Tektronix 2235 Capacitor C904

[jcrubin]

After searching for intermittent issues with the unit I found that C904 has blown....


picture of cap
https://plus.google.com/+JordanRubin6502/posts/iaxUppYJ772

Im trying to track down the value of C904 and where I could purchase a replacement

thanks in advance


Jordan

[alsetalokin4017]

Looks like C904 is a 2200 pF, 250V polypropylene film cap. Probably Tek number 285-1192-00

Your picture link is missing a [

[jcrubin]

I corrected the link.... where on earth did you find a cross reference for C904 and the TEK part number / value???


would this be a good candidate


http://www.amazon.com/Film-Capacitors-2200PF-630V-10%25/dp/B00HKSOLHS/ref=sr_1_11?s=industrial&ie=UTF8&qid=1438525551&sr=1-11&keywords=2200pf


I happen to have an ESR meter on the way so I will be checking others as well.....  especially in the power supply section

[alsetalokin4017]

Your photo helped me to track down the part in a Service Manual for a 2200 series scope, which all have pretty much the same A1 power supply circuitry. Using the Tek SMs can be confusing if you haven't done it before. Well, even if you have done it before, they are confusing.

Yes, the capacitor you have selected will work as a substitute. You really should try to track down the reason for the cap blowing though. I'd look hard at the bridge rectifier diodes CR901-CR904, which are out of sight in your photo, view blocked by the big square metal can. These are 600V, 1A silicon diodes. Something like 1n4007 would probably be fine there.

[jcrubin]

I see your point with the rectifier.  Would you agree however that if the Scope works, then the 4 diodes would not have failed, as they would be either open or short and not self correcting.


I say this because the scope worked about 70% of the time until I red tagged it.  somehow, that capacitor C904, which I believe is nothing more than a filter, allowed the scope to work if it acted as an open, or caused the quirks when in close proximity, may have been arched or shorted.


From the diagram, C900, C903 or C902 in the filter circuit (as well as EVERY other cap in the power supply) should be tested for ESR.  That is something I will do before swapping any components out.

Funny, do you think there is a reason that the existing cap in the diagram shows one of a polarized nature?   Im lead to believe either the original was electrolytic, or attention should be made with regard to the shield end of a non polypro cap?

your thoughts?


J

[alsetalokin4017]

Sure, with a scope that is that old it is probably a good idea to check all capacitors you can get to, if you have some reason to suspect them, like power supplies off-voltage or etc.

Tek seemed to use that same schematic symbol for _all_ capacitors, regardless of type. The exception is that electrolytics are marked with the + symbol on the positive side. The parts list, however, shows the capacitor type. C904 is listed in the Parts List as "PPL" type, which is polypropylene or polyester film. These are non-polarized.

Intermittent faults can be difficult to diagnose. I mentioned the bridge diodes as possible causes for the C904 explosion. Since switch actuation could stress the circuit board mechanically in that area, I'd look for cracked traces, bad solder joints and etc. around the bridge diodes. Just guessing at this point.

[tautech]

where on earth did you find a cross reference for C904 and the TEK part number / value???

When I dug out a saved SM copy and went looking, i thought you might have given us a clue to where it was in the scope. 

But it's ended well with alsetalokin4017 finding it for you. 

[jcrubin]

Figures...... I just rewired my hallicrafters S38C for a polarized plug and moved the switch to hot instead of B-.   (see AC/DC death circuit)


Now I have a hum in the system but no scope.      I ordered the cap and will have to wait on the arrival of the ESR meter before I continue.   I will post ESR findings as soon as i can


J

[alsetalokin4017]

where on earth did you find a cross reference for C904 and the TEK part number / value???

When I dug out a saved SM copy and went looking, i thought you might have given us a clue to where it was in the scope. 

But it's ended well with alsetalokin4017 finding it for you. 

The photo showing it right behind the main power switch was a dead giveaway.... it's on the A1 board but in the power supply schematic (900-sequence part numbers).

[tautech]

(900-sequence part numbers).

Yep, I had a hunch 900 was a clue, more so for those that are working with Teks frequently. 

[jani]

Hello,

I got the same problem a few days ago with my TEK2235/C904 and also looking for an replacement. 

However, I recall the C904 being 0.068uF. (need to doublecheck when i get home)

The service manual and it says 0.0022uF.

Found C904 referenced as 0.068uF here  : http://www.diyaudio.com/forums/equipment-tools/252125-dead-tektronix-2235a-2.html

Anyone know if it should be different in euro-land ?

Regards
Jani

[Deathwish]

yep It is a .0022uf +-20% 250VAC. I looked in the service manual

http://www.ko4bb.com/manuals/86.157.57.172/2235_sm.pdf page 8-6

[markce]

 C904 in my 2235 servicemanual is PPR, which is a Rifa paper cap. They are self healing, and finally self destructing  .
Replace with a modern polyprop cap.

[jani]

Hello,

my C904 is a 0,068uF X cap.

Regards
Jani

[markce]

X and Y caps can be paper and from the looks of the first photo, it seems to be a Rifa which often fail this way at end of life. Polyprop Y2 (Wima for example) lasts much longer. Ordered a set myself last month for preventive maintenance on a 2236.

