[BrackenMan]

I've got a Philco 41-601 on the bench. It has plenty of hum, and the audio amp is working with signal into the turntable input. The tuner side, not so much. The alignment of the IF was off, down around 421kHz, and I have adjusted it back to 455kHz, but my usual loop-to-loop transmission from my signal generator produces no sound at all anywhere on the dial.

The local oscillator is producing the expected range of frequencies. 

All the capacitors have been replaced, but not by me. 

I'm having some difficulty understanding where the AVC voltage is and what it should be. This is both similar and different when compared to the AA5 circuits with which I am slightly more familiar. 

Your help will be greatly appreciated.

[morzh]

The AVC is the 7C6 diode's plate, the lower one on the sch.

[MrFixr55]

Yes, this is kind of interesting but IMHO, here is what happens:
Usually, AVC is developed from either tthe detector anode or AVC Anode.  this will be a negative voltage.  It is usually impressed on the secondary of the ist IF transformer and on the "grid" of the convertor, either through the antenna coupling transformer or the loop antenna.  In this set (The 42PT94 Transitore and other sets are similar), the AVC is impressed on the RF section of the convertor like most other AA5s, but instead of impressing on the secondary of the 1st IF transformer, Philco impressed the AVC Voltage on the Suppressor Grid of the 7B7 IF Tube.  Impressing a negative AVC Voltage on the IF Suppressor will have the same effect as impressing it on the other lead of the IF input transformer secondary as RCA and most other manufacturers did.  Putting a 3V battery across the 0.1 uF Cap Item # 5 on the schematic, with the+ terminal of the battery to the B- side of the Cap and the - side to the antenna coil side of the Cap will force max gain.

Where your real problem may lie is with the radio-phono switch, which is quite unique.  If you look at # 36 on the schematic, it is an SPDT Switch that switches common B- / neutral (we hope) between the phono motor (for Phono) and the cathodes of the 7A8 Convertor and 7B7 IF.  In other words, with the pickup raised, the phono motor is turned on and the cathodes of the Convertor and IF tubes are disconnected from B-, therefore rendering these staves of the radio inoperative.  It seems that the volume control may be incorrectly drawn.  The instructions in the 1941 yearbook describe a "New type volume control".  I assume that there are parallel pots.  If you recapped the radio and still have hum, disconnect the Audio output of the tonearm from the radio.  The crystal pickup is likely open.  

I hope this helps. (I hope I'm right).

[BrackenMan]

Thank you both. I'll have it back on the bench today or tomorrow for another look.

[BrackenMan]

I got it back on the bench. I haven't really gotten to look at the AVC, but I did get much of the hum to go away by disconnecting the pickup cable. I traced the connections to the volume pot and it's drawn correctly. It's a center tapped pot with the tap tied to B-.

I know about the B- switch, and I have confirmed that it's connecting correctly when it's closed.

Back with it tomorrow.

[Radioroslyn]

https://philcoradio.com/library/download...20Book.pdf

Hi Gents

Philco used on some of their radio/phono a 4 terminal volume control (14). Two controls using a common carbon track. Turn it all the way one way it controls the radio volume the other way the phono. On the schematic it look's like a 1/2 meg pot and a separate 1/2 resistor. But it isn't. It has connections at the top, wiper, tap, and bottom. Just a quick perusal you may not catch it. I didn't the circuit didn't make any sense. Mark Opatt may have a replacement that he put together for you if it is needed.

Happy Troubleshooting!

[MrFixr55]

GE did a similar thing on several AA5 radios including Models 547 and 574. The wiper of the pot was tied to the input of the AF amp, but there was a center tap to the pot that was connected to B-. One end leg of the pot went to the radio, the other to the phono. This was a single pot as opposed to the dual pot used by Philco. Center was min volume for both radio and phone. Thirn CCW for Phone, turn CW for radio. A lot simpler than the Philco method.

[Radioroslyn]

John,

What I was describing is the same thing you did. Just I did it a bit clumsier.  It's just 1 pot w/4 with connections.

Sorry for the confusion.

[MrFixr55]

Hey Terry,

Being clumsy and using too many words is my special job. Since you write concisely and with great intelligence, brevity and clarity is your job (Insert all in good humor emoji here) Since I forgot to click the detailed reply, I will make another entry with the GE 574 clock radio schematic which uses the same scheme. It can be compared to the Philco schematic. A picture = 1,000 words, 1,000 of Terry's words and about 15,000 of mine.

