Motorola ts907 low hv

[timmy]

Ok well low voltages can give all kinds of problems things won’t work so at 113 volts ac b+ is 360v and the source voltages are 275 is 251 v 190 is 131 v 145 is 175 v and if the 2 voltages were swapped they would be functional and my problems would probably be solved and yet I went over my work with the power supply caps and appears right so this does look like I have 2 power supply caps reversed. I did this with chassis out horizontal tube caps off no hv but these source voltages I would think should be up to par without hv because it’s a source voltage.

[Username1]

Hi Timmy, I believe you have a Power Supply Problem.
You may even have multiple problems, but you need to
focus on the power supply if the below voltages are
actually what you are reading. Check this stuff
in this order.

385V Source - Your reading 360V - too low.
275V Source - Your reading 251V - Too Low.
190V Source - Your reading 131V - Too Low.
145V Source - Your reading 175V - Too High.

So lets figure out what's going on with the 385V. source. First off I assume you
already changed all the power supply caps, So be sure they are the correct values,
and correct voltage ratings, and in the correct spots.

If you do not have 395V at Doide X2 you should then check for 143V AC at the output
of the power transformer that feeds the Voltage Doubler circuit. If it's LOW
than you need to check the Thermister R211 to be sure it's ok.
You can check the other do-dads on that side too, be sure nothing
has an excessive voltage drop across it, Switch too.
Be sure the circuit breaker does not have high resistance across it.
L54 should be around 12 ohms. Be sure to check that W, Z Stuff at the
lower winding on the power thansformer. That 275V source goes through
a few caps & goes to those HV Reg Filiments. You remember having
a Short in that circuit on another tv you worked on with Low HV.

If Ok;

At L4 the choke in the power supply, there is a current reading there of 380ma.
You should use your current meter to be sure this is at 380, if high there is a
excessive current draw. If low you have something open possibly.
Be sure to cut, or remove the input line to the choke, put your current meter
in series with the choke & Diode, use a high current scale, Like 10A, because
the resistance of the current meter will be very low, like 1/10 Ohm, and will
not cause a voltage drop across it.

The 385V source flows through a 550 Ohm resistor R214 and becomes the 275V
Source, you should check this resistor.

Also check R 215.

Finally, Your 145V source is too high...... Please check R 216. And be sure to check all
your wiring in this step.

For your 145V Source to be too high, there is either a wiring mistake, or a possible
shorted capacitor between two voltage sources that can possibly be bringing some
higher voltage to the 145V source. This can be anywhere in the tv.

Next step is - you can see below the power supply is a small box that tells you
exactly how many connection points each source goes to, You will need to start
checking these.

The box is called B+ Distribution

If you read more that 380ma. then you need to disconnect each of the 9 - 385V
sources and try and see if one has a high current draw. Disconnect one, turn the set
on and check 385V source. If it goes up, that point can have a problem.

You need to do this stuff with everything hooked up & ready to work as usual.
You can't do Power supply stuff with parts of the tv turned off, or disconnected.

Good Luck I Won't be around much this week......

..

.

[timmy]

Yes most of what you mention I already checked the choke 11 ohms a new thermistor all the power supply caps even checked them parallel them I even checked the sams for where the 275 190 and 145 voltages go and traced them back to what I did in the power supply, checks fine. At the diodes it’s the same 360v the doubler caps have 180v on each cap diodes clipped there is 143v ac present thermistor is new the 550ohm resistor checks perfect all the ps caps are exact or higher in capacitance. I’m going to check line voltage going to the primary of the power transformer tomorrow and hopefully rule out a thermistor problem or transformer primary problem. And also the 15k and 5.6k both are perfect. All this is with 114v or so line voltage.

[Username1]

Be sure also to double check the polarity of C1 & C2, See how they are hooked
up in the circuit. The 2 160Mfg Caps in the Doubler Circuit.
And check the 380ma current.


.

[timmy]

Polarity is good I went over that several times along with everything else double checking. My digital meter can check amps not sure of ma or if I can get it on the wire. C1 don’t go to ground iirc. Well maybe I’ll pull tubes one by one maybe one is defective in such a way that it would cause a drop in voltage.

[zeno]

Wait a minute. Did you say a thermister was bad ?
Did you change it ?
If you did it could be the wrong type. Some go up in ohms when
hot & some go down.
Also does this use a relay for DGS ? Moto was one that did
but I dont remember how they worked. That was the early 70's
that I last changed one !

