Wavetek 188-S-1257 Repair: No DC Offset- - Page 1
Forum section: electronics / repair

author: ErichKeane
author_rank: Newbie
timestamp: March 08, 2023, 03:02:44 am
content: Hi all-Absolute newbie here , I'm pretty handy, but have very limited electronics knowledge (and mild soldering experience!), just a software guy by trade. I have a half-decent "lab" put together as I'm trying to learn a bit (including a couple multimeters, oscilloscopes, power supply, and a function generator).I found the Wavetek 188-S-1257 cheap on ebay, and picked it up. On the bright side, it seems to work basically perfectly! Internally it is really clean, and it doesn't look messed with (additionally, the board doesnt have any Surface mount stuff, and is really nicely laid out for a newbie). The only problem that I am knowledgeable enough to know is a problem: The DC offset knob does nothing.Here is a manual that I've been using (note it is a slightly different model, and I've noticed a few small differences, but the important parts for this discussion seem to be right):https://www.pearl-hifi.com/06_Lit_Archive/15_Mfrs_Publications/38_Wavetek/Wavetek_188_Sweep_Fctn_Gen.pdfPage 43 in the PDF is the schematic that I've been looking at. Lower right you can see the DC offset switch in the output amplifier section.I've tested everything around the switch, I get -15 and +15 volts (or there abouts) on the resistor input, and when i rotate it, half of the "SW15" part of the switch gets that volts. However, I never get continuity between the two lugs. IF I short those two pins, the DC offset seems to work perfectly.This has lead me to suspect the switch inside the poteniometer. Taking it apart looks like it might be destructive (I think it is riveted together!), so I'm tentative to try without a backup.However, I'm having no luck finding a replacement. From the manual I see the part number as:Wavetek 4602-01-0300 (this number is ALSO on the pot itself)CTS GH-1879Additionally, it has written on it R1379209, which LOOKS like a CTS part number, but google/mouser/digikey don't seem to know about it.I DID find this page:https://nationalstocknumber.org/nsn/5905-01-214-0612which seems to have info about it, but searching that NSN also doesn't find anything.Is there a secret to finding a replacement part here? Alternatively, is there any magic techniques for how to fix this without taking it apart? Even better if I don't have to take it off the board,as the pot-side of the wires have other things in the way that would need desoldering to move.Thanks all!-Erich

author: ErichKeane
author_rank: Newbie
timestamp: March 09, 2023, 03:21:31 am
content: So I spent a bit more time with this today. I am still completely unable to find a replacement potentiometer. However, I convinced myself to just go for it, and desolder the potentiometer (and a few wires in the way!), and put it on the bench. It recreated the 'always open' on the bench. I sprayed an unhealthy amount of CRC QD Contact cleaner in every open hole on the potentiometer and let it dry. Then, it worked perfect! As soon as the switch 'clicks' over, it goes to ~.2!I soldered it all back together, and it seems to work perfectly! So I believe that was the problem, and I was able to fix it without taking the potentiometer apart.

author: donlisms
author_rank: Frequent Contributor
timestamp: March 09, 2023, 04:36:27 am
content: We are pleased with your efforts.(Nice work!)

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 09, 2023, 05:07:04 am
content: So that is mission accomplished! My kudos are on their way to you sir!