Category Archives: Test Equipment Repairs

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

Micro-Tel SG-811 Signal Generator: a second unit

By luck and coincidence, I found another Micro-tel SG-811 generator on eBay, at a very reasonable price – sold as not working. Even non-working, these units are great because of the many microwave components contained: YIGs, filters, GHz-capable relais, SMA cables… and a lot of old-fashioned analog circuits.

First check – the fuse! Someone recklessly put a 10 Amp fuse in, because the smaller fuses would blow. That’s never a good idea. Most probably we will have to deal with a power supply repair.

After detail assessment – the 24 V tantalum cap is shorted, maybe this triggered a sequence of faults: the main primary transistors (MJ12002), the rectifier, and two thermistors that limit the inrush current.

Micro-tel didnt safe on screws when they designed the power supply!!

These power thermistors are hard to get – I just desoldered two similar ones from old switchmode power supplies.

A dead rectifier – easily fixed.

All the parts labeled – also replaced the 2N2222 driver transistors, and two tantalum caps that were leaking current.

The most precious parts – the RF section.

A most complicated arrangement of oscillators, switches, couplers, and so on

Some of the oscillators originally used in these units required a variable supply voltage to get stable power output, but strangely enough, the YIG oscillators fitted have built-in voltage regulators, and the supply voltage has no effect at all on their output. Still, the power supply board caused issues – end even overheated, because the voltage is set by very sensitive trimmers, and drifted above 18 Volt…

The YTO has a voltage protection diode – it was completely fried when I received the unit. Checked some good Advantek YTOs, these have 18 V 1.3 W Zeners for voltage protection.

With power back on, and the voltage at the YTOs OK, still no good output – how can it be? Some issues with the oscillator driver board that sets the current of the main coil, and without a proper magnetic field, there won’t be any oscillation.

The precision resistors, seems they were hand soldered with some bad solder (traces of corrosion, and high melting point).

First, some trouble to find the dead part – thought it is one of the opamps, LM308, replaced it with a OP02. But no luck.

So I changed it back to the old LM308, just to keep all in original state.

The bad guy… a 4051 multiplexer CMOS, these are notorious!

Another interesting assembly, the reference assy – the 1N 827 reference diodes where still very accurately set, only a few ppm of the 11.000 V, and -11.000 V!

After these repairs, and some adjustment, all is back to working condition!

Checking out the signal on a 8566B analyzer. All good!

The pulse generator, also a great feature of this unit… 1 ms pulse.

down to 1 microsecond, no problem.

… 10 microsecond pulse…

The attenuator, a really high quality HP device.

The manual has some remarkable comments – use a 2 kbyte memory, just in case a “really big program” would be needed in the future.

Still, I will do some alignment of the oscillators and filters… but that’s no big deal.

HP 3580A Spectrum Analyzer: Digital display fix, and ancient CMOS circuits

With most of the 3580A functions working again, we still need to fix the digital display. Essentially, the 3580A uses a digital scope circuit, similar to those use in digital oscilloscopes of the 70s.

First, some study of the ADC. The 1973 HP Journal has all the details, it is successive approximation, peak detecting ADC.

Checking the input to the digital display board, blue trace, and the comparator/approximator input to the ADC, yellow. Seems something is wrong with the ADC ciruit, or it’s timing-counter control systems.

After considerable checking and probing, I found the issue, a dead 4019 CMOS, 4×2 multiplexer. Replaced it with a “new” part, taking great care to avoid any static discharge to the board.

The dead part, it is almost a historic piece! 1974, only a few year after the introduction of CMOS circuits by RCA!

That’s the full board. Multiplayer construction. Plenty of precision resistors that are needed for the ADC circuit.

Another working antique part – the 2102 S-RAM, Intel, 1 kbit per circuit. 8 pieces – a total of 1 kbyte of SRAM!

Working display…

10 kHz reference display… Great!

Even the log scale scan is working.

One tip – put all the screws and parts in a box, and check that it is empty afterwards. So many instrumented I receive here in the workshop are missing some screws or other parts.

