Motorola ts907 low hv

[Yamamaya42]

=^-^=

[Yamamaya42]

sneaking onto here helps with the boredom @ work!

" OH GREAT, THANKS! 25 MORE PRODUCTION BOARDS FAILED, YOU WANT THEM DEBUGGED BY WHEN!?!?"

[timmy]

Lol ��

[timmy]

This 150 ohm resistor if it burned up down to 95 ohms would this alone cause the tube to go full on ? If zero ohms at pin 2 will cause the tube to go full on then how can the tube go full on if there is any voltage getting past c55 and c56 that would have voltage at pin 2 so I would think if it’s not zero and there’s voltage present then the tube wouldn’t be full on. I checked the volume pot it measures 2.4meg wiper varies ok. So just changing the burned resistor should this fix this at all.

[Yamamaya42]

The 150 ohm cathode resistor is for limiting current, even if it was not there, it would not overload the tube.

The only thing that could is too much positive voltage on pin 2.
And as stated before, W/O the volume control plugged in, there is nothing to stop the grid from creeping up and turning the tube on, this is a design flaw.

[timmy]

Ok well maybe at some time of the history of this set maybe someone had unplugged the harness that carry’s the volume control and it burned the resistor so I’m still not sure about the tube. That’s the only one wire on the volume control that goes to pin 2 so with all hooked up the tube shouldn’t run away but if it does then I don’t know from there. I changed all those caps you mentioned and the booster leg with the 1.5 meg resistor is good. Or maybe between the plug connector maybe a bad mate within the plug itself.

[Yamamaya42]

You could always hook your DVM to the center post of the volume control ( wiper ) with the set in operation, and move the control through it's entire range, you should not see a voltage much higher than 0v, a few millivolts are not enough to cause a problem, but 2-5v or more, PROBLEM! :O

[timmy]

I checked continuity of the wires on the volume control as well as the changing ohms when I rotate the knob it all looks great.

[Yamamaya42]

Originally Posted by timmy

I checked continuity of the wires on the volume control as well as the changing ohms when I rotate the knob it all looks great.

The method I described is an easy access way to monitor the voltage on pin 2 of the 6gk6 with the chassis in set, the center of the volume pot is more or less directly connected to that pin, by watching the voltage there, you are also watching the volts on that pin.

[timmy]

Yes that’s the green wire goes to pin 2. So now I have to change 2 .005 discs and got another tube coming hopefully Friday. Then put it all back together and hope no more load pulling down the b+.

[timmy]

Well putting the chassis back in so if I still have this b+ being pulled down then all I can come up with is a bad cap to ground I don’t think a resistor would do this because it just seems like maybe one b+ voltage is bleeding over to another via a cap to ground. Pulling all tubes one by one with chassis in hooked up didn’t show anything positive.

[timmy]

All back together and the audio output tube is new and still getting really hot fast voltages still low and the green wire on the volume pot goes directly to pin 2 and there is 280 ma at that pin the volume pot is 2 meg it measures 2.4meg if pin has to be zero I have no clue why there is 280ma there. Still looks like this audio circuit is pulling the b+ down b+ down means all other voltages go down with it. The volume pot has 3 connections green blue and ground which ground is on the body of the pot and one end of the pot maybe the ground is picking up stray voltage from somewhere else and back feeding the pot. I don’t know at this point. If the tube is at zero volts then it will run full but with 280ma then the tube should not run full even chassis out pin 2 still showed around 50-60ma. Now I have discovered that the 1st video 6aw8 and the video output 6hb6 are getting hot fast within 30 seconds being on so I’m off now to look at that circuit.

[Username1]

Hi Guys, It is good o see you have made progress on this set. So If the audio tube is
getting hot and is possibly the current draw, it should be easy to figure out what might
be wrong. First turn it on and hook up a quality audio signal to the volume control,
make sure you put it through a .1 uf cap and see how it sounds... Good quality or not?
Does the audio output transformer measure 930 Ohms? Is there approx 145V on pin 8?
If you are reading .25V on the grid & Center line of the volume control, check out R80,
R81, R82, be sure they are right.
Also there are other goodies to think about., like crudlies on the tube socket pins allowing
little bits of current to flow and give you small voltage at pin 2 the grid to the tube.
I Assume Timmy that your 280ma. at pin 2 is actually .28V small typo.
I Also found I kept making the same typo earlier in this thread saying 380ma. for the
power supply current, when it's actually 580ma, as others have correctly written......
Also you should be able to ground pin 2 without causing any harm, and see if the tube
runs cooler, & Your PS voltages return to normal.....

I'm a big fan of voltages being pretty close to what the schematic says, but .25V is kinda
small for that spot.

As it is, when you run the set do you get sound, or snow from the sound when you turn
up the volume ? does it sound like it should? Clear not distorted?

Also remember single tube audio output circuits like that are Class A Amplifiers, so the tube
is biased on anyway, and always, otherwise you would hear distorted sound as each
sound would be turning the tube on and off all the time. Class A Amplifiers are always
conducting, and the input signal smoothly changes the grid voltage & smoothly raises, &
lowers plate - cathode current to give you a nice smooth sound.



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[timmy]

I could answer all you wrote if you want my number let me know it will only take a few minutes so much typing lol. The audio trans checks perfect 80-81-82 resistors perfect and after having the set on the last time I had an antenna on channel 3 fades alittle probably out of tune but channel 4 sound was perfect but heard no snow with antenna off and yes I made a mistake pin 2 is a green wire is the wiper set on it measured 280 mv But the tube didn’t red plate until I had the chassis out and found the 150 resistor burned. The tube just seemed to get really hot fast now I find within 30 seconds the 6aw8 and the video output 6hb6 get really hot fast I have read that low b+ is not good for tubes maybe things change with the tube conducing at low voltage. My initial finding was I had no video so maybe something in the video area causing all this.

[Username1]

It is always possible you have more than one problem. If you suspect the Video Output
tube is using too much current, then you need to do voltage checks at each pin & see
if you find any burnt looking parts in that circuit. It's also sourced to the 385V.

The video amp 6AW8A is also sourced to the 385V.

But if the audio out tube accounts for all the extra power supply draw, then we may be
getting ahead of ourselves.


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