HP 8116A funct/pulse gen fan, battery - Page 1
Forum section: electronics / testgear

author: jonpaul
author_rank: Super Contributor
timestamp: December 23, 2020, 11:07:27 am
content: Bonjour a tous:Working on HP 8116A, funct/pulse gen, opt 001, with many fine characteristics: max 16 V out OC and 8V into 50 Ohm, very low F to 50 MHz, pulse and burst, ext VCO but no sweep functions.A few notes and questions:1. Unused for 10-20 years, date codes 1983.freq, ampl in spec, ~ 1-3%. Suggest use of external freq counter on TRIG out to chack freq is higher accuracy is needed.Surprisingly fine audio distortion ~ 0.35..0.50% on Tek AA501.2. rather loud fan, 80 mm high RPM, early brushless with Buhler module drive.The linear power supply and especially several HP ICs with heatsinks depend on the cooling.Old forum posts mention damaged units with disabled fans.Anyone with experience on disassembly of fan/PSU, and replacement of fan for lower noise?3. Backup battery for recall of last setup on power off, rather than for CAL or other parameters.NiCd 3 cell, 3.6V axial. dead and was leaking, Replaced with modern NiCd. Anyone tried NiMh instead of NiCd?Many thanks!Jon

author: MadTux
author_rank: Frequent Contributor
timestamp: December 23, 2020, 09:39:18 pm
content: Without or with bad fan, the ASICs in there will fry.Battery isn't really necessary, IMO more trouble than it's worth, so I'd just remove old battery and use it without one.Or perhaps replace with an ultracap, so it remembers the settings for a few hours or so.

author: jonpaul
author_rank: Super Contributor
timestamp: December 24, 2020, 11:12:31 am
content: Hello again found fan controller is a 3 phase Buhler module and not sensitive to input VC V.WOuld have to replace the fan or modify the Buler module, both big jobs to disassemble as PSU, heatsink and fax are in a complex mechanical.Anyone else removed or replaced the fan or power module?To Madtux: Had this instrument for decades, what is the issue on the HP ASIC the three with HS?Overheat? Bad airflow? High dissipation? Anyone ever placed a thermocouple and measured the temps?Many thanksJon

author: jonpaul
author_rank: Super Contributor
timestamp: December 24, 2020, 12:12:18 pm
content: I just measured the 3 ASIC heatsinks, with boards in test position and no output load.~ 25 - 35 deg CMadTux are the ASIC failures heat related?Your idea the normal HS temps?Many thanksJon

author: precaud
author_rank: Frequent Contributor
timestamp: December 24, 2020, 01:43:39 pm
content: I pulled the fan in one of 'em for cleaning/lube years ago but have forgotten the details. It did require disassembly of the rear of the unit.Mine still has the original NiCd and holds settings for a few days. Fan noise on it is still tolerable. As Madtux suggested, they do run hot inside, specially around the output amps. Cooks the 'lytics. Since mine is ugly and I'll probably never sell it, I drilled more/larger holes on the underside to get more airflow into that area.I love the user interface. That's why I keep it.

author: jonpaul
author_rank: Super Contributor
timestamp: December 24, 2020, 04:23:02 pm
content: Rebonjour a tous,The user interface is indeed very fine, and the range of freq is useful. My unit has the 001 opt and is very clean, I doubt if it had much use. Only flaw is the sticking rocker switches, ISO alcohol flushes have not fixed that, nor contact lube.The Buhler fan driver is connected to the +5/-5V unreg power thru a 47 Ohm 1 W resistor. The module wires go to A1 PCB on 0.32" round pins.I removed the pin connection that goes to the -% side and placed an R sub box and ma meter, it draws 156 mA.The fan speed is proportional to the current and a 39 Ohm added series R reduces current to 113 ma and the fan noise to acceptable level.I have a GR strobetac, easy to check RPM vs current.To check, the ASICs and output transistor HS should be at worst case temp for highest load/Vout and highest freq.Shall put #36 type K thermocouples on the parts and use external R sub, then reclose cabinet and take some temp data.Open to your comments and advise.Jon

author: bdunham7
author_rank: Super Contributor
timestamp: December 24, 2020, 05:38:04 pm
content: Quote from: jonpaul on December 24, 2020, 04:23:02 pmThe hybrid ICs are a common failure in the 8112/8116A, and while I can't say definitively that they age and fail because of temperature, I can tell you that marginal ones will often work if they are overcooled (unit in service mode, fan right on ICs). But then they will go out of spec and cause boot-up errors if they are used normally. Sort of like the Tek U800 chips. I'd be interested to see what you get for temperatures, but since there's no spec that I'm aware of, I don't know what can be concluded from any results you get. My observation is that they don't really get all that hot compared to semiconductors in general, so traditional notions about what a good operating temperature is may be incorrect here.What I've done on two of these is replace the fan with:https://www.allelectronics.com/item/cf-408/12vdc-60mm-cooling-fan/1.htmlThe first, which was for someone else, I just left the 47R resistor in place and it ran right about 12 volts and seemed to blow a bit more air with less noise. Certainly not quiet, but the noise was less annoying than the rattly, whiny old fan. On the second, with a marginal IC, I tried putting in a 22R resistor, which increased airflow and noise, but didn't completely solve my IC problem. The unit is usable, sort of, but you need to have a scope to adjust the output as the calibration limits have been reached and the timing is still off. This is the acid test as to whether you actually have a working unit--being able to calibrate it.An issue you may have with the timing IC (there's one on the 8116A, three on the 8112A, IIRC) is that as the hybrid timing IC gets warm, it may not completely fail but you will have trouble getting the unit into calibration or staying in calibration. On the 8116A, you also may have real trouble getting a non-distorted sinewave (although the triangle waves look great!). My recommendation would be to go for absolute maximum airflow and hope for the best. The fact that yours is in spec and low distortion is all the more reason to try and preserve it.

author: jonpaul
author_rank: Super Contributor
timestamp: December 24, 2020, 08:08:09 pm
content: bdunham7 many thanks again for the info.Indeed the unit is in fine cal on freq, amplitude and risetime. Just testing at max Vout, and max Freq, I am impressed.The unit has the OPT 001 so log and lin sweep and burst.Sine THD is ~ 0.35..0.50 % in audio band on TEK AA501.I concluded the unit is in cal and was lightly used or had been CALed by previous lab or owner.I am sure the noise of the Buhler is due to worn bearings, etc. so fine idea to replace the fan.Enjoy,Jon

author: jonpaul
author_rank: Super Contributor
timestamp: December 25, 2020, 04:43:12 pm
content: bdunham7I ran it a few hours closed up on the reduced fan speed.If I notice any issues I will do the further temp tests.With fan quieter and rockers fixed, its OK for now.On to many other projects!Enjoy!Jon

author: HarryDoPECC
author_rank: Regular Contributor
timestamp: December 26, 2020, 01:18:21 am
content: The NiCd in my 8112A died and leaked. Lots of blue oxide on nearly IC legs. Cleanup with vinegar->distilled water-> isopropyl worked well and no major conductor damage was revealed.Battery was replaced with 5v 1F supercap. New charge resistor calculated to be 200R (NiCd has 2k in HP schematic) but was found to already be 200R!Cap charges in 5-10 mins and holds settings for at least ten days.This works very well for me. I only use the 8112 sometimes, I set it up and it keeps the settings while I am doing that particular job. Power on every week or so and settings stay just fine. When job is done I put the 8112 away and next time I use it, weeks or months later, cap has discharged and it is automagically back to factory defaults.