Keyboard Shortcuts
ctrl + shift + ? :
Show all keyboard shortcuts
ctrl + g :
Navigate to a group
ctrl + shift + f :
Find
ctrl + / :
Quick actions
esc to dismiss
Likes
Search
SC504 Gain Knob Removal
|
I have a SC504 scope module with a damaged Channel 1 knob skirt and I have a Tek replacement skirted knob. I can remove the small red central knob by loosening a set screw.
How is the skirted knob removed? I've tried gently pulling on it without success. The knob has a long plastic shaft with some internal ribs to fit over another shaft but I don't know how the knob is attached. Anyone done this? -- Best wishes, Larry McDavid W6FUB Anaheim, California (SE of Los Angeles, near Disneyland) |
|
tom jobe
Hi Larry,
toggle quoted message
Show quoted text
Your question got me wondering about this, so I took the side cover off of an SC504 to have a look. The service manual mechanical parts diagram shows the knob has a longer part that goes back into the Channel switch so I looked to see if there was a screw or other fastening device back inside of the scope. I removed the plastic rod that goes through the knob assembly by disconnecting it at the aluminum collar on the VAR switch, and then the knob just fell out for some reason. I put the knob back in and then it would not come out again. So I nervously pried and pulled and got it back out again without damaging it. A study of the knob shaft showed no locking mechanisms of any kind, so I rubbed a very thin smear of Johnson's paste wax on the knob shaft extension and put it back in. After that the knob slid in and out nicely and the only thing holding the knob in is the 1/8" fiberglass VAR shaft and small VAR knob in the center of the knob in question. I guess the knob was just stuck from being in there for 30 or 40 years plastic on plastic? Do be careful about a very thin aluminum washer that is on the knob's rear shaft extension, as it would be very easy to drop that washer into the scope. My guess is that this washer is part of the spacing arrangement that keeps the lettering on the back side of the knob skirt from hitting the front panel and getting damaged, and it also might keep the two plastic parts from rubbing on each other at the knob extension / attenuator surfaces? tom jobe... PS I wonder if one drop of water could be put in from the side where the knob shaft extension goes into the attenuator assembly to break it free as you wiggled the knob to spread it around? You would want to be very careful about anything you put in there, as the short and long term effects on the plastic parts could be devastating. On 7/6/2018 11:30 AM, Larry McDavid wrote:
I have a SC504 scope module with a damaged Channel 1 knob skirt and I have a Tek replacement skirted knob. I can remove the small red central knob by loosening a set screw. |
|
Thanks, Tom, for this reply, the only one I received.
toggle quoted message
Show quoted text
I did loosen the knob set screw and remove the red VAR knob. My replacement skirted knob does have a long shaft that goes through some kind of (cam") switch near the front panel but it seems to have no means obvious to secure it inside the scope. I did not even think of removing the small diameter plastic shaft to which the red VAR knob is attached. I just now loosened the set screw in the aluminum collar on the pot to which the plastic rod is attached and easily removed the red VAR knob and its shaft. I tugged as much as I thought wise and the skirted knob still did not come out. The shaft on the replacement knob is long enough to enter the gray plastic collar that protrudes forward from the cam switch. Rotating the skirted knob, I see no screw to secure that knob shaft. I'm still puzzled. I note the replacement knob has a shallow "V" groove that ends up somewhere inside the other (cam?) switch close to the front panel. Maybe there is some kind of detent spring inside that switch; I can't take that switch apart because I'd need to remove the bottom panel or side beam and I surely don't want to do that. The red VAR knob and its shaft would hold the skirted knob in well enough but it might rub on the front panel without something to position it. The tolerances make the "V" groove seem unlikely to be what positions the knob. I don't see the thin washer you describe. Tell me again where that is located, please. What do you mean by "...pried and pulled" to get the knob back out after first putting it back in? Pry? On what? This seems like a high-risk task! But, that one knob has damaged nomenclature on the skirt and I was able to buy the exact replacement knob on eBay. The interior of the end of the skirted knob shaft has some details that engage the shaft inside that switch thing near the front panel. That detail will keep the knob clocked but can't hold it in. It may be that shallow "V" groove that holds the knob in. I surely don't want to damage something inside the cam switch or that other switch near the front panel. And, I really don't want to dribble anything down the skirted knob shaft in hopes of getting to that "V" groove. I'm feeling kinda stymied... By the way, I'm a mechanical engineer with a lifetime of small, precision instrumentation design. I'm not intimidated by this problem, just really cautious! I'm in Anaheim, California, near Disneyland. Good Grief! It was 115.1¡ãF here this afternoon! Larry On 7/6/2018 4:18 PM, tom jobe wrote:
Hi Larry, --
Best wishes, Larry McDavid W6FUB Anaheim, California (SE of Los Angeles, near Disneyland) |
|
tom jobe
Okay, I will make one more attempt at this explanation.
toggle quoted message
Show quoted text
This morning I went back to the same SC504 that I played with yesterday. I removed the small knob and 1/8" shaft assembly that goes through that same channel switch knob I freed up yesterday. The main knob will almost fall in and out of the scope, but you can feel a very light detent action which probably happens at that groove you speak of. The aluminum washer I mentioned cannot be seen until the main knob comes out part of the way, as the washer sits on the step of the knob's extension where the diameter changes from just under .250" to a little under .300". The hole in the front face plate will barely let the aluminum washer through, so it might get pulled off and missed if you are not watching all of this from the side. As for how I 'pried' on the knob yesterday: I pulled and wiggled on the knob and got it to move outward maybe 1/8" extra, but then it would not move any more, so I used a small flat blade screwdriver to reach in from the closest edge of the front face (which I used as a leverage point) to apply a steady outward force while I wiggled and pulled on the knob. Where the actual sticking point was, or what caused the problem are all unknown to me. You say this is a high risk task. I felt the same way as I tried to find you an answer with my perfectly good parts. Best of luck! tom jobe... On 7/6/2018 11:41 PM, Larry McDavid wrote:
Thanks, Tom, for this reply, the only one I received. |
|
On the SC504, the gain and sweep knobs just pull out after the red variable knobs are removed from their shafts. The SC503 is similar but the knobs are not interchangeable.The variable knob shafts do not have to be removed. You will have to pull hard on the knob to get it out. The knob is keyed with two flat pieces on the inside end that are different widths so it can only be reinserted one way. Best to turn the knob all the way to one side so it is easier to line up when putting it back in. I did not see the washer mentioned in the other post, it may have stayed inside the attenuator module since I removed the knob with the unit vertical. I posted a couple pictures in the SC504 Gain Knob album in the photos section so you can see what the knob shaft looks like.
ed |
to navigate to use esc to dismiss