Murphy V204

[Radio Tech]

Hi folks, I can't seem to get the line generation and line output transformer to produce anything, what I was hoping for was to hear that line whistle but nothing, the components around the line generator seem OK as far as I can see but without disconnecting each resistor in turn the values can't be checked properly. I have put a couple of low value resistors in series with the bridge rectifier of 22 ohms each, so a bit stuck at the moment.

regards

Ken

[FERNSEH]

Does the V204 employ a self oscillating line timebase? A favourite circuit feature in many Murphy TVs. At the time of writing this the only information I have to hand is that of the V214. That receiver employs a 20L1 triode as the part of a multi-vibrator, the 20P4 and the line output transformer forms the other part.
To find the nature of the line timebase, trace the wiring from pin 5 of the 20P4 line output valve. If it returns to the line output transformer, then it is a self oscillating timebase. If pin 5 connects to another valve via a 0.01mfd capacitor it's likely there is a separate oscillator.
A self oscillating timebase will not work if the HT is too low.

DFWB.

[Radio Tech]

Hello DFWB

The circuit in the V204 employs a 20P1 for line o/p and a 6L1 for the line generator and yes the line timebase is self oscillating, there is a capacitor that couples V8 a 6F1 to grid 1 of V9 the 6L1 and grid 2 has a 220pF cap one end linked to ground via a resistor and the line hold control, the other end of the 220pF is connected to the primary winding of the LOPT this then via a divider network. I did find the 82K resistor reading 109K and the 100k out of range though, I will look a bit more at it during the week.

Best wishes

Ken

[Radio Tech]

Hi Dave or anyone else

Just a thought, have you got a schematic of the top view of the chassis that indicates the valve reference number of the position of the valves, this would be helpful to have. see the attached picture of the chassis I have.

Best wishes

Ken

[FERNSEH]

The circuit diagrams of the line and frame timebases of the Murphy V204.
The double triode V9 functions as a line sync pulse inverter and part line oscillator It is essentially the discharge valve for the ramp forming capacitors C44 and C46. V9B conducts only during the flyback, supplied with a positive going pulse from the line output transformer.
The cathode bias resistor of V10 has a high value, 330 ohms, which leads me to believe that the valve is not biased by grid current which is common practise in later Murphy TVs.
Note the unusual method of supplying the heater of the boost diode V11.

The frame timebase also employs a 6L1, again like the LTB one section is the sync pulse inverter and clipper.

HT is supplied by a full wave rectifier. There is some good engineering in this set.

DFWB.

[Radio Tech]

Many thanks for that info I will have to study this circuit a bit more, I found yet another resistor a bit on the high side, the 68k R67 reading 82K, there are no doubt a few more like this. I also found the cathode of the 20P1 being 30V higher than it should be.

Regards

Ken

[FERNSEH]

R67 is the anode load resistor of V9B and is supplied from the HT line via the focus coil plug.
The focus coil is energised by the anode current of V15, the 6P25 sound output valve. The focus control R96 varies the cathode current of V15.

A year later Murphy introduced the model V214.
There is lots of Murphy strangeness in the line output stage.

DFWB.

[Radio Tech]

Hello DFWB

I thought as much that the 6P25 served as a dual valve, I found the reason why there was over double the voltage on the cathode of the 20P1, the 330 4 Watt resistor was O/C I didn't have the correct value in one resistor so had to use two in parallel to give the same value. The voltage has gone up to 350 on the anode of this valve now, got to find a way of measuring the volts on pin 7 of the scan and focus socket to see if there is 450V
progress slow but never mind. Thanks for the info so far though, appreciate it.

