[David]

400uf wow I would think about replacing that with something closer to 40uf.  David

[jcassity]

yup david, the 400uf will be gone,,, i will do things up like the diagram shows.

not sure why there was a 400uf but after reading up on this set prior to turning it on, i see a few minerva experts saying the filter caps are oddly wired and dont match the schematic many times.

if it will work like the prints show, thats how i will install them.

[jcassity]

FULLY MISSION CAPABLE!!! FMC
READY FOR ISSUE (RFI)

working, done, in service!!!!

the one remaining cap was a direct short , its value was .25uf, i replaced with a .33uf.

it was the cap on the second 6sk7 pin 6.  if you short it on the schematic, my rectifier dc output would have a direct shot to ground with the only load being the inductor off pin 4/8 of the 25z6.


order of happenings........
i got the set free
i recap and retest on cap at a time
i kept the .25uf cap from the second 6sk6 installed as i did not have a replacement yet,, did not check it either.  it physically appeared as though it never once saw any harm or battle, looked really new and i made an assumption.

as i was on the phone and the set playing,, it self destructed out of the blue.

i ordered up caps & 2ea spare tubes.

i muddled through making a correction to the input ac power section & installed the missing 25ohm resistor on the cold side of the on/off sw.
undid & corrected the previous work done that compensated for the lack of this 25ohm resistor.
added correct size filter caps.
powered up and no sound at all even when i touched the tone & vol pot leads.

eventually my power up and power down events started to cause my rect tube to almost instantly glow blue.
found that the one remaining cap .25uf was shorted.

replaced the cap, powered up and like a tesla coil , the rectifier tube provided a quick show of force,  i swear i heard the whole radio even vibrate!  but...... it was in a linear way getting stronger and stronger like sumthin's gonna BLOW!

i unplug the cord and consider putting it up for a while...but no.

i used my last resort redneck troubleshooting tactic.  i power up one more time , my plan is to look for the smoke and let the darn thing show me where it hurts,, and it did.

the rect tube took another breath of life then went dark.

still having no idea what to do since my short to ground was gone and i wired  the set corectly, i consider another trick which was a risk to my wallet.

i installed all brand new tubes.,,, thinking that the affect the rect tube had on other things, putting new unbeaten down tubes in would help reveal the "weaker link".

power and and there she is,, playing all good and fine.

i backed out one at a time my new tubes in trade for the orginals that were in the set(excluding the rectifier).  i migrated all the way down to both 50L6's and all the originals were working great.  my new spares are all mostly metal type and even though the metals are supposed to be better, i like the look of being able to see the tube working.

all is well in my small collection of working sets.

missing in this pic of my repaired sets (notice i dont say resto cause im not really doing purist work), are 4) s38's, 1) s53, &  1) philco 46-1226 which is in the master bedroom.

heading out to add an antenna relay to the minerva and a fuse on the power cord.

all is well.

