#16
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I pulled the CRT with the chassis as 1 piece out of the cabinet. Then separated it from the chassis.
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#17
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Power seleniums can be replaced by modern silicone diodes, just check the current rating of the original (usually listed in sam's) against the new one (replace with the same or higher) and make sure the PIV is a good bit higher than what it appears the circuit can produce.
I suspect M3 is the Horiz AFC (I don't have the schematic) those are usually good, but I've encountered a dud or two....The symptom of a bad H AFC diode is soild Vert sync, and horizontal can be made to almost sync, but will at best roll slowly either direction without locking. The AFC diode is three leg because they put 2 diodes in the same case and tied two legs together internally. If the H AFC diodes are on the board and the board is a royal pain to change parts on (like many claim the predicta boards are) then it may be wise to replace the H AFC diode pack as preventative maintenance. You can use 1N4007s for H AFC. Some claim you need fast shottkey diodes, but in my experience the 4007s work fine in the sets I've needed to do AFC diode swaps on.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
#18
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Pix looks good, audio is somewhat buzzy depending on screen content... my Zenith DTV converter produces less buzz. Will watch for a couple of hours tonight before putting it back in the cabinet. Likely I will mount a fuseholder in the area above the rectifier assembly this afternoon, before extended viewing (still have orig electrolytics). Thanks for your help, jr . Last edited by jr_tech; 05-17-2017 at 05:38 PM. |
#19
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OK, back on this TV after a trip, company, and honey-dos.
I'm recapping the "backside" of the chassis and there is 1 couplate. In the process of recapping, I puled the couplate and measured the pins. I measured between 2 and 3 and get 1.13M and pins 1 and 2 and get 5798pf. The cap should be 5000pf. Only a difference of @16%. But the resistor should read 1M. A 13% difference. Looks like it is already out of tolerance. I'm assuming 10% like most of the resistors used. I'm including a picture of the schematis of where it fits into the circuit along with a close-up of the internal schematic of the couplate. My question is what kind of caps are used in these things? I just might try and build a replacement. |
#20
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I had a Philco "Briefcase 19" portable TV from 1961 back in the '70s. I believe it was a successor to the Seventeener. My set worked quite well, and in fact was a temporary replacement for my Sears Silvertone roundie color set (RCA CTC12 clone, IIRC) when that set developed serious problems (the video output tube socket broke out of the video PC panel with a sickening crunch when I tried to replace the tube).
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Jeff, WB8NHV Collecting, restoring and enjoying vintage Zenith radios since 2002 Zenith. Gone, but not forgotten. |
Audiokarma |
#21
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jr |
#22
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+1 - if that copulate were in the vertical or horizontal stages, it would improve sync-etc but as a blanking pulse network, you would see some definite symptoms if it were to drift too far out of tolerance.
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"When resistors increase in value, they're worthless" -Dave G |
#23
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+2 - Also I've cracked open some Philco couplates and they are ceramic caps. Nothing special. They used them to cut assembly time not because they were precision parts.
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#24
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+3. Those values were not that far off from the original. I would concentrate on other issues. Couplates are often one of the last things to go.
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#25
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OK guys, I'll just clean it up and re-install it. Thanks.
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Audiokarma |
#26
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I'm pretty much done with the backside. I re-stuffed the E-caps, replaced all the resistors, and film capacitors. I cleaned and redressed the HV leads and used some heat shrink on the 1B3GT socket. I replaced the 40 ohm 20 watt sand resistor with a 60 ohm 25 watt resistor because of the higher line voltage.
I then replaced the rectifier diodes with 2 1N5408 3A diodes. I used s 3 prong terminal strip. Worked out great. I also replaced the .1uf bumble bee cap with a .1uf safety cap. I then pulled the IF board to do the recapping. A lot of the wires were soldered onto the stakes. It did come off fine though. A bit tedious. Once per TV will be enough thank you. The board had a coating on the foil side. When I removed the resistors, it just ignored the coating. Once the part was removed, I cleaned the area up with my solder wick, then used a Q-tip and acetone to clean away any "varnish". Once the parts were soldered back in and the flux cleaned off I did a real good inspection and the used some clear corona dope to recoat the board. The last picture was before the re-coating. |
#27
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BTW, I'm getting ready to do the main board. I have the film caps, resistors, and the 2 micas will be replaced. Is there anything else that I need to be on the lookout for? These boards look very similar to the Predicta boards. I'm concerned about the networks. Should I just shot gun them and be done with them? Which ones are usually bad if any? As with most, I only want to pull this board once.
BTW, I'm leaving all the ceramic discs alone. Right? |
#28
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While recapping the main board I ran across this:
This is K2 and it has a 500pf cap added to pins 5 & 6. Here is the schematic of K2 and I penciled in the added cap. Here is the part of the schematic that K2 is used in. It is in the audio section just after the volume pot. Again I penciled in the added cap. Any idea why this was done? If you look closely you'll see K2 is missing some coating on the face and on the top in several areas. I'm going to build a new one. Any idea what I should use for caps? Thanks. |
#29
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I have a Zenith 4 tube portable radio with the color case. I saw a set just like mine go on the the E-place for $100+. That's Quite a while ago. |
#30
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Looks like an attempt to "improve" the audio, by increasing the low frequency response. Mine did not have the added cap... perhaps it was added later by a serviceman. Nothing critical there, I would just use ceramic caps to replicate the circuit.
jr |
Audiokarma |
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