Tag Archives: 10811

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

HP 8566B Spectrum Analyzer: YTO/YTX tuning, flatness adjustment, and an OCXO

The 8566B I am dealing with here as parts from at least three units, so no wonder that the YTO/YTX drivers are all completely out of adjustment. So much that the LO sometimes locks on an incorrect multiple of the reference, or that it doesn’t lock at all.

Well, the adjustments are all well described in the manuals, rather straightforward.

8566b corr coeff
The amplitude offset at 10 Hz, a bit more than I want it to be, but this is related to the A4A7 assy of the 85662A, not to the 8566B itself.

8568b flatness adjusted
Might still tweak it a little bit, when all repairs are complete, but for now, a quite satisfactory result.

The OCXO, it is mounted in a set of 3 rubber isolators, here are the rough dimensions, if you want to fit a custom OCXO….
0960-0477-1 osc 49-61c dimensions

8566b 0960-0477-1 osc 49-61C 10 mhz ocxo

From the service manual – there are at least two versions of the 8566B, one using the HP 10811, and the other, using an Ovenaire OSC 49-61C.
8566b a22 assy ocxo

As far as I know, the oscillators have more or less the same performance level, but the connectors on the motherboard are different (still carrying the same voltages – the 10811 has small add-on regular board, 5062-1909), and there may be also differences in the holding brackets.
5062-1909 10811-60111 board
Notice the different plug! I have a spare 8568B motherboard around that supports this connector style.

One of the many test results, the 22 GHz noise floor.
8566b baseline noise 22 ghz

Not bad at all, about -118 dBm. Also checked the power line spurs and the noise characteristics, all considerably better than specified.
The only downside: total weight, of the 8568B: 112 lbs, and two strong fans.