Note to anyone who see's one of these.............. if your a armed forces supporter!
-If the set has the letter "M" in the speaker grill *AND* is occupied by the schematic showing series 50L6's,
~~~civilian sets had a louver grill over the speaker.
-then the set was only found for sale in a military exchange army/navy.
**this means that the set "should" have been on the front lines as the moral radio of the minerva line.

if the radio schematic has parallel audio output push/pull's & other rectifier tubes, then it was civilian ordered.

[jcassity]

all is not well.
lesson- dont turn your back on this set,,, 

when you look into this ones eyes, you can see it working things out!! 

--tubes---
i have two spares of all tubes that all checked out fine during which time the set was playing for over 4hrs just fine.
exception is i have only one spare 25L6 rect since one out of the box seemed to have failed.
most all the tubes i got from play things of the past were metal type RCA tubes ,, yet all were installed as a group and in various stages individually while i was troubleshooting.

I set up the set in its home , go in the house leaving it play and return to nothing but crackle noise.
all caps replaced to this point.
i take it apart again and back to the bench.

symptoms:
tubes light up 
in about 10 sec speaker comes to life with crackle -static - crackle -pop.
the volume out of the speaker seems to default to a simulated 30% of max vol
rotation of band switch offers no speaker noise.
rotation of vol ctl knob offers no increase or decrease in speaker output.
rotation of the tone does affect the speaker noise i hear going from treble to a base / muffled sound.

tests i have done so far during this particular event............
Grounded the center tap of the vol control to chassis and the noise remained.
bench tested each tube to dongle off to the side jumper wiring only the filiment. see picture(s
--this was done to establish when the noise went away compared to which tube i had out of the ckt.
--after all tubes were dongle bench isolated & tested, the noise remained still!
----two of my tubes have the filliment on pins 7 & 8, the rest are 2 & 7.
--the above dongle test excludes the 25L6 and the 2) 50L6's.
replaced filter caps from 450v 47uf to 160v 47uf
-- i did the above because during my prior steps to geti it working one of my rectifier tubes finally set off a light show so i figured the filter caps might have taken a hit.
up above i mentioned i grounded the center tap vol ctl of the 6sq7 and the noise remained which i consider the "input" side.
I lifted off pin 6 of the 6sq7 the .01uf cap and the 220kohm Resistor and the noise goes away.
---with the above lifted leads, i am barely able to now tune to a station and i can hear things but very little vol is avail at max.
I used a plastic tool to jiggle connections around while the set was on doing a wiggle test to see if i could affect the noise in the speaker one way or the other.  each connection on the bottom was wiggled and no change is speaker output
i examined the band switch under a magnifying glass to verify contacts were being picked up as the BS switch was rotated, i even bent all of them slightly down and watched them make contact.
i started shot gunning resistors and replacing all of them working my way though the 50L's to the end of the set. there are 2) 2.2mg, 1) 10mg and another that i did not replace due to access difficulty and they looked good and read within acceptable specs. so... nearly all resistors are also replaced.


****When i install all 3 of my 6sq7's, the set still has noise ,  the odds of all three tubes being bad is questionable.
when you ground the vol ctrl input to the 6sq7-2 and the noise remains, then lift all leads on the output of the 6sq7-6 and the noise goes away, your supposed to say you have a bad tube,, but all three are bad?????????????

*******************What makes less sense is that when i dongle bench tested and got to the 6sq7 where i  only had filliment power to the pins 7 & 8,, my noise should have gone away...... yet the noise remained!!!!!!!!!!!!!!!!!  

to be real honest, it sounds like something bad in one of the IF's , with all the poping and cracking going on. There is one single mica cap on the bottom ,, if this type of cap is inside my IF's, then i might have silver mica disease again? like what happened to the IF on my hallicrafters s53.

I do own a signal gen capable up to 150MC with bnc to gator clip leads and a frequency counter with bnc to gator clip leads that i purchased to do alignments with.

today....... i will.......
pull up the print, zoom in on the voltage and resistance tables ,, copy / paste to paint, then save as a jpg then print and jot down my readings.

[jcassity]

here is a pic of the current view of the bottom of the set showing some replacement resistors and temp connections in various spots while testing.

[jcassity]

here are the voltages i just took compared to the OEM data sheet suggested readings.

i seem to have a couple problem areas around my 6SC7???

[Radioroslyn]

