Failing encoders in a Tek TDS-320 scope - Page 1
Forum section: electronics / testgear

author: Calambres
author_rank: Frequent Contributor
timestamp: June 09, 2022, 09:40:22 am
content: Hi there!My Tek TDS-320 scope recently developed a courious syndrome: all four encoders (Vert. and Hor. position, Trigger level and multifunction encoder) behave randomly and "jumpily". The other two rotary knobs (VOLTS/DIV & SEC/DIV) work perfectly but they're not encoders but rotary switches.I extracted the panel, cleaned all encoders with generous amounts of IPA as well as the connectors for both flat cables to no avail. They still work randomly. To my surprise, when the scope is on for some time, all encoders begin to work OK againI've seen this youtube from a guy that's having the same issue with just one encoder (the multifunction knob) and how he found that there was some internal short and how to "zap" it out:I'm refraining from going that way because it does not seem to me coherent with everything working OK again once my scope is running for a while and hot. Don't see how the operating temperature can make such an internal short to disappear in all four encoders at onceDon't know what is really happening here... Any ideas?This is one of the offending encoders (img. taken from the aforementined youtube)Code: ESZ0D-S00-ES0002 311-2534-00-0103X

author: wasedadoc
author_rank: Super Contributor
timestamp: June 09, 2022, 11:01:19 am
content: What do those four encoders have in common? Eg All ground and voltage fed by same PCB tracks, all are handled by the same processor?

author: Calambres
author_rank: Frequent Contributor
timestamp: June 09, 2022, 11:38:17 am
content: Yes, I guess so...

author: nctnico
author_rank: Super Contributor
timestamp: June 09, 2022, 12:44:23 pm
content: I'd check for leaky capacitors first. Maybe the electrolyte has eaten away a via or caused a short.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 09, 2022, 12:51:34 pm
content: I forgot to tell that I replaced the couple of SMT 30uF 10V lytics in the encoders board. One of them was leaking and damaged one pad but I managed to arrange it. Once desoldered the leaked one measured below 100nF. The other SMT lytic in that board was 20uF and no trace of leaks but changed it too just in case...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2022, 02:55:44 pm
content: Was that same or similar design to the TDS460? The early ones (pre -A?) are notorious for leaking caps; go through and do them all, and wash the boards if you can.I'm also having occasional encoder trouble with mine (TDS460), where -- I assume something like, a mass train of pulses is generated, overloading the input MCU (interrupt driven IO?), crashing it. Else, glitchy comms (back to main CPU) or power could do it I suppose. Then only the power button responds (which is routed directly to the main CPU or PSU, something like that). They seemed to be better after using contact cleaner on them, but that also only lasts so long. (Similarly, I contact-cleaned my function generator around a ballpark similar time (~5 years ago?), whose I think silvered contacts are beginning to tarnish again -- crusty pots and switches.)Tim

author: Calambres
author_rank: Frequent Contributor
timestamp: June 09, 2022, 03:39:42 pm
content: Quote from: T3sl4co1l on June 09, 2022, 02:55:44 pmVery similar design. Mine has a 1994 date label so it's now 28 years oldI already thought about reccapping it all but I am afraid that it has nothing to say with it working properly after 10/15 minutes from power on. That's what's driving me crazy...

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 09, 2022, 03:55:46 pm
content: Well, caps warming up tend to reduce their ESR. Could be marginal caps elsewhere in the system.Tim

author: Calambres
author_rank: Frequent Contributor
timestamp: June 09, 2022, 05:17:55 pm
content: Very true!

author: jonpaul
author_rank: Super Contributor
timestamp: June 09, 2022, 05:20:45 pm
content: Leaking SMD lytics corrode the PCB and contacts.IPA cleaning helps but damaged traces and corroded contacts need more work.PCB internal layers are also suspect.Warmup>>better>> clew it is related to corrosion residue.Jon

author: Caliaxy
author_rank: Frequent Contributor
timestamp: June 10, 2022, 04:05:56 am
content: Quote from: Calambres on June 09, 2022, 09:40:22 amI have a similar story. Years ago, I bought an entry-level 150 MHz Keysight oscilloscope off eBay (that one that it was rumored to have been manufactured by Rigol). None of the rotary encoders were working (but everything else worked fine). I dismantled it and cleaned all of them to no avail. I ordered quality replacements from Mouser. Why they arrived, I replaced one (only one) and all of them started working again. Never figured why... The oscilloscope still works fine today, though I rarely use it.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 10, 2022, 08:08:53 am
content: It seems these enconders are no longer available

