[cknob]

Hi all...brand new to this site and vintage radio message boards in general.  I have just picked up this hobby, buying non-working radios from the 1930s and 1940s and getting them back to cosmetic originality and reliable function.  I'm 61 years old and I live just outside Philadelphia PA.  I've been something of a "professional hobbiest" for over 40 years!  I've been a prolific woodworker and turned by attentions to acoustic guitar building over the last 15 years (over 50 built!).  I've worked on, repaired and restored mechanical watches for longer than that.  I'm a scientist by profession so I know a little about electricity and theory...so I thought I might try my hand at this! 

I'll confess, my restoration experience so far is a few AA5s and a Philco 37-650 and amounted to nothing more than cleaning, recapping and replacing resistors on a one for one basis.  I may ask some very stupid questions.  I also intend to stick around...not swoop in for a few answers in my first and only 10 posts and then disappear!

Here's what's presently going on.  I bought a beautiful and all original Philco 90 lowboy from 1931.  It had stayed in one family and had been stored for the last 40 years.  Sometime during its life, it was refinished with a very thick and dark stain and was practically black.  The veneers were delaminating and the top had a broken edge.  I pulled the chassis and got to work on the cabinet.  After a month of work, this is how it turned out:

   


I started to attack the chassis next.  I was thrilled to see Bob Andersen began a 90 restoration on his YouTube channel (Bob and shango66 are my electronic heroes!).  I did what I do...bought tubes, re-stuffed the Bakelite blocks (even bought Bos's recommended block rebuild tool kit...very easy to use) installed the line safety caps, put in a few terminal ties to make things a little easier and replaced the electrolytics.  It worked!  The radio came to life and sounded OK.  I then went about replacing the dog bone resistors on a one for one basis.  I turned the radio back on and...nothing.  Complete silence.  Then...the dreaded smell and...smoke!

I traced the smoke to the end of the coiled, long resistor mounted to the inside of the chassis below the AC line filter cap block.  To orient you, if the chassis is flipped up on its short edge with the bottom of the transformer closest to the bench and to the left, this resistor runs the length of the inside wall at the bottom.  The smoke and heat is all at the left end of the resistor.  At this end, there's lots going on.  There's input from the center taps, lines 4 and 12 from the transformer.  I have the negative end of one of the electrolytics connected there and a 250K ohm resistor that runs to a tie terminal on the Bakelite above.  I looked at a photo from B. Anderson's thread on his 90 restoration on this site and what I've done looks correct.  As anyone who has worked on a 90 knows, there's lots of available schematic and component info out there but to be honest, my experience in interpreting schematics is pretty limited.  I have included a photo below as to what I've done.  I hope I've uploaded that correctly!  I've checked voltages on the 80 rectifier and the 47 speaker amp tube, plates and filaments, and they are fine.  They run off separate wires out of the transformer.  The speaker is quiet....very slight hum if you put your ear right up to it.

   

Any and all help will be very appreciated!!!

[Arran]

Here are some photos of some original model 90 lowboys from the gallery: https://philcoradio.com/gallery2/1931b/#Model_90_Lowboy As you can see they used shaded lacquer on several portions of the cabinets, such as the legs.

[Bob Andersen]

I'm a little confused by the added terminal strips and the new wiring. Especially on the right side where the AC line bypass cap is.
Did you replace some of the old wiring ?

[Arran]

I'm scratching my head looking at this as well, one terminal strip looks like it is soldered to one of the lugs of the voltage divider resistor with a jumper across the terminals. Then there is the one attached to the bottom of the line cap block, not sure what is going on with that or if it's tied in with the caps inside the block in some way, the two caps inside that block are connected between the AC power line and the chassis, so why is there a filter cap placed there?

[cknob]

Hi Bob...pleasure to speak to you!  Yes, it looks pretty messy.  I'll try to explain what I did and what you're looking at starting from jump.  Yes I did replace some of the wiring.

