mica capacitors
 

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. :scratch2:

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

how many watts on the 100k ?

well when the verticle is fixed i will try this and see if those lines are gone. the circuit was for an admiral set with a 7jp4 and not a motorola. so ill try yours. the admiral one was to cut the ground and just run that pin 3 with the cap and resistor to pin 5 of 6sl7.

you say verticle 6sl7 tube , this tube is marked verticle sweep output tube and the 12sn7 is marked as the verticle sweep oscillator tube. i didnt see anything on the 6sl7 as an oscillator just making sure.

Even 1/10 watt will be OK, so pretty much any size you have available. Typically one would use a 1/2 watt resistor.

Sorry for giving the 6SL7 the wrong name. It's the vertical output, and that's the one you want to connect the cap to.

ok tom , thanks alot for your time and effort in helping me out with this set.

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.

There's no need to experiment with values. Motorola added retrace suppression to later revisions of TS-18 chassis as shown in this schematic. Earlier versions had pin 3 grounded. This version has pin 3 going to the junction of a 250pF (C-70) cap and a 220K resistor (R-90). That couples a pulse during the vertical retrace to the grid and blanks the raster.

I got this from Riders volume 4.

https://farm9.staticflickr.com/8688/...5cb6a133_z.jpg

ok now i have more problems , i got the caps and i turned the set on to see if i was still where i was when i left it well when i left it i turned the vert size down and shrunk the pic and was able to stop the roll. so i turned it on befor doing the caps and now after leaving it alone doing nothing the vert size just makes it smaller and wont bring it big and i dont know why. i changed one cap at a time and with each cap changed the vert size still didnt work and i did this to make sure i didnt put a defective cap in and is always a possibility . so im done with caps except for the 900, 680, and the 200 mmf. what could this be the size worked when i left it waiting for caps and now it wont stretch the pic top to bottom to see if the caps i put in even helped. the other 3 caps will come tomorrow. what does this sound like ?

Well, I would target resistors as the likely cause at this point. On the Sams schematic at ETF (not Bob's above - the numbering is different), I would check the following resistors:

R58, R59 - 2.2 meg
R61, R63 - 6.8 meg
R80, R81 - 2.7 meg
R82, R83 - 2.2 meg

In many of these sets, I find that some or all of the above have drifted to a larger value. Some of these can also affect the focus.

Also, you may have a whisker inside your vertical height control. Try removing it, cleaning up both the inside (remove the back cover) and outside of it, and reinstalling. Or sometimes something as simple as running a toothbrush between the chassis and the terminals of the control will do the trick (if the whiskers are between the chassis and terminals on the outside of the control). You might even be able to see evidence of whiskers by measuring resistances to chassis from the terminals (you may have to disconnect from the circuit to do so).

I presume you've swapped out both of the vertical tubes to make sure they are not the culprits.

A common problem on sets is infant mortality....Some parts test fine, but after a short time under rated load fail...Our sets usually have two periods of infant mortality, initial deployment when new, and during/soon after restoration. If you left carbon comp resistors in it, then one or more may have drifted or outright failed since you last ran it. Modern parts usually have a low enough infant mortality rate that you are unlikely to experience it. Another thing to be wary of in electrostatic sets is the tube sockets...They are often ran close to their insulation dielectric ratings in the HV and deflection stages, and thus internal arcing, carbonization (SP?), etc. can form and be hard to detect.

EDIT: Looks like Mr. Albrecht slipped a post in under me.

ok ill check those resistors but chances are i did already. i have tried another pot no difference. maybe you know , should there be around 250 volts dc on the frame of this set to positive of the b+ . i thought this was not right so i had look everywhere for maybe a shorted cap of some kind to the frame ground as there is an isolated ground on this set. i thought maybe this was causing problems. i have tried several tubes.

R84 is a soft connection from the circuit common to the chassis. If you measure with a high impedance meter (like most modern meters), the chassis will behave more or less as if it is connected directly to ground in terms of measured voltage. So relative to the B+, you might see something like -250 VDC on the chassis. I would check the resistance from the chassis to the circuit common and see if you get the expected 470K from R84. If the resistance is lower, there is a short somewhere (solder blob, some wire touching, etc.).

Even if you checked those high megohm resistors before, as Tom C points out, they may have suddenly died in recent days. It's actually fairly rare to find those particular resistors still good in these electrostatic sets.

Also, check the voltages on the vertical oscillator and output tubes to see if they match the Sams chart reasonably well. Do you have a scope? If so, check to make sure that both triodes of the vertical output have a nice output waveform on the plates (huge sawtooth at 60 Hz, with opposite phases).

no i dont have a scope. and the resistance from both grounds are something like .530 meg is all i get between the both. the voltages look low where they should be high and others that should be neg 25 i get neg 12.50 so i dont know whats going on with this set.

