc over at maximum RF output power.

Figure 2: 
Some "blobs" on several of the rotor plates of the TUNE capacitor.
 Click on the image for a larger version.
If I operated the amplifier in the "CW" position with just 2.4 kV or so on the plates (at idle) everything was fine, but if I switched to the "SSB" position with 3.2 kV (at idle) then the capacitor arced over, causing signal distortion and high grid current - not to mention a loud hissing and the smell of ozone.  In popping the cover (with the power removed and the shorting bar doing its job!) I could see a few "blobs" on some of the capacitor plates which meant that when this had happened to the previous owner, it had probably been in sustained operation - obviously long enough to cause parts of some of the aluminum plates to be melted, further decreasing the distance between plates and increasing the likelihood of even more arcing!

Figure 3: 
In the center of the picture, a rather serious "blob" on one of the stator
plates.
Click on the image for a larger version.
After having had this amplifier for several months and operating it only at reduced power I finally got around to taking a closer look at what it would take to extract the TUNE capacitor and effect a repair.  Even though it is slightly cramped, it wasn't that difficult to do:  Remove the front-panel knob,  the left-hand tube, disconnect the blocking cap from the TUNE capacitor, remove the rear screw and nut holding it down and loosening the front screw and nut and pulling out the capacitor.



Disassembling the capacitor:

Fortunately, the capacitors used in these amplifiers are constructed from lots of small pieces rather than, like some "high-end" capacitors, press-fit into finely-machined rods and brazed.  What this meant was that simply by undoing a few bolts and screws the entire tuning capacitor can be reduced to a large pile of spacers and plates!

Figure 4:  A pile of parts from the disassembled rotor.
The still-intact stator is in the background.
Click on the image for a larger version.
The capacitor itself was disassembled in a shallow cookie sheet that I also use for assembling SMD-containing circuits:  It was fairly likely that any small part will be trapped in this pan rather than wander off elsewhere, such as onto my (messy!) workbench or, even worse, disappear into the carpeted floor!  Because this capacitor has several small parts and many spacers I felt it prudent to take this precaution - particularly with respect to the small ball bearings on the main shaft and the single bearing at the back end of the capacitor:  These smallest of parts were carefully sequestered in a small container while I was working on the capacitor.

Once the capacitor was "decompiled" all of the plates were very carefully examined for damage and it was found that there were two rotor plates and just one stator plate with large-ish blobs and some very minor damage to one or two other plates.  As is the nature of these things, it was the blob on the stator plate that was the most serious as it was the "weakest link" in terms of breakdown voltage and was always the smallest distance between two points no matter the setting of the capacitor (rotor) itself.

"Fixing" the damage:
Figure 5: 
The most badly-damaged capacitor plates, with an undamaged stator plate
(upper-left) for comparison.  The surfaces show evidence of oxidation
due to arcing.
Click on the image for a larger version.

If the damage is comparatively minor, as was the case here, then the "fix" is fairly simple:
  • Identify all plates that have any sort of "blob" or sharp edges.
  • Grind down any raised surface so that it is flush with the rest of the plate.
  • Using very fine sandpaper, eliminate any sharp edges or points.
If the plates are hopelessly melted you have the option of finding another capacitor on EvilBay, making our own plates, or simply cutting away the mangled portion and living with somewhat reduced maximum capacitance:  It is unlikely that the loss of even one entire plate would make the amplifier unusable on the lowest band, and it is also unlikely that more than two or three plates would have sustained significant damage, either, as this sort of damage tends to be somewhat self-limiting.

Placing a damaged plate on a piece of scrap wood, a rotary tool with a drum sanding bit was used to flatten out the "blob" on each of the three damaged plates.  Once this was done the plate was flat, but it was not particularly smooth, the rather coarse sandpaper having left marks on the plate, so I attacked the plates that had been "repaired" with 1200 grit wet-dry sandpaper and achieved a very nice luster where the grinding had taken place.  I also took special care to "ease over" the edges of the plates to remove any sharp edges - either from the original manufacturing process (stamping) or from the grinding that was done to remove the blob:  This is important as sharp edges are particularly prone to leading to ionization and subsequent arcing!

