Philco Model 20 Lowboy - Printable Version
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco Model 20 Lowboy (/showthread.php?tid=16291)
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Philco Model 20 Lowboy (/showthread.php?tid=16291)
RE: Philco Model 20 Lowboy - morzh - 05-14-2017
Terry answered​ it all.
#4 says "without the rectifier". The rectifier IS the 80 tube so it means, without it.
Also in #1 you tested everything, field coil and the output xfmr primary, so all is good. The output secondary always shows as zero (zero means short, not opsn) and it is OK.
RE: Philco Model 20 Lowboy - WallaceRoger - 05-14-2017
This is perfect, now I get it. I'll be back this week with the results of the other tests. Thanks again, guys.
RE: Philco Model 20 Lowboy - WallaceRoger - 05-15-2017
Update:
The filament continuity test was easy enough, so I did that and everything seemed to pass. However, my replacement bulb for my tester came today and I decided to test everything following the booklet instructions: one of my 24s failed the cathode short test. Of course it had to be the really cool, old Philco branded globe style which is probably original. Additionally, my 27 tube technically passed, but in the final test, the needle was in the "?" area, so guessing that one is iffy.
I know I at least have a bad 24, so I'll need one of those.
RE: Philco Model 20 Lowboy - morzh - 05-15-2017
? Area still works, just get the 24A.
RE: Philco Model 20 Lowboy - WallaceRoger - 05-16-2017
Ok
RE: Philco Model 20 Lowboy - WallaceRoger - 06-18-2017
I'm finally back after getting sidetracked by work and vacation.The new tube came, so I figured I'd run these tests; results are below:
80 filament voltage: 5.4v
71A filament voltages: 5.5v
27 filament voltage: 2.7v
24 filament voltages: 2.7v
These above look more or less ok from the charts.
Plate voltage test for 80 (#6 from list): I got 364 at pin 2 and 366 at pin 3. (I think I may have done this test wrong, I'm thinking I was supposed to probe pins 2 and 3 together like in the filament voltage test rather than place - probe to chassis and + probe to pin 2 or 3... if so let me know and I'll re-do)
The B+ test at tube 80 (#7 from the list) gave me 272v.
Plate voltages (#8)
27: 101 (chart says should be 120)
24: 215, 215, and 95 (chart says they should be 250, 250 and 35)
71A: 203 and 217 (chart says they should be 215)
Any thoughts?
RE: Philco Model 20 Lowboy - Radioroslyn - 06-18-2017
Measure the dc voltage from pin 3 of each of the 71's to the chassis. Should see some -voltage there on both unless the interstage transformers secondary is open.
RE: Philco Model 20 Lowboy - WallaceRoger - 06-19-2017
Ok, will do that tonight!
RE: Philco Model 20 Lowboy - WallaceRoger - 06-19-2017
Ok, one reads -8.7 the other reads +0.7 and when I touch the probe to it, the speaker crackles and the hum gets louder.
RE: Philco Model 20 Lowboy - Radioroslyn - 06-19-2017
Seems like 1/2 of your interstage transformer is open. This is why the plate voltage on your 71's isn't more even, one tube has no grid bias and it's causing the plate to draw more current. Just to confirm measure the resistance from each 71 grid back to the center tap of the transformer. Should see a fairly low resistance 1K or less.
RE: Philco Model 20 Lowboy - WallaceRoger - 06-20-2017
That makes sense as I've read on other threads those are problematic on this model. By "grid" you mean pin 3 again, right? I'll check resistance tonight. Thanks again, Terry.
RE: Philco Model 20 Lowboy - Radioroslyn - 06-20-2017
> By "grid" you mean pin 3 again, right?
Yup
RE: Philco Model 20 Lowboy - WallaceRoger - 06-20-2017
Ok, one of them is 3k, the other is 7.2M. That sounds like a problem : ) The center tap is the wire that connects to point 1 on the filter can, right?
RE: Philco Model 20 Lowboy - Radioroslyn - 06-20-2017
Grab you one of these  https://www.tubesandmore.com/products/transformer-audio-interstage-10-ma
and call me in a month[Image: http://philcoradio.com/phorum/images/smilies/icon_lol.gif]
http://www.philcoradio.com/images/phorum/schem/20b.jpg
Point B is connected to the ct of the interstage transformer
as is the ct of the hv winding of the pt.
RE: Philco Model 20 Lowboy - WallaceRoger - 06-21-2017
Ok, thanks Terry.