Riddled TV Repair Forum

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

Trying to fix an old Toshiba. Model #CF2768B - Chassis #TAC9200
Manufactured May 1992
The picture is 2 inches tall, right in the middle of the tube. Color, sound,
and other functions OK. Don't have a schematic but I see VERT on the front of the PCB next to an IC mounted to a heatsink labeled IC301.
Is this the place to start? If so how to check it short of gambling on replacement? The solder joints looked good, but I resoldered them anyway - no difference.

Google

JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

Most likely culprit: C308

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

Swapped out C308 (220mf @ 35v) - no change.

JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

Sams # 3194
Less likely C305, C306 and C311 ... a crack in a foil is also a possibility
VCC pin #1 = 12 volts ... pin #8 = 27 volts
Otherwise IC301 LA7837

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

Haven't checked the caps yet. Voltages check 12 vdc @ #1 and 29.2 vdc @ #8.

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sdtv · Joined: 31 Mar 2008 Posts: 12

Sounds like a bad cap or caps.

We used to change the red ones ? There was 2 of them one by the big chip and the other closer to the vertical chip.

Those caps with the red plastic cover, the only 2 on the board. (it's a Toshiba thing)

An old trick is to jump the caps from the bottom of the board while the tv is on with a 10uf at 200v or so cap and see if the picture changes.

Discharge it before you try it on the next cap.

If you see a big change then that is most likely the bad cap.

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

What type of caps do I use? Mylar? Monolithic?

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

This thing is going downhill fast. I replaced C305, C306, and C311 - no change. Found out my local electronics shop sells the NTE replacement for IC301 for only 4 bucks. Replaced it and no change. After seeing the idea to jumper the caps I went back to the TV and now it won't even power up .
Thats all I get is a relay clicking closed and then open?????? Starting to wonder if it's time to take it to the curb!

JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

Presuming you're following standard practices when working on the set, the worst that should happen is "no change" in the symptom.

ALL the caps I've mentioned are "electrolytics."

Capacitors being paralleled (if both are within spec and both are the same value) will double the capacitance. If the one in the circuit is open, (and that one capacitor is ALL that is bad) the picture will magically return.

Red Caps C301 & C305 (sometimes 1ufd 50V, sometimes 2.2ufd 50V) I don't believe are used in this set.

Start by seeing if you've blown any fuse and if RAW and STANDBY voltages are present.

1968RT · Joined: 13 Mar 2010 Posts: 51 Location: michigan

To JTS1957 - I know all the caps you mentioned are eloctrolytics - I was asking SDTV what type of cap to use for his jumper test. There are 2 large caps as he described, where he described on the PCB. They are brown in color and not polarized. They are labeled C440 (marked 752h, 1.25khp) and C445 (marked 124j, 200). I am a novice at TV repair but not at electronics, so while I understand how various components function and how to test them, I have no knowledge of standard television repair
standards. Thanks for your patience and for sharing your expertise.