mica capacitors

[timmy]

i have to get several mica caps however the schematic shows all the micas but only certain micas show the voltage and others dont, that space where the voltage would be is left blank, does anyone know why some cap voltages are left out ? how would i know what the acceptable voltage would be for a mica if its not listed in the schematic.

[Tom Albrecht]

Not sure why they left the voltage off -- sounds like an oversight.

A good way to figure out what the voltage rating needs to be is to operate the set and monitor the DC voltage across the cap, both during warm up and while it is running. Then add a nice safety margin of something like 20%, and choose a cap with that voltage rating or higher. If you have reason to believe there is a large amplitude AC signal across the cap (not merely coupled through it), you'll need to increase the margin.

Micas are generally available in 300, 500, and 1000 V versions. In most circuits, 500 V is fine, but there can be a few key exceptions -- in particular if a low value mica cap is used as a feedback path from the flyback transformer to the horizontal AFC. That one needs a 1500 or 2000V rating, which is not so easy to find. Usually I don't replace that one unless I have a clear reason to do so. It is not used in all sets, but many do have a small value high voltage mica in that particular circuit.

Unfortunately these days I see lots of failures of mica caps, so the general idea that most of them can be left alone is becoming less and less true as more time passes. When I'm restoring a set for someone else and want maximum reliability, I replace all the micas in the sweep circuits, and also any in the IF or video circuits that have any significant DC voltage across them -- those seem to die faster than those in tuned circuits with no DC across them.

[timmy]

well it makes sence but the voltages are all over the place in the circuit that i believe needs micas, the verticle section, cant get the pic to stop rolling and the only way to do so is turn the vert size down to a small pic and max the vert hold only then will it stop rolling. i have seen in several sams with voltages left out for micas. ok update, the sams left voltages out but the original for motorola set im working on shows all voltages, so im good now. i should have known how bad sams can be and never really count on those diagrams. imagine how bad it was in the early days when techs really depended on the sams, lol... one problem i have is 3 micas i need are listed and marked on the cap as being 2kv but it looks like all i can get is 2 @500v and 1 @ 1kv.

[Tom Albrecht]

What set is this? I'd like to look at the circuit.

In troubleshooting the vertical section, definitely replace all the paper capacitors first, and then also check every resistor value. Then check the potentiometers. Last, check any micas, although micas are not very commonly used in the vertical circuit, so that's why I'd like to see it.

[stusnyder]

Timmy don't mix and match mica's to get proper voltages, only asking for more problems. Check Mouser or Digikey.

[timmy]

the set is a 1949 motorola tabletop 7 inch the chassis is a ts18. i already replaced all electrolytics, all the wax paper caps and several resistors in and around the verticle and horizontal as well as checked the pots and even swapped some pots as i have a parts chassis from 1948 but the pots are the same nothing worked. when i got done with this set the verticle worked but only if the hold pot was maxed to one end and got worse and eventually now it rolls so bad its got a white line and the pic can be seen but its rolling to much. resistors around the vert size and hold, nothing else bad all that is left is the micas and the ceramics in the horiz and vert circuit. i have run out of things to look for, there is nothing else. maybe you guys can shed some light on this problem that maybe i overlooked. the 900mmf cap measures 1.01 when it should measure 0.9 so that cap may have something to do with this and i dont really know if this is a criticle cap for the vert circuit. i measured the 680mmf and i got .069 so this one is up alittle. so i decided to get all the caps , micas , ceramics . i think i ruled everything else out at this point and now im pulling my hair out with this problem as i didnt expect the micas to be the blame but this set is 66 years old and getting older and these caps do go bad i have read. so yes any help would be great at this point,lol,. actually i think the caps in the verticle are ceramic but i decided to get all of them because the horiz has a little bit to do with verticle instead of only doing half.

[Tom Albrecht]

Don't worry about the 680 or 900 pF micas. Those are in the horizontal section, and have no effect on your vertical rolling problem.

There are quite a number of parts that can affect the vertical oscillator frequency with this design, including C62, C63, C65, C66, C67, R56, R57, and R59 (all on the Sams schematic available on the ETF website here: http://www.earlytelevision.org/pdf/M...-Sams-83-6.pdf ).

If the vertical hold control is just out of range, the first thing to try is to adjust the value of R57. If the vertical hold control is closest to locking when it is turned to minimum resistance, then try substituting 1 M instead of 3.3M for R57. If it is best at maximum resistance, try substituting 5.6M instead of 3.3 M. Either of these simply have the effect of extending the range of your vertical hold control.

If that works to stop the rolling, then see if you can find what component is out of whack to cause the problem in the first place. Try replacing the micas in the list above, and confirming the values of the tubular caps you installed. Or, if it is working fine, just leave the altered value of R57 in place and be done with it.

