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Old 28th Dec 2016, 3:17 pm   #161
ms660
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Default Re: My first valve radio project - Philips 462A

Nicola, apologies for the delay, kids visiting etc, anyways as promised a simplified block diagram of the Philips 462a receiver, I've drawn it in a basic form, it's designed to give an idea of the signals journey through the various stages from the antenna to the output, there's also another diagram to give typical waveform/descriptions at the various points along the signals journey, again this is just in basic form with a brief and not to complicated description of what's what, you'll have to excuse the scrawl.

In the 2nd sheet where I've said "Audio Modulation (still the same)" that refers to the shape of the audio modulation, the amplitude of the whole envelope of course does increase as it goes through amplification.

Any errors then guilty as charged.

Lawrence.
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Old 28th Dec 2016, 3:27 pm   #162
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by ms660 View Post
Nicola, apologies for the delay, kids visiting etc, anyways as promised a simplified block diagram of the Philips 462a receiver, I've drawn it in a basic form, it's designed to give an idea of the signals journey through the various stages from the antenna to the output, there's also another diagram to give typical waveform/descriptions at the various points along the signals journey, again this is just in basic form with a brief and not to complicated description of what's what, you'll have to excuse the scrawl.

In the 2nd sheet where I've said "Audio Modulation (still the same)" that refers to the shape of the audio modulation, the amplitude of the whole envelope of course does increase as it goes through amplification.

Any errors then guilty as charged.

Lawrence.
Thank you so much for that Lawrence - this pulls everything together for me. I shall digest it over a cup of coffee this afternoon N
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Old 28th Dec 2016, 3:31 pm   #163
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Default Re: My first valve radio project - Philips 462A

Now that the PSU is working, I suggest that the next step is to check the anode, screen grid and cathode voltages on the output valve.
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Old 28th Dec 2016, 3:35 pm   #164
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Default Re: My first valve radio project - Philips 462A

After those checks it is about time to put a length of wire, bared to make contact, into the A socket and find out if the radio actually works.
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Old 28th Dec 2016, 4:14 pm   #165
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Default Re: My first valve radio project - Philips 462A

There shouldn't be any voltage on the output valves cathode, not with respect to chassis, the cathode connects directly to chassis according to the Trader sheet, grid bias via HT -ve, not sure what the grid bias voltage should be, 6 to 7 volts -ve maybe?

Lawrence.
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Old 28th Dec 2016, 5:05 pm   #166
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Default Re: My first valve radio project - Philips 462A

After a good nights sleep I am now back on the job and ready to tinker with my radio set again I've remeasured all the key voltages and things are looking good (all measurements taken with the disc set to 245V and spec data in brackets).

AZ31: Vh = 3.6 VAC (4); Va = 265 VDC (285); HT output ; 259 VDC (270)
EBL21: Vh = 5.9 VAC (6.3); Va = 230 VDC (245)
ECH21: Vh = 5.9 VAC (6.3); Va = 235 VDC (240)

My intention for this set is to use it as an amplifier and speaker with signal input through the 'PICK UP' connections rather than from the radio antenna. This is because I want to use it for FM stations rather than SW/MW/LW and I can do this by playing the FM stations picked up from my phone through the set. With this in mind I have directly wired in an audio cable across the 'Pick up' with a 3.5mm jack at one end so I can connect my phone or a bluetooth device. I have also removed/bypassed the switches S16 and S17 so the radio input is permanently muted. I have tested the new set-up and audio from my phone plays through the speakers nicely and at a satisfactory volume to listen to comfortably. So good so far

I have noticed an issue with the volume control (it occurred before I wired in the input cable so I have not introduced the problem). The volume pot has to be turned up to max before I get any audio rather than a gradual increase. Also, the tone control seems to affect volume. At its lowest setting (full left turn of the tone control) volume is audible but low volume. As I turn tone control to the right the volume increases.

