HP 5381A Frequency Meter

[beanflying]

Not a lot here on these but they date back to the early 70's and until the early 80's. Introduced as a bare bones simple accurate frequency meter (long before sub $20 delivered and evilbay). This one cost me a touch under $50 delivered in an Auction.

This one has the fairly common LED display issues and looking at the manual carefully reveals at least 6 different LED combinations were used all HP branded. The units aluminium case gets fairly warm which wouldn't help.

Apart from the display issue it is reading within good tolerance against my 34401a from 1k-100kHz at low frequencies below 10hz it really falls over which is as per it's spec. Upper end to 30MHz aligns with what signal generator I currently have (better stuff on the way). Where I got it for was higher frequencies where the 34401a and my handheld meters won't go and 80Mhz is plenty as I don't do any RF.

The manual for any of the 5381a, 5382a and 3a makes for some good reading on how 'proper' electronics was done back in the day (IC's are date coded late 1973)

So worth keeping an eye out on evilbay, shipping will be more than the cost of the unit.

Mods coming up for mine:

1MHz OCXO (units timebase) internal oscillator using a divided 3.2Mhz Polytron. Not in a rush for this as the unit is stable and with an IR gun the current XO is running over 35+ degrees so the stability is good.
Fix LCD display.
Use for the next 4 decades 

Opening the case was a good/bad surprise. The LED plastic baggy and paper around heat and sensitive bits are not a good combo. Bonus there is one LED of two needed for the display.

[beanflying]

The Best laid plans of Mice!

Seems the LED in the box was WRONG. 12mm into a 10mm width doesn't work.

The current LED installed on mine is HP 1990-470 and there is no datasheet anywhere I could find.  Following the trail and the circuit diagram lead me to second hand pin compatible parts 'HP 5032-7731' scattered around evilbay secondhand for to much money.

More searching of datasheets and eventually turned up Broadcom via Mouser with a '5082-611' as Broadcom used to be Avogo who used to be a division of HP ......... 

So if I have this correct Current link to Mouser is https://au.mouser.com/productdetail/broadcom-avago/5082-7611?qs=sGAEpiMZZMvkC18yXH9iIsSFKp4VzurGsCKCtjcFOB8%3D

Seems I might be making an external reference or dropping the double insulated OCXO back to a single can. 

Huge current XO next to a more than warm Transformer = Fairly Stable tempco.

[Q2A]

I just came into possession of one of these with some segments out. This is VERY helpful. Thank you!