Category Archives: Test Equipment Repairs

For some, it is like solving crossword puzzles: fixing defective test equipment. Preferably, mid-70 to early 90s vintage.

Dual CV 1600 Stereo Amplifier: a real HiFi classic with some capacitor smoke, and a very large dropout voltage regulator

This one is a real gem, a stereo amplifier, Made in Germany, and nicely constructed in the 1970s, using mostly discrete parts.

cv1600 front

Some issues are common to all old amplifier, like, defective switches, aged contacts, and so on, but these can be fixed with good contact spray, or by (mechanical) repairs. For the CV 1600, most common fault is the X2 mains capacitor, 47 nF, which will eventually turn into smoke and stench. Be sure to replace this part, if you have a CV 1600.

The 47 nF X2 rated cap is located on a fuse board, and to get access, you need to remove all the transistor wires. This is best done with great care, and without damaging the wires, otherwise, it is quite laborious to connect everything back up again.

sdr
sdr

This CV 1600 also had another issue, no signal from the distribution amplifier board. Some first check showed issues with the +15 V rail being stuck at ~7V. Probably a defective Tantalum or other capacitor? Probably not – not much current flowing either, so it is not related to a short at the output.

dav
dav

Inspecting the regulator – it is a TO-202 LM341p15, pretty rare nowadays.

dav dav[/caption]

No heatsink on the regulator, so I got a bit worried with the dissipation of TO-202 vs. TO-220 devices – all I have in stock are TO-220 regulators, LM78xx series.

cv1600 to202 vs to220

lm341p15 to202

lm7815 to220 heat resistance

After inspection of the datasheets – nothing to worry about. Pretty similar heat resistance, junction to ambient. Voltage drop is about 10 V, current roughly 50 mA, so, 0.5 W – roughly 30 K temperature rise.

A quick test with a 5.6 Ohms, 250 W dummy load showed no further issues (except an incorrectly installed signal cable, probably from an earlier attempt of someone else to fix this unit?).

Finally, some performance data of the CV 1600.

cv1600 data

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

HP 83572A RF Plug-in 26.5-40 GHz: only a fuse away from the highest frequencies

Not one of the most preferred things to repair – a rather rare 26.5-40 GHz sweeper plug-in, not producing any output. Despite its rather simple function as a signal source, typically, not easy to fix if any of the microwave parts are faulty. New, close to 18 kUSD, so it is nothing you can easily replace from a hobby budget, and 26.5-40 GHz sources are not easy to come by.

40ghz-1

40ghz2

Inside, all full of heatsinks, and a few waveguides.

40ghz-waveguid

The modulator.

40ghz-modulator-45211h-2900h

The YTO. Not many companies around that can manufacture such devices.

40ghz-0960-0670-wj-5610-25

First thing to test, with no output present – the YIG oscillator. This has two main items: the bias supply, which is more or less just a variable voltage power supply which is tuned along with the frequency sweep. Secondly, the main tuning coil current, providing the magnetic field for the YTO. Checked both – and found the bias supply at 0 Volts. No wonder there is no output.

40ghz-a7-bias-assy-schematic

Upon inspection of the schematic, I noticed the fuse, which is rather hidden down on the motherboard. And, it was blown. No idea why – maybe just because of its age? Sure enough, HP did not use just any ordinary fuse, but a BUSS GMW model.

40ghz-buss-gmw

USD 9 per piece – that’s a steep price for a fuse.

40ghz-fuse

Cut the fuse open, and connected a 5×20 mm European style fuse. All protected by a piece of shrink tubing.

40ghz-fuse-replacement

Well, an about 1 hour later, the YTO is oscillating again, and you can see a nice and strong signal, well over 90 dB useful range, to test attenuators, or whatever 26.5-40 GHz device you want to put to test.

40ghz-transmitting

40ghz-receiving-again

HP 8566B Spectrum Analyzer: A19 board, YTO unlock, bad precision trimmer(s)

Not the first HP 8566B on the bench, and not the first at all showing the famous “YTO unlock” message. Most of these YTO unlock message issues can be traced to defective capacitors, but not this time.