author: Calambres
author_rank: Frequent Contributor
timestamp: June 19, 2022, 08:26:23 am
content: My TDS320's front panel died definitely. Now none of the controls or buttons are working at all and all the LEDs are permanently on. The scope boots OK, even giving a PASS condition. I see the familiar image on the screen but I cannot control anythingUpon a detailed inspection I see some vias corroded near the leaked capacitors in the front panel PCB. I'm afraid it is multilayered and thus, very difficult if not impossible to repair. The Tek served me well for some 15 years, more or less half of its life (noisy bastard!), but now it's time to move on.I just bought a Siglent SDS1104X-E. The arrival is expected tomorrow.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 19, 2022, 10:40:16 am
content: Eh, could trace the connections; could get a replacement board; could control it over GPIB (but, not sure anyone's made a direct clone (or improved) GUI version of the instrument panel; or maybe Tek did, but it requires VISA supported GPIB or something, not just any serial/USB adapter to it?). But those will all certainly take more effort, and provide less value, than the Siglent; such is life, a part of which is death. Sounds like it had a great run.Tim

author: Calambres
author_rank: Frequent Contributor
timestamp: June 20, 2022, 08:55:51 am
content: I didn't fully give up on this issue but it's not looking good at all. You're right it was a good piece of gear while it lasted, albeit quite dated now. It was cheap when I bought it second hand some 15 or so years ago.Quote from: T3sl4co1l on June 19, 2022, 10:40:16 amIs it possible yet with such an old scope? I've seen ebay replacements for other Tek TDS scopes but unfortunately not for the TDS320

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 20, 2022, 10:29:22 am
content: Oh, maybe not... can be hit-or-miss with old parts, yeah.And yeah, fixed, could still be handy to someone looking for a starter scope.Tim

author: simba15
author_rank: Regular Contributor
timestamp: June 20, 2022, 01:22:15 pm
content: There is a front panel on Ebay, cheap ~65usd or best offer and free shipping.looks clean but shows 1991 date.https://www.ebay.ca/itm/275290173152?hash=item401892aee0:g:VhQAAOSwUIxiaSDF&amdata=enc%3AAQAHAAAA0DnanvZHQaQFghT%2FTIssv%2BecK1v6Jb9dTxfHyog7Dv6MsFfoFY9iHHNm5ObsKafpw%2Bgymgf6oXRqVG4oJ4tBsPRra2TOplujkqXC93HA4RvcuBzDowN8E92IsF0b6BObHIUmuMQxSu3cidlZDSOKor3h7%2FpZKz8b2istvzTF5%2BpaIi39sQ46GtHR2L%2BcAlF0yR2gEvxypo0fru0gIPwDC8%2FkPn%2BO7px3W%2BuJ71F3PEHg1WxCQZl1OAFRsizZuBsPFfW9rZcXmG6310p7Dmsf%2BXo%3D%7Ctkp%3ABFBMxpmglLBg

author: Calambres
author_rank: Frequent Contributor
timestamp: June 20, 2022, 03:21:02 pm
content: Somehow it didn't showed on ebay when I was lookin for it yesterday. I have to think it over because the price plus customs fees may well be more than the total I paid for the whole scopeThanks for the clue!

author: voltsandjolts
author_rank: Unknown
timestamp: June 20, 2022, 03:41:52 pm
content: Whatever you do, don't put the scope in the bin!Sell it as spares / repair and say its the front panel, someone will take a punt on it.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 20, 2022, 06:05:56 pm
content: Don't worry... I know myself and I'm not scrapping this. I'll do all I can to repair it. I've got used to that noisy beast

