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Old 26th Apr 2020, 12:01 pm   #1
Dick Glennon
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Default Voltage drop on Philips 730A

Hi again, I am trying to figure what drops the voltage from the dial bulb (6volts) to the valve heaters (4volts) excluding the rectifier of course, if they all come from the same two wires on the transformer. I am sure there is a simple logical answer. Regards, Dick.
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Old 26th Apr 2020, 12:20 pm   #2
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Default Re: Voltage drop on Philips 730A

Hi Dick

Nothing drops the voltage.

The Scale lamps are run at 4V same as the valves..

6V lamps are used under-run for long life.

This was quite common practice.

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Old 26th Apr 2020, 12:27 pm   #3
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Default Re: Voltage drop on Philips 730A

The simple answer is that Philips decided to underrun the 6V dial bulb at 4 Volts. It should last more or less for ever.

The heater winding that feeds all the valves bar the rectifier and the panel lamp is a 4V winding, centre tapped to chassis/earth to stop hum problems from the directly heated output valve filament/cathode.
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Old 26th Apr 2020, 3:18 pm   #4
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Default Re: Voltage drop on Philips 730A

Hi again, The voltage on the heater/ dial bulb winding is 4.6 volts but when I put a 6 volt 300ma bulb across the winding it lit for a few seconds and blew the bulb. Is the winding doing this. Thanks again, Dick.
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Old 26th Apr 2020, 4:13 pm   #5
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Default Re: Voltage drop on Philips 730A

No idea. Sounds like a dodgy bulb. Are you sure it was 6V and not *.6V with * rubbed off or hidden under the solder blob?

Are you sure you picked the right terminals on the transformer?
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Old 26th Apr 2020, 4:26 pm   #6
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Default Re: Voltage drop on Philips 730A

When you say "across the winding" do you mean that or do you mean in the lamp holder?

The filament supply should not blow the bulb, either the bulbs are faulty (wrong voltage or just plain faulty manufacture) or you have considerably more than 6V across it.

You could pre-test the bulb on a known 6V supply (battery for instance).

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Old 30th Apr 2020, 10:20 pm   #7
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Default Re: Voltage drop on Philips 730A

Hi again, I had a bulb holder connected across the pilot lamp winding (l15) . Could the amps in the bulb (300ma ) be too low.?. I have 42 volts across I14 that is A1 to A2 of the rectifier. This is AC voltage. Any more thoughts or does this mean the transformer is faulty. Regards, Dick.
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Old 1st May 2020, 12:03 am   #8
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Default Re: Voltage drop on Philips 730A

Is that with the rectifier removed, I presume?

And what voltage were you feeding into the set? 42VAC across the anodes sounds very low to my mind, I'd have thought it would be up over 500VAC or maybe more with no load. Are you 100% sure which windings you're on, and why is the dial lamp holder not in circuit currently?

I'd have probably measured the AC at the dial lamp or a valve socket to see what it's sitting at. Are the valve filaments intact? (don't install them as yet in case there is a problem with the xfmr or wiring from it, since these are on the same winding as the lamp)
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Old 1st May 2020, 9:17 am   #9
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Default Re: Voltage drop on Philips 730A

Hi again, the dial lamp was open circuit so I inserted a standard bulb holder in its place. The mains feed was 240 volts. I do not have any valves in circuit. Regards, Dick.
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Old 1st May 2020, 9:23 am   #10
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Default Re: Voltage drop on Philips 730A

How can you replace a dial lamp with a bulb holder? Surely you need another lamp?

The lamp is specified as 6V 3W or 6V 500mA. A lower power lamp will still work, so long as its rated at 6V.

If you're measuring 4.6VAC on the winding and a 6V lamp connected across the winding blows then either your meter is reading incorrectly, you used a lamp of the wrong voltage or the lamp was faulty, ie incorrectly labelled. The laws of physics can't be broken.
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Old 1st May 2020, 10:13 am   #11
Dick Glennon
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Default Re: Voltage drop on Philips 730A

Hi again, I put a standard bulb holder in place of its own and inserted a 6 Volt 300mA bulb. It lasted only a few seconds. The voltage reading across the bulb holder was 4.6 volts.

Regards, Dick.
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Old 1st May 2020, 10:20 am   #12
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Default Re: Voltage drop on Philips 730A

There've been various suggestions as to how to investigate this. There's nothing more I can add.
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Old 1st May 2020, 10:31 am   #13
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Default Re: Voltage drop on Philips 730A

Are you sure you can trust the meter readings?

42V anode to anode of the rectifier I would expect the transformer to be hot or OC have you done the cold resistance checks on the transformer?

Most Philips sets of this period use single colour (often systoflex) wiring insulation making tracking wiring difficult.

IMHO you need to step back and assess what's real here start by making sure your meter is working properly then do the cold transformer resistance checks.

Check your meter by measuring known voltages such as batteries and resistance range by known resistors you wouldn't be the first or last to be lead astray by a dodgy meter.

