The Fine Things are Always Hand Made
I am bogged down in restoration of a BOL remote control AM Philly of 1940.
It had been partly cap shotgunned years ago and I suspect may not have worked properly subsequently. Chrome only fair, but the rivets look pretty good. I recapped extensively including inside the rear corner bass tone choke reactor 4 cap set, all 3 diode assemblies and the 7 cap .5 mfd's in the soldered box, the IF cans and on top of tuning cap. All tubes tested (Hickok 600) very good or better, many from my stock and few NIB - all G type, mostly Sylvania. Found and replaced a couple resistors and a mica cap all with broken leads - in the audio circuit 1st & 2nd audio where there also are various chokes too. Bad vol control replaced temporarily with a new 500K pot with tap @ improper 15K.
Initial power up I could tune stations but somewhat distorted audio, but only if the 6H6 sensitivity tube is pulled out. Once inserted, the signal fades way. Pretty good dial calibration. Audio hum present. Advancing the Variac much past 90 volts can send the radio into very LOUD low frequency oscillation.
Volume control (& radio-phono switch) not operative with regard to radio signal I was hearing. Grounding the grid of 2nd audio 56 tube silences the radio. Radio/phono switch does not cut off the station audio I can hear. (audio signal bleeding somewhere else?). Yet running a CD player into the phono input, volume control is operative and audio quality is good and clear. So problems evidently ahead of the audio amp sections. B+ and -C bias dividers show reasonable values, given Variac at 90 volts.
The chassis is extremely microphonic about anywhere tapped to the left of the tuning cap. A wood chopstick or finger tap sounds LOUD no matter what I tap on the left/audio circuit side of the receiver chassis: any cap, any resistor, any tube, any tube shield, the chassis apron, the inside pot for the attenuator circuit .... I have swapped the audio tubes between this BOL and my pointer Philly, but either audio tube set is very microphonic in this BOL chassis but fine in my pointer Philly. The expander 6L7-G tube sub chassis have new rubber grommet suspension. Looking for another source of mechanical microphonic condition, I only found a broken ground strap for 6J7 grid cap wire braid, which I fixed. No obvious ground connections issue found with prodding and VTVM measurements on lowest scale.
I can inject gen signal 465 KC into the 4th IF 6B8 tube grid and the tone reaches the volume control and to speaker. Motor drive assembly blocks ready access to the 4th IF adjustment screws. Sig gen to the 3 rd IF 6K7 grid seems blocked. SG to the 1st and 2 nd IF is sort of there. I think IF may be badly aligned. Am suspicious of the IF diode and 4th iF tube circuit. But perplexed by the signal loss when inserting the sensitivity 6H6 sensitivity tube which is another yet AVC tube. Meanwhile I can hear the garbled radio station as though bleeding through from somewhere.
I think my next step is to pull the 2nd audio tube (to block the loud low frequency oscillation audio) so I can run the Variac up to 117 volts in order to get a good handle on voltage readings everywhere. I know there are 3 AVC circuits, which are really automatic sensitivity gain controls for the RF and IF stages. And that the complex AVC grid bias sources to the C- divider.
So this nightmare Philly has several problems.
The extreme microphonic problem has me perplexed. Thoughts anyone?
Microphonic problems often come from bad solder joints. Double and triple check ALL the solder joints, especially those in the AF section. I would also check all connections, make sure everything was replaced correctly. Also, check all the resistors. If one is cracked, it too could cause the microphonic problem. Replace any resistors which are out of tolerance. It is always harder to sort through a radio which someone has recently "worked on." One usually ends up having to find that person's mistakes and correct them. MUCH more difficult than starting with an unmolested chassis. Best of luck to you !
yes - I have bend reflowing solder joints and ground solder joints in the Audio areas. Been looking for any sign of rivet ground connection issues, finding none yet.
Also cleaned up some of my initial recapping work to shorten leads. Only some reduction of hum.
Perplexing issue in the Scratch Suppressor circuit- specifically the 6J7 tube. Screen grid reads the proper 100 volt. The voltage testing documentation says the 6J7 plate should be 75 volts. I read 230 volts, same as before I replaced the 500K resistor off the 250 volt B+line. The tube is super sensitive to anything near the grid cap. So, I am reduced to putting the tube shield cover in place to moderate that sensetivity.
Why would the plate resistor fail to reduce the plate voltage in this application ??
David, the full B voltage is dropped across the plate resistor, the tube itself, and the bias resistor on the cathode. Check the bias resistor and the condenser across it. Also, if the tube is near full conductance it will appear like a low resistance there, which will cause a greater drop on the plate resistor. Check the bias of the tube also.
