HP 4192A LF Impedance Analyzer: another visit to the workshop

The analyzer, I had fixed it 3.5 years back, see HP 4192A LF Impedance Analyer, the instrument has been back to very good shape, and since then been operated at an university overseas. Recently, I got the message that repair is needed, the instrument didn’t start up.

I tried hard to fix it remotely, because of the significant size and shipment cost, and the general risk of shipping such precision gear around the world. But to no avail, the failure seemed to complex to repair by remote instructions.

First difficulty, to get the instrument shipped to Germany, and to get it through customs – quite a task that took several hours, personal appearance at the customs office, and some paperwork, along with a small fee.

Following the old rule, to check the powers supply first, it was quickly seen that there is a short on the -15 rail, and systematically unplugging the assemblies, quickly found the short on the A3 assembly plug, which is also powering the A1 assembly – the location of the actual fault.

Smell and eye are the best methods… to find easy faults.

Once you know the location, easily seen – the burned inductor. I replaced it by a 4.7 µH inductor I had around, and fitted a new capacitor (tantalum cap).

The NEC-branded cap was dead-short.

Now, the instrument powered up, at least the power supply, but no further sign of life. Checked around the CPU board, and strangely, even the first test showed, no clock! the CPU clock is derived from the A3 master oscillator, by a divider chain – and probing there, no signal on the 1 MHZ or 100 kHz lines either.

The diver chain uses various divide-by-2, 74C74 flip-flops.

Hard to see, but to determine the defective chip, I cut the clock pin at the 74C74, because the clock was low. So I was not sure if the clock generator/amplifier was defective, or just overloaded by the 74C74.

With no 74S74 (guaranteed to run at 75 MHz, typically up to 115 MHz clock) at hand, I replaced it by a 74F74 (which easily handles the 40 MHz clock).

Interestingly, both 74S74 (HP part 1820-0693) had failed. Maybe both were suffering from some transient when the power supply sorted. We may never find out.

Finally, I noticed some unreliable switch-on characteristics that could be traced to some flaky resistors on the power supply board – this board had corrosion issues that damaged some resistors.

A little box of replaced parts… not too many.

Finally, put the instrument to a 24 hours tests, and also run some calibration of DC bias, which had drifted a little. Otherwise all well in spec and well tuned.

Packing it all up: this time, a package to Saudi-Arabia. The instrument wrapped in bubble wrap, then a layer of styrofoam, then a wooded box re-inforced with metal parts and screws, and a cardboard layer all around (without cardboard, DHL will rank it as “special handling”, at a significant additional charge).

After about 2 weeks, the instrument safely arrived in Saudi, and it is indeed working again. Recipient is happy, me too!

Metalworking: strange defects of long-used tools

Recently, I found two strange defects in my workshop, noteworthy to write it down.

Ever since I expanded my workshop I notices some losses of compressed air, i.e., when coming back after a business trip, the pressure of the compressed air pipework is down from normally 6.5 bars, to only 2-3 bars, or even less after a longer vacation. I had attributed it to some leaking air gun or something like that, but finally I did a search and found various leaks at one distribution point.

(1) the water mist filter had a leak at the drain valve — fixed it by closing it altogether. There is not too much water collected, it is installed only to avoid dust and such getting into the system further downstream.
(2) one of the fitting was leaking, because strangely there are some air fittings available that neither have a seal surface, nor conical thread, to it is hard to get it tight by tread tape in the first place (sometime I resorted to gluing-in those fittings with Epoxy glue).
(3) Still, when checking with some soapy water – bubbles at the distribution fitting (5 connections 1/2″). Strange, strange, strange.

Clearly, a defective casting with a hairline crack leak. Maybe the temperatures or the mold or the liquid brass were not right, or the pouring wasn’t steady enough.

