Keeping 45-yr old HP Power Supply working

[nevetslab]

Some time back, having substituted my HP 6227B Dual Tracking 0-25VDC power supply in place of a higher-voltage source in one of the repairs, it was in Dual Tracking mode, and apparently the wire that straps the Master to Slave supplies on the front panel came loose without my knowing it. It shut down on me, and after disconnecting it from the amp on the bench, I discovered I no longer had the Master supply working. Set it for Independent mode, the Slave side worked fine, but the Master side, just cracking the Course Voltage knob made the meter first go full scale, then clamped down to nearly 0V.

I finally got some time, opened it up on the bench to dig into it. Finally found the first pair of Series Regulator xstrs had shorted, so removed those, replacing with a pair of 2N3773 TO-3 Power Xstrs, parts I had on hand, more than suitable for the task. When I went to power it back up, after reassembling it far enough to check, I got the same results. After reading thru HP's trouble-shooting guide, I came upon one caution....DO NOT TURN VOLTAGE CONTROLS FULLY CCW....THAT COULD DAMAGE THE CONTROLS. SET BOTH FULLY CW DURING TROUBLESHOOTING.

I got thru the steps in the trouble-shooting procedure, which, after disconnecting the crowbar circuit, switching on in that full CW control settings, the meter went full scale. After getting to the 5th step, it suggested R27A/R27B may be open circuit. I searched thruout the main PCB, and other boards, then finally found them on the schematic. One was the 5k 2W WW Course pot, the other the 50 ohm 2W WW Fine pot (dual concentric panel controls). The 5k pot was open. Damn! Little chance of finding that direct replacement control these days. In looking thru the pot, while pondering my options, I found one of their product ordering options was the use of 5k 10-turn Pot in place of the dual-concentric pot. Checked the size and depth, finding Bourns and others make 5k/2W WW 10-turn pots that will fit into the space.

Found a few sources, price ranging from $13-$17 plus tax/shpg. Then, just for the sake of looking, checked ebay, finding a seller in Torrance, CA (local to me) having Bourns 5k/2W WW 10-turn pots (5 pcs) for $7.95, free shpg + tax. Sounds like a solution, so place an order for those.

[patlaw]

Let us know how it goes. The old stuff usually works so well that it's hard to let go.

[nevetslab]

This thread needs to get moved to the sub-forum Tools and Test Gear under Music Electronics, but....I can't figure out how to move it (and other related posts I've made in the Test Equipment maintenance subject).

Meanwhile, the 5-pack of brand new Bourns 3590 5k 2W 10-turn wire-wound pots arrived this morning. So, having already substituted my 10k helipot in parallel with the OPEN circuit existing Voltage Control pot to verify the repair was successful and the supply works, delivers rated current/voltage, today's task was to figure out how to open the front panel assembly in order to extract the dual-gang 5k/100 ohm 2 watt wire wound pot assembly, and then install the new 10-turn pot, strapped for 0-5.00k in its' place. No exploded views in the HP manual, so removed the upper and lower screws that hold the front of the left/right mechanical frames together. That allowed the meter assembly, mounted to an aluminum extrusion to be moved up and out of the way. Then, removing the flat-head screws holding the frame to the removable power supply assembly in the middle, it gave me enough clearance on the front to move the crowbar PCB assembly out of the way. I had to first unthread the machined threaded sleeves from the front panel that threaded into a large 1/2" hex standoff, threaded 3/8-32 at the back end to mate with the crowbar adjustment pot...the sleeves on the front panel takes a 1/4" hex wrench to unthread those. Then, that PCB assembly could be extracted and folded out of the way. Once the mounting hardware for the broken Voltage Pot assembly was removed, I was able to fish that pot out of the cavity.

I strapped the wiper to the bottom of the new pot, so at CCW, the pot is at 0 ohms and at fully CW, it's 5.000k Then, one wire at a time, I unsoldered each wire and moved them to the new control. Then, using the large ITL washer on the pot bushing, mounted that into the front panel, and added the flat washer and hex nut, tightened it to the panel. From there, it was reassembling everything in reverse order. I had a used HP knob that matched the style of the other knobs, but...only had the one, so I didn't swap out the other Voltage control assembly. While I had it apart, I also seeped contact cleaner into the open frame Volts/Amps slide switches, and the Independent/Tracking switch so they moved freely...along with spraying Teflon/Silicon spray onto the actuator from the front. Once I had the supply reassembled, I powered it up, finding it working just fine. I trimmed up the full scale meter calibration pot. Did the same on the Slave channel. I didn't take photos while I had it apart, which I should have done. When I find another HP knob to match, I'll change the Slave side and do that then.

[Tom Phillips]

Hi,
I have some of the HP knobs you mentioned. I sent you a PM with the details yesterday.
I rarely use the PM message service so I'm just checking in case you did not receive the message.
Cheers,
Tom