Category Archives: Various

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.

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.

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.


Field test!!

HP 85685A: another mains filter failure – Schaffner FN 376

With the 85685A repair complete, the instrument was subject the an extensive test, to make sure all is in good order and working stable. Well, it did work well for a while, then – PUFF! The mains filter blew, one of the infamous Schaffner filters that is designed to blow up after about 20 years of service. Schaffner is one of the only companies I stay away from for any design – it is a Swiss enterprise, but too many of their devices failed in my hands – their filters are often the first parts to fail, in high grade test equipment.

The 85685A, like most other HP gear, has the mains filter combined with a voltage selector switch.

Cutting it open, you can see the Wima MP3 cap, 47 nF, 250 VAC. The MP3 are metallized paper capacitors, rated for X2 (mains) service. All embedded in some black resin.

Copper wires of the choke showing trough.

New filters that match the FN 376 are hard to find, and new-old-stock, well, these filters might fail again. So I decided to go for a new filter, a Schaffner-free solution.

This will be the new filter – a ID-10AC-S, available for little money, and seem to be pretty good for their current rating.

The internals… the filter elements are nicely encapsuled in a two-shell plastic case. No potting compound!

The X2 capacitor, and the choke…

Transplanted to the Schaffner filter body… and wrapped with Cu tape, soldered closed, for EMI shielding. All well grounded!

From the datasheets, you these filter should have 20-30 dB loss at 1 MHz. Let’s put it to a test!

For the new assembly, tested with a 3585A, about 12 dB loss at 100 kHz, 30 dB loss at 1 MHz, should be good, and no modification necessary to the 85685A.

HP 85685A Preselector Repair: faulty attenuation

The HP/Agilent/Keysight 85685A Preselector is a great addition to any 8566B or 8568B spectrum analyzer. The preselector converts the analyzer into a test receiver, by increasing its dynamic range by 30 dB, down to very low noise levels.

Recently, I got a 85685A for repair, only knowing that it doesn’t work as it should. With some checks, it was very quickly evident that there must be a issue with the RF attenuator, or its driver.

This defect is clearly seen when looking at a test signal at various attenuation levels of the 85685A. The signal should stay at the same level, irrespective of the attenuator setting, but as soon as you go from 10 dB to 20 dB, the signal vanishes almost completely. This is not good.

This is the RF attenuator, a Wavetek OEM part. Unfortunately, there is no service manual for the 85685A available, so we need to figure it out by ourselves.

First, determined the switch matrix for the attenuator controls, by probing the control inputs at various attenuation settings. Pretty clear, there are 10 dB – 5 dB – 20 dB – 20 dB segments inside, which are activated by pulling the respective control input low. Easy enough.

After some disassembly of the case (removing the rear panel), you can get access to the four screws holding the attenuator to the case.
Notably, the case of the 85685A uses Torx screws, unlike most other HP equipment using this style of enclosure.

Underneath the label, there is now hidden screw to get to the internals of the attenuator, all is glued closed and sealed with silver epoxy. To break it open without destruction, I milled a small slot from the side of the unit. Probably could have milled a bit shallower, and a bit less, but OK.

With the slot, the lid is easily removed using a screw driver. Make sure not to bend the lid too much.

Looking inside, it is pretty obvious that someone must have fired a lot of power into the unit, when set a 20 dB input attenuation. Checked all other segments with a 8752A network analyzer, and all good, except for one of the 20 dB segments, as expected.

How does a 20 dB attenuator work? There are several topologies, Wavetek used a so call pi-arrangement of resistors. Only two of the resistors are blown, the output resistor is OK (this is also clear from the fact that most power is dissipated in the left two resistors).

The switching of the attenuator segments is done with miniature RF relais, similar to these. At over EUR 40 a piece – glad these are all good.

The relais are DPDT switches, soldered flush to the board (which is a PTFE composite board), for best RF performance.

For repair, we need to replace the resistors with good new parts – but there are hard to come by, with not even a Wavetek datasheet available for the attenuator, let alone, these parts.

Several attempts were made to get the best (lowest) SWR, and the best flatness, at very close to 20 dB attenuation.

First, used a combination of 1206 SMD resistors to get close to the values needed.

This is the flatness of the “good” 20 dB segment:

This is the flatness of the “1206 repaired” 20 dB segment:

Another style of repair, with the same parts, now soldered directly between the legs of the relais:

… no improvement, still quite some reduction of attenuation above 2 GHz.

Now, tried with a series arrangement of 0805 resistors for the 250 Ohm resistor (giving lower stray capacitance).

… quite some improvement!

Red is the good attenuator section, blue is the repaired section, at 0.2 dB/div scale!

I would call it good enough!

A quick SWR test (non-calibrated) for “through” and “actuated” setting of the repaired segment (and terminated in a 15 dB precision 18 GHz rated attenuator at the output) showed low SWR (keep in mind, the 85685A will only work up to 2 GHz anyway).

All sealed up with silver epoxy – a bit old stuff around here, but still working. And, used some Cu tape (with conductive glue, 3M type 3313), to make sure all is sealed well and forever.

Now, with the attenuator fixed and working, one more thing to consider – the power handling capacity. The 85685A is rated for up to 30 dBm (1 Watt) average power. Not sure if the SMD resistors used can handle it – they are a bit smaller than the original Wavetek parts. So I decided to swap the control lines for the two 20 dB segments. This way, the “good”=Wavetek segment No. 3 will always take most of the power, and the repaired section (SMD resistors) will only be needed for the highest attenuations, and never see any more than 10 dBm of power, even at the maximum allowable input. Still, I will keep a search going on a spare 0955-0235 programmable attenuator, for a reasonable price (some of these being offered for USD 100, which is a bit more than I want to spend for a 25 year old part of unknown nature and condition).

Finally, all assembled back together, and performed a flatness/attenuator test, by supplying a signal at -40 dBm from a 8642B generator. Measured amplitude at 1 kHz resolution BW is pretty flat over all attenuator settings and frequencies.

Let me know in case you have any 85685A units for repair….

Rigol DS1052D=DS1102D Oscilloscope: encode issue fix

For about 8 years, I have been using a Rigol DS1052D 50 MHz scope (which works up to 100 MHz with a well known software fix), but recently, it has given me some grieve: the knobs turn, but the settings jump, both for scale and time base. This is quite annoying – trying to fix something, with broken tools or scopes, is no fun.

Already expecting the worst, like, mechanical failures of the scope of encoders, or some strange software bug (because all encoders were affected at virtually the same time), I decided to open up the scope. Easier said then done. There are 4 screws, two of which are under the handle, and you have to remove the powder on button with some bent wire (don’t scratch the case!). Then, move around the back cover, eventually, it will come off! Don’t give up! Don’t remove the screws of the power input socket!

Once inside, you need to take off the two screws for the D9 connector, and then, several more screws to take out the power supply, before you finally get to even further screws holding on the front panel.

Before taking any soldering iron to replace the encoders, as suggested by some folks on google, I tried to use the magic DeoxIt D5 to clean up things and to get it going again. Just spray some of the stuff into the encoders, there are some small openings (circled red on in the image). Turn the encoders to make it work. After a while, clean of any excess D5, don’t let it get in touch with the rubber of the DS1052D buttons – most rubber can handle D5, but better not try your luck!

And, it worked like magic. Not only are the encoders working again, but also the feel of the knobs is much better, not as sticky as it used to be.