Post by trombone on Jul 25, 2023 12:10:16 GMT 12
An airtight cake tin is a domestic asset,keeping cake fresh and biscuits crisp.Depending on the tin's construction getting the lid on may require some pressing inwards on the sides to allow the lid to fit.This pressing may , as the sides bend in ,produce a bow outwards in the bottom of the tin ,perhaps only a few thousandths of an inch ,but a bow nonetheless.
The particular "cake tin" upside down in this story , is the chassis of a flip dial Pacemaker battery electric model 5155AB. My set is the red and white one with the tone control ,the initial glamour of which may startle sufficiently to forgive the disappointment of the small chip missing from the cabinet.Lucky is the man who has a pristine model.
The set had been primped and tweaked and was going well,notwithstanding having still the original selenium rectifier for the moment, still delivering 80+H.T. The set was ready to go back in its case. I refitted the "cake tin lid" onto the bottom of the chassis,squeezing the sides in slightly to slip the lip over and did up the two p.k. screws and switched it on - to make sure it went prior to final installation in the cabinet.I turned it on. It did not go. Screws out, "lid" off - it goes. Hmm. A short or open somewhere.
Some time and some words and much poking and prying later I had a g- cramp on the upside down chassis and could reproduce the "short " or "open" with a quarter turn of the g-cramp handle as the chassis was squeezed inwards (no go ) or released outwards (go).
Eventually attention focused on the 1T4 r.f. amplifier, now with its grid isolated from a.v.c. and bias and disconnected from frame aerial, aerial tuning coil and aerial tuning condenser. Only the 100 pF cap remained on grid one and supplied with a modulated r.f. signal of about 700k.c. The valve was essentially isolated from the rest of the set except , of course for its filament - part of a series string. High tension of 83 volts was supplied to the anode of the 1T4 via the primary of the r.f.coil and grid 2 via the usual resistor.
The signal tracer was hooked to the top of the secondary of the r.f. coil. Release the g-cramp handle 1/4 turn -signal heard in tracer. Increase 1/4 turn - silence.There was no visible movement anywhere in the chassis.
All the usual suspects were tried - new valve (why?!!),de oxit in the socket,de oxit on the valve pins,reheat socket tags against a dry joint moving.The screw holding the r.f.coil to the chassis was undone allowing the coil to float on its connecting wires so that its rigidity was not a suspect.There was no change in voltages as the clamp was applied or released,no change in plate current of 1.5mA with application or release. Filament voltage remained the same on or off.
Twist one way -signal heard ,twist other - nothing .
Sometimes we can disappear down a rabbit hole and nothing avails. A chat with a colleague helps in this situation . Alan suggested ,with voltages and currents remaining the same ,that an " open " on the signal path might be the problem. So back to the shed.
Then the eureka moment.This was a radio frequency signal passing through the 1T4. What was tuning it when it could be heard,since all the aerial tuning circuitry was disconnected? A twiddle of the sig gen dial confirmed that the signal did indeed get louder at one point.Rotating the tuning condenser of the set confirmed this conclusion.The 1T4 was supposed to be now acting as an untuned r.f. amp . But whence this tuning?
Maybe the secondary of the r.f.coil and its gang of the tuning condenser,circuitry now essentially passive and disconnected from the 1R5 mixer grid was now effectively tuning the plate circuit of the 1T4.Do I have my theory ducks in a row? Anyway here was a signal path not so far considered - and vulnerable to potential shorts and opens.
The tuning condenser in this set is mounted in 73 year old rubber grommets- doing quite well for their age - like this would be service man-but no longer whole or perfect (ditto).On this Polar tuning condenser the three sets of fixed plates rest on six thick white ceramic washers but two gangs have extensions soldered down through these ceramic washers through the bottom of the tuning condenser to form the connections to two mica insulated trimmer caps mounted out of sight on the bottom of the tuning condenser.(The adjusting screws are accessible through holes in the chassis). Because the grommets have got a little tired ( that damned gravity )the tuning condenser has drooped ever so slightly bringing one of the soldered gang connections (the 1R5 one !) to within a whisker of the chassis.Squeeze the chassis sides in to fit the "lid" ,the "bottom" bows out, the whisker disappears, a short ensues.
I've come across tuning condensers waltzing drunkenly around the chassis before because of perished grommets but poking , prying and jiggling of this tuning condenser had not persuaded it to reveal its secret. After all it was the chassis moving not the condenser which was causing the short.
Corrective measures in place.
Case closed.
