[Ralph]

Hello, 
I have recapped the radio. 
1st time powering it up the speaker does thump-thump-thump when I place my finger on the grid cap of 6K7G the thump goes away and the radio is silent.

The other problem is there is no power going to the pilot lamp which is connected to the 6A8G. I have checked all the resistances for the transformer and they are all within spec. 

Not sure what to look for on these 2 problems.

Ralph

[morzh]

Your pilot is connected to all the tubes' (sans 5Y4G) filaments in parallel.
If all tubes glowing, then there is a problem with the wire to the lamp (the other contact is connected to chassis).

Start with your voltages. Plates, filaments...

[Chas]

Thump could be a wiring error, splash of solder, wire snip or an open coil in association with the 6K7G.

[Radioroslyn]

https://philcoradio.com/library/index.ph...lot-lamps/

[Ralph]

I did some voltage readings this morning and they are all over the place. I don't know what to look at. I did not rewire anything. Here are the voltages I got measuring pin to chassis. Any help would be great.

6F6G plate was 260v should be 175
6A8G anode is 121v should be 220v Pin 4 grid 3&5 is 14v should be 90v Pin 6 is 110v should be 175v
6K7G plate is 280v should be 270v thump stops when measuring pin 4 grid 2 is 14v should be 90v
6Q7G plate is 120v should be 175v pin 4 diode 13.5v thump get louder pin 5 thump gets louder but no reading


Ralph

[RodB]

Hi Ralph,

I'm looking at the Riders schematic and when the voltages seem out of whack as you describe, I look at the power supply wiring. First thing to check is the negative side of filter cap 56. It should not be connected to ground. If it is, then the negative bias voltages set up by candohm resistor 54 don't exist and unbiased tubes will produce funny voltages. So the point where 54, 56 and pwr transformer CT connect should be measuring around -12 to -20 volts to gnd.

BTW 6K6 should have 265v on the plate, not 175v, so the reading you have is normal.

[Ralph]

Hi Rob,

The negative of filter cap 56 is wired correctly. I am getting a reading of -15 volts at the center tap, 54 & 56.
I have also checked all of the ohm resistances of the power transformer are all within the specs of the schematic. Is there something else? Could a bad ground someplace be causing this ( I have looked for a bad ground but see one)?

Ralph

[RodB]

If resistor 57 is too high then the plate voltages for the 3 tubes you list as low would have low voltages. The two audio tubes would have normal plate voltages like you show. Resistor 57 should be 9k ohms, no higher than 10k by my standards.

BTW 6K6 should have 265v on the plate, so your reading is normal.

[MrFixr55]

I am a big believer in "divide and conquer. Inject a signal at the center of the volume control (the terminal with cap # 41 connected to it). Strong audio will show that the amplifier is good. Agree with the others who state to check the power supply wiring.

Did the radio work before recapping? If yes, recheck the wiring and the caps used.

Check the caps that were replaced and the wiring. Check the resistors. There are several B+ "bypass" caps for screen grid and 1st AF plate voltages. If they short, or leak, they will affect screen voltages. Check voltages at resistors 57, 23 and 22. these act as a voltage divider that sets voltages for plates and screens for the converter (6A8), IF (6K7) and 1st AF (6Q7). Consider just replacing these resistors.

Low screen voltages or tubes that don't draw current can cause high plate voltages.

One other issue that can cause "motorboating" (The thump - thump noise, when fast enough sounds like a motorboat) is the AVC cap, #8 on the diagram. Note that it is on the other side of a ganged wafer switch, but should be in-circuit for all positions. I don't know why they placed the cap on the other side of the switch. Ohm this section to ensure continuity.

Hope this helps.

[Ralph]

I did find the 9K resistor #57 was bad reading 37 K I replaced it with a 10K
I checked #23 it was reading 23k it should be 20k the only resistor I have to replace it with is 22k so I left it in, #22 that read 52k and spec is 51k so I left that one in too.
The pilot light is now working, I also found a ground issue so thumping is gone.
There is sound when touching all 3 grid caps but no stations.
6A8 pin 4 is 63V should be 90V and 6K8G pin 4 is 63V should be 90V Where should I look for this problem?
What is the correct antenna set up for a indoor antenna to listen to Broadcast?
Thanks for all the help so far. Ralph

[MrFixr55]

Hi Ralph,

A 22K resistor instead of the 20K resistor should be no issue, as the error is only 10% I am concerned that the resistance increases as the resistor heats up. The 9000 Ohm resistor, #57 drops voltage from raw B+ to create the plate voltages for the 6A8 and 6K7. Note Cap #9, the 16uF capacitor. This is the filter cap for the main plate for the 6A8, and, through resistors # 23,22 and 47, the triode plate of the 6Q7. Was this replaced? If not, do so. The drawings in the schematic show these resistors of the "dog bone" variety, These almost always are defective, even when they read OK under no load. Are the plate voltages for the 6A8 and 6K7 (Pin 3) within 20% of the documented voltage (220V for the 6A8, 270V for the 6K7)? Is the 6Q7 triode plate voltage near spec (175V)? Is the Osc Plate (Pin 6) of the 6A8 near spec (175V)? With power off, is there a low resistance (less than 2 Ohm) between the osc grid (Pin 5) of the 6A8 and chassis ground?

