Réela Radio Dauphin "7 Luxe"

[PJL]

Can you confirm the voltage readings were taken with your DMM on DC voltage with the -ve taken from chassis.

At the moment the output transformer is looking suspect. You need to measure the DC resistance of the output transformer primary.

[kentplums]

Thanks PJL
Sorry to be so dumb but where would I measure resistance on this TX?
Cheers
R

[kentplums]

I am so confused!
As I have said I replaced caps exactly as I found them.
So...I do not see the smoothing caps -ves going to chassis - at least not directly. I don't know if this is apparent from the photos. Could this be a fundamental issue...
Page 4 now - plese let me know if I'm becoming a bore.
cheers
R

[Herald1360]

Easiest way to find primary of O/P TX is to find the connections that do not go to the loudspeaker, then measure resistance between them. There is a possibility that a DVM may not like the inductance and will faill to give a sensible reading, but if it reads open circuit that should be meaningful. If you have an analogue meter, it's easy.

Smoothing caps- were the originals in a can by any chance which was clamped direct to the chassis? If so, the mounting would have been the common negative connection to chassis. As it is, I can't imagine any reason for there to be a 330K resistor as the only connection to chassis- that sort of value in that area of the circuit could only be a "bleeder" to dump the capacitors energy completely at switch off (unlikely in a typical radio), or part of another circuit altogether- maybe the grid leak resistor for the output valve or part of the tuning indicator circuit perhaps.

Have you got any decent "before" pictures of the area in question? (I suspect the horse has already bolted, though.... )

Does the PSU/Output stage circuit match up at all with the generic one I sent you? Don't worry about exact component values, just the topology of it.

[PJL]

The speaker in this set appears to have a field coil. I think we need to understand the circuit before we can help much more.

We will need some better pictures.

[kentplums]

Chris
re

Have you got any decent "before" pictures of the area in question? (I suspect the horse has already bolted, though.... )

the only picture is on an earlier post, the 2 red electrolytics replaced with the 2 black ones in exactly the same place. The red ones were probably replacement as there is a hole in the chassis for a can (which appears in other people's photos).
Dare I bypass the existing set up and run the 2 negs direct to ground?

R

[kentplums]

Originally Posted by PJL

The speaker in this set appears to have a field coil. I think we need to understand the circuit before we can help much more.

We will need some better pictures.

Thanks PJL
hope these help a bit...
R

[kentplums]

And these...
R

[PJL]

Blue-White is the TX primary
White-Red is the field coil

The UL84 cathode is earthed (pin 3).
The grid bias is created across the 150 ohm resistor in the -ve rail.

Try the following first:
1. Check the resistance between blue-white and white-red
2. Switch on and measure the voltage from chassis to a) the blue wire b) the junction of the 150 ohm resistor and the 450V capacitor -ve's

[PJL]

The calculation based on your previous grid voltage of -13V across the 150 ohm resistor indicates the total current is 87mA which sounds high.

I want to change my guess at the tx and field coil connections:
TX primary: red-blue
field coil: white-red

[Herald1360]

There's no UL84 there, it's an EL84.

There's something odd looking about the output valve wiring. Is there a touch perchance between the lead from the 12K grid stopper to pin 2 (grid) and pin 3 (cathode)? This would short out the bias and cause excessive current in the valve.

Otherwise, that 330K is the grid leak for the output picking up the negative (to chassis and cathode) voltage to provide the bias for the output valve. The blue wire on pin 7 should go to one end of the O/P TX primary and it looks like the brown wire from pin 3 of the rectifier should go to the other end of the primary.

It's not clear how the second electrolytic gets its HT feed- maybe from a tap on the primary of the O/P TX or via the speaker field? It's clear the circuit actually isn't much like the generic one! It's more like a prewar design executed with postwar valves- some of the resistors look pretty archaic too for a '50s set.

[PJL]

Ah yes, EL84. The brown wire is in fact white at the other end! I can only assume the field coil is conventionally wired and goes from white to red.

The odd thing is the anode is reported as almost 0V suggesting a faulty output transformer primary yet the total current is high. I am thinking that the voltage reading on pin 7 was wrong.

[kentplums]

So...

Blue / White 1.38k
Red / Blue 358
White/ Red 1.02k

voltage across 150 ohm (actually 148 ohm) is 23v


Blue to chassis 2-10v irregular variation
pin 7 EL84 same as above

Chris, pin 2 and pin 3 are not touching. However, pin 4 goes to chassis, plus green wire to speaker and also to pin 3.

Cheers

[PJL]

1.02K ohm field coil and 358 ohm output tx primary - sounds reasonable.

Are you sure you measured the voltages with the DMM set to DC volts?
What is the voltage on the red wire?

It is taking far too much current so DO NOT LEAVE IT POWERED UP FOR LONG. At the moment I am thinking it might be a short in the output tx to chassis.

When switched off measure
1. The resistance from chassis to the red wire.
2. The resistance from chassis to the blue wire

Another possibility is the tone correction capacitor has gone short. This is the capacitor connected to pin 7/blue wire.

[kentplums]

Yes - DC

Red wire to chassis 69v

resistance chassis - red 385 Ohms
chassis - blue 28.3 ohms
chassis - white 1409 ohms

Pin 7 white capacitor is 20000 pf as far as I can make out. Can I simply dissconnect to test? Would this be about the right value for a tone cap?
Thanks PJL

R

[PJL]

There is a short to chassis.

Try disconnecting the white capacitor and measure the resistance from blue to chassis again. Hopefully the capacitor is at fault. 20,000pF (0.02uF) sounds about right but it needs to be 1000V working voltage due to transients.

At least we now know why the mains transformer died!

PS: It is best not to run it without the capacitor as it prevents the transients from breaking down the output transformer insulation.

[kentplums]

On 2000k setting the blue to chassis now starts at 0 then goes up and up and up! Actually checking again it starts now at about 1700k and goes off the scale.

I have a 1kv 0.01µf ceramic cap to hand. would this be of any use temporarily?

[julie_m]

Yes, try that in the circuit. It may sound a bit funny -- too much treble -- but will be fine just for testing.

[kentplums]

Actually I'm not getting any constant readings now - blue - chassis goes up and down like a yo-yo, sometimes reading nothing. DMM probs or ?

[kentplums]

Yahooo!
I replaced the 20000 with the 1kv cap and I have reception....unbelieveable!
Thanks Chris and PJL and the rest for your kind help.
Now to check it over and clean a few switches.
Thanks again
R