HP 3580A Spectrum Analyzer: a few mechanical repairs, and sweep test

With the basic functions of the 3580A restored (at least, it is sweeping again), some attention needs to be paid to the mechanics. Fortunately, all is good with the intricate tuning assembly and digital display, but the knobs have some cracks, probably, a combination of age and stress.

Everything taped up, and the cracks filled with rapid-set epoxy resin.

To apply the resin to the small cracks, you can use a piece of stiff plastic foil, cut to a tool of appropriate size –

Here, a few close-ups of the tuning mechanism. It has fast tuning, and fine tuning, a clutch, several gears – all good old analog technology.

Finally, some test of the sweep circuit – but how to test a 200 second per division (i.e. 2000 second per screen) deflection for accuracy and linearity? Well, I connected it to a 34401a multimeter, and recorded the values for several hours by GPIB interface.

As you can see, the sweep is very linear, only some minor deviation at low voltages (maybe connected to some offset voltages or similar effects of the operational amplifier), at least, we can’t see any leakage current of the capacitor, which would show up as increasing sweep time with higher voltage/later divisions.

Also interesting, see the accuracy of the sweep speed, with warm-up of the instrument (each measurement is 2000 second). Still, after all these years, well within the 5% specification of the sweep time! Amazing!

HP 3580A Spectrum Analyzer: a non-working marvel of engineering

The 3580A is a audio spectrum analyzer of the 1970s, and not only useful for audio, but anything that can be converted to audio frequencies (e.g., noise analysis of GHz sources, provided, you use the appropriate mixers). This marvel is not a FFT machine, but a discrete audio “received”, using a low-noise local oscillator, and covering a frequency range from 5 Hz to 50 kHz. The resolution filters are quarz filters, with bandwidth down to 1 Hz! Dynamic range is over 80 dB.

The device, it comes from my old university, and has been sitting there on the shelf for a while, not working. And in fact, it shows not many signs of life, it is not sweeping properly, and even in manual mode, it is not working reliably (not showing any reasonable signal, but there is some activity on the tracking output which suggests that the instrument is not all dead, also the “overrange” LED is working).

After some study and test it became clear the the issue is with the ramp generator. Unfortunately, it is not a simple ramp generator, as you can see below.

The main circuit is a capacitor being charged by a current source (mechanical switch with resistors).

The voltage at the main capacitor, a 10 µF polyester hermetic cap (really high end with glass seal and metal case), is charged and its voltage amplified by a FET-opamp (the FET input constructed from a discrete FET pair, and a PTFE stand-off to keep this all really high impedance).

All the sweeting action is controlled by a state controller, more or less, a hardwired program with several TTL chips. It took me quite some study to understand how it is supposed to work. But fact is, it doesn’t. Clearly, the issue is with the A3 assembly. This must have been quit an expensive assembly at the time, with all the FET pairs and opamps. Still today, not an easy thing to fix.

At least, it is a beautifully arranged board, all gold plated and really smells like quality. So it is worth some time and effort to fix it.

Key for such repair, at least in any reasonable time, are a set of good schematics. Fortunately, I have a set around and printed out really large copies – it is worth the effort, because without making some notes, you will struggle to keep all in your brain and still work on the circuit.

With no extender board available, just soldered some wires to the board to monitor the state of the main state counter, and some of its inputs.

Hmmm, after a lot of probing, I was almost tempted to replace a good part of the TTL chips, because it is really hard to find the defect in such a complicated and loop-wired logic circuit, including its analog parts.

But after a bit more consideration and test, I decided to try a step-wise approach, starting from the most likely parts causing issues. One of the 7473 dead, no problem, there are spares around. But the next one – a 7472! This is an AND gated J-K flip flop, with three inputs to each AND gate… in simple words, something old, exotic, and rarely used. Went through all my piles of old boards and ICs, but no 7472 to be found! Quickly arranged a temporary 7472 – from a 7411 3-input AND gate and a 7473 flip-flop.

