After a quick trip to Germany (and with a set of spare parts in my suitcase), I directed my attention to the sick 4192A again.
(1) First, the A7 power supply assembly. Fixed the defective signal diode, and ceramic capacitor. Discovered that the line frequency signal is not working, and that the 5 V supply (analog section) linear regulator isn’t regulating, but passing through about 6 volts. Also noticed that the digital 5 V supply is not working (running at 2-3 volts).
(2) After about 20 seconds of operation, there is some smell around the A7 power supply assembly. The input caps (10 uF, 350 volts) running hot.
(3) No display at all. Is the controller assy dead? Or the EPROMS corrupted?
Let’s tackle it step by step.
Fortunately, we are not alone here, the same instrument had similar issues elsewhere – seems the NiCd batteries weren’t fit for the purpose, and have all leakage issues, as can be seen in below screenshot of a Japanese blogger.
Unsoldered the 10 uF 350 V caps of the A7 power supply – all the positive connections are leaking. Fortunately, not as corrosive as the NiCd electrolyte.
Strangely, HP fitted 85 C capacitors, rather than long life types.
On the picture you can see the temporary fix with some capacitors I had handy. And, you can see the Y-rated capacitors (15 n) RIFA cracked capacitors replaced by WIMA brand new parts.
For replacement of the 10 uF Chemi-Con we will use CFX series capacitors.
These are rated for quite severe ripple current, which is necessary because that’s their purpose in the A7 circuit.
Of course, 2 parallel capacitor 10 uF will have better ESR than a single 22 uF, but fair enough for test purposes.
The digital supply, surprisingly, has no regulation, neither by linear nor by switchmode action (the switchmode circuit is regulated by an independent sense winding on the transformer (running at 29.5 kHz).
The digital ground is connected to analog ground by a 10 Ohm resistor for noise isolation. Other than that, the circuit is rather simple and quickly found the defective part – surprisingly, one of the Schottky rectifiers, a 20FQ040 diode. Maybe it overheated, or it failed just because of age.
For its time, 1980s, it is a respectable diode, with very low forward voltage.
Unfortunately, replacements in DO-4 bolt-mount style Schottkys are very expensive and hard to get. Maybe there are some back home in Germany, in the junk pile waiting to be desoldered, but not sure about it, and no such diodes here in Japan. Anyway, after some measurement, the current of the digital supply will be somewhere around 2.5 to 3 Amps, not too much, for any common switchmode supply rectified diode. Decided on a SBL3040PT. 3 Amps at 0.2 Volts will be less than 1 Watt of dissipation, say, about 40 K temperature rise of a SBL3040PT double diode (such packages have about 40 K/W junction to ambient thermal resistance, probably I will fit a small metal part as heatsink (the existing heatsink may be used after drilling a mounting hole…).
Another issue fixed, a corroded spacer for the 5 V analog supply voltage-limiting Zener (a power Zener, still good!). The spaced could have been cleaned, but not worth the hazzle, so I machined a new one from aluminum alloy.
To the mains sensing and synchronizing circuit – some probing with a scope showed that the U1 comparator (a HP numbered LM339) didn’t work. Replacing it was no easy task, because this area had been affected by the NiCd electrolyte, rendering the solder pads difficult to desolder. Scratching off the corrosion layer from the solder with a screwdriver helped to get some contact with the head, and to finally desolder.
Further probing also showed a few damages to resistors, the wires had come off the main body (seems the corrosion damaged the weld between the resistor case, and the wire).
A quick test – there is a clean sync signal! About 120 Hz, double the mains frequency in this part of Japan. Perfect.
With these basics fixed (except the digital 5 V supply – still waiting for delivery of the diode – just fitted a temporary diode for test purposes), let’s have a look at the CPU board, which is a marvelous piece of engineering, with may TTL, EPROMs, RAMs, etc. – let’s hope we don’t need to fix this complex assembly.
Checking the clock and reset lines – no clock present! After some study of the manual, I understood that the CPU clock is generated from a 20 MHz signal, from the 40 MHz VCO and reference frequency assy. Why is there no 20 MHz? Easy answer, the referency assy has no power except -15 V.
Reason: corroded Molex connector, and broken contacts within. Not easily seen from the outside, but clearly, there is no contact.
Fixed the contacts, at least a few (and ordered more!), and voila, the 20 MHz and 40 MHz are back. No issues with any of the main counters (e.g., keyboard and display scan), good activity on the address bus, etc.
For proper tests, removed the CPU/controller assembly A6 from the unit, and powering it with a lab power supply, to make sure it gets stable power.
After all this, quite some relieve! The unit is starting up, at least as much as it can be told at this point (all the analog circuits disconnected from the A6 controller board – need to first fix their power connections). The startup passes all the RAM and EPROM tests, great! And finally stops at E-50, which means, it can’t find the line sync frequency – which is no surprise, because this signal is currently disconnected.
Next steps will be – (1) Fix the Molex connectors. Quite difficult because these are crimp connections, and the wired have some corrosion making them difficult to solder. (2) Finalize the A7 repairs – the 10 uF capacitors, the Schottky for the digital supply, test the digital supply. (3) Then proceed to the start up and repair-adjustment of the analog circuits, and synthesizer section, many, many, complex circuits, but these don’t have any visible corrosion or damage. Fingers crossed.