85662A | SimonsDialogs

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.

The 8566B/8568B analyzer both use the 85662A spectrum analyzer display section, which is not just a display but also takes care of the IF processing. For the 10 Hz to 1 kHz bandwidths, a 5 pole xtal filter is used. A rather delicate assembly that dates back the the earlier analyzers, 8565A, in somewhat modified form. As a side note, HP had a strong tendency to utilized time proven circuits, some of them, over periods of 20 to 30 years… it helps with the repairs, once you get used to a certain assembly, the same pattern is repeating in multiple instruments. One of the examples it the A4A7 assembly of the 85662A (p/n 85662-60004). It is a rather ingenious design, and is critical for the 10 Hz resolution which makes the 8566B/8566B units so useful to resolve close-in spurs, like mains spurs.

The unit currently on my bench showed issues in the 10 Hz bandwidth – not enough gain. First, I assumed it to be an alignment issue, and spent quite a while re-adjusting the circuit. To no avail (well, it helped to improve the passband shape, which is now perfectly symmetrical again).
Almost wanted to give up. But not quite.

Checked the gain of the A4A7 with one of the stages, at a time, bypassed by a substitution circuit, a 47 n capacitor, in series with a 2.8 ohms resistor (see earlier entry). And, quite surprisingly, the gain of the 10 Hz bandwidth increased dramatically when shorting the 5th of the xtal poles.

After careful inspection, notice the peaking cap. It is at its lowest value – this might be the issue – each of the poles has at least 3 adjustments: center, symmetry, peaking, and 2 of the poles, also a gain adjustment…. Maybe, the 5th stage (which is working at all the other bandwidths), is just not set to peak!!

The 68 p capacitor, it is a factory selected component, and 68-82 p is the allowable range. This assembly had a 82 p fitted, but only at the 5th stage… well, just a few pF too much.

Where to get a 68 p cap (a silver mica…) now, one big ocean away from the well-assorted stock back home in the main workshop? Well, always good to have some old, spare HP boards at hand:

…one of them now is missing a 68 p cap….