Topics

7L12 Reference Level Attenuator contacts


Cliff Carrie
 

I have a later version 7L12 (with LED reference level readout). It works perfectly except that two of the three highest Reference Level attenuator positions are unstable. Jiggling the attenuator knob can make the condition go away, so I strongly suspect switch contact problems. An ohmmeter check shows the normal 50 ohm input resistance is similarly unstable. The attenuator seems totally enclosed and buried behind the front panel. It is almost invisible without major disassembly. Any ideas on how to get a bit of contact cleaner into the switch without tearing the whole front end apart? Any other possible causes for this problem?


Cliff


Ed Breya
 

If the shaft rotates easily without any "crunchy" spots, you may be able to just work it through all the ranges lots of times (maybe up to a hundred) to polish the contacts a bit.

You can take drum step attenuators apart (carefully), but first you have to remove them from the instrument. There may be some instructions for this in the manual. Without some disassembly, I don't think there's any way* to get contact cleaner or anything inside, short of immersing the whole thing for a while, including maybe using vacuum impregnation techniques to speed things up. It's easier to just open it up and see for sure. *On some types you can unscrew just the I/O fittings out of the body, exposing the coaxial spring contacts, and allowing inspection and lubing through the holes, but it's risky if it's not meant to assemble that way - don't force anything beyond reason.

If you have to take it apart, you may find burned or otherwise damaged contacts (bad), or simple corrosion issues (not too bad), or hardened grease (easiest to fix). Good luck.

Ed


Cliff Carrie
 

Thanks, Ed. I have now exercised the switch close to 100 times. The frequency and severity of the problem has not changed at all. I now suspect a cracked connection in one pi section inside the attenuator that is slightly disturbed by movement of the switch. The switch moves very smoothly. I doubt the contacts are burned since the problem does not show up on low attenation settings where overdriving the input would carry a greater risk of contact damage.

I do have the manual (from BAMA) and am considering the teardown to remove and repair the attenuator. I have tested leaving the switch at a low attenuation setting and using an external attenuator. It works fine but it's not very elegant, and the led Reference Level display is wrong.


Cliff
________________________________

________________________________
Reply via web post<https://groups.yahoo.com/neo/groups/TekScopes/conversations/messages/141740;_ylc=X3oDMTJyZm1wamt2BF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBG1zZ0lkAzE0MTc0MARzZWMDZnRyBHNsawNycGx5BHN0aW1lAzE1MDc3NzA4ODY-?act=reply&messageNum=141740> Reply to sender <mailto:edbreya@yahoo.com?subject=Re%3A%207L12%20Reference%20Level%20Attenuator%20contacts> Reply to group <mailto:TekScopes@yahoogroups.com?subject=Re%3A%207L12%20Reference%20Level%20Attenuator%20contacts> Start a New Topic<https://groups.yahoo.com/neo/groups/TekScopes/conversations/newtopic;_ylc=X3oDMTJldWt2MGdvBF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBHNlYwNmdHIEc2xrA250cGMEc3RpbWUDMTUwNzc3MDg4Ng--> Messages in this topic<https://groups.yahoo.com/neo/groups/TekScopes/conversations/topics/141739;_ylc=X3oDMTM4dW4yOTR0BF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBG1zZ0lkAzE0MTc0MARzZWMDZnRyBHNsawN2dHBjBHN0aW1lAzE1MDc3NzA4ODYEdHBjSWQDMTQxNzM5> (2)
________________________________
[https://s.yimg.com/ru/static/images/yg/img/megaphone/1464031581_phpFA8bON]
Have you tried the highest rated email app?<https://yho.com/1wwmgg>
With 4.5 stars in iTunes, the Yahoo Mail app is the highest rated email app on the market. What are you waiting for? Now you can access all your inboxes (Gmail, Outlook, AOL and more) in one place. Never delete an email again with 1000GB of free cloud storage.
________________________________
Visit Your Group<https://groups.yahoo.com/neo/groups/TekScopes/info;_ylc=X3oDMTJlbWw2MHRwBF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBHNlYwN2dGwEc2xrA3ZnaHAEc3RpbWUDMTUwNzc3MDg4Ng-->

