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Old 25th May 2018, 11:12 am   #1
'LIVEWIRE?'
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Question PM24M Puzzle

Having spent a considerable amount of time & effort on a Philips 274A over the past couple of years, I recently powered it up, initially via a lamp limiter fitted with a 40 watt bulb, later, with the bulb switched ou, and then plugged directly to the mains. Result in all cases ' no sound' Taking some measurements, I note that there is ca. -2.5v on the grid of the o/p valve (PM24M), the HT+, relative to chassis is 305v, with mains measured at 240v. It's obvious that the PM24M is drawing very little or no current, since the voltage reading at each side of the O/P Tx. primary is the same, but having checked the o/p transformer primary resistance (620 ohms), and replaced all wax. caps., I'm scratching my head!! I have no easy means to check valves, but it seems to me that the PM24M is faulty. When I first checked the radio I found that someone had wired a 170 ohm resistor between pin 5 of the 24M and chassis, effectively earthing the screen grid. I don't see how this could have harmed the valve, nor do I know why, or when this was done. Should I be looking to source a replacement valve?
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Old 25th May 2018, 12:08 pm   #2
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Default Re: PM24M Puzzle

You're right, your measurements suggest that the PM24A isn't drawing any current. There should be about 20 volts dropped across the output transformer and the grid bias is low at -2.5 volts it should be getting on for 10 times that. The bias is derived from resistors in the HT supply return and the output valve provides most of the current for the bias network and so low bias also supports the suspicion that the output valve is not drawing HT current.

You don't say anything about the PM24A heater; can you see the heaters lit? Also check all the supplies to the actual valve pins before condemning the PM24A, you may have a bad connection on the valve socket or the valve base itself.
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Old 25th May 2018, 1:08 pm   #3
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Default Re: PM24M Puzzle

Colin, the heaters are visibly lit, but you could be right about bad connections between the valve pins(which, incidentally I've cleaned) and the base sockets. The wiring & soldering to the base is good. I hadn't 'thought through' in detail about the biasing arrangement, but a glance at the circuit diagram was all I needed to understand it better. I'll investigate these matters further, including cleaning the insides of the valve sockets. Have to say I've repaired quite a few pre-war radios over the years - all previous ones being superhets, but none has led me such a merry dance as this one!
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Old 25th May 2018, 10:15 pm   #4
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Default Re: PM24M Puzzle

Further checks, plus cleaning the insides of the Valve base sockets (by careful use of a small round file) hasn't brought about any change. I may try carefully expanding the valve pins (they're the kind with slits in the side,b.t.w.) to see if that brings results. I really don't want to have to change the valve base(s), not because of any objection to using, e.g. ceramic instead of paxolin types, but because of the amount of work involved, but am well aware that this radio is about 84 years old. perhaps see if I can source a replacement valve first, or maybe get the original tested.
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Old 25th May 2018, 10:52 pm   #5
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Default Re: PM24M Puzzle

But are the pins on the valve connecting to the wires from the valve elements??
I've had loads of badly soldered valves over the years.
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Old 26th May 2018, 9:27 am   #6
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Default Re: PM24M Puzzle

Good point, Sam. I'll check with a magnifying glass. I'm not sure if the internal wires are soldered or crimped in this valve. No evidence of solder on the pins
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Old 26th May 2018, 10:50 am   #7
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Default Re: PM24M Puzzle

Never seen them just crimped.
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Old 26th May 2018, 11:42 am   #8
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Default Re: PM24M Puzzle

On closer inspection, Sam, they are soldered, and the joints appear to be O.K. I'll get the valve tested first, before attempting the delicate task of resoldering the pins.
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Old 26th May 2018, 11:42 am   #9
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Default Re: PM24M Puzzle

Illustrating Sam's point from post 5.

The first picture shows the base of a 4 pin valve of mine. The top and bottom pins are as found; the left and right pins I have re-soldered. The solder in the top pin in particular doesn't look all that good although the valve is working OK.

The wire inside the pin is soldered to the pin only at the very bottom because most of the pin length is sprung and filling it with solder would prevevent the spring action.

I remove any old solder with a solder sucker and then resolder with 22 swg solder using an iron with a pointed bit. 18 swg solder is a bit big for this job. It is important to do a tidy job for obvious reasons.
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Old 26th May 2018, 12:50 pm   #10
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Default Re: PM24M Puzzle

Make sure the centre tap of the heater supply is connected to chassis/ground, if it isn't the filament would still work but there would be no anode or screen grid current according to a schematic I'm looking at.

