French Miracle 510 1937 too many valves?

[Wishiknewmore]

The radio has behaved well for the rest of today, reception improve using the earth from the mails supply
However I have yet an other question...
When using the PU setting with an iPod, the volume control on the radio has no effect whatsoever (except for a bit of crackle) is it likely it was designed this way?
Philip

[julie_m]

It would be unusual for the volume control not to be in circuit when playing a record. (That said, it is French, so pretty much anything goes.) Could it be a consequence of the iPod's output stage working into a high-impedance load? Try it with a pair of headphones connected via a double adaptor, to rule that out. If that cures the volume problem, you can connect a low-value resistor (anything from 22Ω to 220Ω will do) across the input for a permanent fix.

Failing that, you will have to bite the bullet and trace out the circuit to see whether or not the PU input goes via the volume control.

[Wishiknewmore]

Thanks Julie
I will try that the headphone test tomorrow
Philip

[julie_m]

To be honest, I think it's pretty unlikely; but it's worth a quick test, just for the amount of grief it will save if that actually is the problem.

[Wishiknewmore]

Tried the head phone splitter and extra headphones ..no change!
On other developments the set has done the same thing this morning, worked with iPod but no radio frequencies, then after about an hour long wave came back, but this time not the other bands, that took another hour or so....it seems to be time/warmth rather than twisting the waveband around?

Any directions!

Philip

[Boater Sam]

Possibly the local oscillator is only running after an extended warm up.
What's the frequency changer/oscillator valve like? Seems it's the 6A7, have you a spare? Check the pins for contact.
But you need to start checking VALVE VOLTAGES instead of guessing.

[Wishiknewmore]

Do these valve voltages seem ok?
6A7
1 0v
2. +271v
3. +46v
4. +233v
5. +.4v
6. +.09v
7. +6.3v

Top pin 1.5v

Valve 6B7

1
2. +97v
3. +44v
4. +1.77v
5. +1.75v
6. +2.25v
7. + 6.3v

Top pin 0.09v

These were taken while the radio was working and receiving ( on all bands)

I also have the values for the two 42's

Philip

[ms660]

Originally Posted by Wishiknewmore

Do these valve voltages seem ok?
6A7
1 0v..h
2. +271v..a
3. +46v..g3,g5 (screens)
4. +233v..g2 (osc. anode)
5. +.4v..g1 (osc. grid)
6. +.09v..k
7. +6.3v..h

Top pin 1.5v..g4 (signal in)

Valve 6B7

1..h
2. +97v..a
3. +44v..g2
4. +1.77v..a(d2)
5. +1.75v..a(d1)
6. +2.25vk,g3
7. + 6.3v..h

Top pin 0.09v..g1

These were taken while the radio was working and receiving ( on all bands)

I also have the values for the two 42's

Philip

Additions to quoted above in red to make it easier for folks to see what's what, electrodes given for 6A7 which is what the unmarked valve was assumed to be in an earlier post.

Lawrence.

[julie_m]

The oscillator grid is looking a bit wrong; surely there should be a negative voltage there? Also, either the anode is high or the screen is low. Unless that's how a heptode frequency changer is supposed to be.

[Wishiknewmore]

I may well be doing something wrong
I have simply taken the voltages at the actual pins on the valves
Multimeter set at an appropriate dc range ( ac for the heaters)
Black probe attached to chassis and red to each pin
Radio warm and running on radio reception
Is this the correct method? Am I missing something?
Assuming my method is correct,If the voltages are wrong is that likely to be the actual valve or the components in circuit to it ...or is that the $64000 question!
Thanks everybody for your patience
Philip

[ms660]

It would seem that you are measuring the voltages correctly...but.... + 6.3 volts for AC heaters?? Also top cap measurements, +ve or -ve with respect to chassis?

Mixer screen volts seem a bit low, could be the feed resistor or the bypass capacitor.
Sometimes the screen is fed via a series resistor or sometimes via a potential divider.

The osc grid should be -ve but sometimes the capacitance of the meter leads can kill or reduce the osc voltage when measuring the osc grid, try measuring it again but with a resistor connected to the osc grid pin and measure the voltage on the free end of that resistor, for the resistor try something like a 100k if using a digital meter and see what's measured.

Lawrence.

[Boater Sam]

Your method is correct. The voltages are not a million miles out but without knowing what they should be its difficult.

However, the 6A7 should have a negative voltage on the grid 1 pin 4 if the oscillator is running which it must be to get any signal into the IF amp. It can't change the frequency if there is no oscillation.

Whether its a valve fault or a component is the next question.

[Wishiknewmore]

Thanks Gentleman
I will find a resistor, recheck and report back later today
Philip

[Wishiknewmore]

I tested the 6A7 voltages again this morning both with and without a 110Kohm resistor

Initially I thought that I had misled everyone with the positive voltages on the oscillator grid as when tested without resistor Pin 5 was -1.54v and with resistor -2.13v

I tested again later and the voltages had gone positive – so a little more investigation! Both the oscillator grid Pin 5 and the cathode Pin 6 and go from a range of about -13v to +5v dependent upon the position of the volume control
The top pin also varies with the volume control

The volume control is a 18 Kohm (it’s marked 18000 ohm)

The two connections from the volume control are:- middle one to the chassis and the other goes to Pin 6 – cathode via a 300ohm resistor in series

The cathode Pin 6 is linked to Pin 5 (oscillator grid) by an 80Kohm resistor

Both are connected to the chassis by a 0.1uF cap

Pin 5 (oscillator grid) is then linked by a 200pF capacitor (I think) to the Wave change switch

Is this correct/normal? Is this the reason why the volume control has no effect when using the PU input?

Comments – advice – suggestions all welcome, Philip

[ms660]

This isn't the normal arrangement for most superhets, most have the volume control as the part of the detector load and any PU input is usually switched and it's level controlled by the volume control.

From what you've described it sounds like the volume control varies the gain of the pentagrid mixer, usually I would expect to see an anode bend detector for that kind of set up but your receiver has a double diode pentode for IF amp and detector, usually one of the diode anodes is used to produce the AGC voltage.

This isn't the same as yours but it shows what I mean by the volume control set up, all three connections are used in the case below but you can see how it varies the cathode/grid voltage just like yours appears to be doing:

http://www.nostalgiaair.org/pagesbym...3/M0005363.pdf

Lawrence.

[Wishiknewmore]

Thanks Lawrence

So it is likely that all is more or less in order and the set is functioning as it should.
As it seems to work reasonably well at the moment and after more cleaning of the wave band switch and valve pins and sockets I shall put it back into its case and see how it performs for a while.

Thank you for your time and patience

Picture will be posted once back together

Philip

[ms660]

No problem.

There's plenty of quirky designs out there, this is one of them.

Lawrence.

[Herald1360]

Even by 1937, the design of a "standard" domestic set hadn't settled down yet to the "nothing much changed except the valve series, size and FM radio additions" state of postwar-1960s valve sets.