author: adam4521
author_rank: Regular Contributor
timestamp: June 20, 2022, 06:43:30 pm
content: Agree with the sentiment. I have even older 2245A, modern DSO is really nice but want keep classic one working too. Recently it developed a fault with one of the channel ‘VAR’ pots - there is a detent switch to indicate when it is in the ‘cal’ position but that stopped working, leaving the orange warning light illuminated. Not a big problem at all, just annoying.This scope well over 30 years old but I was pleased to find a seller offering the individual potentiometers and another selling both front panel boards - the latter a very easy repair option leaving me with spare parts perhaps for ‘next time’. £25 I’m happy to spend, so I would encourage anyone with non-running classic scopes to put them up as spares, to help keep other units running.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 20, 2022, 10:08:03 pm
content: Now that you say it, I've got another old scope which I use a lot, an analog Hameg HM203-6. It developed the same failure as yours, the potentiometer for the Variable Gain/Cal in Channel 1 went south. The pot is quite unusual in shape and heavily shielded. Fortunately the guys at Hameg (Germany) are still in business and sent me one original spare for a few bucks. Now it's working like new again.I love this old junk

author: EE-digger
author_rank: Frequent Contributor
timestamp: June 22, 2022, 05:30:02 am
content: inner traces on a pcb should be pretty well sealed from surface level corrosion, moisture and flux residue. On the surface watch for traces to a pad that originate under the solder mask. In bad cases, I've seen these short segments gone from acidic residue and humid season.

author: snoopy
author_rank: Frequent Contributor
timestamp: June 22, 2022, 12:54:52 pm
content: Quote from: Calambres on June 09, 2022, 09:40:22 amI had a similar problem with a TDS784 but not sure that it was caused by internal shorts. The encoders are pretty well sealed but there is a tiny gap to get enough lube into. I used CRC-2-26 and sprayed through the straw nozzle into the gap between the shaft and the body and enough of it got in through the tiny gap to clean and lube the control. No problem sincecheers

author: wasedadoc
author_rank: Super Contributor
timestamp: June 22, 2022, 03:17:13 pm
content: Quote from: snoopy on June 22, 2022, 12:54:52 pmThe OP recently wrote: "My TDS320's front panel died definitely. Now none of the controls or buttons are working at all and all the LEDs are permanently on. " I think that merely spraying into the encoders will not address that.

author: Calambres
author_rank: Frequent Contributor
timestamp: June 22, 2022, 03:43:41 pm
content: I'm afraid it won't... besides I've already done that!

author: Calambres
author_rank: Frequent Contributor
timestamp: July 04, 2022, 10:07:17 am
content: I quote myselfQuote from: Calambres on June 20, 2022, 06:05:56 pmThanks to Simba15's warning my good ole TDS320 is alive'n'kickin' again:Meanwhile I managed to get a brand new Siglent SDS1104X-E (now a SDS1204X-E). You can see it in this picture as well as my old trusty Hameg HM203-6 analog beauty:

author: Calambres
author_rank: Frequent Contributor
timestamp: July 23, 2022, 07:18:40 am
content: By the way... anyone know where can I find the schematics for this Tektronix TDS 320 scope?I cannot find them no matter how much I try. Even the Service Manual for this scope is lacking the schematics!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 23, 2022, 07:24:51 am
content: They weren't made for component level repair, I'm afraid. AFAIK.Tim

author: Calambres
author_rank: Frequent Contributor
timestamp: July 23, 2022, 11:09:37 am
content: Yes, I too think so. The Service Manual is full of flow diagrams to diagnose faults. They always end up with "replace board, panel, etc."The schematics must be somewhere but I'm afraid they were not meant to be publicly available. The Service manual evan has a "Diagrams" chapter which is... blank!

author: Calambres
author_rank: Frequent Contributor
timestamp: August 28, 2022, 08:36:53 am
content: Well, I answer myselfThe Tek TDS320 Service Manual with schematics is now in the TekWiki... where else?(it wasn't there the last time I looked)https://w140.com/tekwiki/wiki/TDS320Direct link:https://w140.com/tekwiki/images/8/89/070-8570-00.pdf

author: mdoda
author_rank: Newbie
timestamp: February 15, 2025, 12:30:48 am
content: My TDS320 had the same symptoms. I discovered that the electrolyte corroded some of the processor (68hc705) pins around the top left corner. Some of these pins are used by the A/D converter. So I resoldered all the pins that looked crusty and the scope works fine.

author: Calambres
author_rank: Frequent Contributor
timestamp: February 15, 2025, 09:29:06 am
content: I suppose you replaced those 'lytics with a fresh, good quality couple. I did it in the new panel I bought from ebay and now I'm sure that it won't fail again at least because of those two capacitors... at least for the next ten years or more