Motorola ts907 low hv

[timmy]

I’m having a hard time believing those power resistors are bad because if they cannot supply the 100 v to pin 8 but only 60 v then I would think they wouldn’t be hot like they are working hard that’s why maybe the tube is partially on maybe not full but like an idle.

[timmy]

If I pull v1 the voltage go way up on v2 pin 8 but v1 pin 8 voltage stays the same low.

[Yamamaya42]

Originally Posted by timmy

I’m having a hard time believing those power resistors are bad because if they cannot supply the 100 v to pin 8 but only 60 v then I would think they wouldn’t be hot like they are working hard that’s why maybe the tube is partially on maybe not full but like an idle.

R 39,40,41 form a voltage divider, your resistance check of pin 8 found a problem with it, so replacing all 3 is the next logical step.

[timmy]

I don’t understand why the ohms go up slowly from 5k to 6.30k why slow it’s like it’s filling a cap. There is the one resistor that goes to ground and I hope something is not grounding that is not supposed to be. And I have new 1n4007 diodes I had put in and made no difference.

[timmy]

On the Motorola it has 2 rectifier diodes is 1n4007 ok for this or is there supposed to be a fast recovery or anything special about these diodes.

[Yamamaya42]

Originally Posted by timmy

On the Motorola it has 2 rectifier diodes is 1n4007 ok for this or is there supposed to be a fast recovery or anything special about these diodes.

the 1n4007 is a very old common diode that SHOULD work OK in the set, that is if you have a real one...

Problem is, over 60% of the ones out there are counterfeit junk and should not be used.

In fact, a few years back, we scrapped an entire lot of thousands of them cause they were all junk.

general rule is, anything not got from Mouser or digikey is not to be trusted when it comes to semiconductors.

as mentioned, a good choice is, https://www.mouser.com/ProductDetail...2B4kkscA%3D%3D

[timmy]

I bought a bunch of 1n4007 diodes but they look cheap so I’m not going to use them but I did try them they did work and like the output diode getting hot so did the 1n4007 also got hot so I put back the originals. So maybe it’s fine since trying to see what the ma was and it only showed 550ma so it’s not loading up.

[timmy]

What I find odd is with the resistors off pin8 there’s 138v and at the same time there is -7v on pin 8 and just touch the resistors to pin 8 and drops right down to like 63v where is this -7 volts coming from.

[Yamamaya42]

Originally Posted by timmy

What I find odd is with the resistors off pin8 there’s 138v and at the same time there is -7v on pin 8 and just touch the resistors to pin 8 and drops right down to like 63v where is this -7 volts coming from.

This is not unusual, take a look at any given tube grid that is not pulled high to bias on a schematic, and you will note that they tend to have a negative voltage on it, so it stands to reason that if you float it, it will go negative.
Examples.

pin 2 V10B , pin 9 V12A

[timmy]

Well so much for the resistors they made no difference I kind of had a feeling the old ones were not bad. there is 132v there but soon as putting it on pin 8 it drops to 63v so something is going on here that I’ve yet to figure out. On v1 pins 1&3 are 15v and pin 2 7.25v it’s like the tube is locked on full pulling the 132v down to 63v. So I lifted the power on pin 8 it’s not the tube pulling the voltage down pulling the 8.2k ground drops the voltage. Then with the ground off I get the proper ohms.

[Username1]

Originally Posted by timmy

Well so much for the resistors they made no difference I kind of had a feeling the old ones were not bad. there is 132v there but soon as putting it on pin 8 it drops to 63v so something is going on here that I’ve yet to figure out. On v1 pins 1&3 are 15v and pin 2 7.25v it’s like the tube is locked on full pulling the 132v down to 63v. So I lifted the power on pin 8 it’s not the tube pulling the voltage down pulling the 8.2k ground drops the voltage. Then with the ground off I get the proper ohms.

Hi!

Sorry, Our crappy internet service was off for several days.... If you have 15V
on the Cathode of tube V1, 6EH7 the 1St Video IF, then I would be wondering
what the plate voltage is? is the 275V source right, and is it 275V or lower?
Also what id the voltages on each side of R38, a 22 Ohm, R44, a 47 Ohm,
and R45, a 560 Ohm. Also what voltages are on both sides of R37, a 2.2K Ohm?

Please make these readings with everything hooked up.
Also are your power supply voltages back to what they should be?

.

[timmy]

Originally Posted by Username1

Hi!

Sorry, Our crappy internet service was off for several days.... If you have 15V
on the Cathode of tube V1, 6EH7 the 1St Video IF, then I would be wondering
what the plate voltage is? is the 275V source right, and is it 275V or lower?
Also what id the voltages on each side of R38, a 22 Ohm, R44, a 47 Ohm,
and R45, a 560 Ohm. Also what voltages are on both sides of R37, a 2.2K Ohm?

Please make these readings with everything hooked up.
Also are your power supply voltages back to what they should be?

.

V1 pin7 193v
R38 14.92 14.35
R44 13.26 14.43
R45 13.45 0
R37 7.70 7.66
The 275v is around 280v other voltages are good on the ps. It appears that once the 8.2k goes to ground the voltage drops don’t have to be on pin 8 for the voltage to drop.

[Yamamaya42]

things to try.
swap V1 & V2 and see if voltages change.

resistance check of L4

pin 3-2 4-1 3-4 1 - gnd 2 to gnd.

[timmy]

Under 1 ohm swapping tubes don’t do anything I think there is something at ground pulling this down. This testing I’m doing with the tuner plug out chassis on bench tuner plug looks like it would not affect the voltage at pin 8.

[timmy]

If I knew the test path for resistance check I could probably find what’s happening here pin 8 to x2 output.