HP 33321SB aka 33321-60026 Attenuator: HP 8662A Synthesized Signal Generator fix

Finally, I received the attenuator set for the HP 8662A repair. The very special 5+40+40 db attenuator with a side connector (rather than the usual design of these attenuators with two top SMA connectors).
However, a quick test showed that the 5 dB step is not working right. The “through” path is fine, but when I engage the 5 dB attenuator, is is something like 45 dB, unstable. Maybe a blown segment? That would be an easy fix.

So, I opened it up carefully, and found a broken contact. It seems the side connection is pushing a bit on the contact, and over the year, this design caused the plastic to fail. Tough to fix without precision drills and machine tools. So it will need to wait for the virus to clear, and for me to go to the German workshop again, to attempt an repair.

There are two small plastic studs, and they broke off, maybe the plastic got brittle over time with the pressure of the side contact pushing.
Let’s think about how to fix it, if at all possible.

Note – the seller was kind enough to refund half of the price, without having me to ship back the part, fair enough. At least some spare coils and segments should I need to fix other attenuators in the future.

HP 85662A Spectrum Analyzer Display: a quick fix of the 120 V power supply

A 85662A spectrum analyzer display for repair, the symptom (I didn’t take a picture) – a green square in the middle of the CRT -some lines are visible within, but no proper display. So, acceleration, CRT, and focus seem good – at least it seems to be a manageable fault rather. Maybe something with the XY deflection amplifiers – but why would both fail at the same time? In any case, first things first and checked the power supply. All the LEDs are on, on the power supply boards, but there is no output on the 120 VDC supply (well, some output, like 7 VDC) – the supply that is essential for the deflection system to work.

Some study of the schematic of the A1A7 assembly. Note that the voltages differ with the serial, this is a 85662-60235 part number board. Q7 is a current source that is driving the main transistor, Q8. If the voltage is trending higher, some of the Q7 current will be shunted to ground through U2.

It is a bit troublesome and dangerous to work on the life circuit (about 150 VDC at the input!). So, I did a check of all the transistors with a diode tester – and found the B-C junction of Q5 shorted. A HP part 1854-0019.

Some study of cross-reference lists, the 1854-0019 is a simple 2N2369A, found some in the basement parts storage (even a military rated and tested JANTX2369A with golden legs!).

Still, even with this fixed, no success. Further to other parts – replaced the green parts in the picture, an LM301 opamp, and another transistor, with no luck. Finally, soldered a few wires to the board and did careful checked in the circuit with power on, it can’t be helped otherwise it seems.

A few minutes later – the failure found. The Q7 current source is not giving any current, the base of Q7 is not biased properly. An open 110 kOhm resistor! It is quite rare to find defective resistors in HP equipment, but especially high value resistors running at higher voltage are prone to aging and failure, eventually.

With a simple, new resistor added, a metal film 110 kOhm, the supply is working again, and so is the 85662A.

HP 6205C Dual DC Power Supply: a generous binding posts fix

The repair itself, it is not particularly noteworthy, because this supply has served me well in the last years, in fact, it had been switched “ON” all the time to power an experimental setup.
The initial repair of this supply has been documented before, and on the pictures there it is quite visible that this supply had damaged binding posts. Seems that the prior user dropped it on the front panel.

Now the noteworthy facts, a kind reader of this blog, an American fellow, had a few of these posts at hand, from a HP plotter. He kindly sent them to me, free of charge!

So, as a result of the kindness of the reader, and the standardization of the parts HP used in their equipment, the power supply is now in better shape than ever before.

Did a few tests, like, checking ripple current at full load, and electrical safety – ground resistance, but all looking good.