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Philco model 77 am radio. - Printable Version

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Philco model 77 am radio. - Dave c - 09-24-2025

I was wondering if anyone can give my some insight. I picked up a Philco model 77 low boy. 
I have changed all the condenser in the radio. Also picked up a spare set of tubes.
The audio portion of the radio works fine but it will not recieve any stations. In the lower range on the tuning dial, the radio sound is that of a 60 hr low hum. 
I tried to touch the top of the 24 tubes with an antenna wire and I either get a static noise or a wob wob wob noise from the speakers. Does anyone have ant thoughts on were the problem area may be.
Thanks in advance for any advice. It is much appreciated.  Icon_razz

RE: Philco model 77 am radio. - GarySP - 09-24-2025

Welcome to the Phorum, Dave! I moved your thread to Philco Electronic Restoration. Should fet a better response there. Take care, - Gary

RE: Philco model 77 am radio. - Dave c - 09-24-2025

Thanks Gary. I appreciate that.

RE: Philco model 77 am radio. - radiorich - 09-24-2025

Hello Dave,
Welcome !!
Sounds like you have a fun project I am looking forward to seeing your progress .

Sincerely Richard

RE: Philco model 77 am radio. - MrFixr55 - 09-24-2025

Hi DaveC,

First off, welcome to the Philco Phorum, a Phamily Phriendly phorum, pfull of phine pholk and phun Philco Phacts. (See a pattern here?)

Please review the Phorum Rules when you get a chance, and most importantly, the "What not to post in the Philco Phorum" message in each Phorum topic. Take a tour of all of the various categories. Note that we are happy to help on Non-Philco radios; there is a category for that, as well as TVs, HiFi Audio, test equipment and a "Trading Post for items for sale, wanted to buy, etc.

Please write a little about yourself in the "Introduce Yourself" section and don't forget to stop into "Old Ron's Bar and Grill" for a virtual burger and brew, and to shoot the breeze about the various and sundry topics listed in Old Ron's Bar and Grill.

And now about your radio.

Did it work before you recapped it?

Attached below are 2 links, one to the schematic in our Philco Library. (You will find a link to the Library on the Phorum's Home page to the right under the Philco Phorum title bar.) The other is a schematic for the Model 77 from Nostalgia Air. Ordinarily, I direct people to our library for schematics, but resistance and socket voltqage values are not listed on the schematic from our Library.

https://philcoradio.com/library/download/service%20info/wiring%20diagrams/vol1/split/Part46-Philco%20Wiring%20Diagrams%20Vol.%201.pdf

https://www.nostalgiaair.org/pdfs/Philco-Radio-and-Television-Corp/Philco-Radio-and-Television-Corp-77.pdf

This is a rather simple TRF radio, with some sort of neutralizing The 2 major concerns of this radio are the voltage divider and the primaries of the interstage coils.

First check the "voltage divider", consisting of the speaker field coil, resistors #30 and #26 and the volume control, item #1. An open anywhere in this "chain" will affect either B+ or cathode bias. The volume control has 2 sections. One section shunts the antenna coil. The higher the resistance, the more sensitive the radio is. The other section varies the cathode bias on the 1st and 2nd RF amps. Any break in the circuit, be it the speaker field, either of the 2 multitapped wirewound resistors or in the cathode bias section of the volume control will drive the radio totally haywire.

This radio is rather similar to the Philco 20; the main difference being the output tubes.

The Detector is of the "Biased Detector" variety. Putting a long wire antenna on the gird cap should allow the detector to pick up strong local signals. Unfortunately, it will also pick up hash from any LED, CFL, Digital TV, Cell Phone, computer, iPad, Router, WiFi thermostat, etc. plus all the hash on the power lines. Try doing the test in a remote location of your house or even outside, using an extension cord.

Keep us posted, ask questions. We want you to be successful!

RE: Philco model 77 am radio. - Dave c - 09-25-2025

Thanks so much for your thoughts. I will
Defiantly check out your ideas and let you know.
Before and after recap resulted in the same symptoms
I did resolve the issue of the power
Supply 60hz hum.
I checked each original cap upon removal
And most tested bad. I used 630 volt metal film caps as
Relacements capacitors except the power
Supply caps which I used 450 volt electrolytes.
What are your thoughts on the mica capacitors.
Should they be replaced. My cap checker cannot check
Such a low capacitance.
I will keep you informed on my progress.

