Short circuit on power on of HP 608C - Page 1
Forum section: electronics / repair

author: DaneH3
author_rank: Contributor
timestamp: August 02, 2019, 11:07:16 pm
content: So recently I came upon an HP608C Vacuum Tube Signal Generator. I knew when I got it that it didn't work, probably due to being so old and needing parts replaced.Since I wanted a good Signal Generator for my testbench, and this one offers an extremely accurate output from 10-480MHz - I opted to try my hand at repairing it.I replaced all of the bumblebee style capacitors with modern film capacitors of the same rating and replaced all of the old dry electrolytics with modern high-quality ones. I made completely sure that everything was wired properly since this was my first time doing this to such an old machine.Anyway, now on to the problem.After first plugging it in and turning it on - the fuse blew! After replacing it with a slow blow, suspecting that it may need one due a high startup current, that fuse blew - too. So there isdefinitleya short somewhere. I checked all my wiring and soldering against the schematic, and everything looks fine and dandy!That's why I'm posting here now, I'm at a loss when it comes to what to check, and I'm sure people here know plenty more about old tube power supplies than I do.Here are some images of the power-supply section of the machine:https://imgur.com/a/s7rDHRVIt looks to me as though someone replaced the CR6 rectifier with slightly more modern (60's) diodes. That leads me to wonder if metallic rectifiers are a notorious problem? Perhaps I should replace the others as well?Any help or advice is welcome, and thanks in advance!Here is the schematic to the 608C:

author: tkamiya
author_rank: Super Contributor
timestamp: August 02, 2019, 11:22:00 pm
content: Well, I had two of these. One is still on my bench. Something this old may or may not be suitable for your project. For example, you can hook it up to an ordinary reciever and SCREAM into the generator, you can get nice modulation out of it. Being a ham, I had a QSO with my friend with it.Since fuse is blowing, the first thing to do will be to disconnect power section with the rest and see if it still blows. That will cut your problem in half. I can't think of any short cut.

author: DaneH3
author_rank: Contributor
timestamp: August 02, 2019, 11:32:09 pm
content: Quote from: tkamiya on August 02, 2019, 11:22:00 pmI might not completely get what you're saying? It would be pretty difficult to separate the regulated power supply from the rest of the circuit, since most, if not all of the tubes are powered directly off of that section. Separating that many connections would be a hassle and a half.Oh and uh, how did you have a QSO using a signal generator?

author: tkamiya
author_rank: Super Contributor
timestamp: August 03, 2019, 12:04:32 am
content: Well? Remove high voltage. Does it blow? Remove heater voltage, does it blow? Remove bias and misc voltage, does it blow?Easier way is to do it at the transformer terminal.Signal generator is a mini-transmitter. I have receiver and SG hooked up to a common antenna via a switch. I just screamed into air vent. I've done the same think as a grip dip meter. I've done something similar using light bulb as an antenna. Those were fun days.

author: tkamiya
author_rank: Super Contributor
timestamp: August 03, 2019, 12:05:45 am
content: Quote from: tkamiya on August 03, 2019, 12:04:32 am

author: 0culus
author_rank: Super Contributor
timestamp: August 03, 2019, 12:12:21 am
content: If you still want an older looking but way better performing (and solid state) VHF/UHF capable sig gen, try to look for a nice-condition 8640B. It isn't a synthesizer, but it does have a very clean cavity oscillator and a phase lock feature that holds steady pretty well. The big weakness, of course, is the gearing in the front panel. I was lucky to find an extremely well cared for example with basically all the options.

author: JFJ
author_rank: Regular Contributor
timestamp: August 03, 2019, 09:57:29 am
content: Quote from: DaneH3 on August 02, 2019, 11:07:16 pmYou could use a lamp limiter (an incandescent light bulb in series with the mains supply) to prevent the fuse from blowing. Thereby, giving you time to take some live test measurements. Something like this:It would, for example, allow you check the transformer secondary voltages, which might provide a pointer to the part of the circuit that's shorted. Also, it should help to distinguish between an inrush current problem and a short circuit. With a high inrush current, the lamp will initially glow brightly and then dim. A short circuit would cause the lamp to continuously glow brightly.