This came from a swapmeet. I don't need another Racal frequency counter, but I hadn't bought anything and I haven't had this particular model before. The seller wanted £5 for it and it came with a rubber duck antenna and a BNC tee piece, which I thought were worth £1. They usually ask £15 and upwards for them, with no guarantee they work.
It worked on the low range (10Hz - 60MHz) but was dead on the high range (40MHz - 560MHz). I checked the power supply lines (+5V, -5V and +24V). The 5V lines were as they should be but the +24V line was showing 0.3V. The 24 V line is used to power a hybrid amplifier in the high range circuitry. A dead 24V line is common in these counters. There were several different 24V supply circuits used. There was a step up transformer run from a transistor switched at 500kHz, a 7824 three pin regulator and a discrete design. All seem to cause problems. This was the discrete design. It turned out the pass transistor, a ZTX 550, was dead. I had a ZTX 753 in my collection, which is a superior replacement. I fitted it and the 24V line was back and at close to 24V.
The high range still didn't work. I checked the supply voltages in that section and 5V was missing on a 741 op amp. A couple of slide switches on the front panel were stiff. I sprayed contact cleaner into them and worked them back and forth a few times. I tried the counter again and the high range was now working at up to the limit (560MHz).
After it had been running for a couple of hours I adjusted the OCXO against a 10MHz GPSDO. The OCXO is the lower grade one, which is the most common. Some are completely shot and can't adjust to their stated frequency (5MHz) and others are quite unstable. They are around 40 years old, give or take. This one seems OK from what I've seen so far.
It does have a dead segment in one of the 7 segment LED displays. Maybe it's a dry joint. I haven't looked into it, but these things do fail occasionally. I don't find it very intrusive.
It has the low frequency option, which is a frequency multiplier used to give more accurate readings at AF. It's activated by turning the sensitivity control fully anti-clockwise until a switch is operated.
The dead transistor is directly below the right end of the axial electrolytic. These are full of tants, and they often cause problems in the 24V supply. They were OK in this unit and I don't normally change them on for no reason.
Showing a 10MHz signal. The dead display segment is easy to see.
Showing a 561MHz signal. The frequency reference in the RF sig gen isn't spot on.
Racal 9917 frequency counter
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Racal 9917 frequency counter
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