EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: SuperDaveOsbourne on August 24, 2018, 11:32:26 pm
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I have a Sencore Capacitor Analyser Model CA55 that has a bad front panel leak zero adjust potentiometer. See pictures below, the resistance track is 100% destroyed.
This does bring up the first question, can I tell what it is by the numbers on the case of the pot? I see Mouser has all kinds of series but I don't see there is a standard to see what this baby is. The schematic is shown on a YouTube video but not close enough to see. This is just a standard single gang pot with three lugs with two tied together. I would call it a micro panel mounted pot.
Is there some kind of standard?
Stamped on the case 1377913 15B8-5
Shaft dia .1260in 3.15mm
Shaft length beyond collar .21in 5.5mm
Collar length .20in 5.5mm
Collar dia .20in 6.15mm
Case dia .62in 15.5mm
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My Google-fu hasn't been particularly successful - but it seems you can get a hold of a service manual ... for a fee.
Without that, I'd be looking for some other markings. A photo could help as there are a few people here that have seen enough components to be able to offer some useful comments if they could actually see the pot.
With the wiper having fallen off, you might be unable to determine the resistance profile (log, linear, anti-log, etc.) but you should be able to identify its total resistance. Just measure the resistance between the two outer lugs (this will be more accurate if out of circuit).
With a loose wiper floating around, it might be an idea to tip the pot around in different orientations and give it a shake so as to make sure the wiper isn't shorting anything internally and take some more measurements to confirm. As a final step, I'd suggest cutting the connection between the two joined lugs to minimise the possibility of something shorting inside that will affect the measurement.
Once you have this measurement, get a hold of a couple of (garden variety) pots with this resistance (or slightly more) in linear and log (these are the most common tapers) and try each of them in turn to see which functions as closely as the original. Armed with the resistance and taper, you can then focus on getting a mechanical match ... with the quality of the movement you need.
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Images of the pot.
I just ordered the caps to recap this so it will be a little while before I can test it. Also I have to run down the low capacitance leads. I saw a listing for someone selling them for the CA-55 off Google but I can't find it today. They were selling the cables for 30$.
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I found this:
https://www.westfloridacomponents.com/P042APF06/10K+ohm+Potentiometer+CTS+1377913.html (https://www.westfloridacomponents.com/P042APF06/10K+ohm+Potentiometer+CTS+1377913.html)
My best guess would be that CTS 1377913 are all 10k pots and the other numbers specify the type of terminals / mounting style etc.
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I found this:
https://www.westfloridacomponents.com/P042APF06/10K+ohm+Potentiometer+CTS+1377913.html (https://www.westfloridacomponents.com/P042APF06/10K+ohm+Potentiometer+CTS+1377913.html)
My best guess would be that CTS 1377913 are all 10k pots and the other numbers specify the type of terminals / mounting style etc.
I found that too - but I'm not sure I would make the same conclusions.
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If you were a little adventurous, you could take the pot apart and see if the fault could be fixed.
Just carefully bend back the four tabs indicated - but only as much as is absolutely necessary - and the shaft assembly should just lift off:
(https://www.eevblog.com/forum/beginners/identifing-a-potentiometer/?action=dlattach;attach=505991;image)
Refitting just requires those tabs to be firmly pressed back into place. Obviously, you won't get too many cycles before the tabs break off from metal fatigue.
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It looks like a
CTS 450 (https://www.ctscorp.com/wp-content/uploads/450.pdf) - no 24mm too big CTS 260 series 16mm potentiometer. These have survived today mostly in guitar applications. Digikey has some.
I'd take it apart, sometimes the wiper ring is badly oxidized or the spring went soft- so it's repairable.
You can but a new pot and swap out the wafer, to keep the housing/shaft.
The numbering scheme is a mystery, possibly:
137 CTS (Chicago Telephone Company), then date code, then house number
134 CentraLab
147 Stackpole
pic from http://retrofokus.blogspot.com/2016/04/start-pickguard-part-i-pots-switch-and.html (http://retrofokus.blogspot.com/2016/04/start-pickguard-part-i-pots-switch-and.html)
edit: guessed wrong size 24mm vs 16mm.
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Taking it apart finally identified it as a CTS pot. The CTS stamp was under one of the lugs. The current replacement is the 270 series. (https://www.ctscorp.com/wp-content/uploads/270.pdf (https://www.ctscorp.com/wp-content/uploads/270.pdf))
You can see in the picture that the resistive carbon had all come off. The flakes on the paper towel of what was left and there was no continuity anywhere across the graphite ring. From the residue inside the body, it failed because of a poor/bad contact cleaner.
It still doesn't get me to the value, which I know I can set up some trim pots and test. My next move is to contact CTS and see if they can look up the value from the number.
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It still doesn't get me to the value,
I'm probably missing something but why not measure? You can easily measure the total resistance and then with one pin of the meter about in the middle you can see if it is linear or logaritmic.
