Philco 38-7 - Printable Version
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Philco 38-7 - PhilcoPhan1936 - 08-09-2018
In between jobs, I decided to pull a recent freebie I was given, a 38-7 chassis. When changing out the bakelite block, I noticed some of the tar sealer had melted, like it had overheated, possibly from shorting out. This block is directly across the line voltage input. I checked the main transformer. It's supposed to read 120ohms/130ohms to center tap, or 250ohms total. it reads 193/205 or 398 total.
Would you say the main transformer is damaged? I do have another chassis. I may just rebuild that one, and steal parts from this one.
RE: Philco 38-7 - Olskuul - 08-09-2018
The transformer apparently isn't shorted or open and probably still within tolerance.
I suggest replacing the line voltage caps with modern X1Y2 types.
RE: Philco 38-7 - Radioroslyn - 08-09-2018
You can pop out the 5Y4 and measure the hv ac from each plate to the chassis. Should be pretty even +/- 5v or so. If that checks ok leave in on for 15min w/o the 5Y4 and check to feel if the transformer gets hot. Make sure your pilot lamp socket isn't shorted.
RE: Philco 38-7 - PhilcoPhan1936 - 08-09-2018
(08-09-2018, 06:08 PM)Radioroslyn Wrote: You can pop out the 5Y4 and measure the hv ac from each plate to the chassis. Should be pretty even +/- 5v or so. If that checks ok leave in on for 15min w/o the 5Y4 and check to feel if the transformer gets hot. Make sure your pilot lamp socket isn't shorted.
Thanks for the reminder Terry.
I should have thought of that. I attribute my lapse of memory to a headache I woke up with.
RE: Philco 38-7 - PhilcoPhan1936 - 08-10-2018
OK. Paperwork states I should get 315v. I get 331v on both sides. This is with my variac set at 110v. Too high?
Have yet to do the warm transformer test. Will report back soon.
RE: Philco 38-7 - PhilcoPhan1936 - 08-10-2018
Transformer stays nice and cool after 15 mins.
I'm going to go ahead with finishing re-capping and checking the rest of the chassis.
RE: Philco 38-7 - PhilcoPhan1936 - 08-13-2018
OK, so I finished re-capping, replaced out of tolerance resistors, and checked most of the crucial transformers and coils, and did a power-up. No sparks or explosions
.BUT, it is completely deaf. No static, no reception.
I flipped the chassis over, and started taking voltage readings. Here is what came up:
6K7G:
H: 6.5v
P: 251v/240v
S: 90v/100v
6J5G:
No voltages indicated other than the heater voltage, which checked out at 6.5v
6K5G:
P: 187v/170v
H:6.7v
Here's where I suspect there's an issue. If anyone can give me a clue of where to start looking, I'd appreciate the help.
6A8G:
H: 6.6v
OscP: 113v/180v
P: 141v/180v
S: 78v/100v
I thought the audio cut-off switch on the dial might be corroded, so it was cleaned, and SEEMS to be working, but this type of dial/switch mechanism is new to me. If there's anything I need to clean or check, again, I'd appreciate any tips.
Note: I am getting audio. There is a very low hum coming from the speaker, and touching the grid cap of the 6K5G (1st audio) tube does produce a loud buzz.
Here's a couple pages of paperwork for easy reference:
[Image: https://farm2.staticflickr.com/1835/30151573608_1000fcd8e7_z.jpg]
[Image: https://farm1.staticflickr.com/939/44020202411_69ff094e60_z.jpg]
RE: Philco 38-7 - Radioroslyn - 08-13-2018
Ok how about pins 3,4, and 8 or the 6F6.
What happen when you apply a 470kc signal the cap of the IF amp and or the mixer?
RE: Philco 38-7 - PhilcoPhan1936 - 08-13-2018
6F6G:
H: 6.6v
P: 346v/230v
S:258v/250v
Will attempt Signal Gen test momentarily.
RE: Philco 38-7 - PhilcoPhan1936 - 08-13-2018
No signal heard when injecting signal to grid cap of either the 6A8G or the 6K7G.
RE: Philco 38-7 - Radioroslyn - 08-13-2018
Next step is to inject a 470kc signal thru a blocking cap connected to pin 3 of the 6k7.
If nothing heard inject an audio signal to pin 5 of the 6J5 with the tube removed. If
nothing heard you got something miss wired between the 6J5 and the vol cntrl.
RE: Philco 38-7 - PhilcoPhan1936 - 08-13-2018
I think I found the problem. I mis-read the schematic
. The 2.3ohm side of the Ant1-Range 1 is dead . Fortunately, I have a parts chassis. Organ transplant commencing.
RE: Philco 38-7 - Radioroslyn - 08-13-2018
Unfortunately the ant coil/band sw isn't going to effect the IF stage or the det. You should hear the 470kc signal anyway when it's applied the the mixer or IF amp.
RE: Philco 38-7 - Arran - 08-13-2018
That's the line capacitors, if it shorts out it won't do anything other then blow a fuse, or if leaky heat up and melt tar, it should not harm the transformer primary and definitely not harm the H.V secondary. On this set it's ahead of the power switch, so as long as the set is plugged in it has power across the caps all the time, actually between each side of the power line and the chassis, so it can go without the set even being on.
Regards
Arran
RE: Philco 38-7 - PhilcoPhan1936 - 08-14-2018
(08-13-2018, 06:55 PM)Radioroslyn Wrote: Next step is to inject a 470kc signal thru a blocking cap connected to pin 3 of the 6k7.
If nothing heard inject an audio signal to pin 5 of the 6J5 with the tube removed. If
nothing heard you got something miss wired between the 6J5 and the vol cntrl.
Went over the radio and schematic with a fine tooth comb. Couldn't find and mis-wirings. Still no signal or audio on your recommended tests. I connected my iPad using a .1uF cap in line with the tip connector, and the ground hooked to the chassis. This radio is deaf as a post.
Would a bad 6K5G prevent any audio from getting through? It does check out, but it is a bit low, though not excessively.
Would part 29, the audio muting switch be a possible problem? I'm assuming having the switch closed would conduct the audio. If there was a problem with it, is there a way to remove it by rewiring or just disconnecting it?