Because many of the plates showed some oxidation I decided that, while I had the capacitor apart, to polish every single plate - both rotor and stator - against 1200 grit "wet/dry" paper and, in the process, discovered several small "burrs" - either from minor arcing or from the plate having been stamped out of a sheet of metal.  I also took the trouble of "easing over" all edges of the capacitor plates in the process:  Again, sharp edges or points can be prone to arcing so it is important that this be considered!

Once I was done I piled the plates into an ultrasonic cleaner containing hot water and a few drops of dishwasher soap and cleaned them, removing the residual aluminum powder and oxide.  After 2 or 3 cycles in the cleaner the plates were removed and dried yielding pristine-looking plates - except, of course, for the three that had been slightly damaged.

Reassembly:

Figure 6: 
A rotor and stator plate having had the "blobs" ground off, but not yet
having been polished with 1200 grit sandpaper.  A bit of lost
plate material is evident on the left-hand side of the round rotor plate
as evidenced by its assymetry.
Click on the image for a larger version.
I first reassembled the stator, stacking the plates and spacers in their original locations and making sure that none of them got "hug up" on the rods with the last stator plate to be installed being the one that had been damaged.  The rotor was then reassembled, the job being fairly easy since its shaft is hexagonal, "keying" the orientation of the plates.  Because there had been two plates that had been damaged, I placed these on the ends so they were the first and last to be installed:  There is one more rotor plate than stator plate which means that when fully meshed, the two "end" (outside) plates are on the rotor.  Even though I was not particularly worried about it, by placing the "repaired" plates at the ends it would be possible to bend them and increase the distance slightly if they happened to be prone to arc without significantly affecting the overall device capacitance.

Having degreased the bearing mounts and the ball bearings themselves I used some fresh, PTFE-based grease to hold the bearings to the shaft while it was reinstalled, using more of the same grease to lubricate the rear bearing and contact, aligning it carefully with the back plate and finger-tightening the screws and nuts.  Once proper positioning was verified, the screws and nuts holding the end plates in place were fully tightened.

Both the rotor and stator plates are mounted on long, threaded rods with jam nuts on each end and by loosening one side and tightening of the other it is possible to shift the position of the rotor and/or stator plates.  Upon reassembly it was noted that, unmeshed, the rotor plates were not exactly in the centers of the stator plates overall so the nuts on the rotor were loosened and tightened to remedy this.  On fully meshing the plates it was then observed that the stator plates were very slightly diagonal to the rotor plates overall so the appropriate nuts were adjusted to shift the positions of those as well.  The end result was that the majority of the rotor plates were centered amongst the stator plates - the desired result as the capacitor's breakdown voltage is dictated by the least amount of spacing on just one plate.
Figure 7: 
The reassembled TUNE capacitor with a slightly foreshortened
and "repaired" rotor plate at the far end.
Click on the image for a larger version.

Inevitably there will be a few plates that are closer/farther and/or off center from the rest and that was the case here so a few minutes were taken to carefully bend rotor and/or stator plates, using a small blade screwdriver, as needed to center them throughout the rotation.  When I was done all plates were visually centered, likely accurate to within a fraction of a millimeter.

The capacitor was reinstalled quite easily with the aid of a very long screwdriver.  The only minor complication was that the solder joint for the high-frequency end of the tank coil - the portion that consists of silver-plated tubing - broke loose from the rest of the coil, but this was easily soldered by laying the amplifier on its left side so that any drips fell there and not into the amplifier.

"Arc" testing:

After reinstalling the top cover, verifying that it pushed the safety shorting bar out of the way, and installing the many screws that held it and the other covers in place I fired up the amplifier into a load and observed that at maximum plate voltage and with as much input and output power as I could muster, the TUNE capacitor did not arc!

One of these days I need to figure out why the 10 meter position on the band switch isn't making proper contact, but that will be another project!

[End]

This page stolen from "ka7oei.blogspot.com".

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