Attempting to restore this two tone green 14" set. Have a blank raster with very faint audio. Picture and sound seem to jump in and out when tweaking volume control. Contrast control seems to do nothing.

130 VDC supply is low (40 VDC) 255 supply OK
When 130 removed from tuner, brightness control, and IF board,
Pin 1 12CU5 (130 volt source) with C4, & C3B connected is normal.

All filters have been replaced , tubes tested and a few caps on IF board.
Common chassis with some variations used in the 10" thru 17" sets.

If someone could provide a tube layout for this chassis and or the schematic from the sams or admiral service literature it would be appreciated. I have 350-1 but some of my tubes seem to be different on the tuner and IF board. This set also has the UHF tuner.

Maybe a quick where to check tip from you experts could lead me to the culprit.

Thanks

For start:

Replace wires one at a time and measure 12CU5, G1 and cathode to see if any one circuit is loading it. If not, replace the 12CU5.
Bridge the electrolytic cap on the 130 V supply and measure the 130V.
Check the tuner for mechanical damage. The disc or drum may have worked loose from the shaft.

Hi Don,

First may I say thanks for your advice. I have read many posts on the forum and have learned a lot from yours and other member posts.

The predicta ones are great as I own several and all have some sort of quirk, as most predicta's are known for :D

Meanwhile back to the task at hand. What I have done is while monitoring the B+ with a DVM, (Pin 1 12CU5) I attached just the tuner B+ lead and watch the voltage drop as soon as the tubes warm up. I have looked for mechanical problems inside the tuner and don't really see anything obvious. I checked both tubes and they seem OK although I haven't subbed them as of yet. Gotta dig out my tube stash. So maybe something down stream connected to the tuner ?

Next I connect the brightness control by itself (Wiper lead disconnected from IF board) and again watch the voltage drop. What I don't grasp is how can a 100K control drop the B+ by itself. It seems to ohm out OK and I shot some deoxit also. It does vary the brightness on the CRT when it is connected so again I am a little puzzled.

On to the IF board when the 130 lead is connected the voltage drops. Measuring several points on tubes show low readings. Again the 255 is rock solid. I realize a number of components on the board could cause this but the brightness control just doesn't make sense to me especially with no load on it.

I'll move on to G1 and cathode next but for now work is calling. I'll dig out my tubes and look for a spare or order some.

Points to the 12CU5 and/or electrolytics on the 130V line. There are two on the 12CU5 cathode. If they are OEM Sangamo brand, they are likely bad.

Swapped 12CU5 no change 130 VDC still low. Filters C3B & C4 were replaced they were both Sangamo.
C3B calls for 60/150 all I had was a 50/160 would that affect it ? I believe with the old cap the problem still existed.


Hope to pull the chassis again tomorrow. Any one got any ideas where to look next ? Do I really have three separate problems dealing with the 130 supply ?

Thanks

Check the 12CU5 grid for 110 V. Quickly remove the tube and measure again. It should stay there. If not, check the resistors in the grid circuit voltage divider.

Does varying the volume control affect grid voltage, with the tube out? If so, replace the coupling cap.

Could be more than one problem; three I doubt.

Quickly remove and replace the tubes on the 130 V circuit, one at a time, while measuring the 130V supply. There could be a shorted tube loading the circuit.

Well things seem to be getting worse !

Audio xformer primary opened up so I used one from a scrap chassis that had a 50L6 output. Measures 186 ohm vs 220 ohm on schematic, got my faint audio back. Will this xformer be OK for now ?

Measured 12CU5 pin 6 grid and voltage was 265 ??? R43 330 ohm checks OK. Is this right ?? Maybe that's what popped the primary.

This is a series set so if I take out any tube that will open the circuit. I have a few of the tubes for the 130 line so I can swap them out and see if there is any change. All tubes checked good.

This set also has a UHF tuner. Could that be associated with my B+ tuner issues ? Schematic 350-1 doesn't show it do you have 351-1 or for that matter I'm sure someone has 351-1 they could look at and post if it shows the UHF tuner.

You can run it with some of the tubes out if you get creative....For instance you could use alligator clip leads to connect the heater pins of removed tubes with thin wires pluged in to the socket holes for the heater pins. Or calculate an appropriate power resistor to plug into the heater pins...Or jumper the socket pins and reduce the input voltage by the amount that the heater is rated for using a variac.

VT105:

Measure the control grid, pin 2 or 5. Should be around 130 volts.

Screen, pin 6 will measure about the same as the plate.

There are a couple of 1.5 meg resistors on the control grid that set the bias, which in turn sets the cathode voltage. If the audio transformer was open all along, that accounts for the low 130 volt supply. Recheck it now.

First a big thanks to Don Lindsly. :banana:

Changed R41, R42 1.3 Meg resistors on 12CU5 and bingo I was rewarded with a strong 130 volt supply. Re-connected tuner, brightness control, and IF board and now have a picture and audio. Brightness, focus, contrast, & vertical are stable, but has horizontal sync issues. All that for a couple of 5 cent resistors !

Next step is to recap the sweep/sync board

Hope to get some pictures posted when finished.
Bill

Check the sync diode, sync sep plate resistor and horiz osc plate resistor for hor sync trouble before doing a shotgun on the caps. I have better luck troubleshooting a problem before I accidentally build in more.

Once fixed, check frequently as you replace parts to make sure you have not built in a trouble.

I am all for simplicity at this point,:yes: surely don't need to add any more problems.

I'll check the resistors first. I have a stash of 1N914 diodes too.

Thanks
Bill

Well it's been awhile (too many projects !)
but today I did some horiz part replacement to include resistors, caps, and the notorious M3 horiz AFC diode and was rewarded with this.

Camera not the greatest but you get the idea. Another classic saved

Nice job!!