To be sure, I tested to old 7472 – indeed, it is not working.

With the A3 board temporary fix, a quick test of the unit.

Unfortunately, still some issues, but is is sweeping:

Display issue:

Check with a X-Y scope (on the rear outputs of the 3580A) – all seems good from the analyzer section, maybe some issue with the storage display?

Finally, on xbay, found a set of 5 pcs 7472 at a reasonable price, from Spain! NOS (=new old stock), about the same age as the 3580a!

Some fluxing issue with the soldering of the old ICs (clearly seen at the 7473), beware! Use some good flux, or solder from both sides.

HP 8659B Spectrum Analyzer: mostly, the known issues

Mostly, the well-known issues for this 8569B: a bad fan, a bad 5.2 V supply capacitor (see the 5.2 V rail ripple below!), some issues with the display adjustment, and a bad control assembly with contact broken off. The control assembly, interestingly enough, somebody else had fixed some part of it before, from the handwriting, an American.
Still some minor issues with the Z axis control (brightness control), but this will be fixed soon, and then the analyzer will be thoroughly tested and will find good use again.

Meridian 506 CD Player: a hot driver

This report is about a really high-end (made in UK) compact disc player – a Meridian 506.
It had some issues with the drive circuit, with the TDA2030 running hot, and sometimes not reacting to the front panel control.

The drive mechanics, it is a quite simple setup – a DC motor with pulley arrangement, and rubber ring.

The cooling plate – just a small piece of metal. Running all good when when the CD compartment is opened and closed quickly, but there will be issues if for some reason the CD deck is not closing quickly – motor switch-off is controlled by the end switches.

The main driver is TDA2030, and an additional issue is the closeness of the heatsink to the metal case. Just some Kapton tape, which had some damage already. Maybe making contact at times (tab is connected to Pin 3, VS-).

Added a big heatsink the TDA2030, which is now also well-insulated from the case.

All working fine again!

The TDA2030 – it’s not a motor driver, but an audio amplifier by design. But essentially, it is a high power opamp, so it can be handy to control motors, coils, etc.

I also made use of the opportunity, please see the zip file.

meridian cd player 506 firmware

You can also find a collection of Meridian schematics in the manuals’ archive: Meridian Schematics, please request the password by email if you need access.

Auna AV2-CD508 HiFi Amplifier: start-up power supply repair

The Auna AV2-CD508 is a quite nice and affordable amplifier, the case is pretty solid, aluminum front, steel case, and the controls are all easy to operate.

It’s a “600 W” peak amplifier, but won’t take more than 80 Watts, so it is more likely a 2x 40 W amplifier – still, 40 Watts are a lot of sound power.

Unfortunately, this set reached my workshop for repair, symptom: it doesn’t switch on, no signs of any activity. After some measurements, the fault is found it the auxiliary 5 V power supply, this is always on, to power-up the main power supply and the rest of the circuit. The auxiliary supply is controlled by a SF5922S switchmode controller, SO-8. Unfortunately, this part is only available in China, and doesn’t seem to last anyway, so I removed the power supply control, and added some wires to an external supply.

… the external supply, a 5 V power supply. Also used it to measure the current. Turns out, the auxiliary supply only needs to provide some 100 mA of current, not a lot.

With the lab supply connected, and the Auna plugged in, it powers up OK and all working!!

That’s the amplifier board, solder side. The auxiliary pwr supply controller marked in red.

With such a device, better not lose too much time, and I decided to add a completely new 5 V supply, from a leftover 5 V power adaptor.

These are the main amplifiers – CD1875 aka LM1875. These are not bad, and can reach -60 to -70 dB distortion, and are generally known as reliable parts.

Some distortions measurement, at two power levels…

… not too bad!

Gain, it’s not quite flat, but OK for the purpose.

Finally, with the new power supply, the Auna has a second life, most likely, even longer than its first.

EIP 545A Microwave Counter: Option 04, a dead eprom, and a noisy fan

Another one of the EIP 545A counters, that are for some reason very frequently seen at my workshop. The reason for this instrument being here on the bench is simply for the fact that I found it on ebay, for a very reasonable price, non-working, but with the 04 OCXO option.

