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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 8th Nov 2019, 4:55 pm   #1
factory
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Default Crystal sizes?

Can anyone help, I have recently acquired a pair of HP meters (34740A & 34750A) from the USA, I've found out that to convert them to 50Hz operation requires changing the quartz crystal from 3MHz to 2.5MHz.

Measuring the original crystals, they seem to be closest to HC33/55 can size.
The problem is I can only find 2.5MHz crystals in a smaller can size, will a smaller one still work OK in these, or do I need to find the HC33/55 size?

David
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Old 8th Nov 2019, 5:09 pm   #2
G6Tanuki
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Default Re: Crystal sizes?

Can't see how tall it is, but that oval profile looks very much like the HC6/U types we used in Pye two-way radios half a century ago.

A smaller-cased one will work - the only issue is likely to be the load-capacitance: do you know what it should be for the existing xtal? If not, specifying 30pF would probably be OK, since it doesn't look like in this circuit you're needing to-within-a-few-Hertz precision.
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Old 8th Nov 2019, 5:19 pm   #3
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Default Re: Crystal sizes?

The crystal is approx 3/4" tall an the pin spacing is 1/2".

No idea what the load capacitance is for these, the manual only gives HP part numbers, which I can't find much info for.
http://hparchive.com/Manuals/HP-3470-Manual.pdf

The crystal is part of the display module half, which uses a dual slope integrator, the voltage discharge slope is counted by pulses from the "precise timing interval" (as the manual describes it), there are various different function modules that can be attached to the display module.

The 34740A is a 19999 count display and the 34750A is 199999 count.
Apparently these need the references recalibrating if used on battery operation, due to the temperature difference.

David

Last edited by factory; 8th Nov 2019 at 5:44 pm. Reason: Extra Info
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Old 8th Nov 2019, 7:20 pm   #4
Jon_G4MDC
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Default Re: Crystal sizes?

wasn't HC33 some wire ended version of HC6?
I don't know HC55 I'm sorry.

One thing with all crystals - wire ended or otherwise.
No shortening of the pins with side cutters. The shock will shatter the plate.
Use something with a scissor action or wreck them at your peril.
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Old 8th Nov 2019, 7:41 pm   #5
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Default Re: Crystal sizes?

Sorry there was no HC55, it was a typo of HC51.
I had discounted HC6 because pictures showed them being twice the height, which wouldn't fit in the meter, they need to be wire ended too as there are soldered in.

The only 2.5MHz crystals I can find are HC49 size, thanks for the warning about cutting the leads, I might not need to do this as they will need bending to fit the 1/2" lead pitch.

David
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Old 8th Nov 2019, 8:14 pm   #6
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Default Re: Crystal sizes?

I'm almost tempted by some DIP14 crystal oscillator modules I've just found in Europe.

David
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Old 9th Nov 2019, 12:41 am   #7
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Default Re: Crystal sizes?

Should be fine with the full size HC49 (the smaller HC49S version seems to have worse Tempco which you could probably do without).

Don't forget you can't (easily) trim those DIP Oscillator modules onto frequency.
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Old 17th Nov 2019, 11:20 pm   #8
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Default Re: Crystal sizes?

Thanks for everyone's help, I've ordered some full size HC49 crystals, hopefully they will work fine.

David
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Old 18th Nov 2019, 7:42 am   #9
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Default Re: Crystal sizes?

Great precision isn't needed, it's just to get the frequency response nulls inherent in the dual-slope timing to coincide with local mains harmonics. Best check there aren't component changes in the integrator area as well.

David
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Old 24th Nov 2019, 12:34 am   #10
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Default Re: Crystal sizes?

Well I couldn't find any other component changes, there are no adjustments for the crystal frequency, all the adjustments are for setting voltage readings only.

The crystal & related timing circuits are indeed for mains frequency noise rejection, the number of readings per second changes too (5 per second at 60Hz & 8 per second at 50Hz), I wonder what they do for 440Hz operation?

The replacement crystals arrived today and have been fitted, the 34740A is now tested & working OK with the multimeter & thermometer modules (I still need to find out the changes for the thermometer module for 50Hz noise rejection at some point too), the 34750A needs further diagnosis & repair as it currently only displays zeros.

David
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