RE: Philco model 77 am radio. - GarySP - 09-25-2025

Mica caps are the exception to age, somewhat. They usually remain stable in application. That being said, they can go bad. No harm in testing them. Take care, - Gary

RE: Philco model 77 am radio. - MrFixr55 - 09-28-2025

Hi Dave,

You used the right stuff here. In replacing the caps, it is important to not just connect the caps to the terminals on the Bakelite cap assemblies. (There are only a few of these in the set,#19, the input coupling cap to the AF amp is one.) There are 3 canned multiple caps, indicated as #s 7, 13, 14 on the Diagram from the Philco Library All of these caps have a common ground. If the ground is not made, all sorts of nasty noise will be heard, including squealing.

RE: Philco model 77 am radio. - Dave c - 09-28-2025

Hi
hhtjank-you fot your thoughts. I definatly appreciate it. This is a picture of how my grounding has been done. Crimped and soldered the ground lug. What do you think.?        

RE: Philco model 77 am radio. - morzh - 09-28-2025

Grounding is fine, if the circuit is correct.

RE: Philco model 77 am radio. - MrFixr55 - 09-29-2025

Hi Dave,

Yeah, I would definitely call that grounded.

Reviewing your pictures. You will see 2 Bakelite capacitor "blocks". The one only partially in view is Cap # 19 which couples the detector plate to the 1st AF grid and should be 0.01 uF. The one more totally in view is Cap # 11 which is a B+ bypass cap for the 1st audio, detector and RF amps and should be 0.05 uF. It is important that the caps in these blocks be changed. There are 2 ways of doing so. One way is to pass a drill through the rivets for the terminals for these blocks to sever the connections to the capacitors within before "bridging" the connections to the block with new caps. The other way is to remove the contents of the block and replace these contents with new caps. Below is a link on how to remove the innards of these blocks without desoldering the leads.
https://philcoradio.com/library/index.php/shop-talk/techniques/bakelite-capacitor-rebuild-without-desoldering/

Another thing to check are the values of the resistors, which so often change values by much more than the 10-20% acceptable tolerance limits. Resistors #12 and #18 are 100KOhms. Resistors #17 and #20 are 500KOhms. Resistor #20, shown in the picture as the grey resistor with the red, yellow and white bands, is the 1t AF amp grid leak resistor. It is a good idea to change Resistor #20. Changing Caps # 19 and #12 are mandatory.

Resistor #17, combined with Resistor #18 provide B+ to the detector. 100KOhm resistor #12, shown in the picture as the white and orange "dog bone" resistor provides cathode bias to the detector. If any of these resistors or the 250KOhm screen grid resistor, #15 are sufficiently out of tolerance, the detector will not work.

The Nostalgia Air schematic for this set lists various voltages. the voltages for the detector will be much different when measured with a modern DVM, FET VOM or VTVM.

In a previous post, I explained the volume control. It does 2 things. It shunts the antenna, limiting the signal input when lowered, and increases cathode bias as lowered by increasing the cathode voltage. Remember that this set does not have AVC. Turning the volume control full clockwise may cause saturation of the RF amps. the section that adjusts the cathode bias is part of the "voltage divider" resistance "chain".

Hope this helps.

RE: Philco model 77 am radio. - Dave c - 09-29-2025

I was working on this radio this weekend and I did measure these resistors and they are way out of spec. What would be the wattage for these resistos? I was going to replace these resistors with wire wound. The two caps C19 and C20 are already replaced. I opened the ground on the one and then bypassed. The other, I used the one lug to connect all.
If you could give me a suggestion on the wattage of the resistors that would be much appreciated.
Thanks again for everybody's suggestions. It would be nice to see this unit up and running. I enjoy working on these radios.
Dave C.

RE: Philco model 77 am radio. - Dave c - 10-06-2025

Well I replaced the resistors, which measures really high when I took them out. Could not get the exact but was close. 250k were replaced with 240k. They actually read 243k. 500k were replaced with 510k and the 100k were replaced with 100k. All are 3 watt. I decided to remove the condensor box and remove all condensors then replace all caps inside. I removed the original caps I added to replace the ones that failed  in the condensor box. A lot of the wiring to the box was cracking so I replaced with high temperature wire. 
I attached pictures of the work. We power it up tommorrow to see if it functios.                

RE: Philco model 77 am radio. - David - 10-06-2025

Back in the day resistors range +/- 10% some companies +/- 20% . Welcome to the Phorum. David

RE: Philco model 77 am radio. - MrFixr55 - 10-07-2025

Dave C, you did some rather neat soldering and parts replacement there. Great job, hope it works!