HP 8447A RF Amplifier and others bulb for power switch - Page 1
Forum section: electronics / repair

author: W3KW
author_rank: Contributor
timestamp: July 20, 2025, 08:29:51 pm
content: I need a replacement bulb for an HP 8447A RF Amplifier. The white, round power switch pulls out and a small bulb is seated inside. The push button power switch was popular on many vintage HP gear from the same era. Grok search states…”use the Chicago Miniature CM8-683.” I can’t find a source. The base may be a T1? The writing on the bulb reads OAK A1H or AIH. Not sure if it’s a 1 or an I.The amp was a cheap eBay find as the seller assumed it was inop based on the bulb being burned out. I used my RF Sig Gen and Spectrym Analyzer to verify it’s actually working perfectly (aside from the pilot lamp) at 28 dB gain. Nice cosmetics also. If I can find the bulb, it’s a huge win for my collection of vintage HP test gear. (That I actually use in the shop).Thanks in advance.

author: edavid
author_rank: Super Contributor
timestamp: July 20, 2025, 08:49:00 pm
content: Looks like an A1H neon bulb. They are available at high prices from Amazon or eBay, but I think most people stick an LED in there instead.https://www.amazon.com/OCS-Parts-Flanged-0-625-Inch-02-Pack/dp/B0CYR6V7G7https://www.ebay.com/itm/256461588870

author: W3KW
author_rank: Contributor
timestamp: July 20, 2025, 08:57:23 pm
content: Wow. Thank you. That was super fast. I ordered two at Amazon and yes, you are correct, they look just right. I will update the thread with a confirmation when able.

author: W3KW
author_rank: Contributor
timestamp: September 20, 2025, 09:33:08 pm
content: Ok. Some Deoxite got the 8447A power switch working including the lamp, but only temporarily. It's still intermittent. It has to go. So, the manual states S! = SPDT -DB. Ok, Why DT? What am I turning on or shorting out when I turn off the switch? Another head scratcher. Why are there 6 contacts on the switch?? Clearly, two are for the internal lamp. I get that. But that leaves a SP-DT switch with 4 contacts. Huh?Oh, I did measure the output (before the power switch crapped out) as a solid 20 dB gain so I'm pleased with that at least.The schematic clearly shows the switch being SP-ST?? The label on the side of the switch shows DP-ST plus the LED. Hence, six contacts. It looks original. Maybe someone has been inside fiddling? Because the wires are undocumented and the switch isn't working, it's impossible to use an ohm meter to determine the wiring, though looking at the lamp wire with a 27K resistor on one terminal gives clues.Does anyone have an 8447A in the lab who could pull the bottom cover and solve this mystery for me? If not, some serious digging, prodding, and disassembly will be required.

author: wn1fju
author_rank: Frequent Contributor
timestamp: September 21, 2025, 12:55:18 am
content: I happen to have a hard copy of the 8447A manual. Inside were the change sheets. They changed the switch to 2-pole at some point. Revised schematic is attached. In case the wire color numbers on the schematic aren't easily read on my picture, the top row is 908, 918 and 8, the middle one is 928 and the bottom row is 98 and 94 (all left to right).

author: W3KW
author_rank: Contributor
timestamp: September 21, 2025, 01:12:19 am
content: Oh man. Thank you. Pulling my hair out. LOL My manual was missing the changes!