With the 8566B, take my advise, don’t touch any of the assemblies if you aren’t really sure which one is at fault, it is a fairly complex machine. To troubleshoot, a microwave counter is handy, to check the LO frequency.

8566b-repair

Next, the PLL was disengaged by disconnecting the cable from the sampler/LO pll. Still, no good LO frequency output. This leaves two main assemblies to be checked, the LO pretune DAC, and the YTO driver assembly, A19 and A20, respectively.

Quickly traced the issue to the A19 DAC assembly, and luckily enough, had a spare one around, albeit, an older version. After swapping the boards, it was confirmed that the A19 assembly is really the faulty part.

8566b-085660-60164-a19-brd

8566b-085660-60212-a19-brd-old-version

Next – desoldered all the capacitors at one end, but, to my surprise, all useless work, all caps in best working order, even after 25+ years!
Checked various components, and finally, found some issue with the precision trimmers – seems a cold or aged solder joint. To be sure the the fix is as permanent as possible, all the trimmers were removed, the solder connections cleaned, and all installed back in. Easy fix, all working again.

With this unit, there was no intention to do a full calibration, but as an extra service, I checked the power at the reference signal outlet – see below. Quite amazing how accurate, and pretty sure that this unit hadn’t been at a cal lab for at least 15 years….. this is really a superb level of lasting precision and quality, and ingenious engineering.

8566b-pwr-ref-test

EIP 545A Microwave Frequency Counter: another dead tantalum repair

It’s not the first fix of an EIP counter here, and I have to say, these units are still very useful tools around any RF and microwave workshop, despite their age. The unit under repair, judging from the date code of the parts, is about 25 years old.

The symptoms – the EIP is just not counting. Showing 000000 zeros in the display, but no count. To confirm, you can use the build-in test function 01, by pushing test-0-1, and normally the counter will show “200 000 000”. But no indication of any counting activity for the current unit.

First thing you do, checked all the rails, and turns out the 12 V rail is dead. Starting from the right, removed all boards inside. And soon the defect was traced to the A107 board, gate generator assy.

Checking with the schematic, there are several ceramic and one tantalum cap.

eip-cap-desoldered

Desoldered the tantalum, and it is no cap any more, just a short.
With all the various tantalum shorts repaired in the past years, there seem to be to cases, some tantalum go fully short and activate the power supply protection circuit, with little powder dissipated over the tantalum. This is the good case. Some other tantalum seem to develop a short with some resistance, leading to considerable power dissipated in the cap, causing stench and risk of fire. The latter more seems to be more common with SMD tantalums, for reasons unknown. Also, the tantalum story is one of the many reasons why you should design current limit circuits, and power supply protection circuits, even in low power equipment, especially, if there are any valuable components that you might want to protect from a power supply failure.

eip-cap-a107-gate-gen-assy

eip-cap-replaced

The 10 uF replaced, with a new orange cap, but same “drop” style.

eip-counting-again

After carefully installing the A107 assy back into the counter (take care not the damage any of the wires), all is good. Counting at 200 MHz with the test function 01 activated.

A Thermal Fuse, and HP 10811-60111 Repair

Usually, I don’t care much about high precision oscillator options being fitted to frequency counters, etc., because in the lab, any critical equipment is anyway connected to an external well-controlled 10 MHz reference, locked to DCF77. However, this time I need to install a OCXO (HP 10811) in a HP 5335A counter for service outside of the lab.

The only thing that needs to be done is to remove a jumper on the board of the 5335A (see red box in picture below), and mount the 10811 in the slot already prepared for the OCXO inside.

ocxo 5335a jumper

While such installation is fairly straightforward, it turned out to take more time than expected – simply because of the OCXO not showing any stable output signal.

ocxo 10811-60111

After a few quick tests, the cuprit was found, a defective (open) thermal fuse. This is apparently a quite common issue for the 10811 oscillators, and you might get away with just putting in a wire jumper. However, I didn’t want to take any risk of overheating in case of a failure of the 25+ years old OCXO circuits. An exact match for the thermal fuse could not be found, so just soldered in (very carefully, cooling the case and leads!) a 10 Amp 109 degC fuse.

ocxo fuse picture

This is the OCXO with the new fuse installed.

ocxo new thermal fuse

This style of fuse as a non-insulated outer shell, so a shrink tubing sleve serves as insulation.

ocxo insul sleve

Finally, a note found in a datasheet of a common thermal fuse – it clearly states that lifetime will be limited when operating the fuse to close to the cut-off temperature. So clearly, thermal fuses are not the best protective mechanism for the OCXO case. Maybe better would be a bimetallic switch (self-resetting, but at least no subject to any significant aging), or some other device like a PTC.

