EH Scott Radio Enthusiasts

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?

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If it is not a microphonic tube, a loose connection can definitely cause the microphonics.  Check the grid circuits in particular. 

Thanks. Never before encountered such a microphonic condition like this.

Yes - been looking for loose connection. With everything seeming microphonic to touch really tough to pin down. I already spent time looking for a bad ground.

Discovered that the one original 100 Pf mica cap lead broke at the case, and the other end lead broke too just as I was starting to replace it. Like the wire had crystalized where it entered the plastic case. An identical one is buried across the 6J5 tube socket lugs lugs adjacent. I wonder ...

Guess will start somewhere with solder gun and reheat solder connections.

I'll second the loose connection theory.  Bad solder joint or poor switch contact included.  Being a remote control set inspection and diagnoses may be very difficult.

Norman 

Definitely look at mica caps...I had two in a Northern Electric "Vimy" hi-fidelity set I recently rebuilt, that were causing microphonic problems. I finally found them while using cold spray on components in the affected area.

Thanks all.

Sort of like a bad connection could work like a condenser mic and appear as the microphonics I hear.

I do have a spray can of "component coolant " to try if reheating all solder joints fails me. 

And yes, remote control mechanics does block access, especially around the front most tube sockets.

Receiver is now playing.

Substituted my other Philly amp which allowed me to advance Variac to 117 volts so I could take voltage readings. Amp substitution eliminated the sudden loud low frequency oscillation. I had to build up the the BOL amp from a rough parts chassis with Scott Philharmonic amp parts. So I have an amp issue to resolve after I have the receiver operating. My suspicion involves the interstage transformer - in particular the wiring of the secondary. The interstage transformer has two separate secondaries, one for each pair of 6L6's in the quad of output tubes. Maybe I crossed the two sides at the ear phone jack.

More later when I get back to the BOL amp.

I dismounted the volume pot and motor assembly obstructing access to the front left tube sockets, and substituted another 500K volume pot.

Discovered an old wiring error involving the old replacement 500K volume pot in this radio.  A dark wire was soldered in error to the high side (input) lug of the volume pot.  Supposed to be using the ground end lug of the volume pot from the nearby sensitivity control. This dark wire traces to the ground end of R33 cap which appears on the circuit diagram near the 6L7 converter tube from the sensitivity control. The old replacement 500K volume pot has the proper 50K tap and two working switches and fortunately seems ok on my VTVM. Second switch is for the 24 volt circuit powering the two electric motors. Incidentally, for those having a remote control Scott, Kent loaded his copy of remote/robot control operating instructions in the Scott Sixteen set folder.

Found a broken connection at the 6J7 tube socket lug end of the grid cap wire for the 6J7 scratch suppressor tube - thus resolved the extreme sensitivity of my hand anywhere near that tube. (This break was in addition to broken ground connection on the braid cover for the grid cap wire already fixed earlier). Appears that the grid cap wire must have been really pulled hard sometime in the past. I also replaced the other mica cap across the 1st audio socket, but it tested ok after removal. At this point I discovered the vague, distorted station reception improved somewhat if I had a finger on the 3rd IF tube and moved my arm close to the 4th IF grid cap. Curious. Kent dropped by and helped me find a wiring mistake at 1st audio socket, restoring proper volume control action. The extreme microphonic condition for the receiver has lessened somewhat, but persists  anywhere on the audio side of the chassis.

Proceeded to try IF alignment for 465 KC per documentation. Started at 4th IF - had to sweep the signal  generator to find the secondary (surprise) was set near 450KC. I don't think I disturbed any air caps replacing all the caps to the IF coils. Continued  worked back towards one stage at a time touching up the others. which we had been much closer to 465 KC. Re-inserted the osc tube and had a playing radio! The radio now functions with the 6H6 sensitivity tube inserted. But the Selectivity control was acting funny, like almost backwards. Repeated the IF alignment, this time following all the control positions per instructions, in particular that the selectivity control is set to minimum (narrow). Much better results, the selectivity control functions properly, but seem to have a 3rd IF secondary not wanting to peak.