The option 04 provides 10^-9 level frequency stability, per day.

The actual OCXO unit is an Ovenaire 49-38c model 10 MHz oscillator. Very similar OCXOs are used in various HP equipment of the time.

The first item to fix – the very noisy fan. An ETRI model 126-LF-182. Fortunately, a very common 115 V model.

Replacing fans, you can’t just go for the flow rating, but you also need to consider flow direction, static pressure (mostly related to the blade shape and RPM), noise level, lifetime, and bearing type (ball bearing).

Found a very similar NMB fan, which is available from new production, at a reasonable price.

For comparison, the ETRI data put into the NMB pressure vs. flow characteristics, as red dots. Indeed, quite similar.

When changing the cable, from the old to the new fan (EIP uses a special connector, so I needed to re-use the old connector) – seems EIP didn’t trust their crimping, the cable was additionally soldered to the crimp pin.

The new fan installed, make sure to use some insulation tubing (not present in all EIP counters, but was present in this one, and a good idea, because the fan cable runs directly underneath the top cover, and over time, could be damage and expose mains voltage to the case (not a good thing!).

Next fix – a broken tantalum on the front panel (capacitor was OK, but one wire broken off) – fortunately, it could be soldered from the top, because disassembling the from panel is quite a time consuming task.

Finally, all these things fixed, but still a non-working EIP. It would not even count low frequency, or show any reaction on the display. All voltages OK. That’s strange – most likely something with the CPU, data bus, or similar. So, swapped the main CPU/control board with a know-good assembly, and the EIP came back to life. After some checking and probing, found one EPROM that had corrupted data – no wonder it didn’t start up.

While cleaning the power supply (removed the card from the main board), another issue showed up: EIP didn’t use sufficient thermal grease to make good contact of the assembly cage (used as a heat sink), and the heat conductor of the power supply assembly. All screws were tight, but no contact.

So I cleaned up everything, and added some more generous amounts of a good thermal compound.

This is the top view: you can see the OCXO auxiliary power supply to the left, and the OCXO in the right upper corner.

Also quite interesting, this unit has seen some pretty famous owners, including, Bell Labs, AmerSatCo, and Verestar, Inc. – probably, it has seen good use, also judging from the noise fan, which has 50 khours+ lifetime.

One all had been fixed and confirmed running, I could not resist to also add the 02 option Power Meter Upgrade to this unit, it is in the end just few eproms, and some additional parts for the A107 assembly.

… counting at 10 GHz, and showing the power.

EIP 545A Microwave Counter: El Salvador’s brittle leg disease

One more EIP 545A made it to my workshop, all in good shape, but not counting any microwaves, or other signals. The “gate” LED stays on permanently, and no counting also with the test function 01, which directs a 200 MHz signal to the counter chain.

Looking at the schematics (fortunately, there is a full service manual available), the issue needs to be with the A107 gate generator assy. And, easy find, no 100 kHz signal present. So something must be wrong with the 10 MHz to 100 kHz divider. Soldered a wire to some of the signal points, and needed to go all the way back to pin 1 of the 1st divider, which is a decade counter. No signal present.

The EIP 545A has all-socketed ICs, so pulled the IC from the socket, and 1 leg missing! The leg number 1 that needs to take the 10 MHz and put it into the silicon chip. Temporarily soldered a wire onto the IC, and it solves the issue, but not a good permanent solution.

Also the other pins are very brittle, when you touch them, and bend a little bit, they break off, rather than bend. This is quite common for some old Texas Instrument TTL chips, not quite sure way – maybe some precipitation hardening of the copper material they used, or an interaction of the copper core with the tin/lead top layer. We don’t know, but it seems to be particularly common with El Salvador’s chips.

Here, a close-up of an (intentionally) broken leg.

Also the LS175 on the same board, although it was all good electrically, shows quite severe brittleness. I replaced with right away (with a 1977 date coded LS175, also from Texas Instruments!).