I noticed that the Phorum somehow promoted me to expert. If that was a moderator's decision, I am humbled and honored. They made a big mistake in doing so, but I thank them. If it was an Algorithm's decision, then I still thank the Phorum

[MrFixr55]

Hi Pholks,

This is a copy of the GE 574 Radio - Phono Volume Control.  
   


It is a single pot with a grounded (to B-) center tap.  Resistance between the 2 ends of the resistor body is 1MOhm.  It is likely that the resistance between either leg and the grounded center tap is 500KOhm.  I assume that the unit acts as a back to back audio taper.  The midpoint of control rotation puts the wiper over the grounded center tap and is minimum volume for both.  Unsure why Philco did not draw the control in the same manner.

[MrFixr55]

Hi BrackenMan,
When you aligned the IF, where did you inject the signal? Was it modulated? If you inject 455KHz into the antenna input, does it pass through?

Measure across Cap #5 with a DVM. There should be a DC Voltage if between 0 and about -5V. It should get quite negative if you put a modulated 455KHz signal through the IF stage.

This is an interesting issue, but this is a rather simple radio. One other issue may be that the 7A8 is oscillating but not converting. Apparently, Loctal tubes and sockets are known for poor connection. I don't know why this is an issue with Loctal tubes but not with 7 or 9 pin miniatures. Diameter of pins? The Shop Talk section of the Philco Library discusses Loctal tube issues. It is reproduced below with thanks to the anonymous author:

***********************************************************************************************************************************
From the Philco Library-> Shop Talk-> Techniques

Having replaced all of the paper and electrolytic capacitors in a Philco 41-605, power was applied for the first time. The radio played well with good volume – only it had a very loud hum!

The next day, the radio suddenly began to make loud static noise, like the sound of a close thunderstorm. Remembering an experience with the Type 75 tube (which can be notorious for producing loud static even when it tests good), the decision was made to go after the 7C6 second detector/first audio tube. Wiggling this tube not only made the static stop, but also ended the hum.

Thoroughly cleaning the tube socket with contact cleaner, and replacing the 7C6, cured the trouble.
Bad loctal tube socket contacts can produce a multitude of symptoms, including intermittent or no reception.

Loctal tube socket contacts should always be cleaned; in this case, they had not been. Many repairmen use a very tiny file, found in welding supply shops, to carefully clean the contacts. Another approach is to carefully spray each contact in the tube socket, accompanied by the use of a tiny brush intended to clean between your teeth, in each socket contact hole. Spray contact cleaner into all eight holes of the socket, one at a time, then insert and remove the brush several times to work the contact cleaner in before it evaporates. Do this to every loctal socket in the set.

Important: Wait a while before you apply power again, however, to allow the cleaner residue to completely evaporate. This is especially important if you use DeoxIT instead of regular contact cleaner.

While you have the tubes out of their sockets, carefully inspect the pins. If they look blackened, then carefully scrape this corrosion off each tube pin, or use sandpaper sparingly on each pin, until they are all shiny. The corrosion can also interfere with making proper contacts between tube pins and socket contacts.

[BrackenMan]

Thank you one and all for your help and for following along. I do have more information, but I'm too tired to be coherent with it, so I'll wait to post until I can get some sleep and get the camera images available.

[BrackenMan]

Wrote a reply last night and it disappeared somewhere. 

So let's try this and see if it works: 

Input signal is 1000kHz

Signal shown in photo is what I see at pin 5 of the detector.

[BrackenMan]

Okay, not sure where the reply went last night, we'll do it again. 

As shown in the last reply, I'm testing with a modulated 100kHz signal. I'm seeing the IF, the original signal, and the local oscillator. What I'm not hearing is any output from the detector. 

The test signal being sent to the loop is shown in cyan. The output from the mixer (pin 2) is shown in yellow. The input to the detector (pin 5) is shown in magenta. I'm not sure what the periodic bursts from the mixer are - their frequency is about 300Hz. 

Looked for AVC: DC at pin 5 of the detector is -0.47V, at pin 6 is -0.82V. Haven't tried the 3V battery thing yet.