Category Archives: Various

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!!

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!

Marantz Stereo Amplifier PM-14 MkII: explosive silicon

A heavy beauty, a Marantz Amplifier, PM-14. Certainly worth a detail look – according to the owner, the left channel is not working, and some smoke escaped along with burned smell.

No wonder – the two main transistors of the left channel are blown, say, exploded. And, upon further inspection, also the traces leading to the transistors, at least some of the traces – evaporated.

bty

This is the full view of the left channel amp (right channel is quite similar, but mirror image – some components placed at slightly different positions.

bty

bty

Some burnt resistors replaced…

bdr

The board is double sided, but not through-plated, and the quality, well, is not all that good – needs to be soldered with care, otherwise the traces will lift.

After some further checks, fitted a new set of transistors (ISC brand, China – still better than some fake “Sanken” transistors), but to the biggest disappointment, when switching on, these exploded in a loud BANG when adjusting the idle current. Well, turns out that also one of the earlier stages had a short transistor 2SA1145. More testing revealed another defect, a dead Zener (exchanged both Zener, to make sure that there are no issues with temperature drifts etc.).

With all these things fixed, time for a test, using a HP 8903A, and a scope. The HP 8903A is really handy, if your are into any quality audio repairs.

The test setup –

nor

Distortion measured at several frequencies, and several volts RMS at the output (using a 5.6 Ohm dummy load – just because I didn’t want to take out the 4 Ohm precision load for these power levels). -80 dB total harmonic distortion+noise, not bad.

bty

bty

bty

The front view – it has the classic Marantz indicator, for best sound, wait until it has warmed up.

bty

nor

As a reference, just in case you need it, the service manual Marantz PM-14 Service Manual. Or, just send me a message, or the come along with your dead amplifier.

HP 8640B Signal Generator: A new divider

The veritable 8640B generator, a marvel of engineering and still remarkable design today, it has reached an age where repair is often hampered by obsolete parts – sure, you can search for a donor unit or some NOS parts in some far away places, but in other cases, it is more practical to replace the circuitry with some new parts, especially, if the circuit is easily tested and verified.

This applies to the pre-scaler/frequency counter, which is an essential part of the 8640B, at least, if you want to use its internal counter (the 8640B is not PLL controlled).

Thanks to a kind contributor, Bodo, here a brief story of the repair:

(1) Symptom: the 8640B showed irregular frequency counts, completely unrelated to the expected output frequency. First consideration was a defective band switch (which has a cracked Delrin gear), but test with a spectrum analyzer revealed a perfectly good signal. Connecting a 0-10 MHz signal to the external counter input also gave perfectly good counting.

(2) After study of the schematic, the issue could be traced to the RF scaler, which is located in a die-cast cavity. Note that there are various versions of this board, but all feature some ECL logic ICs, with high power consumption.

prescaler 8640b assemblly

(3) Further tests showed that the first divider, :2 was defective.

(4) There are several ways to fix this. Here, the complete divider chain was replaced by a U644BS, available in DIP8. This IC is quite common in old TV tuners. There were also several projects in popular electronics magazines of the 90s to use the U664 as a pre-scaler up to 1.3 GHz.

Datasheet U664B U664BS

Picture of a random tuner from the web, using a U664B:
prescaler tuner

With the external input, the 8640B is counting up to 900MHz, sensitivity is better than -25dBm.

Note that the U866BS is self-oscillating (not a problem, because the 8640B oscillator is permanently attached for internal counting).

The modified prescaler board under test:

prescaler 8640b u664bs under test

One side effect is the much lower temperature of the RF scaler (much lower power consumption of the U664BS vs. the old ECL logic).

Finally, this is the schematic, the signal is connected from a-INPUT to b-OUTPUT.

prescaler 8640b schematic

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