I have taken some resistance readings for the two potentiometers. The volume control looks strange. The tone control appears normal and on spec. Any ideas what can be happening to the volume pot here? Is this characteristic of a faulty component? The variable output of R8 is connected to C18 (a black Hunts cap).

R8 (volume):
From ground tag to signal input:
240K ohms (this is very low as the spec is for 700K)

From ground tag to variable output
Zero @ fully anticlockwise; 260K @ fully clockwise
However at about three quarters clockwise it reaches 400K before reducing to 260K at the end. Interestingly the audio is only audible when the dial is past this 400K position.

R15 (tone control):
Across terminals 1 and 3 = 466K ohms (spec is 500K ohms)
Across terminals 1 and 2 (variable output) = 0 to 466K ohms
Across terminals 3 and 2 (variable output) = 466 to 0 K ohms
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Old 28th Dec 2016, 5:37 pm   #167
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Default Re: My first valve radio project - Philips 462A

Was the resistance measurement for the volume control made with your lead plugged into the pickup sockets?

The reason I ask is that (using Trader Sheet references), when you plug in the cable, S16 opens and S17 closes, effectively putting R11 in parallel with the volume control. That would massively reduce the measured resistance (lower than you get).

But given the resistance netween the wiper ('variable output') and ground, I wonder if the volume control resistive track is broken at around the 3/4 point. It would be worth disconnecting the 'signal input' and 'variable output' wires and testing the control again.
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Old 28th Dec 2016, 6:22 pm   #168
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Default Re: My first valve radio project - Philips 462A

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Was the resistance measurement for the volume control made with your lead plugged into the pickup sockets?

The reason I ask is that (using Trader Sheet references), when you plug in the cable, S16 opens and S17 closes, effectively putting R11 in parallel with the volume control. That would massively reduce the measured resistance (lower than you get).
Yes, you are right Tony. I have removed the switches and wired in a cable for the pick-up and so the R11 (the pick-up shunt 100K ohms) is permanently in parallel with R8 (volume pot) and R9 (only 22ohms). This combination of Rs means the measured resistance should in theory be 87K ohms.

So, in fact the 240k Ohms that I measured is higher than would be expected if everything was performing to spec. This means that the actual resistance of the volume pot must be much higher than its spec of 700K ohms.... suggesting a break somewhere perhaps....
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Old 28th Dec 2016, 6:30 pm   #169
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Default Re: My first valve radio project - Philips 462A

"From ground tag to variable output
Zero @ fully anticlockwise; 260K @ fully clockwise"

If you have removed S16 and bypassed S17 then as has been said, resistor R11 (100k) will be connected across the volume controls track, if the track is as per the schematic (700k) then with R11 (100k) across it the reading across the track would be less than 100k not 240k as you have measured, it's possible that R11 (100k) has gone high, the quickest way to prove that is to un-bypass S17 and measure the resistance directly across R11, at the same time you could measure the resistance across the pots track.

"However at about three quarters clockwise it reaches 400K before reducing to 260K at the end. Interestingly the audio is only audible when the dial is past this 400K position."

That might tally up with the fact that the volume pot will have a logarithmic track and not a linear one and also in conjunction with the possibility that R11 might have gone high.

This post might cross.

Lawrence.

Last edited by ms660; 28th Dec 2016 at 6:45 pm. Reason: word change
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Old 28th Dec 2016, 6:57 pm   #170
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Default Re: My first valve radio project - Philips 462A

I've removed R8 from the circuit and also snipped one end of R11 so I can test everything independently.

R8: Total resistance is 1M ohm (higher than 700K spec). As I move the wiper resistance changes from 0 to 1M ohm on a log scale so most of the increase in the final quarter turn. But I don't see the peak resistance at the 3/4 position like I do when its in the circuit. This still doesn't explain the 240k ohms I measure across R8 in situ (would increase expected value up to 90 K)

R11: On spec at 108 K ohms (100K)

R9: On spec at 23 ohms (22)
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Old 28th Dec 2016, 6:59 pm   #171
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Default Re: My first valve radio project - Philips 462A

This volume pot also had the on/off switch attached to it. I have removed the on/off part yesterday as I was attempting to mend the connection. This will not affect the functioning of the pot will it (i can't see how it can but want to check).

