Weird issue with an RCA 730TV2

[Username1]

Call the repair shop...... Is it still under warranty.....?

.

[Kamakiri]

Back on this one. Found a problem! The 3.3KOhm 1 watt resistor off pin 1 of the 12AU7 video amp that I replaced overheated and was open! Heady in my discovery, I soldered a 2 watt in place, and put the set back together.

Powered it up, and the horizontal output tube lit up like a Christmas tree. And then the smoke started coming from underneath the HV cage. Powered it down as quick as I could.

Dammit

[jr_tech]

Originally Posted by Kamakiri

Powered it up, and the horizontal output tube lit up like a Christmas tree.

Bright red plate, bright purple/blue ion glow or sparks and fireworks flying around inside?

jr

[Electronic M]

Originally Posted by jr_tech

Bright red plate, bright purple/blue ion glow or sparks and fireworks flying around inside?

jr

Neon letters that spell the word 'BANG!' ?

[Kamakiri]

Bright purple/blue ion glow AND sparks and fireworks flying around inside. Wasn't too confidence inspiring. I don't get it.

Time to pull the chassis and see if I can see what went wrong here, but damned if I know offhand what it could be.

[Kamakiri]

Just pulled the chassis, and the damage doesn't look too severe. Appears that a couple resistors on the chassis smoked, as shown on the circled schematic below.

Not really sure where to go from here, hopefully someone has a suggestion....

[kvflyer]

You might want to check wiring. Could a part have been connected to the wrong terminal? Blue light dancing around inside of a tube is sometimes indicative of a short circuit down stream.

I remember in an earlier life, I plugged in a Knight-Kit amplifier. The 6CA4 had quite a light show inside of it. Yes, it was a shorted filter. Never made that mistake again.

"Just sayn'"

[Kamakiri]

Oh believe me, I'm not above making a wiring mistake. What KILLS me is that I had the sucker working and was trying to work through a vertical issue when I encountered this chain reaction failure last night.

I didn't change anything under the chassis since then, and nothing that I can see got bumped into anything else.

[N2IXK]

My usual power-up procedure leaves the horizontal output tube out until I verify the following:

Negative grid bias
Grid drive waveform
Screen voltage
DC plate voltage

Making sure that all of these look reasonable without the tube installed greatly reduces the chance of fireworks when the tube goes in.

[Kamakiri]

Right, but I didn't have any reason to suspect any other problem. This is what I was trying to fix when the raster dropped out. Found an open resistor on the 12AU7 and thought I nailed it.

[dtvmcdonald]

I suspect you got bit by a random failure of some part, or a tiny
piece of solder from a recap lodged somewhere it wasn't supposed to be.
I had one of those each happen with my CT-100. No fireworks, however,
just no picture.

[jr_tech]

Wouldn't be the first time that a NOS or well used tube has suddenly cracked/gone gassy or shorted on turn on, especially if it has been subjected to a few bumps 'n thumps such as turning the chassis over for repairs. That's my hunch anyways.

jt

[Kamakiri]

I did check the tube. Although nearly flatlined after that meltdown, the tube wasn't shorted.

Anyone wanna fix this for me? I'll ship you the chassis and pay dearly

[jr_tech]

There is a possibility that the sparks/fireworks sparks flying around in the tube were the remains of whatever shorted (grid wire perhaps) and the short was cleared at the expense of 2 smoked resistors and a dead tube. I guess that I would replace the resistors and tube (and at least check the cap between the two resistors) and try again. carefully!

jr

[Kamakiri]

I can do that. At least I'll know what to half-expect

Naturally I'll run through the tubes, and any component connected directly to the 6BG6. Anything else I should test or look for before another attempt? Aside from wires touching