[dporcaro]

Hi all,

I am new to this forum. I am at a loss trying to refurbish a friends Philco 46-1203 radio/phonogragh. A little bit about me - I am a retired mechanical and manufacturing engineer with a hobby of refurbishing older audio equipment. Mostly amps, receivers, tape decks, etc. I'm not an electronics expert but have been educating myself in diagnosing and repairing audio stuff.

My experience with tube gear is limited. I successfully refurbished a McIntosh tube amp and preamp for a neighbor so this Philco is not my first. The unit belongs to a friend and is a family heirloom for her. She just wants to get it working again. The unit was intact and in decent physical shape. 

Upon receipt I brought the unit up slowly on a variac and dim bulb tester. No drama, the dial light came on but the turntable didn't spin. The needle is intact but no sound from tapping on it. The radio also has no sound - not even static. The turntable motor was gummed up and frozen. I was able to get it freed up and the turntable now functions - but no sound. 

I started by replacing the caps.  They were basically all bad and I replaced all except a few square shaped caps with round dots. They tested good and are not likely failure points from what I understand. After replacing the caps - still no sound. Next I took the tubes to a local electronics store (blast from the past) that still has a tube tester. They tested on the weak side but useable. Since I get no sound from either the radio or the phono - I focused on the audio output and rectifier tubes. I got replacements for those just in case. Still no sound. FYI - this unit came with a 50Y6 rectifier tube not the 50X6 listed on the schematic. The 50Y6 is the correct tube per the diagram on the inside of the cabinet. 

I'll try and shorten the story and give a bullet list of what I've done so far.

1) replaced all electrolytic and wax caps.
2) Replaced all tubes 
3) checked the speaker for function - ok
4) checked all carbon resistors - all checked good.
5) checked the main switch and volume control resistance values - ok
6) checked the output transformer - removed it checked primary secondary coil resistance - ok (it also passed a signal from signal generator).
7) removed the IF transformers and checked coils - ok

I'm not sure what to do next. The only sound is a vary faint hum. You have to put you ear to the speaker to hear it. It doesn't appear anything is shorted - no dim bulb light. I seems something is open in the amplifier section but I cannot find what. There really isn't that much to this thing. I downloaded the Philco service training manual from the library and tried applying a signal to various places to find where it's dying. I was able to get sound out from all 3 test positions but only when the negative lead from the signal generator went to B-. Sound came out but it lit up the dim bulb ( it also cooked my signal generator). 

Anyway, if anyone has some gentle advice on what to do next it would be greatly appreciated.

[462ron]

Hi, The first rule whenever working on a set without a power transformer which your set is this type, is to use an isolation transformer for safety reasons. Most of these types of sets have the chassis or a floating B- connected to one side of the AC line. This set is a little different as it uses what is known as a voltage doubler for the B+. I know you said you are using a variac but many of those aren’t isolated from the AC line. If your signal generator has a 3 prong plug so it is grounded and you connect the ground from the signal out put to either the chassis or B-, you have put a direct short to ground depending on which way the AC plug is in the receptacle. Thus the need for an isolation transformer and is the reason why it cooked your generator and made the dim bulb light up.

Ron

[dporcaro]

Got it. Thanks Ron. Am I correct in thinking that the test equipment (scope or signal generator) and the equipment under test should be plugged into the isolation transformer?

I will get one.

Dino

[morzh]

OK,

First things first:

those " square shaped caps with round dots" should not even be touched for testing. Unsoldering them may damage the leads attachment at the potting point. Those are micas, they are presumed good until proven otherwise. All other caps should be replaced.

Now. You never said you tested the voltages.
We need to establish the voltages are present.
Also: do the tubes (all) light up or otherwise it could be established with sufficient degree of certainty that they do (if not visible)? Like measuring at least 7.5V (with the pilot lighting up should be 10V) across the R2, the 50 Ohm section?

[Chas]

462ron...If your signal generator has a 3 prong plug so it is grounded and you connect the ground from the signal out put to either the chassis or B-, you have put a direct short to ground depending on which way the AC plug is in the receptacle...

462ron's comment on the direct short can also occur to the live of the generator test lead. Many service instruments do not protect the signal attenuator thus the resistor/network gets roasted, occasionally destroying any wafer switch as well. For RF generators, loose coupling often works well via just a couple of turns of the RF lead. An alternate is a small coil of ten turns in the vicinity of the antenna coil or mixer output.

There seems to be a trend to ground service grade instruments by adding a grounded 3-wire cord system. The AA5 (typical) live chassis receiver was very popular at the time these instruments were made. Servicemen routinely used isolation transformers, some did not   depends how much they trusted their knowledge of fault circuits and their work environment, UGH!

Replacement of components without a through triage can often be problematic. Resulting in an inoperable receiver, that will now require stage by stage analysis to repair as well as finding the initial fault.

"Triage" is simply verifying that RF/AF coils are viable, tubes good, speaker functional. Then replace only the power filter condensers, any clearly destroyed components as well as any AF coupling condensers. Carefully power the set and there is a good chance it will work. Note any peculiarities. Then, replace capacitors/resistors as required but two at a time and then power the set. At any point the set quits, retrace to the last two components. Be aware on occasion, a defect can occur on an aging component during a test. This is often caused by leaking bypass condensers overloading a voltage dropping resistor.

