Tag Archives: 6115a

HP 6115A Precision Power Supply: repair complete!

No repair can proceed, without sufficient time, and without the right spare parts. Time was very much occupied by other business recently, spares took time to ship from Greece to Germany… the 2N6211 transistors.

Here they are – the 2N3442 are China-made (ISC) transistors of rather recent production, the 2N6211 date back to 1992.
Not much to write about the further repair, mounted the transistors on the heat sink, soldered-on the cables, replaced the Zener diodes of the main board (series regulator bias), and switched the 6115A on. Success! Some minor calibration of the panel meter. Other than that, all in good shape.

6115a-spare-transistors

Various things can be measured to verify the correct operation of the 6115A, here just a quick test checking all the ranges, linearity, and deviations. In short, it is very much more accurate than the 0.025% + 1 mV output accuracy specification.

6115a-deviations1

6115a-deviations-ten

6115a-deviations-ones

After a full cool-down, checked the stability/drift after a cold start, with the output programmed to 10 volts. It is ramping up nicely, with some very minor “instability” during the first hour, but then stabilizing to almost perfect level. This is with no load. Sure it may degrade a bit under full load, fair enough (horizontal axis shows measurement number, period is about 0.3 s per measurement).

6115a-drift

Finally, this is the working precision supply.

6115a-working

HP 6115A Precision Power Supply: not just one defect mixed with a lot of precision

The HP 6115A is a really great power supply, 0-50 V @0.8 Amp, 50-100 V @0.4 Amp, 0.0005% load and line regulation, 0.01% current regulation, 100 uV p-p ripple, 0.0015% drift over 8 hours, 0.025 + 1 mV accuaracy of output voltage, all in all, challenging design objectives still today. Unfortunately, the unit discussed here has not any of these characteristics, it is dead, and missing the current adjustment pot.

6115a-as-delivered

At least, it is a reasonable clean unit, and at a first glance, nothing major, like a completely melted board or smoking transformer. Judging from the soldering, someone already tried to fix it but gave up mid-way. Sure, I will not give up with this supply too soon.

First things first – the current pot, a 10 turn 1 k, HP part 2100-1864 (Bourns 3540S): missing. Looking around in my parts collection – but no luck. At the very bottom of a stack of old electronics junk, a WTW 610 pH meter from the 1970s. This is used to convert pH electrode signals, to proper voltages, but even more important, it has 2 pcs 1k Helipot 7276 series 1 k pots. These are 20 ppm tempco, even better than the Bourns (50 ppm).

6115a-ph-unit

6115a-helipot-1k

6115a-front-view

With the 1 kOhm pot fitted, still no function. About 60 V at the output, and the current limit LED lit, regardless of current or voltage setting.

Looking around inside – a few issues found. There are several uncommon parts, like, a STB523 = 1N4830 voltage stabilizer, which is more or less a stack of 3 Si diodes. These parts are not commonly available anymore, so I replaced the defect regulator diode with a series assembly of three 1N4148 diodes.

1n4830-stb523-1901-0460

6115a-3pn-diode

Found a few more issues, several blown Zener diodes, all around the Q1-Q4 transistors. This is not good, because it may indicate some blown power transistors. And if fact, it did not take long to find out that Q1 has a full B-E-C short, and Q4 is C-B short, E-C, E-B open. No wonder that this disturbed the bias network Zeners, VR3, VR4.

6115a-dead-transistor

The transistors, 2N3442 NPN (Q1-Q3), and 2N6211 (PNP, high voltage power TO-66), at least the latter, not quite common – on order from an xbay seller in Greece, 5 EUR a piece of old and obsolete part, OK!
The power transistor board was an awful mix of bad soldering and flux, finally, cleaned and most of the solder removed.

6115a-pwr-transistor-board-cleaned

Found another defect – no voltage on C8, one of the main capacitors. Reason: a blown trace, from tap 16 of the transformer. Temporary fix with a yellow wire….

6115a-trace

After some repairs, at least the basic voltages and supplies are up and working again, all capacitors tested, and working fine. Also reviewed the regulator board and its voltages (not shown in below diagram, red cross means dead part removed, green cross means part absent but tested good), all is fine and working.

6115a-reg-schematic-annot

Note the working principle of the series pass regulator – it is a dual range setup, with a high voltage regulator, Q1 and the low voltage regulator Q2/Q3, all driven by the common Q4. Diodes CR11 and CR12 (a dual-diode element) is directing the current from the appropriate transformer DC supply (2 equal DC voltages of about 80 Volts are generated from two separate windings).

6115a-two-range-regulator

Apart from a 1N829 0.0005% tempco temperature compensate diode, there is another remarkable part used in the circuit, a 10 ppm 10 turn trimmer – not quite cheap, and still available today!

6115a-trimmer-10-ppm

This is the current state of the instrument, waiting for the spare transistors, before I can put it back together, and hopefully, put it back in service.

6115a-board-top-view