Repair of HP 5245L Nixie Frequency Counter - V1 finished , V2 in progress

[Cubdriver]

Will do!   

-Pat

[VK5RC]

An interesting comment re glass enveloped crystals I received from a fellow ham whose technical knowledge I would rate very highly, the 'glass' crystals pick up contaminants through the glass and this eventually can make the crystal off frequency HOWEVER after a few MONTHS of being left ON and HOT the contaminants will often evaporate and return the crystal to frequency. I am not planning to try this as my lab is not close to the house and I leave it unpowered except for 1 or 2 very reliable components. I thought very interesting. it might also be why HP wanted the oven/crystal left powered ON.

[Kleinstein]

The glass is usually quite tight, unless it is somewhere cracked. So nothing serious coming in from the outside, except maybe small amounts of oxygen, hydrogen and helium. The bigger trouble is volatile stuff moving from the walls to the crystal and back - this can happen during heating and cooling.

Having the oven hot will also speed up aging - so long term drift will be higher with a running oven.
Turning the oven off and on may give some variable degree of hysteresis, but I am not sure this old counter will resolve / note it, as most of the time the turn on sequence will be rather similar.

[VK5RC]

In buying a few plug ins for the 5245L I found an "As Is " unit for the price of the plug in.
The 5245L looked a bit rough -  see photo 1 - but not much to lose.
Opened her up - no nasty surprises - all the bits there - no bulging caps. Ohms test of the mains plug gave about the right figures. Fired her up with an AC lab supply slowly over 5-8mins or so. Drawing about 0.6A (with 230V AC).
Looked at the back of the unit - switched the frequency source to its internal unit! A counter always works better with a frequency source!!!!
She works, I had a few close fit knobs, I don't have a "double" knob (I don't know the correct terminology) so put a cheapie on and drilled hole for the small central pot control to stick through. Gave it a 15min clean with IPA.
Looks Good (to me)
Final photo is it counting a GPSDO, early after switch on. It got to 9.999 985 - no adjustment.

[Cubdriver]

Wow, Rob - that cleaned up quite nicely!

I'd be inclined to swap the sample rate knob with one of the double barred ones on the back panel to add a bit more uniformity to the front.  I'm curious to see how accurate relative to the GPSDO it winds up after it's been on for a while.  Congrats on getting the old beast back up and running - looks like it's a 1967-ish one.

-Pat

[bitseeker]

Wow, amazing what a frequency reference can do. All the Nixies look good too.

[VK5RC]

Thanks guys, I am amazed some eBay sellers put so little into selling the item, the clean up took me 20mins, he/she could have asked for another $100-150.
I have some original HP knobs coming but they are a little bigger, I think I will put them on the back, bring the back ones to the front. For the dual knob, I have a spare  little Tek red knob and have tried (successfully) drilling a hole in the sensitivity knob to make a hybrid dual knob.
Rob

[bitseeker]

Knob hacking! I like it.

[VK5RC]

I like the term "Knob hacking" , first coined by bitseeker
I used a cheap knob first,  seen here on the sensitivity control, I used a 6mm drill down the collett to find centre, then drilled a 3mm, then a 6mm holes from behind. The red little Tek knob is 10.3mm in diameter, so I drilled a 10.5mm hole from the front but used a drill press at quite high speed, with a low feed rate. I used a 6mm drill to hold the knob (held on by its own grub screw)
 I think I will try one of the bigger spare HP knobs next.
Below is what the counter settled down to after about 1 1/2 hrs with a GPSDO input.
I have been battling which plug ins can be used in which 'serial numbers' 5245Ls, as there was several changes throughout production , once I think I have it correct I will reproduce it here.

[bitseeker]

Rob, if you hadn't said anything, I'd have thought that concentric knob hack was possibly original. Well done!

[VK5RC]

I am reasonably happy with the hack. See below.

[bitseeker]

Yep, good stuff. Such clean knobs, too!