Motorola ts907 low hv
 

I’m trying to figure out what’s going on with this Motorola I’m obviously missing something here bright up 14kv bright down 20 kv with little to no difference by turning the hv pot. B+ is 375 and sams says 385 I tried cap off reg tube no difference all caps at the oscillator changed and no smell of leaking hv. I have a new flyback but don’t want to change it until I go over everything that I may be missing. Another oscillator tube nothing makes a difference. If anyone has even a clue I’m all ears. I don’t have a scope which would probably help I also checked the ohms of the horizontal hold coil which sets the frequency and the hold and ohms are good.

Off frequency? Bad voltages to Output tube? Are you getting a picture that locks?

what CRT is in the Motorola ts907?

there is very little info out there on this set out there.

Also, why start a 2nd thread? you already have a topic about this set under the fried focus resistor.

Would it not all be easier to follow if it were all centralized under the same one? :D

Sound but no picture full screen missing edges both sides due to low hv

This set has the 21fjp22 and I started another thread because the focus coil resistor I don’t think has anything to do with this hv issue

they could very well be related to each other, as they are both in the same section, and tied closely together to the function of the HOT & FBT.

Had a weird they happen twice I was disconnecting everything to take the chassis out and touching plugs to unplug with one hand and my arm was lightly setting on the hv cage and I was getting lightly zapped from the voltage in the crt now that would be impossible but until I removed the anode wire from the crt I was good to go. This makes no sense because the anode is in the hv cup with the 3at2 tube so how can this voltage make it to ground I have no clue. And there is a ground for the crt and that was good.

Remember, the crazy thing is all one big tuned circuit! :)

From one end to the other, it's amazing how they got it all working so well, but, it DOES mean it's easy to upset the balance, get one key element out of wack, be it the horizontal frequency, something in the damper area, yoke or convergence area, basically ANYTHING that can mess up resonant frequency of the circuit, then you have a big headache on your hands, and like I mentioned, it may very well include the focus circuit. :(

On a bk tube tester and recapped I tested the hv rectifier and it’s new but on the scale it tested on the end of the yellow bar which is around 62 on the scale I would think it’s no good but I don’t know how low it has to test befor seeing a hv problem so I tested the old one the tv came with and reads almost 90 on the scale. I didn’t try it yet but how low can this tube test befor it’s a problem.

Unfortunately, as is often the case with tube testers, especially with the power type tubes, they never give the full story about good/bad because they can't put them in real life load situations that they would be in the circuits they are being used in, in these cases, it's always best to try a "known good" one.

cause they often fool you.

WAY TOO MUCH info on tube testers, but it shows what you gotta deal with. :yes:

http://www.tungsol.com/html/faqs7.html

I don’t know if these series chassis were known back in the day for flyback failures the last Motorola I had I changed the flyback so with this one I have a new flyback and I put it in so I’ll see what happens. I keep going over this chassis and every cap I changed tested new and every resistor I checked tests new as well so if I still have low hv then maybe the yoke. I’m not sure really how to check the yoke coils ohms. I changed the rectifier tube and gained 4 kv but just won’t adjust via the hv pot it will lower from where it is but that’s it.

Boost voltage off the flyback is around 750 volts should be 770 volts after a 10k resistor and b+ is 350volts and should be 385 volts. I don’t know if the b+ diodes go weak from age has anyone had these diodes go weak by not putting out the correct voltage and if b+ is low everything else will be low. And the line voltage is 111 volts. Originally the b+ was 375 volts I don’t know why it dropped.

If the boost diodes are selenium (possible) I’d be looking there.

B+ diodes no boost diodes in this set.

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:smoke:
LFOD !

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.

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.

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

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.

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.

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.

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

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.

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

Not sure what you mean by the cap above gnd

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.

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.

Chassis on off switch jumped on the bench no tuner no hv hot caps off no convergence none of this is plugged into the chassis. Best way I can define unloaded. I checked the voltage on the b+ line no run away voltage I had new caps that seem to be bad 2 more caps seem to fix the low b+ I had.

The only TRUE way to insure that it's really unloaded is to unhook one side of the choak (L54?), that will make certain that there is no slight draw anyplace, but there is most likely no real reason to do so, it's rather normal for those diodes to get a bit warm under normal operation.

2 new old stock tubes for video IF both don’t even read on the tester I still have low hv I don’t think these 2 tubes have anything to do with hv. If I turn the horizontal frequency which is also the horizontal hold in about 5 turns the screen fills and I get good hv but the slug is halfway out of the coil not sure that is correct. I’m lost at this point.

If you have to back it that far out, there is clearly a problem with the horizontal oscillator/AFC , this can include the phasing det and feedback loop ( winding on FBT), SOMETHING is causing it to be way off frequency. :(

If you have a good digital o-scope, it may be a good time to see what your horizontal frequency really is right now.

No don’t have a scope. The flyback is new I put that in and are you talking about the chroma phase detect with the 6al5 tube. And all the caps were replaced and every resistor around the horizontal oscillator was perfect if only I could find a possible bad resistor.

I'm not sure how it's done in this set exactly but the horizontal oscillator is norm always phase locked, look for telltale shark fin waveform on the schematic for the area, you are in a level of complexity now that w/o a oscilloscope you are basically working with your hands tied and one eye shut, you really need to get one to track down problems such as this, they are that complex.

One thought I had was one of the 3 caps in the yoke may be bad I can check NF but not PF so I’m not sure and there are 2 560 mmf 2.5 kv ceramic disk caps on the horizontal side of the yoke.

Keep it painless first. Change hoz osc tube ( 6BL8 ? ) first.
If slug is way out but HV OK the hoz osc is way off the norm. Recheck caps
& VALUES used. In theory at this point the sweep is giving full width & HV
Try hooking up a source & look for clues. Hoz AFC can be defeated by grounding it out at the right point & will give a "floating" pix. I dont
have schematic so cant say exactly where. Never seen H AFC or the feedback from the FBT cause this prob. BUT ??

BTW if this set uses 6BL8's or 8BL8's just swap them out to test.
It was Motos 6GH8 but a better tube.

73 Zeno:smoke:
LFOD !

Horizontal drive circuit