Ctc 15 clone/Curtis Mathes C20 Horz output grid not negative.

[ZenithNut]

Hello guys,

I have this ctc 15 curtis mathes clone. It fires up with B+ and sound but the first grid of the horizontal output tube is not being driven negative. I checked the voltages on the horz oscillator (half a 6fq7) . The grid needs to be -100v.

When I got the set the horz waveform coil was snapped off so I bought one from Moyers and installed it. Would the new coil be throwing the horizontal off frequency to cause this or would this be something else?

[Electronic M]

Could be. If the osc trans is set far enough off freq it can kill oscillation.

Remove your H output tube till the osc is working, pull the chassis, get an oscilloscope, and get the osc making on frequency correct looking waveform.

You can verify frequency by feeding your scope composite video, getting 1-3 cycles of video denoted by H sync pulses on scope screen, count the scale divisions between leading edges of the synch pulses, do NOT touch the scope horizontal time scale till your done adjusting the osc, now connect the scope to the H osc and adjust it to make a waveform the same number of divisions between peaks as the sync...If that gets you good osc signal populate the output tube, and if you then have raster run through the sam's horizontal setup procedure.

If the osc won't run start changing resistors (I assume you've already dealt with caps) and checking voltages.

[Tim Tress]

First of all, check the replacement coil for continuity; stranger things have happened. Next, check your soldering work around the coil. Pull the horizontal output tube, and check the voltage readings on the 6FQ7. If you have an oscilloscope, it will be a big help.

[ZenithNut]

I'm diving back into this set. I'm going to give it a go again. I have my O scope calibrated.

[ZenithNut]

Tim, Tom C I probed around the horz osc voltages with the tube installed. The first have of the tube (Horz Afc) had correct looking voltages on pins 1, 2, 3. The other half of that 6fq7 (horz osc) had a plate voltage of 416VDC WITH the tube installed. The grid was -50vdc. The osc plate should be 260vdc with a grid voltage of -100vdc.

[Electronic M]

Pretty clear the osc isn't working.

The plate voltage indicates no plate current. The grid voltage should cause excessive plate current...unless either the cathode has become more positive by 50V or more, or the plate of the tube is open and the grid getting a lot of current, or the tube is dead on that section. Verify the tube and the components connected to the osc tube.

[ZenithNut]

Originally Posted by Electronic M

Pretty clear the osc isn't working.

The plate voltage indicates no plate current. The grid voltage should cause excessive plate current...unless either the cathode has become more positive by 50V or more, or the plate of the tube is open and the grid getting a lot of current, or the tube is dead on that section. Verify the tube and the components connected to the osc tube.

Alright, I will. I think the cathode is supposed to be like .5vdc

[ZenithNut]

Tom I found an open 150k 5% resistor coming from between the horz hold control and the grid of the horiz osc. Also the cathode is measuring. 016vdc instead of .1vdc

[Electronic M]

That could be the problem in and of itself, but may as well shake down the whole circuit while you are in there... I'm still suspicious of the osc tube and it's socket given the voltages.

[ZenithNut]

Originally Posted by Electronic M

That could be the problem in and of itself, but may as well shake down the whole circuit while you are in there... I'm still suspicious of the osc tube and it's socket given the voltages.

The resistor has been changed and the voltages are spot on. Moving the horizontal hold increases or decreases grid voltage. Osc grid is -100 volts and the plate is steady at 260vdc. Unfortunately the g1 hot voltage is still positive 60volts not -60.

[ZenithNut]

GOOD NEWS EVERYONE. I populated the 6je6 and the high voltage is working!
As soon as I populated the bulb the grid dropped -55 volts. I took a segment of cleaning rod with a plastic handle and drew a good two inch arc in which it then jumped and shocked me!

[ZenithNut]

The chassis is hooked up and it did make a raster. Its scrunched around the edges and has those obvious bars across the screen from bad electrolytics. There's also green and red blobs along the bottom. So basically were looking at a Re cap, horz efficiency coil adjust, purity set up, etc.

[Electronic M]

Glad it is working.

The hot grid goes negative from the action of the output cathode and grid rectifying the osc signal. If the osc voltage and waveform look ok then that signals it is time to populate the output and check it's grid voltage like you did.

[ZenithNut]

Originally Posted by Electronic M

Glad it is working.

The hot grid goes negative from the action of the output cathode and grid rectifying the osc signal. If the osc voltage and waveform look ok then that signals it is time to populate the output and check it's grid voltage like you did.

It was pretty exciting to hear the hv come up and see a raster. It's an ugly raster but it's a raster!