Tag Archives: OCXO

EIP 545A Microwave Counter: Option 04, a dead eprom, and a noisy fan

Another one of the EIP 545A counters, that are for some reason very frequently seen at my workshop. The reason for this instrument being here on the bench is simply for the fact that I found it on ebay, for a very reasonable price, non-working, but with the 04 OCXO option.

The option 04 provides 10^-9 level frequency stability, per day.

The actual OCXO unit is an Ovenaire 49-38c model 10 MHz oscillator. Very similar OCXOs are used in various HP equipment of the time.

The first item to fix – the very noisy fan. An ETRI model 126-LF-182. Fortunately, a very common 115 V model.

Replacing fans, you can’t just go for the flow rating, but you also need to consider flow direction, static pressure (mostly related to the blade shape and RPM), noise level, lifetime, and bearing type (ball bearing).

Found a very similar NMB fan, which is available from new production, at a reasonable price.

For comparison, the ETRI data put into the NMB pressure vs. flow characteristics, as red dots. Indeed, quite similar.

When changing the cable, from the old to the new fan (EIP uses a special connector, so I needed to re-use the old connector) – seems EIP didn’t trust their crimping, the cable was additionally soldered to the crimp pin.

The new fan installed, make sure to use some insulation tubing (not present in all EIP counters, but was present in this one, and a good idea, because the fan cable runs directly underneath the top cover, and over time, could be damage and expose mains voltage to the case (not a good thing!).

Next fix – a broken tantalum on the front panel (capacitor was OK, but one wire broken off) – fortunately, it could be soldered from the top, because disassembling the from panel is quite a time consuming task.

Finally, all these things fixed, but still a non-working EIP. It would not even count low frequency, or show any reaction on the display. All voltages OK. That’s strange – most likely something with the CPU, data bus, or similar. So, swapped the main CPU/control board with a know-good assembly, and the EIP came back to life. After some checking and probing, found one EPROM that had corrupted data – no wonder it didn’t start up.

While cleaning the power supply (removed the card from the main board), another issue showed up: EIP didn’t use sufficient thermal grease to make good contact of the assembly cage (used as a heat sink), and the heat conductor of the power supply assembly. All screws were tight, but no contact.

So I cleaned up everything, and added some more generous amounts of a good thermal compound.

This is the top view: you can see the OCXO auxiliary power supply to the left, and the OCXO in the right upper corner.

Also quite interesting, this unit has seen some pretty famous owners, including, Bell Labs, AmerSatCo, and Verestar, Inc. – probably, it has seen good use, also judging from the noise fan, which has 50 khours+ lifetime.

One all had been fixed and confirmed running, I could not resist to also add the 02 option Power Meter Upgrade to this unit, it is in the end just few eproms, and some additional parts for the A107 assembly.

… counting at 10 GHz, and showing the power.

A Thermal Fuse, and HP 10811-60111 Repair

Usually, I don’t care much about high precision oscillator options being fitted to frequency counters, etc., because in the lab, any critical equipment is anyway connected to an external well-controlled 10 MHz reference, locked to DCF77. However, this time I need to install a OCXO (HP 10811) in a HP 5335A counter for service outside of the lab.

The only thing that needs to be done is to remove a jumper on the board of the 5335A (see red box in picture below), and mount the 10811 in the slot already prepared for the OCXO inside.

ocxo 5335a jumper

While such installation is fairly straightforward, it turned out to take more time than expected – simply because of the OCXO not showing any stable output signal.

ocxo 10811-60111

After a few quick tests, the cuprit was found, a defective (open) thermal fuse. This is apparently a quite common issue for the 10811 oscillators, and you might get away with just putting in a wire jumper. However, I didn’t want to take any risk of overheating in case of a failure of the 25+ years old OCXO circuits. An exact match for the thermal fuse could not be found, so just soldered in (very carefully, cooling the case and leads!) a 10 Amp 109 degC fuse.

ocxo fuse picture

This is the OCXO with the new fuse installed.

ocxo new thermal fuse

This style of fuse as a non-insulated outer shell, so a shrink tubing sleve serves as insulation.

ocxo insul sleve

Finally, a note found in a datasheet of a common thermal fuse – it clearly states that lifetime will be limited when operating the fuse to close to the cut-off temperature. So clearly, thermal fuses are not the best protective mechanism for the OCXO case. Maybe better would be a bimetallic switch (self-resetting, but at least no subject to any significant aging), or some other device like a PTC.

Sure, we can slightly blame the HP engineers, because it is stated on most thermal fuse datasheets, like the one below, that the operation temp limit should be about 30 degC less than the cut-off, which is not quite the case for the 10811 OCXO. 80 to 84 deg C operation, 109 degC fuse cut-off.

ocxo storage temp

ocxo thermal fuse