Time for a cup of tea. Ahh! Fresh cake.How nice.
I am indebted to John Dodgshun for clarifying the nature of the earthy connection of the r.f.coil's secondary.Thanks John, I have learned something today.
The particular "cake tin" upside down in this story , is the chassis of a flip dial Pacemaker battery electric model 5155AB. My set is the red and white one with the tone control ,the initial glamour of which may startle sufficiently to forgive the disappointment of the small chip missing from the cabinet.Lucky is the man who has a pristine model.
The set had been primped and tweaked and was going well,notwithstanding having still the original selenium rectifier for the moment, still delivering 80+H.T. The set was ready to go back in its case. I refitted the "cake tin lid" onto the bottom of the chassis,squeezing the sides in slightly to slip the lip over and did up the two p.k. screws and switched it on - to make sure it went prior to final installation in the cabinet.I turned it on. It did not go. Screws out, "lid" off - it goes. Hmm. A short or open somewhere.
Some time and some words and much poking and prying later I had a g- cramp on the upside down chassis and could reproduce the "short " or "open" with a quarter turn of the g-cramp handle as the chassis was squeezed inwards (no go ) or released outwards (go).
Eventually attention focused on the 1T4 r.f. amplifier, now with its grid isolated from a.v.c. and bias and disconnected from frame aerial, aerial tuning coil and aerial tuning condenser. Only the 100 pF cap remained on grid one and supplied with a modulated r.f. signal of about 700k.c. The valve was essentially isolated from the rest of the set except , of course for its filament - part of a series string. High tension of 83 volts was supplied to the anode of the 1T4 via the primary of the r.f.coil and grid 2 via the usual resistor.
The signal tracer was hooked to the top of the secondary of the r.f. coil. Release the g-cramp handle 1/4 turn -signal heard in tracer. Increase 1/4 turn - silence.There was no visible movement anywhere in the chassis.
All the usual suspects were tried - new valve (why?!!),de oxit in the socket,de oxit on the valve pins,reheat socket tags against a dry joint moving.The screw holding the r.f.coil to the chassis was undone allowing the coil to float on its connecting wires so that its rigidity was not a suspect.There was no change in voltages as the clamp was applied or released,no change in plate current of 1.5mA with application or release. Filament voltage remained the same on or off.
Twist one way -signal heard ,twist other - nothing .
Sometimes we can disappear down a rabbit hole and nothing avails. A chat with a colleague helps in this situation . Alan suggested ,with voltages and currents remaining the same ,that an " open " on the signal path might be the problem. So back to the shed.
Then the eureka moment.This was a radio frequency signal passing through the 1T4. What was tuning it when it could be heard,since all the aerial tuning circuitry was disconnected? A twiddle of the sig gen dial confirmed that the signal did indeed get louder at one point.Rotating the tuning condenser of the set confirmed this conclusion.The 1T4 was supposed to be now acting as an untuned r.f. amp . But whence this tuning?
Maybe the secondary of the r.f.coil and its gang of the tuning condenser,circuitry now essentially passive and disconnected from the 1R5 mixer grid was now effectively tuning the plate circuit of the 1T4.Do I have my theory ducks in a row? Anyway here was a signal path not so far considered - and vulnerable to potential shorts and opens.
The tuning condenser in this set is mounted in 73 year old rubber grommets- doing quite well for their age - like this would be service man-but no longer whole or perfect (ditto).On this Polar tuning condenser the three sets of fixed plates rest on six thick white ceramic washers but two gangs have extensions soldered down through these ceramic washers through the bottom of the tuning condenser to form the connections to two mica insulated trimmer caps mounted out of sight on the bottom of the tuning condenser.(The adjusting screws are accessible through holes in the chassis). Because the grommets have got a little tired ( that damned gravity )the tuning condenser has drooped ever so slightly bringing one of the soldered gang connections (the 1R5 one !) to within a whisker of the chassis.Squeeze the chassis sides in to fit the "lid" ,the "bottom" bows out, the whisker disappears, a short ensues.
I've come across tuning condensers waltzing drunkenly around the chassis before because of perished grommets but poking , prying and jiggling of this tuning condenser had not persuaded it to reveal its secret. After all it was the chassis moving not the condenser which was causing the short.
Corrective measures in place.
Case closed.
Time for a cup of tea. Ahh! Fresh cake.How nice.
I am indebted to John Dodgshun for clarifying the nature of the earthy connection of the r.f.coil's secondary.Thanks John, I have learned something today.
