HP 5245L counter crystal oscillator

[barrymagrec]

I thought it was 1224 but it was a long time ago when I played with ours. I made a DAC to drive an analogue recorder from the digital output - it sort of worked, then the project went away and a little later I picked up a proper HP DAC for next to nothing at a surplus supplier.
I don`t remember ever actually using it though.....

[cmjones01]

More on this story. No pics because my phone's playing up, but there's progress. I got the oven all reassembled with silicone-insulated wire and a built in DS18B20 temperature sensor. Fibreglass loft insulation from a handy local house renovation packs the space where the mysterious foam used to be. It's not as effective as the original but it'll do the job well enough.

Testing it out with the oven controller run from a bench DC supply, it almost works. The catch is that the germanium power transistor which is supposed to switch the power to the heating element doesn't really do that. It conducts well enough, but when "switched off", still passes about 300mA! The oven controller is definitely trying to do the right thing and responds to the temperature adjustment, but its efforts are thwarted by the faulty transistor.

In this condition, with 300mA passing through the oven element the whole time, the oven would eventually get to over 90C, I think. I saw it as far as 83C and switched off. No wonder everything's got a bit crispy inside. I'm really glad I built in the temperature sensor!

Luckily the circuit round there doesn't look too critical so replacing the transistor with a silicon PNP one shouldn't be painful. It's only got to switch 36V at about 400mA.

Chris

[cmjones01]

The counter is back in action. The errant transistor (a funny looking 2N1183B) is replaced with a TIP32B which works fine with no other modifications. The oven temperature is stable to within about half a degree and it's been warm for about 24 hours now so the frequency has settled down. Relative to my frequency standard it drifted down by about 3 parts in 10,000,000 over the 24 hours of warm up. Let's see how it continues.

Initially it was about 50ppm low in frequency because I'd mis-wired the 'medium' and 'fine' frequency adjusters, shorting them out. Interestingly the EEVBlog thread says the oscillator wouldn't come up to 1MHz - I wonder if that was their problem too?

The oven temperature readout isn't yet integrated the way I'd like it to be but the LED Display hanging out of the back of the case is fine for debugging!

The view of the area round the oven controller shows, just about, the replacement transistor and transformer, the silicone wiring, and a 1A fuse I've added to the transformer secondary to save it if there's another short-circuit mishap.

Chris

[cmjones01]

Hopefully the final update on this one. Spending the odd few minutes here and there I've finally got the temperature-to-frequency converter in there and working. It uses a DS18B20 sensor shoehorned in to the end of the oven, connected to an Arduino Mini which is running a very simple sketch which sets a PWM output pin to 10 x the temperature in degrees C, so 67.5 deg C is 675Hz. The output of this is then wired to the '10V' position on the input sensitivity switch, which I've never used in more than 30 years of owning the instrument, so I won't miss it. Now switching to 10V sensitivity reads out the oven temperature directly. This is probably the world's first 5245L with a microprocessor in it!

Chris

[MrBungle]

Looking good. Nice job with the repairs. I'm regretting passing the one up I saw on ebay now

[factory]

Excellent job on repairing the counter, I like how you have got the temperature sensor modification working as a self check feature using the (unused) 10V switch setting without the need for any external stuff.

Also you've reminded me I need to fix the input on mine, it's gone from working on some of the range switch settings to only usable with input via the plug-ins, found this out after recently acquiring a HP 8640B signal generator.

David

[cmjones01]

Reviving an old thread, I'm still pleased with this counter and it's capabilities, especially as I've just received a 5254A 3GHz frequency converter plugin. Here's a picture of it in us to debug a phase locking problem in a spectrum analyser's 1st local oscillator. The counter reading is added to the 3.0GHz setting on the converter's dial, so the frequency is 3052.028853MHz. Not bad for 1965.

Chris

[MrBungle]

Looking good