Also, here is a photo of the pick-up connection:

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Old 28th Dec 2016, 7:52 pm   #172
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Default Re: My first valve radio project - Philips 462A

Hi
I was just wondering if you have combined both the left and right channels from the phone with a couple of 2.2k or similar resistors, so that you get a mono signal.

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Old 28th Dec 2016, 8:00 pm   #173
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Default Re: My first valve radio project - Philips 462A

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Hi
I was just wondering if you have combined both the left and right channels from the phone with a couple of 2.2k or similar resistors, so that you get a mono signal.

Mike
Hi Mike -I have just connected the right and left audio wires together and soldered the to one end of R11. I presumed this would effectively be mono since the stereo signal will go through just one speaker. Would it be better to include a couple of resistors?
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Old 28th Dec 2016, 8:12 pm   #174
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Default Re: My first valve radio project - Philips 462A

Can't quite understand why you are going to all this trouble, when the gram plug is inserted it opens S16 and puts R11 across the pickup as a load, suitable for a crystal cartridge as S17 closes taking the other side of R11 to chassis, at the same time muting the radio input via R12 by shorting the final IF. The aerial input could also be looped to chassis to stop any RF input. A shame it will never be used as a radio. A preferable way might have been to get a purpose made audio amplifier, and have use of this as a radio.
Usually to couple a stereo signal to a mono input is to feed through a couple of resistors of around 1K to 10K depending on the input level, joining the resistors at the mono end.
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Old 28th Dec 2016, 8:34 pm   #175
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Default Re: My first valve radio project - Philips 462A

Quote:
Originally Posted by indigo.girl View Post
I've removed R8 from the circuit and also snipped one end of R11 so I can test everything independently.

R8: Total resistance is 1M ohm (higher than 700K spec). As I move the wiper resistance changes from 0 to 1M ohm on a log scale so most of the increase in the final quarter turn. But I don't see the peak resistance at the 3/4 position like I do when its in the circuit. This still doesn't explain the 240k ohms I measure across R8 in situ (would increase expected value up to 90 K)

R11: On spec at 108 K ohms (100K)

R9: On spec at 23 ohms (22)
1Meg instead of 700k isn't that unusual for a pot of that age, at the moment I wouldn't think it's cause for concern, the fact that it measures high shouldn't affect the output from your MP3 player.

You won't see the peak resistance spot you saw earlier if R11 isn't connected across the pots track, to explain this then imagine the pot has a linear track of 1M ohm with a resistor of 100k ohm connected in parallel across its track and you were measuring the resistance between the pots wiper and the bottom end of the track, let's call the bottom end of the track A, the top of the track C and the wiper B and the parallel resistor R1, with the wiper B set to A there would be a resistance of 0 (zero) ohms between A and B.

Now we set the wiper to the half way point along the track, we now have a resistance of A--B which = 1M/2 which = 500k which is in parallel with a resistance, consisting of B--C = 1M/2 which = 500k in series with R1 (100k) which = 600k giving a final resistance between A and B of 270k (500k in parallel with 600k) Without R1 A--B would equal 500k.

Now we set the wiper (B) to the top of the track (C) we now have a resistance A--C which = 1M which is in parallel with R1 (100k) giving a final resistance of 90k (1M in parallel with 100k) Without R1 A--C would equal 1M.

So....R1 connected in parallel: zero resistance (0)---> peak resistance (270k)---> less than peak resistance (90k).

R1 removed: zero resistance (0)---> 500k resistance---> 1M resistance, no peak at mid travel.

Post might cross.

Lawrence.