Best wishes

Ken

[FERNSEH]

The line output stage follows the practise which became the norm in all valve line output stages. C51 is the boost capacitor, locate that component and boosted HT can be measured at one end. the other is at HT potential, 210volts.
Pin 5 of the scan coil socket can also be used to measure the boost HT, approximately 450volts when the line output stage is working properly.
The Murphy V204 employs a Mazda U281 as the boost diode. This valve has a limited heater to cathode voltage insulation, so for that reason the bifilar windings on the line output transformer are introduced into the heater supply.
In the event of the U281 needing replacement I'd recommend using the Mazda U301. The heater of the replacement valve can be wired directly to the mains transformer.

DFWB.

[Radio Tech]

Hi folks

After replacing C44 and powered the set up, a strange whistle and wining noise came from the loudspeaker, but still no EHT yet, haven't checked or replaced C51 so this will be the next step, Thanks DFWB for the info, its all helpful, don't know if I have a U301 though, I measured the HT at the main smoothing and this now sits at 235V, I wonder now that because V10 is conducting much more, is this the reason the HT has dropped a bit, at the moment I have a couple of 22R resistors in series with the bridge, I thought of lowering these to 15R to raise the HT to more or less what it should be 245 V

Best wishes

Ken

[Radio Tech]

Here is a little more on the progress of this monster of a set, I replaced the U281 with a new valve and after warm up, I could see a strange but obvious glow from the U25 EHT rectifier, the glow being a blue arc over between anode and cathode, I suspect this poor little valve has done its time and this maybe why I am getting this strange oscillation noise from the loudspeaker perhaps. So now wait for a replacement U25, I ought to get two of them so I have one as a spare.

Regards

Ken

[FERNSEH]

It seems that the line timebase is working. First, unsolder the anode wire of the U25. If the line TB is working there will be a healthy flame like spark at the anode connection of that valve. Apply the insulated screwdriver test to confirm. Also, disconnect the EHT smoothing capacitor. If it is a TCC visconol it's most likely to be leaky. The set will work without it until a suitable replacement is found.

DFWB.

[Radio Tech]

Replaced the U25 and that stopped the screechy noise on the LS but held a screwdriver near the anode of the u25 but only weak spark. I had replaced the visconol cap with a new type but did wonder if the long discharge resistor that comes off this is causing trouble.

Regards

Ken

[beery]

Hi Ken,
it looks like a VDR which is acting as a kind of shunt stabiliser, they are usually quite reliable.

Did you replace the visconal cap with something up to date? I have seen people offering NOS visconals for sale, but steer well clear of such things as they will still fail even if they are NOS.

Is the heater of the U25 lighting up ok or is it rather dim?

Your next step would be to look at the line drive and the boost voltage. I don't have a circuit to have so I can't be more specific.

Cheers
Andy

[FERNSEH]

Refer to the circuit diagram on #25. The resistor R79 is a metrosil, a voltage sensitive device introduced across the EHT supply to function as a variable load in order to stabilise the EHT voltage.
It'll be OK, but if you are unsure, disconnect it to confirm that it is not dragging the EHT supply down.
The CRM152 needs at least 10KV.

DFWB.

[Radio Tech]

Cheers Andy and DFWB, The EHT smoothing cap was a nos which replace the visconol, I will disconnect and see if it makes any difference, the heater on the u25 is a bit dim though, so not sure at this stage what's dragging the voltage.

Regards

Ken

[Radio Tech]

Took the lead off the final anode of the tube and while set was on held the lead away from chassis and there is although a healthy spark but the EHT would not jump any further than a 1/4 so next is to take lead off the EHT smoothing cap and see.

Best wishes

Ken

[Radio Tech]

This was the next step, temporary removed connection on smoothing EHT cap but this made no difference to the EHT voltage, I haven't yet disconnected the voltage sensitive resistor but I did and still wonder about the HT voltage being at 235V after the valves have warmed up, at the moment I have 22 OHM w/w resistors in series with the AC side of the bridge rectifier and thought of lowering the value of these to 15 OHM, not to certain if this will give me the extra 10V that I need, as the circuit specifies 245V for the HT.
Regards

Ken

[Radio Tech]

Mods close post please, set bought as a prop for theatre use

regards

Ken