Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5

Philco 87 grid voltages
#1

Good morning,

I'm new to the Phorum but expect to visit often, hopefully more to improve my knowledge than plea for help.

I completed the electronic restoration of a model 87 and it plays wonderfully.  The only thing that has me scratching my head are the grid voltages on the #26 RF tubes and the 45 output tubes.  Rider's calls for 6 volts on the RF tube grids and 45 on the 45 tube grids. I believe they are all mislabeled as positive rather than negative voltages but that's not the issue. My issue is that I measure 0 volts on all the grids.  I'm measuring from chassis ground. The plate voltages are within range of the schematic.

The radio performs perfectly so I don't think I have a problem but why am I not getting negative voltages at the grids?  The only issue this radio had was an open field coil which I had rewound by the gentlemen at the Alabama Historical Radio Society. They do great work. Oh, it also had sprung tuning cables. That was a joy to fix. Finally decided to give up on the cables and use fishing leaders. 

Thanks in advance for any help on my voltage conundrum.

Scott
#2

Welcome!  Are the cathodes running at a positive voltage giving a net negative voltage on the grids? Would have to look at the schematic to see how they are biased.
#3

Hi Scott and welcome,
Some of those early set w/the directly heated tubes can be a little confusing. It's also not too uncommon for the diagram not to notate + or - voltages, just voltage and not polarity. To get a better idea whether to grid is + or - measure from the filament to grid rather that the chassis. Funny thing is that RCA sez the 45's grids should be - 50-60v not 6v. >Oh, it also had sprung tuning cables. That was a joy to fix. Yes they are! [Image: https://philcoradio.com/phorum/images/sm...n_evil.gif] There was a thread abt it here a week or two ago. A lot of these early sets have open primaries on the rf coils and the audio transformers. No big deal to fix or replace.
Have had a few 87's over the yrs great players for '29 technology.

GL

When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!

Terry
#4

Gents,

Thanks for pointing me in the right direction. I should have realized that the negative point of reference for the grids was the filaments, measuring from there shows that all is well with the grid voltages.

I am impressed by the sensitivity of this radio. With just a 6 foot wire in my basement I'm picking up stations 300 miles away (at high noon). It should be a hot performer upstairs with a proper antenna and ground.

Cheers,

Scott




Users browsing this thread:
[-]
Recent Posts
Philco 91 Speaker Replacement
The spider is glued on the cone and so is the voice coil. I think you can use a cotton swab dipped in acetone and carefu...RodB — 02:42 PM
Restoring Philco 37-604C
Yes I saw that. If I knew, I'd probably try to arrange for getting it myself. But I have just got one. Which does not m...morzh — 01:44 PM
Philco 38-2 Automatic Tuning
When you push the lever, you are supposed to rotate the disk to the desired station. Then the magnetic tuning will acqu...morzh — 01:42 PM
Philco 38-2 Automatic Tuning
I figure out the muting from another picture that helped. Now I need to know how the automatic tuning works. When I push...dconant — 12:11 PM
Philco 91 Speaker Replacement
How would I go about removing the spider from the cone?dconant — 12:04 PM
Philco 38-2 Automatic Tuning
Hello, I am having trouble getting my automatic tuning to stay unmuted. If I play with the tuning handle I can get it to...dconant — 10:51 AM
Does anyone make photofinish replacements?
Here’s the link to the DIY photofinish section of our site: klondike98 — 09:46 AM
Delco car radio Peko vibrator converstion issues
Hi Richard, Thx for posting the schematic.  As I remember as a kid, these were great performers for the time, likely...MrFixr55 — 08:56 AM
Delco car radio Peko vibrator converstion issues
hello mr Fixr, The Electro  Powersupply that I am powering the radio with has a huge choke plus a 10,000ufd electrolyti...radiorich — 10:58 PM
Delco car radio Peko vibrator converstion issues
How about putting a choke in the power supply? The big cap was likely an attempt to stabilize the DC. Kids who cre...MrFixr55 — 09:19 PM

[-]
Who's Online
There are currently 3541 online users. [Complete List]
» 2 Member(s) | 3539 Guest(s)
AvatarAvatar

>