Can you measure the voltages on the tube socket pins for the vertical output tube and compare to Sams? If they are substantially different, tell us what you see, and it may provide a clue to the problem. Note that those voltage should be measured from the circuit common, not the chassis.

ok, will do and ill get back to you tomorrow. ill use the sams schematic voltages.

ok, i checked voltages on per sams, v9 , v11 , v12 ,
v12 6sl7 pin1- neg 1.6 to 1.8 v- pin2 140v- pin3 0 -pin4 -neg.936v pin5- 47.3v- pin6 0 , 7&8 i didnt check.
v11 pin1-neg 12.30v pin2 15v pin3 0 pin4 neg .551 pin5 71.8 pin6 0 7&8 did not check.
v9 pin1neg.7v pin2 17v pin3 0 pin 4neg 1.630v pin5 11.26v pin 6 0 , 7&8 not checked.


it looks like the biggest problem here is v12 vert output pins 2 and 5 measure very low unless i cant use a dvom on these pins. where does pins 2 & 5 get there hi voltages,pin 5 i think comes from the focus circuit and the resistors are fine and i also changed the resistors in that string. does pin 2 get this voltage from oscillations or the hv cap. i am lost at this point,lol,lol... one other thing i still did not change the 900, 680, 200, micas yet.

V12 plate voltage comes all the way down through the resistor divider from the HV output. That's why it's so important that all the megaohm resistors and the HV output be good.

so if the voltage that should be on pins 2-5 come from the focus circuit it should be there since i changed the resistors at the 6sl7 i think they were 6.8 meg and also all the 4.7 k resistors at the hv pots for horiz and verticle. the voltage behind the 6.8 meg resistors is around 900 volts, is this right or should it be higher. because what feeds the 2 6.8 come from the focus pot and also changed the 2.2 megs, 2, and the 2.7 megs. the hv is under 5kv. i dont know if this helps any but recently the hv oscillator tube suddenly the heater got super bright so i shut it down tried another one it was ok then i tested the one that went bright and it too was ok, i dont know why this would have happened.

V12 (vertical output) gets its plate voltage for both sections (pins 2 and 5) from the focus control, which in turns gets its voltage from a resistor chain fed by the HV.

You correctly note that your voltages on pins 2 and 5 (+140 V and +43 V) are too low, telling us that something is wrong.

There are probably two main ways this can happen:

1. Not enough drive signal from the vertical oscillator. If the signal on the grids of V12 are too small, then both triodes will tend to conduct too much, pulling their plate voltages low.

1a. If the grid drive on just one of the two triodes is too weak, it could still cause this problem. One way this could happen is if there is a problem in the network that feeds the inverted signal to the second triode (grid = pin 4). Can you confirm that C65, R65, R64, C67, C66, and R62 are all correct value and good?

2. Supply voltage to anodes is low. This is fed from the high voltage through the following resistors: R4, R5, R80, R81, R82, R83, R3, R61, R63. Have you confirmed that all of these are OK?

Is your HV correct? Do you have a HV probe to confirm that there is 4-5 kV there? If you don't have a HV probe, let me know and I can suggest how to use a microammeter (if your DVM has this function) to read your HV.

(edit: sorry, didn't see the posts on the new page yet when I wrote this reply -- see below)

Around 900 V probably is correct at the junction of R61 and R63 (6.8 M resistors on the plates of the vertical output). Your HV looks like it is fine. You mentioned 4.7 K above, but those should be 4.7 M (just a typo? or wrong value in there now?)

If you've got indication that some filaments are too bright, check the AC voltages on the filaments and see if something is wrong there.

r61 63 are new and checked again. the 4.7 is meg if i put k my error. i checked the filiment voltage and its good so maybe the tube had an aging moment but tried another and no change. i checked the voltage at the hv oscillator pins 3,4,5 there is almost nothing there. omg... its a mess....

i checked pins 3-4 should be 225vdc so i used my dvom and its showing me 100v then milli volts so i tried another meter and its showing me 110volts both at those 2 pins and pin 5 should be neg 37 im getting neg 0.47 sorry i didnt say what tube its the hv oscillator v14

strangely now i just noticed that the image is upside down, turned over and it was not when i had originally put it away until i got the caps. so between the image and the verticle size, didnt have that when i left it to wait for the caps. unreal.... well weather or not the 200, 680, 900 micas will make a difference ill change them when they get here, still waiting.

Looks like your HV is fine, so no need to check the HV oscillator. Right now I'm suspecting scenario 1a in my post above (copied again here):

1a. If the grid drive on just one of the two triodes is too weak, it could still cause this problem. One way this could happen is if there is a problem in the network that feeds the inverted signal to the second triode (grid = pin 4). Can you confirm that C65, R65, R64, C67, C66, and R62 are all correct value and good?

Also check for any possible wiring mistakes in this area (in case you accidentally made a connection wrong on a replaced part. Check against schematic).

Since you're showing such a low voltage on pin 5, I have a hunch that the grid to that triode (pin 4) isn't getting a proper drive signal.

As for the picture being upside down after putting in caps. Were any of those caps the 6KV deflection caps? If so swapping which cap connects to which terminal on the VO tube will reverse the vertical deflection.

the weird thing here is i originally had a full screen and the verticle did work but only if the hold pot was maxed at one end and gradually it got worse until i laid it up waiting for caps and all of this started after turning it on. when all it was was it seemed that the hold pot was out of range then i had to replace the rest of the disc caps and a few micas. the caps were 1kv disc caps 1kv micas and the others are 1kv also. i had also changed the vert centering thinking this may be harming something but no good maybe i reversed the 2 caps there for the pic to be up side down. ok ill go over a few things here including the caps and resistors you mention as well as go over what i did around the v o tube. i would hate to think that one of the 5, 6kv caps may be bad if this would cause this i dont know. all this started befor i put any of these caps in but i put them in anyway thinking there were a few bad but it never helped.