[timmy]

ok, looks good thanks for looking into this it will sure be a big help. as for the vert hold pot IIRC the pot was at the minimum resistance when it did seem to me that there was not enough range on that pot but i did change the 2- 6.8 resistors at the 6sl7 and will check them again and the resistors on the hold pot and the vert size pot both were good. and every resistor i checked were very little out of tolorance certainly not enough to cause this. there is a 20 mmf cap c74 at the horizontal oscillator trans that went bad and the oscillator would not run and nearly burned up the horiz size pot so i put a good pot in and tacked in a used .02 ceramic disc that measured 17 and not 20 so i have a 20mmf mica and a .02 cap coming but i dont know if this cap is supposed to be a mica or ceramic . the motorola schematic says that cap 20mmf is molded whatever that means i dont know as i want to put the right one in since i am alittle short on the horiz width and the width pot is maxed but still short of filling the whole screen with no extra range to spare. the caps you mention i have coming except the .1 cap i changed already. as for the tubular caps they are the right values and in the right place. im just concerned as to what cap is the right cap to use in place of the 20mmf c74 mica of ceramic.

[Tom Albrecht]

It looks like you're thinking that .02 uF is the same as 20 mmF? Actually 20 mmF (20 pF) is .00002 uF. You might want to check if there are any incorrect values among the ones you replaced.

[timmy]

yes 20mmf is .02 nf but which one would be a molded cap, a mica or a ceramic. i should have said the .02 was nf. and the 20mmf was pf. this 20mmf cap was about a quarter inch long and looked like a small dog bone. the motorola schematic for this set shows , molded , mica , ceramic, this is where i get confused because its the horiz oscillator transformer and it needs to be right, the cap type.

[Tom Albrecht]

The dog-bone shaped ones are cylindrical ceramic caps, which generally do not fail and usually don't need to be replaced. I would generally use a mica capacitor to replace one that failed, although a ceramic cap could be OK. Modern ceramic caps have many variations, and people often don't know much about the differences between them. Some have a lot of drift; others have very wide capacitance tolerance that varies drastically with temperature and applied voltage. Micas as generally very well behaved. Bottom line is that in almost every case a mica cap is OK, whereas ceramic disk caps may or may not be well suited to the particular circuit.

[timmy]

ok great info because i was in the dark on this so ill use the 20mmf 500v mica in the horiz to replace the bad dog bone cap that went bad. i was told if i did use a disc that it should be an NPO i guess this has a temperature rating or something along these lines. thanks for this info.

[timmy]

there is a blanker circuit out there and was wondering if there is one for this motorola or maybe this one would work to get rid of those lines when the brightness is turned up. the circuit i saw was for an admiral set with a 7jp4 crt. its got a 250pf 1 kv cap and a 220k resistor in series from pin 5 of the 6sl7 to pin 3 of the crt. the crt pin 3 goes to ground and this wire should be cut off ground and attached to these 2 components to pin 5 of 6sl7. does this sound right for the motorola ts18 chassis. this setup is said to eliminate those lines with the bright up.

[Gleb]

Originally Posted by timmy

there is a blanker circuit out there and was wondering if there is one for this motorola or maybe this one would work to get rid of those lines when the brightness is turned up. the circuit i saw was for an admiral set with a 7jp4 crt. its got a 250pf 1 kv cap and a 220k resistor in series from pin 5 of the 6sl7 to pin 3 of the crt. the crt pin 3 goes to ground and this wire should be cut off ground and attached to these 2 components to pin 5 of 6sl7. does this sound right for the motorola ts18 chassis. this setup is said to eliminate those lines with the bright up.

You'd better use a variable resistor of a bigger value, adjust the brightness control to get an acceptable picture, and then turn the variable resistor to eliminate the retrace lines. After that you can measure the value and use a fixed resistor of the closest value. The voltage rating of the capacitor should be at least 1000v.
Blanking intervals in modern video signals often carry some extra information (teletext, copy protection, etc), causing it's visibility on the retrace as bright dotted lines. It may be difficult to supress them completely, causing you to set much deeper blanking.

[Tom Albrecht]

Yes - I have done the blanking modifications on many electrostatic sets like yours (most recent case yesterday, on a Hallicrafters). All you need to do is add a 100 K resistor between pin 3 (control grid) of the CRT and its existing connection to ground, and then connect a 150 pF 1 kV minimum ceramic disk capacitor from the CRT pin 3 to either pin 2 or pin 5 of the 6SL7 vertical oscillator tube. Try both pin 2 and 5 (the two anodes) -- one of them will fix the problem, and the other will not. Leave it connected to the one that fixes it. If you have a scope, you can identify the right pin by choosing the one that has a rising sawtooth with a vertical falling waveform. The other will be the opposite. If the circuit you found is correct for this particular set, pin 5 is the one. The values of capacitance and resistance that you note above will also work OK - you can try those values if you like.