Sure, we can slightly blame the HP engineers, because it is stated on most thermal fuse datasheets, like the one below, that the operation temp limit should be about 30 degC less than the cut-off, which is not quite the case for the 10811 OCXO. 80 to 84 deg C operation, 109 degC fuse cut-off.

ocxo storage temp

ocxo thermal fuse

EIP 545A Microwave Counter: another power meter upgrade

This is just a brief update on an earlier post, related to the power meter option for EIP counters, EIP Power Meter Upgrade. The current 545A came without any options, even without GPIB, and I was not quite sure if the firmware power meter option firmware would work, even if no GPIB option is present. Well, easy enough, put it to test.

545a upgrade eproms

It didn’t take much, a few (rather old 2516, and 2532) EPROMs, a resistor, some spare ICs, and a few inches of solder…

545a board

Here, the result: all working just fine, counting away at several GHz, and power reading appears to be resonably accurate. I would not substitute it for a proper power meter, for calibration purposes, but the power meter option is a handy feature for any quick test and confirmation.

545a upgrade test

HP 3325B A26 Assembly: low byte SRAM, and an incorrect signature

A quick comment on the 3325B service manual – Thanks to a kind contributor.

3325b kernel sa test u27 pin10

The kernel SA test is a bit flaky on A26U27 PIN 10. This is an inverter output, and can’t be same as the input… correct signature is A70F. Appears to be a misprint in the HP service manual!

3325b a26u27 74as04

In the present case – the culprit was one of the SRAMs, the lower byte. Fortunately, not really a rare part, and easily sourced and replaced, once you know that this is the circuit at fault. You might want to check this very carefully, because there are multiple versions of the 3325A/3325B control boards.

3325b srams

HP 8662A 8663A Power Supply A7A3 Assy: base transformer defect

A little note, Thanks to a kind fellow sharing this repair info with me, related to the A7A3 assy of the 8662/8663A generators: the 9100-4018 base drive transformer. There are two of these on the 08862-60289 board, protected by a fuse and diodes, nothing should acutally happen to them, but things can go wrong.

Below, the two versions of the A7A3 boards, left, the older 08662-60289, and the more recent 08662-60604. This post only refers to the -60289, the base drive transformers of the newer units look the same, but have part number 9100-5291 – don’t know if the can be exchanged.

8662 8663 pwr supply

A snipet of the power supply circuit, there are two identical base drive transformers, center tapped on one side.

08662-60289 base drive schematic

Cross reference – this is a NSN part, the most prominent manufacturer seems to have been Fil-Mag, which use to be a Sprague company long ago.

9100-4018 master cross ref

5950-01-267-1279 aka 9100-4018 transformer

Note that these little beasts come at a quite hefty price! That’s well over USD 100, just for the two base drive transformers – I hope, HP did not pay the list price, or anywhere close to it.

Here is a rare view of the interals, after heavy work with sandpaper and other means (these transformers are potted, but as with all potting compounds it can be removed, if you have plenty of time, a good supply of tools and don’t mind the dust and dirt).

8662 8663 pwr fil-mag 42z994 internals

It uses an OJ41408 bobin/pot core, PC14/8 size. Wire is about 0.1 mm size. The pot cores are still available from Magnetics Corporation, mag-inc.com, and the material is just a regular mid-frequency ferrite. So you might be lucky with just using any average good pot core with about 5000 permittivity (e.g., N30 ferrite).

8662 8663 pwr oj41408

mag-inc 41408ug pot core

mag-inc j material

Fingers crossed that you will never need this information, because it is quite a laborious effort to reverse-engineer the internals, and to fabricate a new transformer manually. Tempted to say, I could manufactur them well below list price, if someone would need a 1000 pieces….