Motorola ts907 low hv

[zeno]

B+ rectifiers almost always DEAD SHORT
B+ 375 vs 385 not a problem. Only 3% difference.
Boost low probably due to low HV. Boost is added ON TOP of the high B+.
Comes from FBT pulse then filtered & sometimes rectified.
Boost feeds the CRT G-2 controls & often the Vert & Audio.
If you pull the plate cap OFF the HV rect tube you should be able to draw a nice
1" arc into the air. If you can things are probably good to there.
Pay attn to the hoz out voltages. The G-1 HAS to be very negative per
the manual.
73 Zeno
LFOD !

[timmy]

Well that 375 volts I posted now dropped to I think was 350 volts the b+ diodes are not shorted but one side of the diodes is only 179 volts that comes to 358 volts it’s to low and all the big resistors for b+ test like new so with having low hv I would think it because of the low b+ and the reg tube cap off does nothing the hv should jump to like 30 kv and I had a new flyback that I put in and that made no difference. I’m kind of lost at this point. The g2 settings red low blue and green about half.

[timmy]

Ok so this is baffling chassis out on the bench nothing plugged into it except line cord to check b+ transformer putting out 143 vac just what’s listed in sams checked diodes they are good diodes in circuit no load goes up to 358 after 30 seconds voltage drops to 340 there should be 385 or 390 volts cannot figure why it’s so low I thought maybe I got a bad cap even though I checked them befor using them and checked good.

[zeno]

Bad cap about only thing left with PS unloaded. Be sure the choke
isnt shorted either.
You can bridge them to test.
Take about a 50mfd 450 volt and hang it right on the old one
+ to + , - to - see if any improve it.
As you try different sections DISCHARGE it between.
Get in the habit of that cause if you do a SS set you can charge up 150 volts then dump it on the 24 V line, not good.
This was a "fix TV 101" subject. Also for radio. Best use was quickly get the
TV going to evaluate for est.

Zeno

[timmy]

Do you mean the chokes on the ac input there are 2 I don’t think there is a choke on the dc line. I have only 2 caps that are responsible for the b+ voltage they are both 250 v 160 uf one cap tests 175 uf the other tests 202 uf so I don’t know if that indicates the cap is bad.

[timmy]

Ok so piggy backed a 470 uf 450 volt cap and it jumped up to 390 volts and it backed down then started rising up having to allow 30-40 seconds for the thermistor to heat up. New caps are bad.

[timmy]

This set has 2 250 volt 160 uf caps wired in series so that means it only gets 160 uf but if parallel then it would be 320 uf somehow it don’t make much sense to do this it’s like having and extra cap for really no purpose if it’s only 160 uf why not just have the one cap.

[Yamamaya42]

Originally Posted by timmy

This set has 2 250 volt 160 uf caps wired in series so that means it only gets 160 uf but if parallel then it would be 320 uf somehow it don’t make much sense to do this it’s like having and extra cap for really no purpose if it’s only 160 uf why not just have the one cap.

Are you referring to C1 & C2? I can't see the schematics very clearly cause the ones given are so low res, but if so, they make up a voltage doubler circuit, and are thus critical to get the correct bias voltage.
In such situations, it never hurts to go slightly over like 180uf or so.

[Yamamaya42]

example, a pair of these should last forever!
647-UCY2D181MHD3
It's what i would use.

[timmy]

Ok yes it does make sense voltage doubler and yes c1 and c2. Well I have 2 more caps coming and if the voltage is not up to where it should be then it may be time to replace the diodes with 1n4007 diodes.

[zeno]

The doubler cap thats above GND needs to be same temp rating or
higher. They take a beating.

[timmy]

Not sure what you mean by the cap above gnd

[Yamamaya42]

It's pretty much a standard that both caps in a voltage doubler should be of the same size and type, and for best results, 2 an exact pair is best.

[timmy]

Does anyone know if the b+ diodes are supposed to get warm with the power supply unloaded.

[Yamamaya42]

Originally Posted by timmy

Does anyone know if the b+ diodes are supposed to get warm with the power supply unloaded.

define "unloaded"

how did you unload it, and if it's truly unloaded, with the sec - > diodes -> caps, it could lead to a runaway condition where the bias voltage goes over the rating of the caps.