HP 5350B Microwave Counter Teardown

[AlphZeta]

I have not seen a teardown of this one anywhere before. More high-res pictures and information can be found on my site: (http://www.kerrywong.com/2014/06/29/hp-5350b-microwave-counter-teardown/). Enjoy!

[Vgkid]

Thanks for the teardown, i wonder if the ocxo has a bad connection to the connector.
The A5 synthesizer board seems out of place, compared to the others.

[AlphZeta]

I am pretty sure that the oven's thermal fuse is open as the oven does not draw any power at all. I will have a separate oven teardown and repair for that

[daqq]

Thanks for the teardown! I love seeing the insides of specialized tech.

[sportq]

If your 5350B has a UK serial number xxxxUxxxxx then there is a very good chance I did the factory test and calibration on it. There were only two production lines, one in the US and one in South Queensferry, Scotland. Since it was a low volume unit there was one assembler and one test engineer (me) and I was frequently moved to other products when demand was low.

Peter

[AlphZeta]

Well, just checked mine. The serial number on mine is XXXXAXXXX, so looks like it was made here in the US.  HP 5350B must be super expensive back then. Even by today's standards, this frequency counter is still pretty impressive considering its range and resolution.

[AlphZeta]

Good news. I opened up the OCXO and found that the reason the oven weren't working was due to a blown thermal fuse. Now everything is fixed!

A teardown of the OCXO is on my site: http://www.kerrywong.com/2014/07/01/hp-5350b-ocxo-repair/.

[Vgkid]

Congrats on the ocxo repair.

[AlphZeta]

Thanks! I will try to find a proper thermal fuse and replace the jumper shortly.

[SeanB]

Thermal fuses tend to fail either from old age or from a leaky transistor keeping heating the unit up. If it keeps regulation it was old age, but check and measure it is at the correct temperature. Interesting fact is to look at the ratings, where the tolerance is always +0 degrees, but up to -20 degrees on the tolerance. They have to have opened at the marked temperature, but can fail at lower temperature depending on how much current is flowing as this adds extra heat. So the actual operating temperature is always selected below the rated as the 10A current adds extra heat to the element.

[AlphZeta]

Thanks Sean,

I think old age is the most likely reason in this case. The temperature seemed to be fine (well, I haven't checked the precise temperature yet, but judging from the power consumption the regulation seemed to be working).

[SeanB]

Actually the green device you showed is a simple wire ended fuse, 125mA rated. It looks like a thermal fuse, but is a current operated fuse. Likely that one was operated hot for a long time and simply failed as open. I have changed a lot of those in assorted equipment. You can replace it with another of 125mA or any small wire leaded fue that fits.

[AlphZeta]

Interesting... The service manual referred to it as the thermal fuse (F1) and that was the only fuse inside the oven oscillator. But you are right, by the marking it looks just like a standard 0.125A fuse. I have plenty of fuses lying around and I am sure I have one close in spec and size.

Thanks!

[Raindog]

Just acquired one of these counters myself. Mine looks new, no missing feet, bails or trim pieces. Downloaded the service manual to try to locate the "fine" adjustment for the 10811 reference oscillator. Come to find out there is none, makes precise adjustment a little more difficult but not impossible. Thanks for the teardown by the way, saved me from breaking the cal stickers just to see what's inside.

[Turbinedave2003@yahoo.com]

Thank you for all your teardowns.  This will come in handy, as I have one still in the box, an Ebay find for $95.