Tag Archives: 436a

HP 436A Power Meter: smoke and stench – X-rated cap failure

By coincidence, another HP 436A power meter – this one, emitting smoke and terrible stench! The culprit was easily found, a defective X-rated cap. One of the known-bad epoxy covered capacitors that tend to blow after about 30 or 40 years of service.

436a x2 capacitor 100 n

The residue, oily stuff, terrible smell. Use plenty isopropanol or methylated spirits to clean – otherwise, the stench will stay with the instrument for years, and I can’t say that it is a healthy smell.

436a 100n oily

The cap is of the well-known PME271M series. Still available, but hopefully, with improved construction.

436a pme 271 m 610

436a pme271 series

A replacement is easily found – taken from an old switchmode power supply. Make sure to take a “X2” cap, not an ordinary cap. Only X2 caps are specified for mains voltage service, and self-exinguishing, anything else will present a major fire hazard, don’t compromise on the choice of capacitor!

436a 100n x2 replacement

Fix complete – new cap soldered in, and insulated with some electrical tape. In general, I tend to avoid electrical tape where possible, but in this case, it appears to be the only viable solution.

436a fix complete

HP 436A Power Meter: a strange analog ground issue

This power meter had been received with strange defect, a permanent overrange error, irrespective of any settings or input to the sensor. Sure enough, in most cases, this would be because of a dead sensor – but not here.
The 436A is a really simple instrument, at first glance, but with its design dating back over 40 years (mid-1970s), it has a remarkable complex design to achieve the A/D conversion, and to use something close to a CPU, at the time, called a state controller.

What was wrong with this unit? Something with the analog ground driver.

Checking the A2 and A3 assemblies, it turned out that the analog ground was floating, at about -6 V. Strange! And, simple enough, grounding the analog ground on either A2 or A3 solved the issue! For a temporary fix, a wire was added, from the board edge connector, to chassis ground. Need to look at the analog ground driver…

436a analog ground wire

Using chassis ground for general grounding – an indication of the dated design, and some of these board use 3 or 4 separate grounding path to keep noise down…

436a analog gnd schematic

After this fix, working again (still need to check out what it wrong with the analog ground driver).

Update: found the issue – lower right and corner of above diagram, this is the analog ground driver (also supplying analog ground to the A3 assembly (via mother board) – transistor Q1 found dead, a 1854-0003 (which is equivalent to 1854-0637, JEDEC 2n2219A, or any other ordinary 0.8 W NPN transistor).

436a 1854-0003 2n2219a

Soldered in a 2n2219A, and removed the temporary ground wire. Fix done.

436a analog ground circuit

After a full calibration and extended test, the instrument is rock stable, both for zero point, and 1 mW input. Also checked linearity, and it appears to be better than any means available here to check… most likely, better than 0.1 dB.

Output of the 50 MHz 1 mW cal source – cross checked with a calibrated HP power meter, 437B , and in agreement within 0.01 dB – good enough!

436a pwr meter working

Some other issue with this unit – a stuck analog indicator. After disassembling the front panel, used a razor blade to open up the plastic case of the indicator, and some mechanical adjustment of the inner workings fixed the issue.

436a analog indicator

436a front panel

The 7 segment decoders, these use heat transfer compound, for some pretty unusual way for HP design – being pushed vs. the front panel for cooling. To make sure these stay cool, I added some fresh white stuff.

436a dm9374 7seg decoder driver latch

The decoders are quite remarkable anyway, for their time – these are latching decoders with constant current output, high level integration for the early 70s….