The PHILCO Phorum

I'm currently working on a Model 86 and it's got a few oddities that I'm curious about.

Question #1 - what with the 27 tube and it's placement slightly below the chassis?



Question #2 - probably a pretty basic question but why is no shielding necessary / used?

Question #3 - what does the right-most control do? The schematic labels 'range control' but it seems to be a combination of a small variable capacitor and a switch.



Question #4 - Why is there a switch for the power transformer primary winding (labeled 'primary tap switch' on the schematic)? What voltages does this set support? It's got a cover over it on the chassis so seems not normally supposed to be messed with by an end user. You can barely see it just below the filter bank near the mid-right of this picture:



Question #5 - how critical is the grid leak resistor value? I believe it's supposed to 2M based on the labeled schematic but I measure 3.75M. The set seems to work fine and actually is pretty sensitive.



For reference, that's #20 on the schematic (Part #3083). In case anyone is interested, here is a PDF scan that includes the parts list (but not nicely labeled with parts values like Ron's version):
https://drive.google.com/file/d/1JkRvp2C...sp=sharing

My we're a little question box this morning .

#1. It's shock mounted to help lesson microphonices

#2  Couldn't find a schematic w/ a parts list but best guess is that the 27's cathode has a fairly high resistance to ground. This controls the gain of the detector so the tube isn't conducting very much hence the gain or amplification is low. Less chance of hum pickup.

#3 It looks like the small cap is in parallel w/the tuning cap's 1st section so it would act as a fine tuning or peaking control for the 1st rf stage. The switch I'm not sure as it's not clearly defined on the schematic.

#4 The power transformer has a tap on the primary and this switch between the end of the winding or the tap. This is used to set the transformer's primary voltage vs your line voltage. If the switch is unmarked use an ohm meter across the the ac plug to determine the highest resistance of the two positions. This would be the setting for the higher line voltage.

Monday morning laff break: https://www.youtube.com/watch?v=_Q0AAvx8...M_2ZXWs55g

Thanks Terry! I appreciate both your wisdom and humor.  

Here's my answer to your question #5:

3.75 meg isn't too bad, but I believe I would go with the original value of 2 meg.

Back when I was more into 1920s three dial battery sets, I found that the "sweet spot" for grid leaks was 2 meg. With a 2 meg grid leak I would always obtain the best reception and the most signals. This also held true for Philco models 80 and 84, even though they originally used a 4 meg grid leak.

Again, that was my experience - YMMV.

Nathan

When finished, I'd be interested to find out if you have AC hum.

I finshed an 86 this winter; if interested, look up the thread, there might be some useful info there. Like the caps inside the wirewound resistors etc.

Thanks guys.

@Ron - I will definitely try out the 2M value once things are running otherwise.

@Mike - Ah right! I found it: http://philcoradio.com/phorum/showthread.php?tid=17679
I should've looked earlier.

Here's where I'm at so far:

BC Resistor

The 160 ohm and 1K sections of the BC resistor were open. I created a new BC resistor out of 25W resistors. I was lucky in that the three resistors *just barely* fit.

4K resistor w/ tap: 2240 ohm + 1540 ohm sections
160 ohm Philco part
1K resistor



I brought it up on the variac and it actually received quite a bit at ~80VAC.

I was fortunate that the RF coils all tested good.

Recap

I replaced the big filter can caps. Boy that was a pain to get out. The tar had previously partially melted - making it stick really well to the sides of the can.
I relied on the trick described elsewhere on the Phorum of driving a screw into the tar block (to pull on) and then heating the sides of the can with a heat gun until I could get the block out.
I also used a small thin pry bar I have to separate it from the edge of the can. It took a little of everything but eventually it came free.



I also replaced the 0.1uF caps inside the 100 ohm wirewound resistor tubes. Kept the original resistance wire around the tube.

Interstage Transformer

The secondary of part #26 was 1/2 open. I believe that someone long ago someone worked around that problem with this?



The transformer with the open winding is in the top-right of the picture.

It actually did work reasonably well the way it was (decent volume, but a bit of distortion). I was able to find a replacement transformer on oldradioparts.com so I ordered that.
I'm about to swap that in. I did measure ~6,000 ohm for the secondary. I wonder if I'll get that hum you described now that the two caps / resistors work-around is removed.

I hope you won't. I am not sure why but the hum seems to originate in the transformer. It is not there before it and not after. A cap across it killed the hum (mostly). But I am still not sure as to the origin. It is possible a new transformer would not do it.

PS. If you come out with a good way to empty the old caps and tar from the box (it is easily disassembled but then the removal of tar...let's just say that I haven't completed it) let me know. I am not a fan of boiling it and the heating (well I haven't tried the oven....I use it for cooking) by a gun did not do much.
Surprisingly in 111 wit ha similar and actually bigger can I was able to quite easily remove the tar with a large screw and a large vice.

Grid Leak Resistor

I liked the way the original grid leak resistor looks - and it has that nice holder under the chassis, so I just soldered on a small 1/2W 4M resistor to it. Voila! 2M gridleak resistor.

It did seem to be an improvement in reception over the original (drifted) 3.7M value.

Alignment And Neutralization

This was the first time I've neutralized a TRF set. The neutralization procedure is well-documented.  

The first step was to build the described adapter from a junker chassis 4-pin socket and a dead 80 tube base:





Next I did the RF alignment as described and then went through the neutralization procedure for each RF stage.



Reception and Hum

After alignment the set works quite well.

In fact I detuned the 3rd RF slightly because it was just too loud even at minimum volume on the stronger stations. There's a pretty long antenna for the workbench in the garage.
(I checked the volume control and it reads OK at 8K vs 10K expected - and has a good ground connection on one end.)

Mike - here's a video of what it sounds like tuning around the dial.



There is a low (60Hz?) hum that is more noticeable between stations but doesn't really affect listening.

Is that what you heard with yours?

P.S. Ты заметил наш российский канал?

I think that we need to clarify here, the Model 86 is NOT strictly a TRF, it's a Neutrodyne TRF, or a "Neutrodyne Plus" as Philco called it, as opposed to the Screen Grid Plus chassis which are also a TRF but do not require neutralization thanks to the screen grids in the RF sections. Philco had a Hazeltine license, as well as an RCA one, and Hazeltine Labs did the original design work for most of the early Philco models such as the 511, the 86 is sort of a hold over except that it uses a pair of #45s rather then 71A. The small variable cap is probably used as a form of gain control on the front end, on strong stations you detune it a little so the set doesn't blast your ears out. The rheostat on the top is a hum balance control, it's wired in parallel across the #45 filaments, you could try adjusting it if you can't find another source of the hum.
Regards
Arran

Thanks for that clarification Arran. I was muddling together 'Neutrodyne Plus' with the other TRF sets.

One small correction though - the Model 86 still uses a pair of 71A tubes for output. Model 87 uses 45 tubes.

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I have adjusted the hum control to minimize hum. It does do that, although there's still a minor audible hum present.
I don't really think it's a problem but Mike ran into issues with hum on his Model 86.

Nathan

No, my hum was present all the time.

Speaking of neutralizing procedure: one of the tubes was a dud with open filament. This makes any adapter unnecessary.
In fact, old neutralizing procedures, as noted by Ron, called for a dud. I would add, preferably from the same manufacturer and the same time periode, to keep the similarity as close as possible. I had exactly that.