Zeno

[timmy]

Originally Posted by zeno

Wait a minute. Did you say a thermister was bad ?
Did you change it ?
If you did it could be the wrong type. Some go up in ohms when
hot & some go down.
Also does this use a relay for DGS ? Moto was one that did
but I dont remember how they worked. That was the early 70's
that I last changed one !

Zeno

Changed the thermistor already exact one for the set moto number and no relay

[timmy]

Would it be a bad idea to jump the thermistor to at least see what the voltages show.the strange thing is first turn it on b+ will show 390v but steady it drops and lands at just making it to 360v.

[Username1]

Originally Posted by timmy

Would it be a bad idea to jump the thermistor to at least see what the voltages show.the strange thing is first turn it on b+ will show 390v but steady it drops and lands at just making it to 360v.

You may cause excessive current to flow. You can check the AC voltage drop
across it.

It might be a good idea to check line voltage, & Compare it to the primary
of the power transformer. I think it should be a small difference maybe
10VAC no more......??

113V at cheater cord, Maybe 108VAC at primary???? I don't know what to
say about that, I have no idea what THAT Thermister's specs are???

The Thermister at 25 Ohms cold, My Guess is it would go close to
Zero when it warms up, So Very little voltage drop across it.
when it's hot....

L46, & L47 appear to be line filters, There appears to be no Degaussing
circuit on the schematic. The Thermister must be to prevent a surge.


.

[Yamamaya42]

You are most likely chasing a phantom here.

Remember that no 2 sets are exactly alike, and tubes are VERY forgiving about their bias sources.

If it says "385v" on a SAMS, this does not mean that the MUST BE that voltage there ABSOLUTE, there will always be variance, sometimes +/- 10% and this does not necessarily mean there is a problem, this is in fact normal.

but by all means, check the PWS area, but you will most likely find, there is nothing to worry about there.

[Username1]

If you first get 385V and it drops as the set comes on, then it's possible a tube is
conducting too much current, and dragging down your 385V Source. It can be
ANY Load connected to Any Spurce point, But start with 385V.
As for the starting voltage, The formula for figuring Voltage from a doubler is
approx 2X(V-Peak) - .7V(Diode Forward Conducting Voltage.) From the
Transformer ( 143V X2 ) -.7 = 185.3V

If it's drawing more than 380ma. it is reasonable to believe a circuit may be drawing
more current than it should.

If your transformer secondary is less than 143VAC I would first go forward
and look on the primary side. The 143 V is with the tv on and all stuff
hooked up as in the schematic.

Not a Phantom, He has several PS voltages too low, and one too High.

You better double-Check that Thermister too.....

.

[timmy]

As for the thermistor I got the same results from the new thermistor as the old one. None of this make any sense to me tracing the 145v to specific tubes and trace back to the power supply and there’s 175v. It still seems to me that something is crossed within the power supply cap network so I’m going to go back to this area and look again because it make no logical sense to have 175v at a 145v spot this voltage is coming from somewhere. Bottom line is I believe something is back feeding another circuit and I need to find it. I would think if something is pulling it down then all the voltages should be bottoming out but there not the circuit has no real amperage to keep voltage up during a short of some kind.

[Yamamaya42]

You are jumping too far ahead of yourself, don't focus on the 145v issue yet, for that is load dependent, IE if the 2 loads on it are insufficient, it will drift up, no matter what you do.

Go through the steps that Squirrel boy described, especially the current check at L54 (380ma ).

[Yamamaya42]

go through the checks he gave.
AC V@ plug before L46/L47 and after.
AC V@ primary of power xformer (blk-red/black)
AC V@ secondary of power xformer (red/red-green) before & after M7, 143ac
DC V& Ma@ L54 395V 580ma.

145v is sourced via 5.6k resistor (r216) from the 275v rail, and goes to L25 & v9, as mentioned, if things are not right in other parts on the set, the load on the 2 won't be normal, and it will drift up.

[Yamamaya42]

The 145v seems to be primarily used for Audio, pin 8 of the audio output, and power to the Audio det/limiter, but it also goes to pin 5 of the tuner power connector to provide 130v B+#2. If the tuner is not connected/ inactive, the load on the branch for 145v won't be enough, and thus read high.