HP 105B - Page 1
Forum section: electronics / general

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2020, 08:33:25 pm
content: I got one of these and I opened it up. I was a little concerned the batteries were removed and the metal ended clips for battery were just loose inside so I taped them up for good measure. I'm hoping this will be testbed to test 10811 oscillators with? I'll post some stats once it's warmed up some.Bill

author: tkamiya
author_rank: Super Contributor
timestamp: September 28, 2020, 02:38:47 am
content: I removed my batteries on purpose. It's just not needed. Some say it's part of the circuit and used as ripple reduction filtering. This is not true. It's only a backup battery. HP105A is same thing just without batteries.Be sure to pull the fiber glass corse adjustment rod and verify the tip is still in tact. Mine was threaded. Used superglue to put it together again and filed it to proper shape.As you work on this, right by fast/slow charge switch area, there is a live and exposed 115V AC. (poor design!), so be careful.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 28, 2020, 04:28:51 pm
content: I have an HP 105B with the old oscillator that is running properly with new batteries installed. What I found was that there have been big advances in NiCd batteries from when this unit was built in 1978 and today. The original cells were no longer available and I replaced them with sub-C size cells that keep the unit running on back-up for over 10 hours, 2 hours longer than the specs say the original batteries would.What happened to the original battery pack was that over time cells started to short out and eventually the battery pack was just a heavy load on the supply and the current limiting resistors in the power supply on the circuit board overheated and burned the board in the area of the resistors. Replacing the old cells with new NiCd batteries required changing the current limiting resistors so the new cells would charge properly and not overcharge/overheat. I also had to replace one zener that had shifted value high by 2 volts.The exposed mains voltage in the area of the fuses and switches on the back panel is worse in your unit because a protective shield with a warning is missing. Check out the proper shield in the photo of my unit below. The older oscillator in my HP105B had a 120VAC fast warm-up connection but this isn’t needed and was disconnected. A more serious design problem already mentioned is that the fast/float light on the front panel is a neon that is powered from 120VAC and it is possible to accidentally hit these leads on the switch not knowing they are hot. What I did is cut the back off the neon holder and put an LED in place of the neon. I then made a couple of connection changes in the power supply area using existing wiring to power this LED from low voltage. The series resistor was chosen to give a reasonable level of light with the LED I used.

author: tkamiya
author_rank: Super Contributor
timestamp: September 28, 2020, 05:28:16 pm
content: Back of the box exposed mains area is certainly missing a cover and dangerous - but one would expect that. On mine, (and I'm sure on everyone), there is a main voltage on front panel. There is no marking, there is no cover, nothing. I wasn't expecting 120V there. I would certainly appreciated a cover or at least a sticker.

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 28, 2020, 05:53:03 pm
content: Yeah 120V at the back of the front panel is pretty sketchy to say the least.I like Arthurs work around for this though... can't hurt having a low voltage LED there instead. Despite the fact of how much I like nixie tubes is one thing but at mains level would bite you considerably more than even a nixie power supply would at 170V.Bill