[macboy]

I corrected the link.... where on earth did you find a cross reference for C904 and the TEK part number / value???


would this be a good candidate


http://www.amazon.com/Film-Capacitors-2200PF-630V-10%25/dp/B00HKSOLHS/ref=sr_1_11?s=industrial&ie=UTF8&qid=1438525551&sr=1-11&keywords=2200pf


I happen to have an ESR meter on the way so I will be checking others as well.....  especially in the power supply section

Absolutely NOT. C904 is across the AC line and therefore must be a X rated safety capacitor. These are not ordinary film capacitors. Fortunately they are not difficult to source. At just 2200 pF (2.2 nF) you may be more likely to find a Y rated capacitor, and these are virtually always X rated as well (look for the ratings, they will be printing right on the capacitor: X1Y1 or similar). You could use as low as X2Y2 for an ordinary household circuit. I think that 0.068 uF as mentioned by others is a more likely and more suitable value than 2.2 nF; yours is too big to be 2.2 nF. Don't worry too much about the exact value, it's just there for EMI suppression... anything from 0.047 to 0.22 uF would do the job.

If you are salvaging one from other dead gear, note that these degrade over time due to AC voltage transients, so don't use a very old one. Also don't use anything with a transparent outer casing; these are notorious for failing, sometimes catastrophically (which, ironically, they are specifically designed not to do).

[Deathwish]

Any use ?.

http://cpc.farnell.com/lcr-components/pc-hv-s-wf-2-2nf-1kv/capacitor-2-2nf-1kv/dp/CA05653

http://cpc.farnell.com/epcos/b81123c1222m/capacitor-class-y1-2-2nf/dp/CA05831

[macboy]

Any use ?.

http://cpc.farnell.com/lcr-components/pc-hv-s-wf-2-2nf-1kv/capacitor-2-2nf-1kv/dp/CA05653

http://cpc.farnell.com/epcos/b81123c1222m/capacitor-class-y1-2-2nf/dp/CA05831

No and no. The first is just a high voltage cap, not safety rated. The second is Y rated but not X (appears to be an exception to my observation that Y rated are usually X rated). You specifically need an X rated cap (X1 or X2) since it goes from AC line to neutral.

edit:
Also keep in mind that others see a 0.068 uF (68 nF) not 2.2 nF fitted there, despite the value in the schematic; this is a better choice given the application.

[Deathwish]

Can i have another try ?. EMI suppression capacitors (MKT) X1 class.  http://uk.rs-online.com/web/p/polyester-film-capacitors/0335044/

[jani]

Hi,

this is the one i am ordering for C904. https://www.elfaelectronics.com/elfa3~ex_en/elfa/init.do?item=65-643-98

edit: i thought X2 is "good enough" ?

...sorry for slightly offtopic, but C906, ELCTLT 75uf/450v. I have seen 100uF mentioned as an alternative, but is 68uF ok for C906 ? . This is the closest elfaelectronics.com has for C906.



Regards
Jani

[markce]

X2 is ok, but 68nF???
68uF for C906 is not enough. It is spec'd at 75uF -10%. I would go for 100uF. Are you sure this cap is defect. It is a top notch Sprague, not cheap to find a decent replacement.

[macboy]

Can i have another try ?. EMI suppression capacitors (MKT) X1 class.  http://uk.rs-online.com/web/p/polyester-film-capacitors/0335044/

Those are maybe a little pricey, but will work.
X2 is good enough. X1 is better.

[jani]

Thanks all,

I am looking to recap the psu part of the scope, thats why I mentioned the other cap.

Regards
Jani

[jcrubin]

Well,   It didnt go as expected

Here was the capacitor after removal

https://plus.google.com/+JordanRubin6502/posts/EdBUxjBjT9L

And here, in both movies is the issue I continue to have

https://youtu.be/PJmj8UKZXlM

and

https://youtu.be/a1lLjaGHnJs

[jcrubin]

Ok so it seems like this issue only exists in Horizontal Mode A (and ALT obviously to the extent A is involved with ALT) However when in B mode the scope works fine with no issues. Id hope this would narrow down the cause, the movie below elaborates on the issue.

https://youtu.be/lTkdPYGIRRU

Apparently This setting deals with which sweep generator is used (page 2-6 of the operators manual)

I believe sweep generator A is BLOCK 3 of the block diagram which may be disassociated with Alternate B in BLOCK 5

From TM 11-6625-3135-40 page 1-4

Al MAIN CIRCUIT BOARD ASSEMBLY (C) — Provides instrument working voltages and processes vertical and horizontal signals.

A5 ALTERNATE SWEEP CIRCUIT BOARD ASSEMBLY (D) — Processes Alternate B Sweep horizontal signals.

From TM 11-6625-3135-40 page 1-8

A Sweep Generator and Logic Circuit (fig. FO-8)
Alternate Sweep Logic Circuit (fig. FO-9)


A SWEEP GENERATOR AND LOGIC CIRCUIT. The A sweep generator logic circuit controls A Sweep generation and Z-Axis unblinking. When the A TRIGGER mode switches are set to either P-P AUTO or TV FIELD (with no trigger signal present), a reference sweep is produced. In the NORM setting, sweeps are inhibited until a trigger input is present. This is useful for low-repetition-rate triggering. The SGL SWP setting allows only one sweep at a time to be generated. The sweep signal is applied to the horizontal amplifier.


ALTERNATE SWEEP LOGIC CIRCUIT. The alternate sweep logic circuit controls the alternate and B horizontal mode displays, intensity, Z-Axis amplifier drive level, and includes the B Miller sweep generator and B sweep logic circuitry. It also provides the B sweep sawtooth waveform and generates signals to control switching between the A and B displays.


From TM 11-6625-3135-40 page 2-21

Monitor A4R701 (161) pin 1 for approximately -8 vdc ±50 mv, as switch is turned through all TIMING ranges. ? If voltage varies more than 5 mV, replace A4 (para 2-20).


From TM 11-6625-3135-40 page 2-47

A4 TIMING CIRCUIT BOARD ASSEMBLY REPLACEMENT