The screen voltages are low. If the 6Q7 and 6A8 Plate voltages are also low but the 6K7 plate is normal, then the voltage divider formed by resistors 23 and 22 is being loaded down. Change Resistor 23 even though it reads OK. if all that you have is a 22KOhm resistor, don't worry, use it. If the screen voltages (pin 4) for the 6A8 and 6K7 are still low, then pull 1 tube at a time and measure the screen voltage. replace the tube, then pull the other tube. If the voltage goes to normal with one tube in but not the other, then either the tube is shorted, gassy or something in the circuit is loading the tube down. If the voltage is low with either or no tube in place, then check cap #22A. If you did not replace it, do so.


When touching the 6Q7 cap there should be a hum. BTW, I apologize, I forgot to state that placement of the grid lead (called lead dress) is important, especially for the 6Q7, as the grid wire should be either shielded or run as close to the tube shield as possible. I believe that the 6Q7 in this set should have a shield around it, and the 6K7 also should have a shield.

Generally, when touching the grid cap of the 6A8 (the pentagrid convertor), you should receive stations. However, if in your basement with fluorescent, or even worse, LED lighting, your computer, smart TV, etc., you will usually get nothing but noise.

It is possible that the oscillator circuit of the convertor is not working. One way to tell this is to:
-Hold another radio tuned to a midband station in your area near this radio with the radio in question turned off.
-Confirm that you receive a station on the radio being used to test the radio in question.
-Power up the radio in question, set to the broadcast band. Slowly tune from the lowest to the highest frequency on the radio in question. If the radio playing the midband station starts squealing or the station disappears, then the local oscillator of the radio in question is at least oscillating. It may be off frequency but it is oscillating.

As mentioned on my last post, there is a rather complex Band Switch for the 6A8.

If the radio being used to test cannot be tuned to a midband station in the location where you are working on the radio in question, then move to another location. There are times that I must move outside the house with some radios in order to receive anything other than my home AM transmitter.
Most pre-WWII Radios, including this one were designed to be used with an outdoor long wire antenna going to the antenna lead, and the ground lead connected to a cold water pipe or a "made ground" such as a ground rod driven into damp ground. However, if you have strong local stations, a 25ft long wire stretched out even temporarily on the floor (but not touching concrete or tile) should work.

Do you have any other vintage radios, especially pre-WWII or at least pre-1955 radios? do they play in the same environment? I believe that radios with a "ferrite loopstick" (most radios post 1955 and almost all transistor radios) are more resistant to electrically generated noise than plain coil or loop antennas or sets requiring an outdoor antenna.

One last important note:

Often, a device like a radio fails, it could have several symptoms, all caused by the same failed part. However, with an antique radio that was received in a non working condition, there could be several different part failures. The first failure that caused the first owner to stop using the radio and putting it in the attic, basement, barn or worse, and all the other parts that failed when the radio was stored in the rough environment.


Keep us posted, it sounds like you are making progress.

[RodB]

Resistor #23 should be 20k, no higher than 22k.

For an antenna I use a 6 foot piece of cat5 ethernet cable. There are 8 wires in the cable. Each colored pair, eg. green with green/white, is connected together at one end then connect the other ends to wires of different colors. When finished you have a 48 foot continuous wire antenna. I'm using one right now in my lower level work shop and it screams stations all over the band.

BTW You mentioned the ground connection fix. Check all the tube socket rivets for ground lugs. I discovered 2 lugs measuring over 500k ohms to ground on my 1936 650. The rivets are not a guaranteed ground point.

[462ron]

Hi, I have a Philco 620 which I think is similar to your 37-610 but uses a 75 as second detector/first AF and as I recall not only is it shielded, but as MrFixr suggests, lead dress to the grid cap was critical. I ran mine inside, between the shield and tube to prevent a howl my set had. As for a long wire antenna, I use the two wires out of the four that my home phone uses. That gave me an antenna around 25’ high off the ground and 65’ long to the telephone pole. Works great!

Ron

[Ralph]

Mr Fixr,

I have replaced #57 with a 10k resistor. I will replace #23 with a 22k for #22 I have a 47k instead of 51k and for#47 I have a 68K instead of 70k. Can I use those that are a little lower? If not I will order some. I will do the other checks you mention after I replace these 3 resistors.
The way the radio is now I don't get stations even with my finger on 6A8. I do have the grid wire in the shield of 6Q7. The radio is in the basement and I have in the past worked on and played pre war radios in the basement.
The band switch did have some dirty spots so I cleaned as best I could with DeoxIT D5

Ralph

[Chas]

The Riders/Philco provided schematic indicates the recorded volts were measured with a 1k/ohms/volt meter. With a DVM the readings will be higher because the DVM does not load the circuit.

Soldered and mechanically made connections to either chassis or other electrical parts with hardware are suspect as after 90 years the metals, including solder are subject to inter-granular corrosion. They may measure O.K. with a DC meter but at RF act like a semiconductor junction. Chassis riveted and solder tabs are suspect.

Your series of Philco used solenoid wound coils with forms that use a celluloid insulator between primary and secondary layers of windings. Over time the celluloid decomposes forming nitric acid, that eats past the enamel and corrodes the windings. A winding must have the resistance given measured with a analog ohmmeter. If it is higher it is corroding, if it is open that is a factor in the putt-putt noises but also lack of signal..

To be true, a DVM is NOT your friend servicing an old radio. After these preliminarily checks on coils, all of them. Remedy what is wrong as well as chassis grounding connections. Then, a stage by stage fault check can resume..

Note large grounding connections to chassis can be more easily re-soldered when the chassis is warmed to 140f with a heat gun and a soldering iron with a large copper mass. Raising up the temp of the soldering iron only burns the flux and accelerates oxidation.

GL

Chas