Philco 38-7 Restoration

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Philco 38-7 Restoration

Post by Map63Vette on Tue Feb 27, 2018 1:50 pm

So I'm attempting my first antique radio restoration and decided to start on something a little simpler. I have had a GE F96 in my basement for a while now that came from one of my dad's old houses when he was a kid, but one look under the chassis had me thinking twice about diving in. I have started cleaning up the cabinet at least, but in the meantime a coworker of my dad's had an old Philco 38-7 that he brought home for me.

I just recently went through the recapping process on this set and think I have that all sorted out (no fires or explosions yet), but am not getting any kind of sound out of it. I know about the cone-centric tuning and how it mutes the output and have checked and the output isn't being grounded via that, so I'm pretty sure that's not my issue. I was checking voltages on the tubes via the Rider's manual and they all are close (assuming the difference is because of non-exact modern capacitor values, maybe this is an issue), but on one pin I'm not getting anything. On the 6F6G tube the P (I think, hard to make it out on the Rider's manual) that is supposed to be 230 V shows nothing. I wasn't sure if this was a tube issue or something else in the circuit. I checked the F96 radio to see if I could try any tubes from it, but it doesn't use a 6F6, so no luck there. The only tube tester I have is an old Weston model 566 that can only do 4 and 5 pin tubes, so that doesn't help either. Any suggestions where I should start on checking the circuit?

For what it's worth, it appears this set has been touched before, but likely way back in the day. It had different electrolytics installed than the factory as they didn't fit quite right and some of the caps were different values and brands, but still the old wax types. I tried to put everything back to the original schematic as close as I could buy modern capacitors at least.

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Re: Philco 38-7 Restoration

Post by jukeboxman on Tue Feb 27, 2018 3:02 pm

Welcome to TRF

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Re: Philco 38-7 Restoration

Post by Bill Cahill on Tue Feb 27, 2018 11:36 pm

Welcome to TRF. It sounds as if you may have an open primary on the audio output transformer. Warning: With voltage on pin 6 of the 6F6 the tube will have a melt down without any plate voltage....
Is the radio producing B+ to the rectifier cathode, of, if the set uses a rectifier with the DC voltage on the filament.
Check there. Warning. Philco used the cheapest power transformers. When a short occurs, they go up in smoke. That's known as letting out the smoke, which is destructive...
Bill Cahill

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Re: Philco 38-7 Restoration

Post by Map63Vette on Wed Feb 28, 2018 9:38 am

Okay. I did a little digging last night and looking at the schematic a little closer and it does look like the primary on my audio transformer is open when I tested it with a meter. I haven't had the radio on for more than a few minutes at a time after the recap, so hopefully nothing has gone totally nuclear yet. The F96 I have appears to have a "universal" style replacement audio transformer on the speaker, so I may scavenge that to test with or just see about getting another one. Any suggestions as to what would work well? The schematics list the DC resistance, but most transformers I see online don't show DC resistance, just the impedance. I'd like to make sure I get the right thing so I don't hurt anything else.

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Re: Philco 38-7 Restoration

Post by Chas on Wed Feb 28, 2018 11:23 am

Welcome!

You need to determine if the power transformer is toast...

It is a simple but effective test. There is no sense in going further if a transformer is needed.

Remove the rectifier tube, leave all other tubes in place but look at the pilot lamp sockets. Sometimes these get shorted and can ruin a transformer too.

Then power the radio with the speaker plugged in if it is removable, not that it matters as removing the rectifier disconnects the HV secondary of the transformer from the load of the radio.

If the transformer is defective there will be frying noises and smoke in a very short period of time. If, as time passes allow 1/2 hour if the transformer is just barely warm and still power the filaments of the remaining tubes it is O.K. Use your judgment, the severity of a ruined transformer can go from a dead short that the house circuit breaker will open.

Many restorers like to use the series safety lamp in the power circuit. A 40-60 watt filament bulb in series with the power. If the lamp lights dim on first try, then bypass for full power, save tripping the local breaker... Do not attempt to operate a radio with a series lamp expecting it to work as a radio.

Then go here:

This site explains how to set-up a replacement output transformer. There are numerous vendors, some more pricey than others. Be sure the frame size of the transformer is about as large or larger than the original. Though insulation has gotten better. Basic transformer has not shrunken in design for the job to do.

http://www.radioremembered.org/outimp.htm

Be aware that the first filter condenser after the rectifier is NOT returned to the chassis, but to the bias voltage divider resistor part # 43. Do check this resistor array.

That said, check all the coils in the set, Antenna, Oscillator and IF. Any coil with high resistance or open will have to be repaired or replaced.

If the rubber of fabric over rubber covered wire is stiff, heating the wire with a hot air tool will soften the insulation so it can be moved without cracking. It will get hard again when cooled.

Keep water and cleaners away from the dial ! ! !

