TEKTRONIX 7A24, 7A26, 7000 Vert plug-ins pots, Var switch? - Page 1
Forum section: electronics / repair

author: jonpaul
author_rank: Super Contributor
timestamp: February 16, 2023, 03:49:25 pm
content: Hello all, working on four 7A26, similar question for 7A24...The CH1/CH2 variable knob controls an elegant multipurpose mechanical (second photo)A Push/push on the knob toggles VAR/CAL, red VAR OUT, black CAL INTo CAL each of the channels to the mainframe, a small screwdriver depresses the center knob ( black, slotted) and latches the CAL trim after rotation, so a second control pot can adjust CAL of the channel.Releasing the knob fixes the CAL pot position and returns the channel to CAL.The control shaft has two gaged concentric Bourns MOD pots and the CAL detent mechanism.On the PCB, a switch to select CAL/VAR, according to the knob push/push control in CAL, black, out, VAR, red.In restoring these marvelous 1980s..1990s plug-ins, performed MOD pot cleaning procedure, after MOD pot screws loosened for contact cleaner.PROBLEMS:1/ VAR/CAL pots still scratchy after spraying Tuner Cleaner/control cleaner (not Deoxit!) down pot shaft and thru loosened MOD pot cases.2/ PCB VAR/CAL knob toggle switch is still scratchy after contact cleaner in to the PCB switch3/ on one channel of one plugin , the knob cannot toggle to the OUT VAR red position.Anyone seen these issues? Suggestions appreciated!I Can post a photo album if needed.Mille Mercis et Bon journéeJon

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2023, 01:58:51 am
content: Quote from: jonpaul on February 16, 2023, 03:49:25 pmI take the MOD pots completely apart to clean and lubricate them. If rivets were used, then they will need to be replaced with suitable small screw hardware.QuoteThe switch should be lubricated after cleaning.

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2023, 04:21:33 am
content: David many thanks yes done that....the MOD pots have,screws ,yes loosened.The CAL/VAR push..push toggle mechanism on rear MOD pot is stuck in the IN CAL position ( black)no amount of lube, or forcing frees up the push push toggles action to OUT red knob positionanyone seen this?blue highlights in photoJon

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 17, 2023, 01:48:06 pm
content: I took my 7A26 parts unit apart to see what might cause the VAR/CAL knob to get stuck. The spring action comes from the small black coupling device (see picture). Assuming that has not self-destructed, the only thing I can think of is that the shafts are not pushed in far enough. In other words, when you press the knob on the front panel in a little, the spring mechanism in the black coupling is not pressed in enough to disengage and release the knob. Get a small Allen wrench and play around with all the setscrews along the shaft, making sure everything has enough travel.As far as the scratchiness of the pots is concerned, if they are still noisy after a thorough disassembly and cleaning, they simply have reached the end of their life and is something you will have to live with (unless you want to replace them - and good luck finding the exact replacements!!!).You haven't yet commented on the main problem with most of the 7000-series plug-ins, namely the cam switches. While they were a marvel of engineering at the time, and a favorite of the Tektronix designers, they need maintenance. Soon you will be an expert at disassembling the cams and cleaning them with strips of paper saturated in alcohol.

author: jonpaul
author_rank: Super Contributor
timestamp: February 17, 2023, 02:30:56 pm
content: Bonjour cher wn1fju and David Hess!Many thanks for the very excellent tips!I am fortunate to have a fine stock of 7A26 (not 7A24!) plugins, from Silicon Ham fleas, silent key estate and ebay since last 30 years (!).I can only marvel at the fine engineering, brilliant circuit design and interesting mechanical of the se very useful machines, still going strong after 1980s..1990s.( 7A24 are the 50 Ohm twins and very similar except for the BW).7A26 Inventory ....2 x excellent and in cal, mech and electrically 100%2 x OK but the scratchy pots and controls and the one stuck CAL/VAR. Will try the fix you so kindly mention.David, The pots are less scratchy after unscrewing the Bourns MOD pot screws and using contact cleaner.2 X junkers/parts donors, smashed knobs, serious bugs, but one is a late SN and may still be restorable.The attenuators on all seem OK! Need to recheck them.A few plugins need a compensation adj, transient trim or 2X/invert trim.The 7000 mainframes : 7603 (for 7L5, 7L12 use) 7904 (main workbench) 7104 (high speed use only) I have enough 7A26 for this lifetine even with no further repair.Will update with pix as I have time to finish more work.Bon Journee,Jon

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2023, 06:44:39 pm
content: Quote from: jonpaul on February 17, 2023, 04:21:33 amLater MOD pots use rivets instead of screws. This will depend on the age of the plug-in.QuoteI think I only saw that a couple times, and my solution was to take apart the entire assembly to clean and lubricate it.

author: David Hess
author_rank: Super Contributor
timestamp: February 17, 2023, 06:50:36 pm
content: Quote from: wn1fju on February 17, 2023, 01:48:06 pmI doubt any have reached the end of their life, as I have never found any which were not restored by complete disassembly and cleaning, which includes wiping down the element. The MOD pots use a carbon brush which wears and the particulate gums up the grease until it becomes a cake causing the brush to not contact the element.QuoteIf the cam assembly is not periodically cleaned and lubricated, then it will get stiffer until something breaks.

author: wn1fju
author_rank: Frequent Contributor
timestamp: February 17, 2023, 11:49:57 pm
content: Tektronix cam switches - I hate everything about them. Thank God HP rarely used that approach. Just my opinion....

author: David Hess
author_rank: Super Contributor
timestamp: February 18, 2023, 01:55:52 am
content: Quote from: wn1fju on February 17, 2023, 11:49:57 pmThe earliest 7000 plug-ins used relays instead of the cam switches. The cam switches had the virtue of low parasitics for good high frequency performance.

author: jonpaul
author_rank: Super Contributor
timestamp: February 19, 2023, 12:49:02 pm
content: Rebonjour all: the 7000 plugins are in reasonably good condition, or late SN.Some cosmetic/knobs/sticker and usual control lube and exercise bought most back.Vert CAL and DC VAR bal off on a few.I Performed same tests and service on all my 7A26, 7A24 and also a good 7A18A.Everything fixed except on 7A26, CH2 VAR mechanism.CAL and attenuators are fine, transient and VERT CAL now good after lube of MOD Pots (screws, loosened covers, used standard contrl lube/cleaner)David Hess and W1: All the attenuators seem fine after some exercise, no cing needed.The TEK cam switches are a fine solution to high speed contacts on a PCB, and complex switching patterns.Have not had issues with them on the TEK 7A/B xx plugins,The TM500 are a worse issues eg SB503.Bon soireeJonPSIn the process of working on the 7A18/24/26, discovered the 7B53A H I was using in the 7603 MF has an intermittent TRIG out so traces disappear even though triggered.Whacking the scope MF can restore the traces.After some troubleshooting the issue was isolated to the HOR plugin, swapping with another 7B92A fixed this.