Components in the RF sections such as mica/ceramic condensers should not be replaced routinely, often they are temperature compensating types that may not be available. A receiver that will not align is often a clue there is a failure with these components. Do NOT rotate a mica condenser on its leads as that breaks the seal in the Bakelite housing allowing the entrance of moisture or mechanically break connections and eventual failure.

Classic vacuum tube electronics on a metal chassis has now reached the point that any soldered or riveted connections to the chassis will be problematic... Despite a continuity measurement the connection can act-up when passing RF creating havoc in RF/IF circuits.

These are YMMV suggestions to help avoid "creating" what professional servicemen referred to as a "Tough Dog"...

If the receiver safely stays powered without overheating, consider safely injecting an audio tone into the AF circuits, this is done at the grids via a .01 blocking capacitor, Actually since the AF generator may also be grounded set up a .02 on the return lead and a .02 on the hot lead of the AF generator. That will protect the AF generator from a AC line short. Trace this tone backward into the circuits until it disappears at the detector. From that point switch to the RF generator, modulated, and set for IF frequency and test continuing backward to the mixer grid. If there is still no clear indication of what is happening the Oscillator may be non-functional generally from either a poor connection, failed coil or poor contacts on any band switch. DO check the phono-radio switch for poor contacts.

Tune the receiver to 1mhz inject a RF un-modulated signal that is 1mhz plus the IF frequency the may come alive at 1mhz. An alternate to oscillator test is to use a pocket portable radio and tune for the oscillator radiation at 1mz plus IF. The should be a carrier un-modulated (so called "dead-air") detected by the portable radio. A bit of twiddling is required to sense the radiated oscillator if it is working.

The phono does not make noise may be from a melted salt crystal. The signal can be detected on a 'scope if it is working. The phono cartridge can be rebuilt.

I hope these are workable service methods for you.

Chas

[dporcaro]

Thanks for the comments Chas and morzh. Sounds like good advice. I did measure voltages at the tube pins. I didn't document the results but there didn't seem to be anything way off. I will measure them again and post the results. The tubes do light up particularly the 50Y6Gt and the 35L6GT they get quite warm. The 7C7 and 7A8 don't get very warm but a faint glow can be seen near the base.

I will go tomorrow and see if my local electronics store has an isolation transformer.

[morzh]

I doubt they will.
If they do, great; if not, eBay has plenty.
This is Amazon search.

See if you like anything.

https://www.amazon.com/s?k=isolation+tra...xclick_1_3

[Chas]

The Philco library will have DATA for the model you have and any possible quirks.

How to troubleshoot a "dead" receiver as a technician would instead of "Is it this or that". Go here:

https://www.worldradiohistory.com/BOOKSH...vicing.htm

This is the repository of technical books out of print on how to service. They are generic by nature meaning not specific to a model of radio but for the techniques. You can down load as needed free. The scans are PDF word searchable. A big plus for coverage on a specific problem. The Riders series of hand books are also teaching but at a technician level.

I see that this title may be more useful:

https://www.worldradiohistory.com/BOOKSH...ellman.pdf

Highly recommended:

https://www.worldradiohistory.com/BOOKSH...y-1947.pdf

On the same main WRH page is G-E tube characteristics...

Not present or possible to PDF, I suggest getting a hard copy of most any edition of RCA's Triple-Pindex. That is a vital tool to find your way underneath any chassis.
Such as: https://www.ebay.com/itm/196354059776

A huge online tube manual can be found here:

https://frank.pocnet.net/

Franks web site works well by just typing the tube type into the browser like: 7A7 pocnet then press ENTER. You will get the tube of interest.

This is Frank's Vade Mecum: https://frank.pocnet.net/vademecum0.html 

More tubes than one can possibly imagine...

If one is to continue serving receivers a useful electronic tool is a circuit tracer. This can be a restored vacuum tube unit or a solid state (battery) powered unit insulated for at least 600 volts.

I could suggest simple signal tracing but unless one realizes the risk there is a real danger of shock.

However, an RF/AF generator capacitor coupled makes a good signal injector. A salvaged amplified computer speaker protected with capacitor coupling make a good receiver tracer, create a RF probe with a diode resistor coupling. Plans for a probe are all over the web.

Where you are now seems to indicate a short or mis-wire from the volume control/phono switch forward to the speaker. Some four tubes or so and about a dozen components and a rats nest of wires. Look around for a wire snip, blob of solder shorting, component on the wrong terminal. Oh, maybe a tube in the wrong socket

Be SAFE!

Chas

P.S. Beware that a DVM can tell lies about voltage and the resistance of inductors. If, you have a good VOM use it. Voltage readings printed on a schematic often have a reference text as to the resistance of the meter used to make such readings like 1000 ohms/volt. Such a meter draws a load from the circuit. A DVM draws practically nothing so it will read high and not consistently high depending on the available current from the circuit.

[462ron]

Dporcaro, in regard to your question on what gets plugged into the isolation transformer, plug in only the radio, that will keep it isolated from the AC line then it should be safe to attach the ground lead from your Oscope or signal generator to the chassis or B- of the radio!

Ron

[mancave]

2 suggestions

1. Verify B+ voltage 
2. Check and clean the radio/phono switch.