+1 on Michael. If the tube is near cutoff, the plate voltage will be high.
Yes, as Robert said. Think of this as a simple series circuit. between ground and B+ you have the cathode (bias) resistor, the tube itself, and the plate resistor. When the tube conducts the most, near saturation, it will appear as a LOW resistance, both the plate resistor and to a lesser extent the bias resistor will drop more voltage, since they become proportionally greater resistances in the circuit. Since the plate resistor is dropping more of the voltage, the voltage measured at the plate will be considerably lower than the full plate voltage. When the tube conducts the least, at cutoff, it will appear as a very high resistance. The current flowing through the plate resistor will be minimal, so the voltage dropped across the plate resistor will also be minimal, and so one will see virtually the full plate voltage appearing at the plate itself. It is simply a matter of Ohm's Law, I=E/R; or, in this case the transmutation E=IxR
Appreciate the comments and info.
The 6J7 cathode is grounded per diagram and per actual wiring in my receiver, so no cathode resistor.
Both of my Sylvania 6J7-G tubes I tried test good on my Hickok 600. (My restored pointer Philly works fine.)
The plate voltages off the 250 volt B+ line through plate resistors for other tubes are OK .
So that just leaves the tube and new plate resistor I put in.
The 034 pf mica cap off pin #3 plate socket lug (connected or not) seems not the problem.
The grid wire's braided metal shield is securely grounded by the tube socket and also across the socket to the cathode ground .
I have tried both Sylvania 6J7 -G tubes I have - no difference - so I think that a tube pin solder issue is unlikely.
Scratching my head ??
The bias for the sJ7 is supplied from the -27 supply when the Scratch is off. I measured the grid voltage on my BOL (with DMM) and it is at about -22 volts with the Scratch off. Without looking at tube specs, I would assume this is turning the tube nearly off, raising the plate voltage (I don't want to take it out of the cabinet to measure as it is a lot of work!) With the Scratch on, the negative grid voltage is about -12 volts, probably putting the tube in the middle of its operating range. The plate voltage should drop considerably.
What happens to your 6J7 grid voltage when you turn the Scratch on? Realize that even a DMM will load the grid slightly.
I use an RCA VTVM I built from a kit in high school about 1960.
I also checked that plate voltage socket pin # 3 is active with voltage prod into the socket from above.
The 6J7-G negative grid voltages varies from -20v (off) to -16v (on). Running off the two taps of the -C divider.
There is no 6J7 plate voltage change switching scratch suppressor between on and off.
Touching (and releasing) the 6J7 grid cap with VTVM's DC volt prod produces a tremendous audio pop, as does touching it to the pin 3 of the socket.
Scott - Had not considered the 75 volt plate voltage spec for the 6J7 might be a typo.
One difference between my Pointer and BOL receivers. Touching with my finger the 6J7 grid cap is not a loud audio event for the pointer Philly. But my subject BOL, touching the 6J7 grid is a huge noise event.
I know there were a series of changes made mid 1939 to the Philly receiver circuits. I believe my 8 control BOL is later by virtue of the 1940 Strad red band dial with upper logging scale. However, my chassis was punched for the early scratch suppressor switch below the tuning knob. Kent says my set # RR-112 suggests a much earlier build and speculates mine could have been returned for factory updating, perhaps even to add remote control, the dial and other updates. The volume control had been replaced before I acquired this BOL set, with a 500K control with appropriate 50K tap. My set has a vitreous ceramic B+ voltage divider, a suspected replacement of a candohm.
There were some lengthy Philly restoration threads on the Scott site late 2014 and early 2015. One entitled Philharmonic Record Scratch 3/24/2015 discusses this circuit and issues involving the 6B8 tube. Norman's problem was overacting of the circuit when engaged. I do not note a change engaging the circuit by listening. I have now installed an older 6B8-G hot branded Sylvania - no difference noted. Next I will try a temp shunting a 560K resistor from 6B8 screen to ground as mentioned in this thread.
Audio hum still a problem. Used the screw driver tip to ground on a number of riveted ground lugs and tightened those held by nuts to no avail.
The grid on my BOL 6J7 is very sensitive. A finger on it produces very loud hum/buzz. Also when I measure the voltage.
If I get a chance, I'll pull my chassis and measure the plate voltage. It's kind of pain to get in and out and have it operating when out!
To be honest, I'm not even sure my scratch suppressor even works the way it is supposed to!
So maybe the disturbing nature of my Scratch Suppressor and the 6J7 is appropriate for a later BOL radio?
Robert - Be interested in your voltage of the 6J7 plate voltage with a meter prod into the tube socket from above. Save you extracting your Philly from the cabinet.