The defect line goes all around the fitting, otherwise I would have tried to file it out and solder it tight. Anyway, it is not worthwhile to fix such part at the risk of it failing again. So I replaced it with a new fitting at the cost of EUR 9.90 and at least 3 hours of work to get it all investigated, replaced, fitted and leak tested. Maybe a good idea to fully upgrade the pressurized air system of my workshop, which is now a combination of various distribution pieces, hoses, connectors. But there are trade-offs of perfection, cost, and efficiency — I have actually installed some air pressure distribution pipes for others (like, polyamide pipe, PE-Al composite pipe), but at my own workshop, I will be dealing with a less perfect system, at least, the major leaks have now been found and eliminated.

Another instance of strange failures, I noticed some problems with a bench vice, the smaller size vice I use regularly, and have been using since my childhood. If I remember right, I bought it for something like 35 Deutschmarks way back around ~1990. After all these years, it had some failures, the spindle pin sheared off once (maybe because I tightened it too much), and also a retaining piece of the front part of the spindle wore out (probably very soft steel and insufficient lubrication). Also some part of the casting broke off before, circled in black. Now, I almost injured myself as a part came loose unexpectedly, finally, as they say, always check the tool before you use it.

Didn’t need to go far, a major crack of the dovetail guideway.

It is cracked through, but holding on enough so that the defect is not easily seen without disassembly.

Finally, a cast iron bench vice, after 33 years of service (not always careful use), it may eventually break.

For replacement, I wanted a slightly bigger bench vice, 115 mm width. Also, a solid steel vice that is not too bulky and has a strong grip. These don’t come cheap, but I found a good (but rusty) Peddinghaus Matador steel vice for 35 EUR used, listed in online adds.

After some cleaning, adjustment, and other TLC, it is installed on the bench and working great. Definitely a step up.

Meridian 506 CD Player: after 5 years, in need for more attention

Recently, I had two instances of equipment coming back after some year – with different defects that at their first visit in my workshop. One device is the Meridian 506, a high-class CD player that is popular with hifi enthusiasts, and has been marketed for many years in various different versions. Earlier I had fixed some design issues with the motor driver of the CD deck, see Meridian 506 CD Player: a hot driver.

This time, it wouldn’t detect any CDs, and made some noises. The deck was still opening fine. After looking around, there is no obvious defect, the power supplies are good. All the service modes work – just that I can’t get it read discs or even lock on tracks, it just spins up, then stops.

Reading through all various posts, it seems clearly related to the CD head assembly, this version of the 506 uses a CDM 4/19 Philips laser head assembly, and these are known to wear out over time, i.e., the laser will show aging, and then the mechanism can’t lock on the CD.

Well, easy enough, I found a CD 480 player cheaply as used goods, and this has the desired CDM 4/19.

It arrived strangely packed, but well, better strangely packed than insufficiently packed. Indeed, a lot of styrofoam, chips and paper inside.

Inside, a very simple assembly, much lighter than the Meridian deck. Just some thin plastic.

Easily disassembled the donor.

There are some changes necessary – the hub is different, but can be removed with simple tools (plastic tools) and switched.

Well, all this done, unfortunately —– no success. Still same symptoms. Next, poking around the main TDA chip, there are no signals that are of any use. Strange. Normally it should at least start reading and locking, but it doesn’t (not that the CD drive uses a feedback loop to keep the head on the track, by adjusting the laser arm coil current according the the laser feedback; in these drives, the laser arm is driven by a coil).

After some further study, finally realized that the CD is spinning the wrong way. How can that be? Seems like a defective motor driver! On the small board of the CD deck, there are two L272, and one of them is terribly hot. Not good.

The L272 has long been obsolete, but I would two pieces cheaply on a Chinese website, these were apparently no new but recovered from used equipment. Well, no problem, I rather buy used parts than fake parts.

With these repairs done (not easy because the soldering of the L272 is very strong, need to put a lot of heat to the small board because all the ground pins are firmly connected to a ground plane), the Meridian is working great again, let it play for several hours with no skipping.

And, we now still have a spare laser head assembly should it fail next.