* New Members<https://groups.yahoo.com/neo/groups/TekScopes/members/all;_ylc=X3oDMTJmYjZrY2N1BF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBHNlYwN2dGwEc2xrA3ZtYnJzBHN0aW1lAzE1MDc3NzA4ODY-> 6

[Yahoo! Groups]<https://groups.yahoo.com/neo;_ylc=X3oDMTJkbzJvMGs0BF9TAzk3MzU5NzE0BGdycElkAzIxOTEyNDcEZ3Jwc3BJZAMxNzA1MDgzNjYzBHNlYwNmdHIEc2xrA2dmcARzdGltZQMxNTA3NzcwODg2>
Privacy<https://info.yahoo.com/privacy/us/yahoo/groups/details.html> Unsubscribe<mailto:TekScopes-unsubscribe@yahoogroups.com?subject=Unsubscribe> Terms of Use<https://info.yahoo.com/legal/us/yahoo/utos/terms/>

.


Cliff Carrie
 

Well, I did it. You should be warned: gaining access to the reference level attenuator is not for the faint of heart. Major disassembly of the front panel area is required, including the rat's nest of semi-rigid coax and a bunch of ribbon cables. A real jigsaw puzzle.

Details of my repair attempt follow, but my real question is: Is anybody parting out a late model 7L12 with what is believed to be a good attenuator? Everything else on my unit seems to work fine.


It is a Weinschel Model 3000 rotary, 10db steps from 0 to 60db, Tek PN 119-0871-00, probably custom made for Tek because the shaft is hollow and double ended to drive position sensing and gain switches behind it. After removing it, I did a test setup, with a 50-ohm terminator on the output and an ohmmeter on the input. The resistance at the three highest settings (40, 50, 60db) is unstable, ranging from near 50 ohms to over 100. Contact cleaner did not help. With the rear cover off (3 screws; the front cover is pressed or threaded in place and doesn't seem to come off) I found that further disassembly probably requires removing the input and output connectors first. Failing to do so would probably make it impossible to reassemble because of the spring loaded contacts in the two connectors hanging up on the drum carrying the resistive networks. The connectors seem to be threaded into the attenuator body. I removed the locknuts which were secured with what looks like Crazy Glue. I was still not able to unscrew the connectors (using reasonable force and protecting the threads) even though the glue did not appear to have penetrated beyond the locknuts. I am still at a point where I can reassemble the unit and just avoid the bad attenuator positions, so I don't really want to get much more aggressive. Any suggestions?



---In TekScopes@yahoogroups.com, <test2cal@...> wrote :

I have a later version 7L12 (with LED reference level readout). It works perfectly except that two of the three highest Reference Level attenuator positions are unstable. Jiggling the attenuator knob can make the condition go away, so I strongly suspect switch contact problems. An ohmmeter check shows the normal 50 ohm input resistance is similarly unstable. The attenuator seems totally enclosed and buried behind the front panel. It is almost invisible without major disassembly. Any ideas on how to get a bit of contact cleaner into the switch without tearing the whole front end apart? Any other possible causes for this problem?


Cliff


Ed Breya
 

You should be able to pull out the shaft and drum assembly, even if there are no provisions for separating the front plate. First, inspect the coaxial contact assembly of the back plate that you removed. If all the pieces stay put on their own, that's good. If they don't, then screwing in the connector/contact sets will likely be necessary as the final step in reassembly. The front side connector/contact will likely be the same as the back one.

