HP 8640B Signal Generator switch disk & plastic gear repairs - Page 1
Forum section: electronics / repair

author: TimInCanada
author_rank: Regular Contributor
timestamp: May 23, 2017, 12:19:55 am
content: Hi all,I picked up a 8640B that turned out to have these common problems. Here are cheap fixes that seem to be working so am putting this up in case someone else can use the info.There are four rotary switches that use a plastic disk with two to four metal contacts heat staked on two tiny plastic pins. The pins break, releasing the contact. If you're lucky the wayward contact can be found inside the cabinet. The disks are reversible and contacts can be moved to the plastic pins on the other side of the disk.If not lucky the contacts are gone. My unit was missing two. A card edge connector from the junk box came to the rescue:The grey goop is JB Weld epoxy, but these contacts aren't under much load so other adhesives will work. The disk surface was abraded for better adhesion. The end of the card connector contact was clipped off. The rest was pushed into the glob of epoxy. Once the epoxy hardened the exposed ends of the contacts can be bent to the right positions.The Range and Deviation switches are geared to a differential that operates a third rotary switch whose output is a function of those two switches. The gears attached to the two switch shafts are plastic, probably acetal (Dupont trade name is Delrin), with a brass hub insert molded in. The plastic has residual tensile stresses from the molding operation. Stress concentrations at the setscrew holes cause cracking to start there. The gear on the Range switch shaft eventually splits out a sector between the two setscrews.I didn't know this repair would work so didn't think to get before pictures, but if you've got one you know what it looks like. There's someone out there who will make replacements from brass, but the cost is US$ 225.Measuring the width of the split showed the plastic shrank about 1% once the split allowed the tensile stress to be released.To get the brass hub out I put the gear in a pan of water and boiled the water on the stove. The plastic has about eight times the coefficient of thermal expansion as the brass and you can actually watch the split close up as the plastic warms up. (Take care to make sure there is water under the gear and not just the hot metal pan.) Acetal is rated for use at 250F/120C so boiling doesn't harm it. Once it's hot the hub can be driven out of the gear easily.The brass hub is knurled. To reduce its diameter I used one of the setscrews to clamp it on a 1/4 inch drill bit, then spun it with a drill and filed off most of the knurling. This kept the bore and outside concentric. The plastic gear was clamped together with a wire (see picture below). It was still too tight on the filed-down brass hub so I used some medium (150 grit) sandpaper wrapped around a drill bit to sand the hole bigger. The sandpaper on the drill bit was held in one hand and the gear was rotated over it with the other. A drill bit was used to make a sanding mandrel that was round and just a little smaller than the hole. Care was taken to sand evenly around the bore and it was checked frequently for fit over the brass hub. I wanted just a little clearance between the two so some epoxy could fill the gap and the plastic gear left without tensile stresses.More JB weld and some wire for clamps:When the switch module was reassembled with the gears oriented correctly and the setscrews tightened I put another of these wire clamps on the two gears with setscrews. You can see the green gear has a crack through the plastic at the setscrew hole, but it hadn't progressed far into the rest of the gear yet. If the wire clamp keeps the crack from opening at the hub then it shouldn't propagate through the gear, or at least that's my hope.Adhesives don't develop much strength on acetal plastic unless special surface treatments are used. Fortunately these gears carry very little torque.Another problem with this 8640B was that the Range switch was very hard to turn. That problem turned out to be from the cam shaft located behind the Range/Deviation switch module and connected with a plastic coupling to the Range switch shaft. The cams are inside an aluminum housing and the shaft rides on a bronze bearing at each end. Old grease cause the shaft to seize to the bearings and the bearings were turning in the aluminum housing. Mineral spirits quickly dissolved the old grease. Once the old grease was wiped away and new grease put on the shaft, it turned freely in the bearings.The detent on this cam shaft was also very stiff. It's a roller on a spring. Reducing the bend in the spring by about half made the cam shaft much easier to turn while stilll stopping in the detent positions.Hope this helps keep these 8640Bs going.Tim

author: xrunner
author_rank: Super Contributor
timestamp: May 23, 2017, 12:26:21 am
content: Good job and thanks for the writeup. Interesting approach to the gear repair. There's quite a few of us here that like to repair vintage test gear, so any documentation on repair tips is always appreciated.

author: tautech
author_rank: Super Contributor
timestamp: May 23, 2017, 12:43:55 am
content: Quote from: TimInCanada on May 23, 2017, 12:19:55 amThanks for sharing.Sue had similar problems with those contacts in her SA.Her journey:https://www.eevblog.com/forum/repair/hp-8569-spectrum-analyzer-repairs/

author: jca1955
author_rank: Newbie
timestamp: September 26, 2018, 02:22:10 pm
content: Very helpful discussion!I recently repaired several HP 8640B generators that had lost rotary switch contacts and broken (or cracked) gears.Rather than overloading this reply with images and details, if you are interested you can see what I did (and my recommendations) on this blog post:http://k6jca.blogspot.com/2018/09/repair-log-hp-8640b-rotary-switches-and.html- Jeff

author: Radio Tech
author_rank: Frequent Contributor
timestamp: September 28, 2018, 09:52:33 am
content: My very first YouTube video was on repairing one of these units. It had both problems. Broken gears and missing contacts. But also had a broken switch.Was able to get my first three videos on the repair.I have noticed that the frequency counter dose not work in that mode.Good job on the repairs guys.