Be aware that some DVM can give errors when measuring leaking capacitors and inductances. If the reading seems not to make sense, try an analog VOM or a VTVM with a D'Arsonval meter movement.

This radio does not have a tuned RF amp, expect there will be images in the shortwave when aligning. Be careful that the oscillator is set correctly so calibration of the dial is accurate.

Do not attempt to align by "ear" or before it is determined that the radio is working correctly. Certain capacitors in the RF, Oscillator and antenna circuits are for alignment, do not change these out unless they have failed or alignment cannot be achieved. Do NOT replace by routine.

YMMV

Chas


Last edited by Chas on Wed Feb 28, 2018 12:22 pm; edited 1 time in total
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Re: Philco 38-7 Restoration

Post by Map63Vette on Wed Feb 28, 2018 12:12 pm

Alright, I'll have another look at it and see about leaving it on for a while with the rectifier removed. I did plug it in again last night while trying to sort out the issue and rechecking voltages and the filaments did still light up (with the rectifier still installed), and I've yet to see any smoke or hear anything, so I think my power transformer is still okay, though hard to say if it's just limping along or on its way out if I damaged it with the open speaker transformer issue.

I actually just found that transformer website myself this morning and bookmarked it for quick reference. I don't think I'll be able to determine a winding ratio since the transformer I have is toast, but I can probably pull a rough speaker impedance estimate and work backwards from there. Does the DC resistance of the transformer matter at all? I was thinking it helped to load the circuit or something like that, but I suppose the impedance is the true load in use, so maybe it's not a big issue. Just wanted to make sure I matched as many values of the original as I could. I know that value is marked on the schematic, but I've also read it's not the most useful number.

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Re: Philco 38-7 Restoration

Post by Chas on Wed Feb 28, 2018 12:47 pm

You WILL need to replace all the paper wax capacitors, those that are in any bakelite blocks that also function as terminal lugs...

They will leak and shift bias values and reduce values on screens of tubes as well as overheat and burn dropping resistors...

Look at the chart on the link given the 6F6 is listed. that gives the primary impedance. Do not worry about the plate voltage being part of the equation not the resistance of the primary

You will need a 3-5 watt transformer 7K ohms impedance primary up to 9k. do not go lower. Secondary can be 2-4 ohms, do not get an eight ohm model.

Be careful cleaning the chassis, keep steel wool away, compressed air VERY carefully. Air over mica trimmers will shatter the mica!

Cleanup all solder splatters and wire clippings. Large solder blobs on terminals can short to chassis or adjacent circuits Do no spay an contact "goop" on tube sockets, especially the rectifier..

There are several methods to working with Bakelite blocks, BTW they are nearly ALL different in their content. Some folks freeze them, some use a heat gun. I did mine in gasoline 40 Y.A. that was a sloppy filthy mess... But I did discover one block has a wirewound resistor inside, probably NOT in this radio.

Blocks have a part number and there content can be found in an online chart for Philco help.

I used dipped radial caps as the leads on the side directed into the holes in the block terminals. Use good quality caps in them.
Standard values for the caps and resistor have to be used at 20 or 10%. For example Philco often used a .09mf, the replacement is .1mf... Like, .05 becomes .047, .03 becomes .33 and so on...

Resistors like 50K will be 47K or 51K.

Cap #47 on the AC line should be of the safety type that goes open when it fails.

Caps 37 and 40 should be of a metal foil construction. High peak static noises will open a metalized film cap in this location of tone control. That cap may be leaking and failed, opening the output transformer ... A cap that publishes the dv/dt value here.

Again,

YMMV

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Re: Philco 38-7 Restoration

Post by Bill Cahill on Thu Mar 01, 2018 6:27 pm

In most radios, the resistance is not the impedance. It's thee DC resistance.
If the tubes light up, power transformer doesn't have an open primary. Now, it could have an open hv secondary.
Open primary on audio output primary isn't good, and, at the least, has damaged the output tube, and, possibly the rectifier tube, as well.
Hope this helps.
Bill Cahill

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Re: Philco 38-7 Restoration

Post by Map63Vette on Fri Mar 02, 2018 10:34 am

I don't suppose there are any easy tests for tubes that can be done with a multimeter are there? New/used replacement tubes are cheap enough, but curious if there was any way to check out what I have without having a tube tester.

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Re: Philco 38-7 Restoration

Post by Chas on Fri Mar 02, 2018 12:08 pm

A tube can be checked for filament continuity with an ohmmeter.

Go to this web site for tube basing and other info:

https://frank.pocnet.net/

If the tube has gone to air, generally, the silvery getter deposit on glass tubes will turn white.

If you shop around, a set of tubes in NOS, no box, should be about $50. I suggest that a set of white boxed new would be the way to go, If the set has issues, the tubes would be out of the equation. Original boxed tubes are for "show" and will cost more. White box or no box will be less expensive. Buy used tubes only from a reliable dealer or another trusted collector.