There should be a detent function somewhere with a spring and ball or plunger, either radially riding a cam feature in the drum periphery, or axially riding in a cam in one end of the drum. It also could be around the shaft, as in a common rotary switch. The shaft/drum may be retained by the the detent, if radial, or by a setscrew from the side, or by a retainer ring of some sort around a neck in the shaft just outside of where it passes through the mounting boss threads. The ring may be a spring or a soft metal that's crimped around the neck. If the shaft is gouged up from the knob setscrew, you may have to file it down to restore the shape and finish so it can slide through. There should also be some kind of stops to limit the rotation range - this may be included in the detent or other retention features.

If you don't find any external detent hardware that can be removed, then it's likely inside between the drum and front plate. Be very careful with disassembly because there may be loose parts that fall out. I always work over a tray or box of some sort to catch the pieces of any mechanisms I take apart.

If you find that the connector/contacts have to be removed, you can break down the super-glue-type adhesives with moderate heat like from a heat gun or oven. Keep track of the number of rotations involved in unscrewing them, in case this is an important factor - they may have been inserted to a certain depth to get the right axial preload, or RF characteristics. If the connectors are stubborn even with the glue deteriorated, you can use the old double-nut trick to get plenty of torque. Sometimes barrel SMAs have small flats in the middle for wrenching - use them if present, and if not, then the nuts.

That's all for now - will say more later. Good luck.

Ed


Cliff Carrie
 

This thread is now over 3 years old, and I am very late in updating it, but for anyone who still has problems with the Weinschel reference level attenuator, you may be interested to know that it can be repaired if you are not faint of heart. This is a very well built unit, but it can fall victim to excessively high inputs (see the manual warnings) which will make toast of the tiny attenuator resistances. While the overall assembly was custom made for Tek, the individual attenuator modules in it (used singly for each switch position) are Weinschel standard. You will need some luck to locate a good, similar looking Weinschel 50 ohm unit with 10db steps up to at least 60. First, take note of which switch positions are acting up; you have the option of only replacing the bad ones or doing a full repopulation. Also, set the switch to the 0 attenuation position (fully CW) so you can id the individual attenuators later. Remove the attenuator from the 7L12. It can be done without removing the front panel; I have the instructions somewhere. Note that there is a short flat cable that will need to go back in the same orientation; it has swapped wires so it is physically, but not electrically reversible. Remove the readout switch board (don't rotate it). The input and output connectors must now be removed. They are screwed in quite tightly (more so than really needed, IMHO). I got them out without damaging the threads by using BOTH locknuts tightened against each other and then using a correctly sized spanner wrench on the lower locknut to break the connectors free. Be careful not to lose the spring and center plunger as they come out of the body. Scribe an alignment mark showing the position of the back cover on the body. You can then remove 3 screws and the back cover, remove the detent leaf springs (3 leaves each in 2 locations; they can fly away and disappear so be careful) and slide the internal drum out. There are thin grounding springs on 4 sides of the drum; don't crease or crush them. Take photos as you go or reassembly will be a guessing game. You then need to slide off the gold plated sleeve that covers the entire drum (friction fit). There are individual shields over each attenuator module; don't distort them. Do a similar disassembly on your donor attenuator. Identify the attenuator position you want to swap and remove the shields and the attenuator modules GENTLY with small pointed tweezers. The attenuators have tiny contact springs on each end; don't bend them or break them off (did I say BE VERY GENTLE?). Repeat for any other attenuators; note that they are all different. Install the gold cover cylinder (compress the internal shields gently to get the cylinder started) and center the cylinder carefully between the end commutators. Reassemble the Tek attenuator (or both and mark the donor as defective). Install the grounding springs after the drum is in place; sliding the drum in and out a bit may help here. You should at least do a DC input resistance check with a 50-ohm load on the output at this point; every switch position should show very close to 50 ohms. You could also do a check of the attenuation at each switch position if you are equipped to do so. Reset the switch to fully CW. Reassemble everything else. I made some of the mistakes I have noted here, so I went through this procedure about 3 times, but it worked in the end. Good luck! Sorry that this is so long; I hope it helps someone. As usual, you are on your own here.

Cliff