The PHILCO Phorum

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

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.

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! 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

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