Motorola ts907 low hv

[timmy]

I have been thinking the same thing about the caps not being able to handle ripple. But without the tubes I only did it for around 10 seconds and I did watch the voltage.

[timmy]

As for the caps if one it shorting then it could effect the b+ network since they all go to ground except for one section but I’ll be checking them.

[timmy]

Changed the caps still 190v pulling down I didn’t change the 2 250 160 uf being the initial voltage is little over 400v but slowly drops because of the 190v dropping. Still the dark.

[Yamamaya42]

Put in the 1w wire wound 15k resistor you have for R215, then check on the other end for warm resistors that they are giving power to.

And, if you have not done so at this point, replace disc capacitor C61, that is the only one that is NOT going through a resistor on the farther side of the 190v line, all others have current limiter/ voltage drop resistors that should be warming up with power pushed through them.

This again can be related to V6 cathode or even the socket, as it is the only tube that has a direct connection to 190v w/o going through resistors.

The whole point of putting in the new larger watt R215, is so it will heat up less, and hopefully make the problematic heat more and easier to find, it's bad anyway at 18k.

[timmy]

Yes I already changed the 15k and it to gets warm and c61 I took one leg off and tried it still pulling down and the cap tests good unless the problem is somewhere else and it’s affecting v6 red plating. V6 out still have the draw down.

[timmy]

More then likely this is the whole problem this set has with the oscillator and video and sound it seems maybe the problem started out small and finally shorted out to whatever or wherever it is I can’t check resistors anymore because they are all good so it don’t leave much else but possibly a disc cap.

[Yamamaya42]

You still have no HV, and therefore no boost, so it's a bit odd for it to be red-plating, as there should be no bias on pin 8.

Try voltage check on each pin of V6, with tube in and out, monitoring voltage as the set is turned on.

[timmy]

Ok there was a resistor pin 7 of 6cg7 to pin 1 of v6 which is the 190v source the resistor is a 560k it checked.800k so I changed it and I felt it and was warm so I disconnected one leg and the hv came back and it’s stable at -45 so if I put the resistor back the voltage pulls down so there maybe a problem at v10.I’m thinking the 56pf cap may be shorted since it’s tied directly to ground and the 56k resistor is 61k.

[Yamamaya42]

R93? OK , getting someplace, power has to be going somewhere, leaking out C66 or C65 , or even R94? C66 prob.

[Yamamaya42]

if bad, it must be replaced with another N750 class or better type.
https://www.tedss.com/LearnMore/Ceramic-Disc-Capacitors

or else it wont be stable.

[timmy]

R93 is the 560k after I changed r94 is a 56k it measures 61k so what’s left but the 56pf cap. .022 is new I double checked that one.

[Yamamaya42]

Originally Posted by timmy

R93 is the 560k after I changed r94 is a 56k it measures 61k so what’s left but the 56pf cap. .022 is new I double checked that one.

Well if you have a NPO one, that would be even better then N750 class.

[timmy]

More trouble I clipped one end of the 56pf and pulled one side of the .022 still shorts with the resistor connected but good with it off. This is nuts I thought the cap was it.

[Yamamaya42]

there is only 2 places for the power to be going then, R94 (internally bad ) or V10 has a short grid to cathode. this will be evident by the wrong voltage on pin 7.

[timmy]

Originally Posted by Yamamaya42

there is only 2 places for the power to be going then, R94 (internally bad ) or V10 has a short grid to cathode. this will be evident by the wrong voltage on pin 7.

I’ll clip that resistor and check it again but it can’t be a short grid to cathode since the short is still there with the tube out.