Motorola 17T21 Flyback & Schematic

[Mre12ax7]

By the way would anyone have any knobs for this set?
All knobs are missing with the exception of the power/volume.

[Mre12ax7]

How would I go about adding a dropping resistor to bring the B+ back to the spec 250v after replacing the selenium rectifiers. Should I just add in a 100 ohm 5w resistor or similar after the diodes?
Something like this?

[bandersen]

You only need one resistor and probably less than 100 ohms. An easy way to do it is replace the 7.5 ohm R97 with a bigger resistor.

[Electronic M]

No, you want only one resistor on the line with the arrow pointing to "w". Value of resistor will depend on the voltage you need to drop, the current of the circuit, and the way the load reacts to the change...

[Mre12ax7]

Should the resistor be before or after the field coil? The field is 83 ohms.

[bandersen]

I don't think it matters

[Mre12ax7]

I have placed a order at Mouser for some capacitors for the set.
The transformer has been recoated with the silicone.
I will solve the dropper resistor problem after the set is recapped and working using Ohms law.

[Mre12ax7]

Is this yoke salvageable?
It will have to have a new cover made for the back at the very least.
The rotted portion goes deep inside between the coils and 4 pieces of metal, which when I removed the clamp to remove most of the rust they all fell off of the degraded plastic core.
Two of them now have small chips from when they fell out (i have the pieces) because the edge of the plastic is warped.
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[Kevin Kuehn]

If you still have the centering rings you could make a new back cover to hold them. Most aftermarket yokes won't include those rings or the correct cover to hold them.

[Electronic M]

Unless a yoke in that state is electrically malfunctioning I'd say it is best not to try and dismantle it. Rust normally hurts nothing (except maybe your eyes).

[Mre12ax7]

I will just be making a new rear cover to hold the centering rings.
Do the centering rings have a direction they must face?

[bandersen]

I don't think it matters, but experiment it you can before permanently mounting them. You should be able to get the set working before bothering to make a new yoke cover.

Also it will work fine without the rings at all. You only need them to tweak the image so it's centered in the screen.

I show one way to fabricate a yoke cover with centering rings in this video: https://youtu.be/UXyra3vNYuQ

[Mre12ax7]

I finished recapping the set.
So I tried powering it up. No luck.
No picture, no HV static in speaker when you turn the volume control.
after about a minute and a half I noticed the horizontal output tube was glowing blue so I shut it off.
The output, damper and HV rectifier tubes are all good. Horiz, osc tube tested weak so I swapped it around with the other 6SN7 in the set (I do not have spares).
I have moved all the controls on the back including the horiz hold potentiometer before powerup.
Should I try to mess around with that?

P.S Can a moderator please change the name of this thread to Motorola 17T21 Restoration (I don't see a way to do it myself)

[bandersen]

Blue glow doesn't necessarily mean anything bad. A blow glow on the inside of the glass is common for power output tubes. Now if it's move of a purple and between tube elements then you've got a gassy tube.

Most common cause of no HV is the horizontal oscillator isn't running.

Odd that you don't hear any crackle from the speaker, It's field coil is a critical part of the power supply. But it must have continuity otherwise you wouldn't have any power going to that output tube,

[Mre12ax7]

I will be testing all the voltages on the horiz, output and osc tubes.
I will use my Fluke 87 DMM or my Eico VTVM with the set plugged into a isolation transformer.