Category Archives: Test Equipment Repairs

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

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.


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.

SME Tonearm Series V: a really THIN wire!

A rare guest in my workshop, a high class “SME Series V” tonearm, a great item for the audiophile! Unfortunately, the arm we are dealing with is not working – easily traces the issue to a broken wire. Wait – with the tone arms, you can’t just pull out the old wire and put in some odd wire – it needs to be 0.0127 mm2 seven-strand silver wire!

sme tonarm 1

sme wire

After some careful disassembly action, one of the wires was found broken at the needle end, easy enough, soldered it back on, followed by even more gentle and careful re-assembly of the unit.

tonarm wiring

All in all, nothing too difficult, but not an everydays’ task, and it made a friend of mine, the proud owner, happy again!