[Ron Ramirez]

I have made it an annual tradition for several years now to perform electronic restoration on one specific radio around Christmastime, and then call that my Christmas radio for the year.

This year, I chose a Philco 91 code 126 chassis.

Schematic here:
http://www.philcoradio.com/tech/images/1491.jpg

It was not the best choice, as its power transformer had obviously overheated at some time in the past. There is black tar all over the top of the chassis near the four tubes closest to it, plus a fair amount of dried wax running down some of the leads coming from the transformer on the bottom.

But I dove in anyway, and soon had all of the bakelite blocks rebuilt and all resistors replaced. The tone control was rebuilt, the off-on-band switch was cleaned with DeoxIT, and two of the set's three coils were rewound (the antenna coil measured OK for continuity).

Yesterday, I applied power for the first time. First, without the rectifier tube. The two lamp bulbs, both #46, were bad and were replaced. Then, being mindful of the transformer's meltdown, I replaced the rectifier tube and slowly brought it up on the variac. Fortunately, the power transformer appears to have survived its overheating (most likely caused by bad electrolytics in the past). It came up - but did not play.

So, tonight, I performed voltage checks, which pointed to something wrong at the det-osc tube. Guess what...I had removed part (20), a 700 pF mica cap, and part (21), a 10K resistor - but I had forgotten to replace them!

See, even those of us who have been in this hobby for many decades can make mistakes, too. (Next year will mark my 40th year in the hobby.)

Well, I installed a new 680 pF mica cap and a new 10K resistor...but the set still does not oscillate. So further troubleshooting is in order.

Oh, I almost forgot: How do you guys get the caps off the Mershon electrolytics? I tried to remove the cap from the lone Mershon in this radio. I managed to make it go up about 1/8 inch but dented it up pretty badly in the process. I finally gave up and connected new electrolytics under the chassis.

[morzh]

Ron

I forgot but here on the Phorum someone told how to open Mershons. You should be able to find it unless I am having some false memories.
And it was fairly recently, a few months ago tops.


Now to Det-Osc. I looked at your sch - there is some problem with it. If you think the assembly is factory then it's nothing; if someone tried to fix it before and rewired it according to the sch....well, take a look at the 36 plate and its load, coils 22 and 25. They do not connect to the V+. Same mistake many Philcoo sch have - the cap they connect to goes to GND but there should be a tie-dot to the Right side of Resistor #14 (rectifier filter output).

[Ron Ramirez]

Yes, I think you're right...no time to search right now as it's past my bedtime, but perhaps tomorrow.

[37silverstreak]

Hi Ron,
I'm working on the same 91 126 chassis right now too! I have the earlier version without the bandswitch, just the on/off. I also tried to remove the caps on the Mershons and gave up!!! I did manage to get those covers off on the 90 I restored by very carefully tapping them off with a flat blade screwdriver, but those caps were a slightly different style than the one on my 91.
I fired up her up for the first time last night, but no stations yet. I do have a loud 260KC IF when connected to the grid cap of the 39/44 first IF tube but not from the RF tube so my problem is there. Coils all appear good but the cathode voltage is only 2 volts where it should be 25. Now that my Dad's funeral is over with I'm hoping things will slowly go back to normal and I will feel more like working on it.
I wish you luck with yours.
Kevin

[murf]

Merry x-mas Ron and good luck on the Philco.
murf

[morzh]

Ron, take a look - I added something to my post you might wanna look at.

[tab10672]

I have had luck with the mershons by first soaking the cap seam with wd40 letting it set overnight then using a Popsicle stick as a chisel working it with a small hammer all the way around untill its off.

[TA Forbes]

Sounds like the oscillator coil might be bad! Common problem!! Also the 36 tube can be finicky!!

I am working on a later variant of the 91 (6A7, etc..), and three of the transformers were open!!

[Ron Ramirez]

I'm hip. I rewound the oscillator coil already. Perhaps I made a mistake in that? Well, I'll test the 36 tube before digging back into the coil.

Although it is tempting to perform one of my 6A7 mods to this set...

And Todd, thanks for the WD-40/Popsicle stick advice regarding the Mershons, sounds good, will have to try that.

[Ron Ramirez]

So I worked on the 91 some more last night...

36 tube tests OK...replaced it with another from a working radio anyway...still no dice.

I did find that the primary of the antenna coil was open. Fixed that (a simple matter of a broken wire). Still doesn't work.

I can pass an IF signal through the set just fine now. I did a quick and dirty IF alignment, by ear. Will go back and realign it - the right way, with my oscilloscope - once it's up and running.

I decided not to bother with it tonight. Perhaps tomorrow. I will definitely have to revisit the oscillator coil. And I am still thinking about just converting it to use a 6A7...

[morzh]

Ron,

Does 36 tube have the plate voltage?

[TA Forbes]

Well, the 6A7 IS a better system.

[Mondial]

Ron, you might try reversing the leads to the 36 cathode winding or the plate winding. If you rewound the coil, depending on the phase relationship of the winding you may be applying negative feedback rather than the positive needed for oscillation.

At least it might be worth a try before considering converting to the 6A7?

[Ron Ramirez]

Well, there is good news and bad news tonight...

First, the good news: The 91 lives! I decided that just in case I had made an error rewinding the oscillator coil, I would try another coil - if I had one.

As it happens, I have a small stash of coils. In the box was an oscillator coil! I do not know if it came from a 71, 89 or 19, but it is the same type of coil, with the same pinouts.

The cathode winding needed to be rewound, so I did that.

Installed it and the set began to work!

I've let it play for over an hour and the power transformer is only lukewarm. Considering it had been overheated in the past, to the point of leaking out a lot of wax, I find that pretty amazing.

Now, the bad news:

The tracking is way off. I can not tune in 1400 kc. Tuning the signal generator around, I managed to get 1190 kc at 140 on the dial. My SSTRAN, which is set at 660 kc, comes in at 75. So the entire AM band is shifted upward on the dial from where it should be. Adjusting the high frequency oscillator trimmer only managed to move things back down the dial somewhat before the adjustment screw started coming loose.

No, I have not tried the low frequency padder, simply because that adjustment is supposed to make the tuning condenser track properly with the IF.

Oh, and the tuning condenser seems to be very dirty and the tuning cuts out - loudly - at various spots on the dial. There is no obvious visible rubbing of the plates.

The tuning condenser, incidentally, is the same type as used in the 1933-34 models 14, 17 and 18 rather than the large type used in the earlier 91 and the 71. So this must be one of the last production 91 receivers.

So...am I looking at adding or removing turns on the oscillator coil secondary (the winding which the tuning condenser is connected to)?

[Mondial]

It seems that the inductance of your replacement oscillator coil is too high, as you are receiving stations higher on the dial ( less capacitance ) than normal. In his case you would need less turns on the tuning secondary winding.

With less turns and less inductance, more tuning capacitance will be needed, so the tuning cap will be more meshed for a given receive freq and the stations will tune lower on the dial.