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Old 19th Jan 2021, 4:54 pm   #1
Keith956
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Default SG505 repair

I have a Tek SG505 and the attenuator variable pot seems to be faulty. It does not stop, freely rotating in both directions, with no on/off for the cal setting.

Anyone had any experience of repairing these and got any hints of the best way to get at it (it appears to be sandwiched between two PCBs)?
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Old 19th Jan 2021, 6:17 pm   #2
RogerEvans
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Default Re: SG505 repair

Keith,

I have to admit I have never seen one of these but I have a few other TM500 plugins and they can be a right pain to dismantle. Looking at the manual there appears to be a single PCB unless you have 'option 1 intermodulation distortion' installed. Without option 1 you should see the variable attenuator pot on the back of the 0.125 extension shaft through the step attenuator. There aren't any pictures of a SG505 with option 1 but it looks to be a relatively small PCB attached to the main A10 board towards the front and probably obscuring the attenuator. There is a picture of a SG505 with option 2 on TekWiki which appears to be a larger sub-board attched to the A10 with board with a few screws and with push buttons protruding through the front panel. It looks very much like you undo the fixing screws and any connecting multi-way connectors and withdraw the sub-board to the rear of the plugin to clear the push buttons.

The attenuator pot is R1518, 'Allen Bradley 18M51 5k non-wire-wound linear with switch'. It looks like you would need the exact replacement since it apepars to be soldered to the A10 board and has to line up with the extension shaft.

Regards,

Roger
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Old 19th Jan 2021, 6:22 pm   #3
tony brady
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Default Re: SG505 repair

I have one of these. have you checked the coupler from the shaft to the pot? these can split. take the cover off and rotate the knob while observing the pot if possible - if you are lucky the coupler may just be loose

cheers
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Old 19th Jan 2021, 6:27 pm   #4
Craig Sawyers
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Default Re: SG505 repair

Not on mine it isn't - it is visible and getattable easily. There is no sandwich of board - just a single board.

Do you have one of the odd versions, like the WQ or WR, one of which has another board?

Craig
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Old 19th Jan 2021, 6:39 pm   #5
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Default Re: SG505 repair

I think you must have a mod WQ. That came with a balanced output fed from two power amplifiers on a large daughter board.

It was part of a complete set up with a mod WR SG505 and a WQ version of the AA501. The intention was that there was connections via the backplane between them to make a "Test Set" to make operator use as straightforward as possible.

The whole combination was intended to do IMD measurements as flexibly as possible.

Craig
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Old 19th Jan 2021, 7:41 pm   #6
Keith956
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Default Re: SG505 repair

All I can see is that it says 'Option 2' on the board on the left side (looking at it from the front). It's hard enough to see the attenuator switch and R1518 pot, let alone get at them.

It does have the IM option, I guess it is one of the 'odd' ones! Good point about the shaft coupler; if there is one, I can't see it, but I can't see much of it at all. I guess the next step is to carefully try and get one of the boards off.
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Old 19th Jan 2021, 8:31 pm   #7
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Default Re: SG505 repair

mine has the IM option but is not option 2 which has the balanced output. you have to remove the top board in the first picture, then you will see the pot

you can see it at tekwiki

https://w140.com/tekwiki/wiki/SG505#...5_CompSide.jpg
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Old 19th Jan 2021, 8:45 pm   #8
Keith956
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Default Re: SG505 repair

Quote:
Originally Posted by tony brady View Post
mine has the IM option but is not option 2 which has the balanced output. you have to remove the top board in the first picture, then you will see the pot

you can see it at tekwiki

https://w140.com/tekwiki/wiki/SG505#...5_CompSide.jpg
Ah excellent. I see the coupler, that may indeed be the problem.
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Old 19th Jan 2021, 8:52 pm   #9
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Default Re: SG505 repair

Is there a trick to get the push button extender from the switch? It seems to have a locking mechanism and so far gentle pressure won’t release it. I am taking about the IM button switch
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Old 19th Jan 2021, 8:54 pm   #10
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Default Re: SG505 repair

That's a modular pot assembly. You can take them apart an play mix and match if you can get the right sections spread across one or more parts.

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Old 19th Jan 2021, 9:01 pm   #11
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Default Re: SG505 repair

This is the switch I’m having problems with. The black shaft must detach from the white plastic piece but gently twisting it with a screwdriver blade doesn’t seem to free it.
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Old 19th Jan 2021, 9:05 pm   #12
tony brady
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Default Re: SG505 repair

'gentle persuasion' is what it needs.are you using a flat blade and twisting it between the 2 black pieces while pulling the extender away at the same time?
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Old 19th Jan 2021, 9:23 pm   #13
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Default Re: SG505 repair

Quote:
Originally Posted by tony brady View Post
'gentle persuasion' is what it needs.are you using a flat blade and twisting it between the 2 black pieces while pulling the extender away at the same time?
Yes been trying without any luck. The white plastic part has a groove on at least 2 sides with the black plastic part locking into that groove. With only 2 hands though I can't seem to pry both black sides out of the groove and pull at the same time. At least not without feeling that something's going to break imminently...
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Old 19th Jan 2021, 9:34 pm   #14
tony brady
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Default Re: SG505 repair

that is the problem. I can't remember if there is a raised 'ridge' on the black part on the left side though I suspect there is.

how about a small jewellers flat screwdriver to gently lever between the tip of the switch and the inside or the extender? perhaps with a knife blade to gently lift the black piece on the right hand side off of the white centre a little to allow it to come off?
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Old 19th Jan 2021, 9:36 pm   #15
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Default Re: SG505 repair

Got it in the end. The trick seemed to be to use two very small bladed flat screwdrivers to lever either side of the black plastic piece away and towards the front.

Once the board was off, the coupler was visible. And alas it is cracked, with the halves of one side damaged presumably from having been forced at some point in the past.

It doesn't look easily repairable - if anyone knows of (or has?) a source of a spare I'd be very grateful!
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Old 19th Jan 2021, 9:56 pm   #16
tony brady
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Default Re: SG505 repair

there is one in the states but the postage is quite high. the part number is 376-0051-00
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Old 19th Jan 2021, 10:18 pm   #17
Craig Sawyers
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Default Re: SG505 repair

Quote:
Originally Posted by Keith956 View Post
Is there a trick to get the push button extender from the switch? It seems to have a locking mechanism and so far gentle pressure won’t release it. I am taking about the IM button switch
Firm grip with thumb and forefinger and pull.

Craig
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Old 20th Jan 2021, 11:34 am   #18
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Default Re: SG505 repair

I had a similar failure of this part. Managed to glue it back together: from memory I wrapped some thin tinned copper wire around the outside of the broken collar to reinforce it. Problem is caused in part by over-tightening the grub-screws. File a small flat on the shafts if there isn't one already.

Alternatively I can send you a metal shaft-coupler, but it obvioulsy doesn't have any "give". What are the shaft diameters?

John
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Old 20th Jan 2021, 1:35 pm   #19
Keith956
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Default Re: SG505 repair

Thanks John,

I have attempted a quick fix by glueing it and putting a bit of heatshrink sleeve over it then heating. Seems to be OK for now.

Keith
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Old 20th Jan 2021, 1:45 pm   #20
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Default Re: SG505 repair

That looks like an ITT-Schadow switch. The pushrod will be a Tek custom part.

David
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