The LS490, unfortunately, none to be found in my basement storage of all kinds of electronics parts. Many counter TTLs, but no 490. An the offers, they are pretty pricey, and I want this instrument to be fixed today, rather than waiting days for some overprices NOS TTL ICs to be delivered.

Looking at the datasheet, and schematic, it is a simple :10 divider, there are many of these – including the much more common LS390, and the pin arrangement is almost identical.

We only need to route the output of the 1st divider (a :2 divider) to the input of the next stage – and the LS390 will work like a LS490. The red lines show the additional connection needed (you need to bend up the pins, because they are grounded on the EIP A107 board).

…A107 board with the LS390 installed.

With these fixes, the 100 kHz signal came back! And the gate LED flashing!!

Tested with the test function 01 for some hours, and with some shaking and bending of the boards – just to be sure that the repair is permanent – all counting along fine. Also in band 3, no issues, and very good sensitivity all the way up to 18 GHz.

The last thing to do to before the unit will leave the workshop – adjusted the TCXO to the right frequency, it was only off by a few Hz, after many years of aging.

Oil Temperature Measurement Ni50: several meters of very thin wire

This is a short post about a very complicated and difficult repair. The function of the device is simple, it is an oil thermometer, of an old Deutz Locomotive, based on a resistance thermometer. Nowadays, virtually all resistance thermometers use platinum elements, but at the time, nickel was preferred for some applications, because nickel has a higher temperature coefficient of its resistance, giving about 62 ohms increase, per 100 ohms at 0°C, vs. only about 38.5 ohms, for platinum.

Moreover, the device used is a 50 ohms Ni resistance thermometer (Ni50), which is even less common than 100 ohms (Ni100). To add to the difficulty, also the thermometer itself is faulty, the pointer missing, the front glass damaged. All a bit rusty.

That’s the formula to calculate the resistance at any temperature – this is what we need to get.

Now, we have several approaches to fix this.

(1) Put in an electronic meter, to show the temperature – don’t want to do it, because it doesn’t fit to the locomotive’s age, and probably will fail soon with all the noise, oil, moisture, and vibration.

(2) Use a modern Pt100 element with some extra resistance to get the readings approximately right – this could work, but the electro-mechanical resistance thermometer indicator uses a pretty large test current, about 20~30 mA, much more than the rating of current thin film Pt100 elements, and wire-wound Pt100 are very expensive, especially, the larger sizes.

(3) Buying a Ni50 element, or two Ni100 elements. I tried, good luck, maybe for EUR 1000 you can get a couple made by some specialized company, custom order.

Well, all these options can’t really work, so I decided to wind my own Ni50 elements. Fortunately, I had some Ni wire, 0.065 mm diameter, of a reputable supplier around in my workshop from another project (has been there for about 20 years!), so let’s give it a try.

Some calculation quickly shows that a single layer or wire will be enough. Such wire will easily work with the measurement current.

Winding of course needs to be done with a machine, at about 0.2 mm pitch, you can’t do this by hand.

The elements were then measured to get the resistance corresponding to the workshop temperature, about 56 ohms, and fixed the wire in-place on the machine, with some super glue. Afterwards, the wire was further covered with high-quality epoxy, and fitted into a thin-walled aluminum cylinder, for added protection, and thermal equilibration.

The old sensor housing had still some stuff in it (the old Ni wire, and some stinky resin), and almost impossible to re-use the old mounting case for the sensor. Fortunately, we have a CNC lathe around, so quickly machined a new case as well.

A layer of Capton tape wound around the protected element, just in case of some leakage current developing over time.

The element was then put in the mounting case with some silicon-base thermal compound.

Finally, the sensor completed, connected to the old, steel wire braided cable.

For test, a litte fixture was made, which can be heated up with a 4 ohms, “100 watt” resistor. Well, it easily gets up to 150 degrees C.

As you can see, all working pretty well!

Some hours later, also the instrument fixed, dial and case sandblasted and painted, etc.

Working!!

Field test!!