11v on the 6sq7 is a problem. Measure the resistance from p6 to the chassis should see a high resistance. Perhaps 50K or better. A wrong value resistor on pin 6 or + voltage on the grid of the 6SQ7 could cause this circumstance. Voltage on the 6SC7 are good the one you noted is .95v vs 1.2 not a big deal.

[35Z5]

Pin 6(plate) of 6SQ7 should measure at least 220K ohms which is it's plate load resistor. Through the power supply is its only  ground reference. If it measures lower there is a leaky cap or maybe tube socket.

The likely scenario is resistance will measure high, meaning the 220K has increased in value. If resistor value was low, plate voltage should be high. If all resistance is good, likely the tube has issues. If all is good and tube doesn't fix it, quit worrying, a 6SQ7 will operate with only 11v on plate..

For tube heaters, measure voltage across heater pins, if it's 6v(or 25v on higher voltage tubes) you're golden.

[Electrothaumaturgist]

>>>replaced filter caps from 450v 47uf to 160v 47uf
Hopefully, that's a typo    If not,...

[Electrothaumaturgist]

Ah, yes. now looking closer at the blue caps, it can be seen that you did in fact put 160v replacements in. If these call for 450v, therein lies the problem... (maybe)

[35Z5]

B+ is derived from the AC line, 160v caps are fine.

Considering the voltage drop within rectifier, B+ will likely never be greater than 135V(in fact is listed as 117.4 & 119v on chart).

NOW, since the voltage on pins on 4 & 8 is different, is there a resistor between these two points? Cathodes should be tied together.

[Electrothaumaturgist]

Thanks for that. Couldn't really see the schematics too good from here. Zero chance of anything being that obvious, of course. Never is.

[jcassity]

the filter caps are set up temporary right now.
when i got the set, it had a single filter cap and a modified or repaired power supply (power resistors) end that i had to undo and redo to match the prints.

the set came with a single filter cap that was too big.

i downgraded from 450v 47uf to 160v 47uf dual filter caps.

the big bare sol bar is B+!!


the 6sq7 pin 6 220k ohm resistor is new and is actually 270k and twice the watts than original.. thats what i had.
the original resistor was reading 330k.

i guess i could hunt down a tad smaller than 220k? as long as i am 1w or bigger...? if the 270k is too big??

all other resistors are new excluding the parallel 2.2mg below the printt 6sq7 and the one 10mg pin 4 6sq7 and one 4700 on the 6sc7.



when i power up from a bone cold chassis, the set seems to play well enough until within a full minute the signal is disrupted by speaker crackle, pop, crackle pop.

anytime i power up afterwards there is speaker noise.

the pic above showes a "GREEN" resistor which is the 40ohm 5w power resistor on the AC hot line cord.,,,, this gets very hot, a bolt passes through the center and fixes the resistor against the metal chassis to wick heat,,, prior work shows someone used plastic insulators which tells me the heat cant easily get off the resistor. I temp'd in a 50ohm 50w made for free air and noise remains.

there are two resistors to the left which i had to reconfigure. the blue is 40ohm and the longer brown is 50ohm. i soldered them in parallel to make what reads now as 23ohms which becomes the single resistor for the pilot lamp, the output side of these two resistors feeds the filament string.

i am hopeful the socket is bad,,, measuring this here in a few.


as of right now though i have 3) 6sq7's which by the book says i have 3 bad tubes.
when i install all three,, the noise remains.
when i dongle / jumper wire bench test just the filament of the 6sq7 the problem **STILL** remains,,,, and hopefully what this really says is that i have a bad tube socket.

[jcassity]

the filament voltage on pin 7 & 8 of the 6sq7 reading was about 7vac when set was warm and failing.
so........... that reading is good.

sorry tom,, i am trying to learn / understand jargon, - heater = filament =(insert another like term for the same thing here)

you know,, like
distributor points = hall effect sensor = pick up module ~all perform the same function yet time has changed the design.

i am trying to associate tubes with something i already know and what i can tell is a tube is just like a transistor yet it can have several Bases or "Grids" ,, which unto themselves have all the xxx"ODE" nomenclatures.

[35Z5]

Scott, how many ohms per volt is that meter?? If it's like 20K ohms per volt, it's no doubt loading down the reading. If you were using one of those cheap Harbor Freight Chinese(you love so much) meters that was a megohm per volt, voltage would be higher. The 270K is fine, later radios used 330K or even as high as 470K for plate supply. Current is so miniscule a ¼ watt would be fine.

For noise, disconnect pins 4 & 5 on 6SQ7 to see if it stops(radio should be silent). If it does problem is ahead of audio amp in the RF/IF circuitry, may have a flakey IF transformer or cap inside. These aren't the transformers that are famed for noise problems, but I had a GE that acted exactly same and was the IF transformer.

Actually you can more or less tell with the heater test setup of 6SQ7 pictured. If you have additional leads, connecting pins other than 4 & 5 would be a valid test.