HP 5245L help with manual or schematic for very early model - Page 1
Forum section: electronics / repair

author: rkabz
author_rank: Contributor
timestamp: July 26, 2020, 11:53:00 am
content: Hi I'm restoring an old HP5245L counter and would like some help with finding schematics and PCB layout information. The serial number is 504-03114. I have three boards that are not covered in manuals I have found online. A19, A20 and A22 are early 'soldered in' versions of the boards and do not have the edge connector as shown in the manuals I have found. The circuitry is quite different as well. For instance there are no FET's on the A19/A20 boards, they are all bipolar transistors. This unit was being used as a doorstop in someone's shed and was exposed to a lot of dirt and water. The fan filter was partially eaten through it was so badly corroded. I powered it on at the place I bought it figuring I had nothing to lose and it fired up without letting out any smoke and all displays lit up! Since then I have spent a few weekends plugging away at cleaning the unit up and getting it up and running. It counts up in CHECK mode using START/STOP, but in any other mode the GATE lamp is stuck on and the frequency/period display is wildly out. The 1MHz oscillator and oven appear OK, all dividers work and all power supplies are nominal with no dried out electrolytic caps (have checked ripple). A21 and A23 appear OK. A22 is not gating or passing through a sensible gated signal. A19/A20 marked 5245A-65K Rex A, A22 marked 5245A-65M Rev B. Any help would be appreciated!!! Before and after pics of restoration so far attached...

author: bob91343
author_rank: Super Contributor
timestamp: July 26, 2020, 11:52:50 pm
content: I have spent a lot of time working on this model. Mine was old with germanium transistors. In fact, it would not count at one point and it turned out that a 2N404 was shorted. Luckily I had a spare. The ovenized standard oscillator was very good. I donated mine to a good friend, who still has it, along with some plug in modules.I have photocopies of the manual for this old timer. I could scan any given pages for you if I can locate the files.

author: srb1954
author_rank: Super Contributor
timestamp: July 27, 2020, 08:02:39 am
content: There is some complex logic implemented on the multi-gang rotary switches that controls the display annunciators (MHz kHz etc) and the decimal point positions. This logic also controls the routing of signals for the various counter modes. Check whether the correct annunciators and decimal points come up when you select various functions and timebase ranges. This may give you some pointer towards the fault. Unfortunately, the wiring to the function switches is difficult to access and very difficult to trace, even with a schematic.I also had a serial 504-xxx 5245L that had been stored in less than ideal conditions, although it wasn't in nearly as bad a condition as yours. One problem you may need to look out for is corrosion on the pins of the Nixies and on the leads of the neon bulbs. On some of my Nixies the corrosion cracked the metal-glass seal around the pin and the neon got out or the air got in, killing the Nixies. The leads on the neon bulbs were very corroded and would snap off at the slightest touch, right where the wire went into the glass seal. Access to replace the neon decimal point indicators is also very difficult.

author: rkabz
author_rank: Contributor
timestamp: July 27, 2020, 12:12:15 pm
content: Thanks for your input srb1954. All lamps/annunciators and decimal points work as they should, so switch 'logic' is OK. All Nixies display 0-9 when in CHECK mode with START active and all timebase frequencies work in this mode. I'm pretty sure that the problem is within A22 the gate control board. The gating signal makes it in to pin 27 but there is no gated output at pin 29. Problem is as I indicated in my earlier post, I don't have a schematic that represents the board in my unit. I might have to just trace it out myself if no one can dig up info on the board I have. I am also an Electronics Engineer (48 years in the game and still working at 66) - and early in my career spent two years repairing HP, TEK and other comms type test equipment in a telecom Radio environment, HF, VHF, UHF and Microwave radio and associated test equipment. It's just a bit easier when you have at least a relevant schematic. I have a decent DSO and a lot of other useful test equipment to fault find with, so I'll get to the bottom of this in the end. As you rightly pointed out, the logic is quite complicated in these units and there are a lot of switch contacts and wire mazes to navigate!! Switches have bee carefully spayed and cleaned with cotton buds. Just as an aside, a transistor in the back of the unit had leads corroded so badly the body fell off when I gave t a bit of a prod. Thankfully, nothing else is corroded to this degree.

author: rkabz
author_rank: Contributor
timestamp: July 27, 2020, 12:25:48 pm
content: BTW is 1954 your birth year?

author: srb1954
author_rank: Super Contributor
timestamp: July 28, 2020, 07:25:13 am
content: Correct

author: rkabz
author_rank: Contributor
timestamp: July 28, 2020, 10:30:36 am
content: Me too, born in March.