Dove right in! Help with RCA 21-D-8628

[Electronic M]

Originally Posted by old_coot88

The replacement xfmr also has to have the dedicated 5 V filament winding for the 5U4.

That's in addition to the 6.3 V winding.

If you don't mind not having the 5U4 you could get a transformer without the 5V winding and use a pair of 1N4007s in place of the 5U4. I've been known to make Solid state 5U4 substitutes and use them in sets where the original transformer runs hot to reduce heater load and cool the transformer.

[JohnCT]

Originally Posted by Electronic M

If you don't mind not having the 5U4 you could get a transformer without the 5V winding and use a pair of 1N4007s in place of the 5U4.

Yet another option, and an excellent one if the supply of 5U4 transformers available to the poster proves scarce. There may be more 1960s transformers laying around than '50s.

I suggested a 5U4 equipped donor by considering the skill level of the poster. A 5U4 transformer would take care of all filaments with just the B+ to tweak and likely a couple of holes to be relocated for mounting.

But something like this RCA, that isn't a historic piece, should be put back into operation even if requires some minor modification.

Looking forward to seeing it run.

John

[Kevin Kuehn]

I found this one that I pulled from a late 50's narrow neck RCA. I kept the Sams page with the transformers specs and Sams lists it as the exact same voltage/current ratings Tim posted. I'm not sure if it will physically mount the same, but there's a possibility. I've checked resistances of the winding's and they look good. If you think it will work send me a pm. USPS flat rate box will be the cheapest shipping method. But before you start investing money I'd recommend you follow Old C's advise and get your CRT tested. Also I might be upset if this transformer also goes up in smoke.

https://imgur.com/a/yalufGU


Here is a test with the CRT tester I purchased on eBay. Perhaps you all can figure out if the tube is good.

[Electronic M]

It is probably good. Not sure what the egg timer is for. CRT emission can be asleep for a couple minutes to a couple of hours in some cases but precise timing is not important, all that matters is waiting till emission eventually climbs which it appears to have done.

On all glass CRTs cathode emission is usually the only important metric. (Other defects usually aren't noticable or can be worked around with an isolation "booster" or similar.

Step .1) identify mysterious transformer for possible use.

Hi all,

I took a break to collect information and regroup. I am going tp reinstall all of the original components I removed, it will be good practice for actual replacement. I found a transformer that could potentially have come from an RCA chassis.

I'll post a link to the images, I am trying to identify primary and secondary windings to compare to my KCS116C chassis needs.

I am struggling to identify and measure ohms/volt outputs for needed voltages.

Perhaps the group can help.

Thanks!

Jordan

http://imgur.com/a/LNuBET5

[JohnCT]

Originally Posted by BandDirector

Step .1) identify mysterious transformer for possible use.

Hi all,

I took a break to collect information and regroup. I am going tp reinstall all of the original components I removed, it will be good practice for actual replacement. I found a transformer that could potentially have come from an RCA chassis.

I'll post a link to the images, I am trying to identify primary and secondary windings to compare to my KCS116C chassis needs.

I am struggling to identify and measure ohms/volt outputs for needed voltages.

Perhaps the group can help.

Thanks!

Jordan

http://imgur.com/a/LNuBET5

The *first* thing I do on an unknown transformer is to check every wire to see what other wires (there is at least one) have continuity.

I then put all wires with resistive relatives into groups. Within each group, the wires that read highest resistance to each other are the outside connections, and the third wire will be the tap.

Note, if you find one wire that has no resistance friends, you have a bad transformer...

John

[old_coot88]

Are there 9 leads coming out of the xfmr?

If so, what I would do is this - First identify the center-tapped hv secondary. It'll measure a few hundred ohms. That'll get 3 leads outa the way.

Then identify the 5V and 6V secondaries. Each will read very near zero ohms. Never mind which is which for now.

The remaining winding will be the Primary, and will read a few ohms. Put the juice to the Primary, and measure what voltages you're getting out of the secondaries. Careful - there can be 800 - 900 VAC on the hv secondary.

11 leads I beleive

[Kevin Kuehn]

Originally Posted by BandDirector

11 leads I beleive

Probably just another 6V filament winding.

https://imgur.com/a/E5oIWZz


Top 3:

Tan (right 11)
& Yellow Green (7)
=44.6 ohms

Tan 3
&Yellow Green 7
=26.4 ohms

Tan 3
&Tan 11
=18.4 ohms

Lower:

Tan 2
&Black and Yellow 4|
=.1 ohm

Black 1
& Black and Yellow 4
=.6 ohm

Yellow Green 5
&Yellow 6
=.1 ohm

Black 10
&Yellow Black
=.4 ohms


The numbers reflect a left to right order, they have been paired according to continuity and ohms.

So next step is to apply power to the lowest ohm pairs? The measure output voltage?

Thanks

[Kevin Kuehn]

Things don't quite add up there to me. Do you have a variac so you can bring the voltage up slowly? If I had to pick I'd say the .6 ohm pair is the primary, but with no variac I'd at least wire a 1amp fuse in series with one of the pair rather that just hit it with 120V AC and hope for the best. The 3 top wires seem a little out of balance for a normal center taped high voltage secondary.

[Electronic M]

You wouldn't happen to have a second good transformer that puts out 6V AC would you?

My process for testing unknown transformers is to connect the winding I suspect is the heater to 6V from a powered known good transformer (and measure voltages on the unknown transformers windings)...that way if you pick the wrong windings to power on the unknown you should not be over volting it by a dangerous margin.

Originally Posted by Electronic M

You wouldn't happen to have a second good transformer that puts out 6V AC would you?

My process for testing unknown transformers is to connect the winding I suspect is the heater to 6V from a powered known good transformer (and measure voltages on the unknown transformers windings)...that way if you pick the wrong windings to power on the unknown you should not be over volting it by a dangerous margin.

I have an AC/DC transformer that can run 3-12 volts, I use it for testing other electronics/game systems/devices that need DC current.

Probably not what you mean. This is the first object/appliance that I am doing this kind of work with. I do have an RCA UHF tuner from 1952 that seems to work well. I can match the output signal to a digital tuner and the channel goes from snow to black accordingly. There are tubes in it that require 6v, is that assumption correct? Therefore I may be able to piggy back off of that.

[Electronic M]

Originally Posted by BandDirector

I have an AC/DC transformer that can run 3-12 volts, I use it for testing other electronics/game systems/devices that need DC current.

Probably not what you mean. This is the first object/appliance that I am doing this kind of work with. I do have an RCA UHF tuner from 1952 that seems to work well. I can match the output signal to a digital tuner and the channel goes from snow to black accordingly. There are tubes in it that require 6v, is that assumption correct? Therefore I may be able to piggy back off of that.

You need AC to test the unknown transformer. The UHF tuner may have a 6V heater winding....You can check by looking at the ID numbers of the tubes in it. On most signal tubes made after 1936 the first digit of the tube ID number is the heater voltage...So a 6AG5 is a 6.3V heater and a 12SN7 is a 12.6V heater.

If you use the UHF tuners transformer to power your mystery transformer pull the tubes out of the tuner and if the tuner has only 1-3 tubes don't run it for more than 30second stints as a source for the mystery transformer. Probably still a good idea to use a 1A fuse in series with the connected winding of the mystery transformer....

The UHF tuner (which I suspect is really what collectors call a UHF converter) probably has a small transformer designed to run the heaters of 1-3 tubes...many 5 tube and higher transformers can take a heater winding short for long enough that you'll notice and switch off, but ones designed for fewer tubes are easier to kill.