Lawrence.

Last edited by ms660; 26th May 2018 at 1:07 pm. Reason: clarification
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Old 27th May 2018, 9:07 am   #11
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Default Re: PM24M Puzzle

Checked the Heater winding, Lawrence, and it's centre tap is indeed earthed to the radio chassis. Colin, I might try as you suggest, but if it's not a silly question, how do you support a suspect valve when re - soldering the pins? Probably not wise to clamp the base in a vice.
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Old 27th May 2018, 9:40 am   #12
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Default Re: PM24M Puzzle

No problem, the reason I mentioned it is that if the valve filament has no continuity to chassis the valve will in effect have no emission due to it being of the directly heated cathode type.

Lawrence.
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Old 27th May 2018, 9:43 am   #13
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Default Re: PM24M Puzzle

Quote:
Originally Posted by 'LIVEWIRE?' View Post
.......how do you support a suspect valve when re - soldering the pins?
The picture tells the story.
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Old 27th May 2018, 9:54 am   #14
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Default Re: PM24M Puzzle

I use a Smash tin with a wrap of foam around the valve, same as when gluing the glass back onto the base.
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Old 27th May 2018, 10:20 am   #15
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Default Re: PM24M Puzzle

If I can get into the workshop over the weekend, I'll dig my 274 out that I restored about 10 years ago and do some quick voltage checks around the output valve. It's still fitted with the original PM24M. I do know that a PM24A will work as a replacement (I have one of those) but I don't know what effect it has on the overall bias.

One other thought....have you checked the wirewound 'volume' control? This is actually a tapped wirewound job connected across the HT to chassis that controls the bias to the previous stages. If it's the original control it may be open circuit....I've seen some that resemble a hedgehog when the windings rot and spring off the former. I would need to look at the circuit again to be sure but I think one tapped section feeds the output valve grid.
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Last edited by Sideband; 27th May 2018 at 10:30 am.
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Old 27th May 2018, 11:49 am   #16
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Default Re: PM24M Puzzle

Having just checked the diagram, the o/p valve G.B. is via R7 ((1 Meg), which is connected to the C.T. of the HT Secondary. This in turn is earthed via the volume control (R1 in series with R15/R18 in parallel with R16) this, according to my calculations, should equate to 1390 ohms between the Secondary C.T. and chassis, but I'm getting a much higher reading, so the pot. assy. needs further investigation, yet I'm convinced it did have the correct resistance. Oh, well, I'll have to disconnect, remove and check the pot and it's associated Resistors. Incidentall I wonder why those 1930s designers felt the need for this arrangement rather than a straight 1500 ohm w/w pot. There is a connecting point shown at the junction of R15 & R18, but nothing wired there.
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Old 27th May 2018, 2:34 pm   #17
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Default Re: PM24M Puzzle

Hmmm...I would think it far more likely the pot is faulty rather that the output valve failing since the fault happened between one session and another. It was possible at one time to get new 'inserts' for them supplied by forum member Ben_Dijkman in Holland but whether they are still available remains to be seen. You can PM him and ask but he'll need to know the resistances of the element (he calls them an 'R band') as there were different values made for other models.

To try and preserve the pot (still original) in mine, I unplug the set rather than turning it off or if the set is a 'working display' you could just have a switched mains socket.
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Last edited by Sideband; 27th May 2018 at 2:47 pm.
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Old 27th May 2018, 4:35 pm   #18
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Default Re: PM24M Puzzle

It will be interesting to see what the problem turns out to be. No doubt a faulty gain/volume pot can cause no sound but - 2.5 volts on the control grid will not cut the output valve off, it would need to be of the order of -50volts.

The grid voltage and anode voltage were not measured at the same time of course so it could be that the way the grid voltage was measured altered the reading. Possibilities include a low impedance meter and/or a grid resistor gone high value.

BTW I have belated noticed that I typed PM24A instead of PM24M in post #2.
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Old 27th May 2018, 5:06 pm   #19
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Default Re: PM24M Puzzle

All measurements have been taken with my M3800 DMM, which of course has a High i/p Impedance, so doesn't load the circuits under test. The o/p stage hasn't worked since completing work on the radio then powering it up.
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Old 27th May 2018, 5:40 pm   #20
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Default Re: PM24M Puzzle

I would be wanting to know what the output valves filament DC current is, easy to measure.

Lawrence.
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