Source:

http://findatube.com/   will be the least expensive.

There are many others, unfortunately, Google hits put the most high end vendors at the top, there are other vendors that are far less costly.

Individuals will have more economical prices as well.

Please fill out your profile. The info will help members determine any costs to potentially offering a set of tubes or other needs...

GL

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Re: Philco 38-7 Restoration

Post by Map63Vette on Fri Mar 02, 2018 12:21 pm

Yeah, I found that findatube website and priced out a full set to see and it was only $30 for a full used set. This would be more a novelty to have around the house, assuming I can get it to work vs a true restoration piece, so used would be fine with me. Pulled the universal transformer off my other radio, but need to do some testing to see what the different taps give me. It's old with no real markings, so I have no idea what the different winding ratios are. All I know is there are 6 taps on it, labelled 1-6 at least. The primary side is three taps, so could be push-pull or single ended depending on how I want to wire it.

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Re: Philco 38-7 Restoration

Post by Chas on Fri Mar 02, 2018 1:22 pm

If there are any markings on the transformer, look online. Might be able to find data on it to use it correctly. Shoot from the hip, find a primary circuit that has the greatest resistance. At worst case this is probably 10K impedance. Use that, at best it will be close enough for the 7K impedance needed.

There are several output transformer makers still around; Edcor, Hammond will have what you need.

You can also try, Play Things of Past:

http://www.oldradioparts.com/

If you have used the schematic to check the resistances of all the coils and there O.K. then the worst is behind. Clean the band switch with isopropyl alcohol 91%, let dry or carefully blow dry with compressed air. I did not reference the schematic but if the band switch is transferring B+ then it has to be clean or it could arc. Manipulate the band switch many times, carefully, lube the bushing and the ball detent carefully, but not the contacts. The repeated action will clean them of oxides. Examine the contacts to be sure none are bent. The bandswitch will be difficult to repair/replace be careful.

Do NOT turn on the axis of the lead wires ANY "postage stamp" mica condensers. If you must see the dots use a dental mirror. What happens is the copper wire distorts and opens the seal in the Bakelite and/or breaks down the connections to the mica stack inside.

Do not re-arrange any wiring under the chassis. That may cause hum, detuning, instability... Any wire that has failed insulation should be replaced with same wire size and type, strand or solid. The insulation must be for 400 or better volts, NOT computer 30 volt wire.

It is best to carefully loosen and re-tighten any solder tab to chassis connection. Any riveted to chassis connection is suspect. These may have to be soldered to the chassis.

PTOP will probably have most of the parts, even a band-switch or a doner chassis.

Any old electrolytic capacitor must be replaced but can be left on the chassis for "looks" Solder lug terminal strips are use to mount new capacitors near the old ones. In the DC area wire shifts are generally O.K.

The rectifier, power output and oscillator mixer tube, have a higher failure rate. I IS important to replace all coupling and bypass capacitors, as said the leaking cap can cause coil and resistor failures, a bad cap can cause an output tube to draw excessive plate current, overheat and possibly cause an output transformer to go open. Further capacitors in the output plate area can also short or leak same effect.

I, would not use metalized "general purpose" caps in the plate and tone circuits. impulse and static, lightning noises have a fast rise time and will propagate through the radio to the output, the pules can cut-off the output tube current and the resultant is a discharge from the collapsing magnetic field of the output transformer. An arc-over can occur internally in the transformer, wiring or the tube socket. The wrong type of cap in that area will get the pulse with a fast rise time that will exceed the Dv/Dt rating of the cap, the metalizing will breakaway from the internally cap construction and it will loose its capacity, it may short. That is why I would install a cap with real foil and most any polymer construction. Such a cap will have its dv/dt rating in the listing of manufactures values.

I n RARE cases two foil/poly caps are placed in parallel to satisfy a dv/dt condition as well a mfd rating, where one cap of twice the dv/dt rating would be ruined. I find that this is what is need in tube auto radios to replace a similar wax/paper cap. pair...

I mention these little oddities as I imagine because the radio has some significance it will be shown and played often and static may be present...

Note any glass output tube that has gone red plate is suspect for grid emission, cathode emitter material can come loose during the event and stick to the signal grid. A tube tester may not show this issue. At full voltage the tube will be unstable, cause distortion and may "run away" with excessive plate current, possibly taking out the output transformer.

YMMV

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Re: Philco 38-7 Restoration

Post by Map63Vette on Wed Mar 07, 2018 1:16 pm

It lives and makes noise! I just took a guess for the time being on the transformer taps to pick (used the same ones that the radio I took it from used) and managed to pick up a few stations, though it wasn't amazing (not that AM ever was). Definitely have some tuning and clean up work to do, but was great to actually hear stations from the old girl.

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Re: Philco 38-7 Restoration

Post by Chas on Wed Mar 07, 2018 2:00 pm

Yeah!
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Re: Philco 38-7 Restoration

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