Progress.  

"Learning happens when things don't go as expected." Applies to me with this radio.

Receiver lacking sensitivity with sensitivity control on full.  And eye tube barely registering the local station tuned in. Nowhere near the performance of my restored pointer Philharmonic. 

Repeated the alignment of IF. The trimmers for the 3rd secondary and 4 th IF not peaking. Intermittent and pressure sensitive trimmer for the primary side in 4th IF can. Appears the primary side trimmer in the 4 th IF can had broken.

Opened the 4th IF can to find the variable part of the trimmer dislodged.  The press fit "nut" around the adjustment screw slot shaft had let loose. The "nut" is under pressure from a brass spring on the top side of the trimmer's ceramic base

Comparison:  a pointer Philly IF trimmer and AW-23 IF trimmer appear the came. But my BOL Philharmonic trimmer is a bit different. A little bigger and more plates. Found a match in a junker FM Philly chassis.

Attachments:

The replacement  4th IF primary trimmer brought the receiver back to life, way better that before.

Attempting to calibrate the BC band, the BC trimmer broke - same problem, the press fit adjustment nut broke free shorting the trimmer, silencing the receiver, except if trimmer was fully open.  A replacement from junk chassis allowed calibrating the BC dial. Finished Osc, RF and Converter stages alignment. Receiver working well with plenty of gain and tuning eye action, finally. Volume Expander now working appropriately. The SW bands needed only slight touch up. Finally working and sounding like a Philharmonic should.

However, the receiver is still very microphonic on the left/audio side of the chassis. Almost anything I tap gently comes through loud - including several tubes, the IF diode, 10K attenuation pot, many resistors, ...  Any 6J5 I try in the 1st audio is very sensitive as well as the 2nd audio, expander 6L6's,  6H6 sensitivity, IF diode 6B8,  4th IF 6B8,  tube shields ...

Not finding an obvious source of the super microphonic condition.

Next, returning to the amp issue, so I can return the other amp to my pointer Philharmonic.

That is not a common problem and is one I have never encountered.  Thanks for reporting so we can look out for the problem hereafter.

Norman 

The BOL Philharmonic amp is now fixed and working nicely. In building up the amp I made a wiring mistake.

The interstage transformer has a center tapped primary off the 3 audio pair of 56 tubes.

This interstage transformer has two separate secondaries - one for each pair of 6L6 output tubes.

For each secondary, one lead goes through the complex earphone jack and on to a one pair of 6L6's and the other lead goes to one end of a long terminal strip in the middle of the quad of 6L6's. Same for the other secondary, with the one lead going to  the other end of that long terminal strip. Carefully studying the amp diagram (colored pencils tracing the circuits helped). Realized  I may have the long terminal strip connections backwards, crossing the two secondaries and causing the bad low frequency oscillation. Confirmed it by running continuity checks comparing my working pointer dial Philly amp with the misbehaving BOL amp. Fixed the amp which is now working well on the BOL receiver. 

That leaves the microphonic issue to try to address.

Then I can move on to the remote control aspects and re-mounting the volume control and motor assembly, which I am already somewhat familiar from my remote control AM/SW Phantom Deluxe.

I've been pulling the replacements from my parts sets. It is odd and two in one chassis is really something. I am beginning to wonder if this is a failure we will see more of as these sets continue to age. I've seen the two failed units, just fatigue of the pressed retainer - I hope it was just a bad batch and not a harbinger of the future!

Norman S Braithwaite said:

That is not a common problem and is one I have never encountered.  Thanks for reporting so we can look out for the problem hereafter.

Norman 

In my most recent comments, I should have mentioned 6J5 audio tubes, not  56 tubes (of earlier Scott models).

Kent and his junker Philly chassis have been invaluable for the replacement trimmers I needed.

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