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I'm probably missing something but why not measure? You can easily measure the total resistance and then with one pin of the meter about in the middle you can see if it is linear or logaritmic.
He stated that the carbon track is damaged, so he cannot measure it:
You can see in the picture that the resistive carbon had all come off. The flakes on the paper towel of what was left and there was no continuity anywhere across the graphite ring.
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He stated that the carbon track is damaged, so he cannot measure it:
Ah ok, I was still with the first post " The wiper fell off"
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Ah ok, I was still with the first post " The wiper fell off"
Yeah, I was a bit puzzled by that at first also... :)
I'm assuming that even though the area still looks dark in his photo, the actual conductive layer is gone by his later descriptions.
As others have stated, it seems that the 1377913 is 10K linear, the other numbers denoting the style of shaft, mounting, etc. I expect that virtually any 1/8 watt or greater 10K linear pot that will physically fit will work sufficiently well in this application. There should be no need to repair this one with a new wafer part from another pot or replace with an identical part.
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Since it came from a Capacitor Analyzer ,chances are very good that its a Linear pot .Logarithmic pots are generally used for volume control because of the response of the human ear to volume. Take a measure from center to 1 quarter of just the carbon track and multiply by 4 that will give you a ball park measurement . Judging from the number and description it's a 16mm 1/8 shaft 15K linear. 15K seems odd ,but try the measurement to see what you get.
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Take a measure from center to 1 quarter of just the carbon track and multiply by 4 that will give you a ball park measurement . Judging from the number and description it's a 16mm 1/8 shaft 15K linear. 15K seems odd ,but try the measurement to see what you get.
Once again, the OP stated that the carbon track is damaged (the carbon layer is gone), so he cannot measure it:
You can see in the picture that the resistive carbon had all come off. The flakes on the paper towel of what was left and there was no continuity anywhere across the graphite ring.
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Mikes Radio Repair has the Sencore CA55 service manual, seen in the youtube vid. Ask him. Or contact Sencor, they do sell service manuals.
It's front-panel R9 100k on the schematic and has a R1061 500k Zero Center trimpot in series.
But not listed on the parts list, that I can see; just these:
R20 15C7-26 Control, 1M Vert. PC Mount
R22,R101 15C7-17 Control, 1k Vert. PC Mount
R24 15C7-9 Control, 100R Vert. PC Mount
R61 15C7-6 Control, 500k Vert. PC Mount
R71,R74 15C7-13 Control, 100k Vert. PC Mount
https://www.patreon.com/MikesRadioRepair (https://www.patreon.com/MikesRadioRepair)
https://www.youtube.com/watch?v=zFt52G3-w2Q (https://www.youtube.com/watch?v=zFt52G3-w2Q)
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Did email him last week with no response. I did download the video and try to do screen grabs but there just isn't enough resolution at 720 to get much. What I do know is that any of the stuff not on the main board or display board isn't on the parts list. I am replacing the 2000uf and the 1000uf back on the power supply and they aren't in the manual/parts list or schematic he showed, at least that I could see.
He said forcefully he would not scan the manual or the schematic in the video and not to ask. I just asked if he would let me the know the value. From the parts list I try and eliminate all the "controls" as they call them. It leaves me with 500k and a 100k which might just be some rework on the board.
Sencore emailed me back that they don't have any manuals or parts and they are out of that business. At least for the CA55, might have other stuff.
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Take a measure from center to 1 quarter of just the carbon track and multiply by 4 that will give you a ball park measurement . Judging from the number and description it's a 16mm 1/8 shaft 15K linear. 15K seems odd ,but try the measurement to see what you get.
Once again, the OP stated that the carbon track is damaged (the carbon layer is gone), so he cannot measure it:
You can see in the picture that the resistive carbon had all come off. The flakes on the paper towel of what was left and there was no continuity anywhere across the graphite ring.
From the appearance of the photo some of the track is still in place only the to ends are broken free so check the unbroken part like I described in previous post .By appearances theirs not enough carbon flake on the towel to make up a whole track.If indeed the whole track is missing then your S.O.L.
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From the appearance of the photo some of the track is still in place only the to ends are broken free so check the unbroken part like I described in previous post .By appearances theirs not enough carbon flake on the towel to make up a whole track.If indeed the whole track is missing then your S.O.L.
I was thinking the same thing and enhanced the photo a bit for a better look:
(https://www.eevblog.com/forum/beginners/identifing-a-potentiometer/?action=dlattach;attach=506780;image)
My annotations are just a guess, but it seems like there might be a chance that the section to the right (between the two yellow lines) is intact. If that is the case, then do a resistance measurement at the point of those yellow lines. Since those lines represent about half the rotation of the pot, just double that measurement to give you a pretty close value for the pot - presuming it is linear. If this section of the track is good, you can verify it as a linear taper by checking the resistance from each yellow marker to the green one - and they should be roughly the same.
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The most resistance from any two points I could get was 5 ohms.
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.... on the black carbon track? :-//