I replaced the AC line caps by restuffing the box with the safety caps you recommended.  I tied a lead from both to the ground pin and the other lead from each capacitor to the individual remaining tabs and reinstalled the block in the chassis.  I then created a terminal strip to make wiring to the safety cap block a little more accessible for me.  I soldered it to the center tab on the safety cap giving me 3 point access to the center tab.  The AC line coming in was split.  The hot line went to the center tab terminal strip and the neutral line to the remaining, non ground tab on the safety cap block, the tab to the left side of the center tab.  This neutral tab has a line off it that runs up into the transformer.  Now here is where the funky mess I created begins.

I drew a line off the center tab of the safety cap thereby replacing an original wire that ran from the center tab to the on/off switch.  That's the new wire in the picture coming off the safety cap center terminal strip that runs up to the left, out of sight to the switch.  The other side of the switched wire was replaced as well and spliced to the line that goes into the transformer. 

Now, the resistor on the bottom.  Again, I created 3 point access to the tab all the way to the left end of the resistor coil, tab 1 according to the schematic.  There are 2 wires coming from the transformer connected to this tab...what I am assuming according to the schematic are center tap line 12 and the center tap line from the A circuit, line 4.  The negative side of the first electrolytic is tied to this first terminal as well as is the 250K ohm resistor that runs up to the the Bakelite block above.  If I ground one side of my volt meter to the chassis and measure voltage on this first coil resistor tab when I turn the power on, it bounces all over the place, oscillating between 300V AC, 0V AC, 35V AC etc.  After a minute or two...burning smell and smoke.

Thank you Bob for offering you insights into my difficulties.  I can post pictures of any of my dirty work on this radio you need for diagnostics and possible solutions.  I'm probably doing something extraordinary stupid!!!!

Chris

[cknob]

Thank you Arran for your thoughtful inquiry. If you look at the response I gave B. Andersen above, I hope it explains some of what I've done. As I said to Mr. Andersen, I'm probably doing something very stupid. The wires I replaced were replaced on a one for one basis.

Thanks again!
Chris

[RodB]

Hi Chris and welcome to the Phorum. I've read your explanation of the wiring you did. It appears the bakelite block for the transformer primary is correct. From what I see on the schematic I have, which is for the single 47 output tube, Tab 1 is establishing a negative DC grid voltage to bias the last two audio tubes. Connected to Tab 1 is the center tap terminals 4 and 12 of the secondary windings. You should be able to check the continuity of these connections using an ohmmeter with the power off. From your explanation it looks like it is correct. I assume the negative of electrolytic cap on the right side of your photo is attached to the ground lug of the bakelite block. Recheck your connections for a short or solder splash. Another point is to power up these old beasts with a dim bulb system or variac to avoid damage to the power transformer. BTW, we all do stupid stuff and keep doing it; it's the nature of the beast.

[cknob]

Thank you so much. I have the single 47 model. Sorry I didn't make that clear. I know this will further expose my "lower than entry level" status with this incredibly knowledgeable community but when I check the continuity of these connections (I assume you mean the leads 4 and 12?), what exactly should I do to do that? Should I ground my multimeter to the chassis, de-solder the 4 and 12 lines from the terminal and check each of the center tap lines individually?

[cknob]

I will check the electrolytic caps when I get home from work and take more detailed photos. I'm almost certain these are in as you have assumed but at this point, I'm doubting everything I've done! The ones in there now are kinda' junky ones I had in my supplies but tracking indicates 2 new Nichicon 50uf 450V electrolytic caps were just delivered to my house. I don't want to hard solder them in until I get this sorted out!

[GarySP]

Welcome Chris! You did a great job on the cabinet. Trust me, some of us (well OK, me!) fly by the seat of my pants, and rely heavily on the gurus here to keep me from burning down the house. Bunch of very helpful folks here. Take care, and BE HEALTHY! Gary

[cknob]

They're here!  Hope I get to use them!

[Phlogiston]

Chris,

Your goals are admirable. I have been working in that direction for - well - a very long time.

Take a look at my site. You might find the "Before & After" interesting as well as many of the blog posts. Some are on refinishing, many more are on chassis restoration. 

Russ

[morzh]

cknob


A question: where do you intend to use 47uF 450V caps?

[cknob]

In place of the electrolytics currently in the radio.

[morzh]

Well....there are no capacitors of this value in the whole of the 90 radio, independently of the chassis code.