Last edited by ms660; 28th Dec 2016 at 8:39 pm.
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Old 28th Dec 2016, 8:51 pm   #176
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Default Re: My first valve radio project - Philips 462A

Also with some pots there is a low resistance conductive film at the end of the carbon tracks, it's the interface between the carbon track and the solder tags external to the pot, if that film has an oxidized layer on it's surface it could present a high resistance to the wiper when the wiper is at that far end. In other words measuring the track end to end might read ok but measuring the wiper to the end might not, not saying it's that but it's a possibility.

Lawrence.
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Old 28th Dec 2016, 9:10 pm   #177
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Default Re: My first valve radio project - Philips 462A

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You won't see the peak resistance spot you saw earlier if R11 isn't connected across the pots track,

R1 connected in parallel: zero resistance (0)---> peak resistance (270k)---> less than peak resistance (90k).

R1 removed: zero resistance (0)---> 500k resistance---> 1M resistance, no peak at mid travel.

Lawrence.
Ah, so in fact all my measurements are quite normal for this circuit.... so if the pot is OK then why do I only hear audio at the top end? Surely it wasn't designed that way?
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Old 28th Dec 2016, 9:19 pm   #178
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by Hetrodyne View Post
Can't quite understand why you are going to all this trouble, when the gram plug is inserted it opens S16 and puts R11 across the pickup as a load, suitable for a crystal cartridge as S17 closes taking the other side of R11 to chassis, at the same time muting the radio input via R12 by shorting the final IF. The aerial input could also be looped to chassis to stop any RF input. A shame it will never be used as a radio. A preferable way might have been to get a purpose made audio amplifier, and have use of this as a radio.
Usually to couple a stereo signal to a mono input is to feed through a couple of resistors of around 1K to 10K depending on the input level, joining the resistors at the mono end.
Each to their own I guess - for me I like to listen to FM and I also like the visuals and history of an original bakelite radio. This combines the two

What you say about R11 being suitable for a crystal cart is interesting... I presume this is something to do with impedance matching. If I'm using a bluetooth device across the pick-up I presume R11 won't necessarily be optimal....

Also, what is the rationale behind adding resisters to the ends of each stereo input before joining them to the mono input?
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Old 28th Dec 2016, 9:28 pm   #179
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Default Re: My first valve radio project - Philips 462A

Hi Nicola, the 2 resistors are to give a small degree of isolation to the stereo channels. They also act as safety resistor if something go wrong on the radio and it will not incinerate your player.

These sets were often designed to run from high output record player cartridges, perhaps up to 1 volt. Do you know, or can you measure (scope preferably) the output of your player?
Does this drop significantly when connected to the radio?
Some players have been used with a small IC amplifier to boost the sound.

Not sure of your circuit, but check to see if the cap in series with the tone control is OK, if it has gone low resistance, it will shunt the output transformer and reduce volume as you describe.

Note that some sets used the on/off switch on the tone, not vol control (to save wear and tear.

Ed
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Old 28th Dec 2016, 9:56 pm   #180
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Default Re: My first valve radio project - Philips 462A

"Ah, so in fact all my measurements are quite normal for this circuit.... so if the pot is OK then why do I only hear audio at the top end? Surely it wasn't designed that way?"

The pot is a variable potential divider, slider near to the top equals more signal, slider near the bottom equals less signal, the amount of signal needed to produce a given power to the loudspeaker depends on the strength of the input signal and gain of the amplifying stages, there's two stages of audio amplification as you know, audio amp and audio output, so far there might be an assumption that those stages are both working as they should, the 1st port of call for that is measure the valves voltages, the triode section of V2 (AF amp) and the pentode section of V3 (AF output) Measure the voltage on V2's triode anode and V2's triode grid (called the control grid) Measure the voltage on V3's pentode anode, screen grid and control grid, the control grid voltages should be -ve, all other voltages +ve, the cathodes of those valves are connected to the chassis so there will be no cathode voltages to measure.

All voltages measured as DC with respect to the chassis.

There might also be other capacitors that might need replacing or resistors that might need replacing, but voltage checks first.

Lawrence.
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