Philco Experts: Model 87 Grid Condenser? - Videokarma.org TV - Video

Winky Dink · #1 03-21-2013, 01:42 AM

I'm supposed to be working on a Radiola 28, so how did I end up with a Philco Model 87?
Well, I started working on the cabinet of the Radiola, and found that all four feet were dry-rotted.
I found a local woodworker who could fix the feet, and he just happened to have a Philco 87 that he's listened to since he was a small child. But now it doesn't work. So I have the opportunity to work with a fine old classic and save a few dollars on the woodworking.

This set is in beautiful condition. The cabinet appears to be undamaged, and as far as I can tell everything is original except the power cord. So far I have rebuilt the filter condenser block and replaced the bypass condensers. Now I'm stuck on the grid leak resistor and condenser because this is an early model with some variations from the Model 87 data that I have found.

The information I have is that the grid condenser is part #3082, a Philco body-type 3 mica condenser, 250 mmf.

The condenser in my set is an Aerovox, no part number, no color code, which is currently measuring 350 mmf (by my Radio Shack multimeter). It matches Philco body type 1 in the catalog illustration. The catalog lists hundreds of condensers types 2, 3, 4, 5, 6, but only one type 1. The listing for the single type 1 condenser indicates that it's 600 mmf and color coded yellow.

My inclination is to leave the original mica condenser in place since it's probably OK and I don't know what to replace it with. Anyone have a better idea?

Reece · #2 03-21-2013, 07:29 AM

See if it works and leave it in. You could always cut loose one lead and tack in a new 250pf mica and compare how they work.

Winky Dink · #3 03-21-2013, 02:08 PM

Thanks, Reece. I'll do that. Tonight I'll power it up for the first time.

old_coot88 · #4 03-21-2013, 02:51 PM

Quote:

Originally Posted by Winky Dink

Last year I couldn't even spell capassiter.

There used to be an old techie who called them capacitators, and referred to the TV sweep axes as vertical and horizontical. A bit of a goofball.

Reece · #5 03-22-2013, 08:48 AM

They're still condensers to me. Gotta check my stock of Leyden jars....

Winky Dink · #6 03-25-2013, 02:11 AM

Philco 87--Looks pretty, doesn't she? Can't imagine why anyone would hide this beauty inside a wooden cabinet. Anyway, I'm bumping this back to the top for more advice.

I've replaced or rebuilt every questionable component. That includes the filter block, all other paper caps, and bad resistors. The test data looks good with one exception. But so far I have weak reception which can probably be described as good sensitivity and poor selectivity. The weak audio is competing with a loud 60-cycle hum. My next step would be to look for faults in the filter block, but I'm going to seek counsel first.

Here's the data. My figures in red. What stands out to me is that the detector plate is getting a small fraction of the specified voltage.

The hum is steady and unchanged by volume. It does decrease when the AC line voltage is decreased. The hum is unchanged when I pull the detector tube. When either 45 in the 2nd AF is pulled, the hum is reduced equally. The only tube I've changed is the 1AF 26 globe. I only replaced that because the globe went microphonic whenever the set was jiggled.

I've marked up this schematic to keep track of what I was doing. Blue numbers are my measurements, green numbers are new components, and red arrows indicate that I've checked the continuity of coils.

As I said, I'll take apart the filter block next. When the hum is conquered I may have to learn how to neutralize the Neutrodyne. Beyond that, I'd sure appreciate any ideas on what else I should check into.

Thanks much.
- The Eternal Novice

Reece · #7 03-25-2013, 07:44 PM

What voltage do you get at E? What happens when you pull the first AF?

DavGoodlin · #8 03-25-2013, 08:39 PM

That looks really nice. Good luck with the filter block. I cant imagine tackling that without a schematic.

Nice deal on those feet. That's the way to work it, no money involved!

Winky Dink · #9 03-25-2013, 10:41 PM

Reese,
As it looks to me, "E" is the detector plate voltage. At the filter block I read 8.9V (versus 30V specifiication) and at the power transformer I read 6.5V (versus 30V specification).

If 1AF is pulled, there is no sound.

I went carefully through my filter block reconstruction, and it seems to be OK.

I also found some Philco "field service data" that gave several causes for Hum. Two items were defective condensers, which I think I've ruled out. One item was bad friction contacts on the range control, and I've ruled that out. The final item was neutralization, which I haven't dealt with yet.

Dave,
The diagram at the lower right of the schematic is a filter block schematic with numbers for the terminal strip to the chassis. It was that easy.

Thanks, All.

old_coot88 · #10 03-26-2013, 11:45 AM

OOPs! Double post.

!@#$%^&*

old_coot88 · #11 03-26-2013, 11:47 AM

Quote:

Originally Posted by Winky Dink

Reese,
As it looks to me, "E" is the detector plate voltage. At the filter block I read 8.9V (versus 30V specifiication) and at the power transformer I read 6.5V (versus 30V specification).

Um.. "at the power transformer"

Don't you mean at the plate of the '27 detector?
What happens with the point E voltage if you pull the '27? If it stays low, then something is lugging it down. There is a cap in the filter block that goes from point E to ground. Is there any chance that cap could be installed backward? (That's assuming you've replaced the original caps with electrolytics.)

Winky Dink · #12 03-27-2013, 12:26 AM

I don't know why I wrote "at the power transformer," except it was way past my bedtime. Yes, I did mean the '27 detector plate. With the '27 pulled, I get the same plate voltage--about 6V, and about 9V at filter block terminal 6.

The replacement caps are all nonelectrolytic, and they still look OK. To be sure, I'll take them apart, reassemble on the bench, and make a temporary reconnection before reinstalling the block.

The Rider's schematic is marked up with eight point-to-point voltages in the filter block. I'll check those voltages.

And I'll stop writing posts at 3 am.

Thanks.

old_coot88 · #13 03-27-2013, 11:50 AM

Quote:

Originally Posted by Winky Dink

...With the '27 pulled, I get the same plate voltage--about 6V, and about 9V at filter block terminal 6.

Since you indicate the 70K resistor feeding B+ to term.6 (point 'E') is good, you want to look for a low resistance leakage path from point E to ground.

Quote:

The replacement caps are all nonelectrolytic, and they still look OK. To be sure, I'll take them apart, reassemble on the bench, and make a temporary reconnection before reinstalling the block.

Check the replacement 2mf cap for leakage.

Winky Dink · #14 03-28-2013, 02:33 AM

Checked the 2 mfd cap--multimeter reads 2 mfd and infinite resistance. I don't know if it would be different under load, but I'll replace it with another new cap tomorrow and see if it makes a difference. The 70K resistor is still 66K. I checked a few other things but this time of night that I seem to start writing gibberish, so I won't mention those at this time.

Winky Dink · #15 03-28-2013, 02:49 PM

From Old Coot's advice to look for a low-resistance path to ground I spent some hours checking continuities and retesting several components. Although everything seemed OK, I did a temporary replacement of three components (below) which would give a low-resistance path if they failed. Lo and behold!...the detector plate now gets 27V!!!

Now I have a decently-functioning radio with a moderate hum which the hum adjuster reduces to an acceptable minimum. I'll reconnect the original components one at a time to find the culprit(s), reassemble, and then, if I don't screw up, return it to the owner.

Heartfelt thanks to all my fans out there.