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Old 1st May 2020, 12:28 pm   #14
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Default Re: Voltage drop on Philips 730A

Quote:
Originally Posted by Dick Glennon View Post
Hi again, I put a standard bulb holder in place of its own and inserted a 6 Volt 300mA bulb. It lasted only a few seconds. The voltage reading across the bulb holder was 4.6 volts.

Regards, Dick.
There's only 2 ways this can happen, faulty bulb or faulty meter reading. As suggested I'd want to verify the meter being used at this point by checking against known voltages/resistances, and then follow the suggestions of Mike T to verify the cold (unplugged) resistances of the transformer windings.
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Old 1st May 2020, 1:18 pm   #15
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Default Re: Voltage drop on Philips 730A

Hi again, The cold unplugged resistances of mains transformer primary total is 39ohms (38) Heater secondary 0.5 ohms (0.05) Rectifier heater secondary 0.4 ohms (0.1). H.T. Secondary total 560 ohms (560) .The readings in brackets are from Trader 647. Sorry about my stupidity. Dick.
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Old 1st May 2020, 2:39 pm   #16
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Default Re: Voltage drop on Philips 730A

Hi Dick
That's good so the transformer may still have shorted turns but seems less likely, on the face of it looks good.

Are you using a lamp limiter on the radio's primary side?

If you are is the lamp limiter lighting up?

That Anode to Anode voltage is a mystery it should be more than 10 times what you were getting.

Were you measuring the A to A on the rectifier valves socket?

Is the transformer getting hot?

Going back to your bulb problem.

If you take an "ideal transformer" the output voltage will not vary by load so if the bulb were 100mA or 1000mA the voltage would be the same.

Real transformers are not "ideal" and changing the current to a degree changes the voltage, this is the transformers "regulation" the higher the transformers secondary current the lower the secondary voltage.

However providing you don't exceed the transformers ratings this effect is relatively small on anything but the cheapest nastiest transformers and is taken into account during the transformers design and build. This is why the 4V measured high because the transformer is not fully loaded. When all the valves are in and the radio is working that voltage will fall by around 10%. But what ever the current of the bulb was not what made it blow. a 6V bulb connected across 4.6V should be lit but not to full brilliance.

I suspect you had a faulty or mis-marked bulb.

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Old 1st May 2020, 3:14 pm   #17
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Default Re: Voltage drop on Philips 730A

Agreed with above, only measurements that concern me slightly are the heater sec. 0.5 (spec of 0.05 so 10 times lower resistance than measured) but that could well be in the leads of the meter or less than perfect connection to the probes.

Certainly the 4.6v measured should not have blown a 6v bulb so I'd suggest trying another from a different source (to rule out a mis-marked batch from one supplier) and also testing the bulb on 6v from a battery first to make sure it's good. Any way you look at it, 4.6v will never ever blow a good bulb rated for 6v so the bulb must have been faulty.

Again my main concern here is still the voltage measured across the anodes of the rectifier socket. If this is definitely a correct measurement (I'd be inclined to double-check terminals and meter range to be 100% sure) then there has to be a problem somewhere. Good advice above to use a lamp limiter to check if the transformer is drawing excess current, and if nothing shows on that then perhaps let it run for 5-10mins (under supervision in case it starts showing issues) and then unplug it and feel if it's heating up at all. I'd expect an unloaded transformer to remain cold during this time.

EDIT- Thinking about the voltage measurements, have you verified that your meter is reading correctly with a known voltage source? I have to wonder if the voltages were somehow off by a factor of 10, seeing as some other measurements also appear that way)

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Old 11th Sep 2020, 8:37 pm   #18
Dick Glennon
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Default Re: Voltage drop on Philips 730A

Hi again,first of all I want to thank all you good people who helped me out in getting the Pilot H665 working again.I have decided to take on the 730A again and on looking at the schematic I see that H.T goes from R18 to R 21 to Ex LS to C25 to positive C27. In reality C 27 has come from the opposite end R3 R5 junction. What I would like to know is do I connect L8 output transformer, C25, ext LS and R21 to R18 to complete the HT line. Any thoughts would be very helpful. Thanks again Dick.
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Old 11th Sep 2020, 9:26 pm   #19
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Default Re: Voltage drop on Philips 730A

Hi Dick looks like you are using the ERT sheet this time.

Had me fooled for a while thought it was trader

These points all connect electrically to the same point and all of the resistors you mentioned are on tag strips and the factory wiring should connect these tags together.

Has something happened to the factory wiring such that these points are no longer connected?

Philips normal practice at this time was to mount the output transformer on the basket of the loudspeaker C25 is mounted across the external speaker sockets.

All these points would be connected by Systoflex covered wire so I think at this point it would be an advantage to see what you are working with.

Perhaps a couple of under chassis pictures?

Cheers

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Old 22nd Sep 2020, 11:46 am   #20
Dick Glennon
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Default Re: Voltage drop on Philips 730A

Hi again, I am now almost ready to power up the 730A. What I would like to know is if there is a way that I would be able to give as much protection as possible to to the valves as they warm up. All filaments are wired correctly. I do have a lamp limiter. Any advice would be greatly appreciated. Regards, Dick.
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