View Full Version : Arvin 4080T

05-15-2017, 05:01 PM
Didn't want to post about this till I knew if it was worth saving or not, and as it turns out it's definitely worth some effort. :thmbsp:

This was sitting in a pile at the museum, looking rather forlorn. CRT socket nearly ripped off, cabinet had been dropped, it was sort of a mess. But it grew on me the more times I walked past it, it's got this funky little 8" metal cone CRT with a triode gun in it for nerd appeal (8AP4). I made Steve an offer, and took it home.

Tried to variac it, but all that got me was hot capacitors and nowhere near enough B+ to spool up the horizontal section. So I plugged some lytics into it and tried again, this time it showed signs that it was waking up so I decided the chassis was probably salvageable. But that left the question of the CRT...

I can't get a reading on it with my tester no matter what I do since it's a triode and to make matters worse, the socket was about to fall off AND it has a cathode/G1 short indicated by DMM. I carefully removed the base and tried the short clear function on the tester, but it didn't do anything. Knowing that feature simply discharges a capacitor through the short to try and burn it open, I tried the same thing with different value caps up to around 600 volts but nothing happened- it still read 300k ohms on my DMM.

Deciding I had nothing to lose, I went for broke and rigged up a variac, momentary pushbutton switch, and a 1.6kv microwave transformer. I tied the secondary of the transformer to the G1 and cathode on the tube and slowly advanced the variac, then hit the button. Nothing happened the first few times, but eventually I got halfway up the variac dial and got a sharp <snap> when the button was pressed and a flash of light. Hoping the tube wasn't destroyed I disconnected everything and checked with the DMM again, this time I read no short so apparently it had cleared- but that still didn't mean it was good! I could have destroyed the cathode in the process.

I glued the base back on with sensor safe silicone, came back the next day after it dried and soldered the pins. Then I put the tube back into the chassis and let 'er rip. I was rewarded with luma stairs!

It still needs a lot of work- there's some old caps that still need changing, most of the pots are crappy, and it's short on vertical height. Over the next few days I'll see if I can't clear those remaining issues up, hopefully it returns to how it should be. Sure is a cute little bugger. :D

David Roper
05-15-2017, 05:34 PM
Is anybody else unable to see the picture within the post above? I had to paste it into a new tab.

Good job reanimating that tube, btw. Cute sets; wish I had one.

Electronic M
05-15-2017, 05:38 PM
Not seeing the pic on this rig.

05-15-2017, 05:41 PM
Yeah I don't know what the deal is, I didn't do anything different than I ever do. Maybe the forum is on the fritz?

05-15-2017, 06:16 PM
Visible for me now.

05-15-2017, 07:23 PM
I have the same set Nick also. Great playing set funny but its true you cant check that crt with a tester for some reason. Found that out from Radiola Guy on the web who has a bunch of these sets.

05-18-2017, 07:43 PM
Got it to the point where the picture fills the screen with decent linearity, which was just a bunch of carbon comps in the vertical section that had drifted high. Cleaned out the remaining pots, so they all behave now. Last remaining issue is no sound, it seems to be detuned somehow because I can hear static but adjusting the coils doesn't seem to do anything. Tomorrow if I have time I'll have a poke at it with the sweep rig, but I have a feeling there's a bad coil in there somewhere.

Kevin Kuehn
05-20-2017, 07:29 PM
Since the 8AP4 is a triode our testers G2 pin is probably wishing it had some load on it? There's ordinarily no connection to G3 using a tester( in this case the plate of the triode). I'm thinking maybe an 8AP4 needs to have the anode(plate) jumpered to pin 10(G2) on the tester in order to read emission? :scratch2:

Sandy G
05-20-2017, 08:02 PM
Keep at it- ANYTHING that is trying that hard to "Live" oughta be given extra effort...

05-21-2017, 06:02 PM
Kevin I tried G2 on the metal bell, no dice. I did find though that if I swapped the grid wires it would read full emission- that is, without G1 bias it simply read like a diode and it would deflect the meter but it's not quantitative. In other words it tells you there's emission there, but won't tell you how controllable it is. Guess something is better than nothing? I still say the ultimate test is in a running chassis, just like with the electrostatic tubes.

Sandy- darn tootin!

Kevin Kuehn
05-21-2017, 06:10 PM
Agree with testing in chassis. It's interesting how so many testers include 8AP4 in their charts, but apparently no one stopped to realized it was a triode. Anyhow looks like you've about got that set licked.

[Edit]BTW I still have the front half of my cabinet if you need some decent looking sheet metal. My set was missing it's metal back, which is another reason I decided to part it out.