HP 3325B A26 Assembly: low byte SRAM, and an incorrect signature

A quick comment on the 3325B service manual – Thanks to a kind contributor.

3325b kernel sa test u27 pin10

The kernel SA test is a bit flaky on A26U27 PIN 10. This is an inverter output, and can’t be same as the input… correct signature is A70F. Appears to be a misprint in the HP service manual!

3325b a26u27 74as04

In the present case – the culprit was one of the SRAMs, the lower byte. Fortunately, not really a rare part, and easily sourced and replaced, once you know that this is the circuit at fault. You might want to check this very carefully, because there are multiple versions of the 3325A/3325B control boards.

3325b srams

YIG Oscillator/YTO Analyzer: linearity, output power, hysteresis

With a good number of YIG oscillators (YTOs) around, time for a few tests. Rather than writing down all the numbers, a quick test setup, with two power supplies, an EIP 545A counter (with built-in power meter), an Agilent 66319D power supply (used as current source for the main tune and FM coils of the YTO), a 14.5 dB attenuator (just happens to be the value I had around) and a few cables.
When performing such test, make sure to put a 10 dB (or larger) attenuator of really good quality (low SWR) directly at the YTO output, without any adaptors or cables in between – some YTOs will show inaccuracies of not properly terminated in 50 Ohms.

This is a view of the setup:

yiga 1

yiga 2

One of the YTOs under test, a S081-0320 2 to 8 GHz Avantek part.

yiga s081-0320

yiga frq vs current

yiga pwr vs ghz
As you can see, the output frequency is pretty linear vs. main coil current.

yiga dev from lin vs frq

The tests are carried out first with increasing coil current, then with decreasing coil current, and the hysteresis is calculated (difference of output frequency, when a approaching from higher vs. lower current).

yiga hyst vs freq

One thing to watch out for are thermal effects, but let the YIG warm-up for 1 hour or so, with the main coil at half-range current. The effects aren’t all that big for the Avantek YTO discussed here, but you never now, for lower quality parts, other manufacturers, and so on.