HMV 900 Pre-War Mirror Lid TV - Radio Fault - UK Vintage Radio Repair and Restoration Discussion Forum

jdwn405 · 23rd Mar 2016, 7:35 pm

Hello

I wonder if anyone can help me with a problem with the radio of my HMV900 pre-war mirror lid TV. I had not switched the set on for some time so I thought I would start with the radio section It powers up no problem HT line is 315V. The only response I was able to get was one fading in and out radio station on Short Wave and on Long Wave an audible tone which increased when turning to the lower end of the scale. Nothing at all on Medium Wave I thought the X41 mixer/oscillator might be the problem and changed it but with no improvement. Here are the current voltages on the RF amp VMP4G and the mixer/oscillator X41 valves

VMP4G
Grid pin 2 - 0.72V
Supressor grid pin 3 +0.11V
Cathode pin 6 +0.11V
Screen grid pin 7 + 8V
Anode (TC) +244V

X41
Osc Anode pin1 +100V
Osc mixer grid pin 2 + 3V
Screen pin3 +40V
Cathode pin 6 +1.53V
Anode pin 7 +247V
Control Grid (TC) 0V

I can't locate my circuit of the Radio chassis at the moment. Not sure if the diagram has the valve voltages but if anyone with an knowledge of this chassis could look over the above voltages and let me know if they are what they should be. The only thing I thought the screen voltage on the VMP4G at 8V was low as the tech spec for the valve says 100V is recommended. I am a bit of a disadvantage because I can't find the circuit but both these valves seem to have the same grid supply feed. They are fed from a 215V line which is dropped to 73V by 3 large resistors in parallel giving an overall measured 22K resistance This 73V line then supplies the X41 screen pin 3 with 40V via a resistor which measures 22K The screen of the VMP4G is fed via a resistor which measures 330K to give the 8V.

Any advice/suggestions appreciated
Ian

PJL · 23rd Mar 2016, 8:16 pm

Without a circuit, the VMP4G screen grid does not look right. Is it supposed to be a 330K or is that the measured resistance?

brianc · 23rd Mar 2016, 8:47 pm

Here's the circuit for the 900 radio section. I agree with the comments that the HF amp screen grid voltage is too low!

jdwn405 · 24th Mar 2016, 7:44 am

Quote:

Originally Posted by PJL

Without a circuit, the VMP4G screen grid does not look right. Is it supposed to be a 330K or is that the measured resistance?

Yes this is the measured resistance with a meter

jdwn405 · 24th Mar 2016, 8:10 am

Quote:

Originally Posted by brianc

Here's the circuit for the 900 radio section. I agree with the comments that the HF amp screen grid voltage is too low!

Brian

Thanks for the scan of the circuit its much appreciated. Do you have a higher resolution scan as I cant quite make out some of the values ? Can you confirm the values of R72 R73 R76 for me please. Thanks

However I can make out from the circuit that the top end of the 3 parallel resistor network that feeds the grids should I think be much nearer in value to the 315V HT main line than the 215V at present It is connected via choke CM2 to the HT line via flying leads. Could it be mounted separately on the speaker? Anyway I don't think there should be a 100V drop across it - should there??
Ian

peter_scott · 24th Mar 2016, 9:05 am

Hi Ian,

You can download the Omnibus service manual from Jon's site:
http://www.thevalvepage.com/tvmanu/hmv/hmv.htm

Scroll to the bottom of the page.

Peter

jdwn405 · 24th Mar 2016, 9:41 am

Hi Peter
Thanks for that valuable info The manual does not seem to be available for direct download from the website anymore but I have sent Jon an email request
Ian

brianc · 24th Mar 2016, 12:34 pm

You have a PM, Ian. The omnibus data is in my dropbox and I've sent you a link.

turretslug · 24th Mar 2016, 1:27 pm

Just as an aside, a couple of rather nice and rather similar circuit pre-war radio chassis from this stable (Marconi 559 and HMV499) have been on the bench recently and the 1600ohm loudspeaker magnetising coil does indeed lose around 100V-worth of HT. I'm struck by the general similarity of circuit approach (apart from the TV sound part of the bandswitching, obviously)- perhaps design effort in common.

The RF valve does seem to be almost muted by that low screen voltage, judging by the very low cathode volts- could the bypass cap be leaky with that high feed resistor giving low screen volts?

PJL · 24th Mar 2016, 1:38 pm

With a circuit, I am not so sure it is wrong as the screen grid voltage is changed according to the selected waveband to alter the RF amp gain. Usual trick to test the aerial and RF amp circuit would be to attach a wire aerial to the mixer grid (TC).

The other odd voltage is the X41 oscillator grid which should -ve wrt the cathode. One of the problems with measurement here is that the meter capacitance may be enough to kill the oscillator so detecting the oscillator using another radio is the best check.

hilitevr · 24th Mar 2016, 1:44 pm

Hi, Ian

There appears to be some confusion re the screen grid feed to the VMP4G.

What you see as three resistors in parallel is in fact a represntation of three positions of switch S9, part of wavechange switch.

Try rotating W/C switch to see if screen grid voltage alters, if not check S9 for good contacts.

Ht switching on yaxley switches was never a good idea.

Hope this helps

turretslug · 24th Mar 2016, 3:11 pm

R41, feeding the screen grid rail from the HT supply, is indeed three parallel 23k resistors, noted as 7670ohms on the circuit, the lower section of the screen grid potentiometer being R42 consisting of two parallel 7k5 resistors and noted as 3750ohms. Re. post 1, they ought to measure less than 22k in situ.

jdwn405 · 24th Mar 2016, 6:28 pm

Quote:

Originally Posted by hilitevr

Try rotating W/C switch to see if screen grid voltage alters, if not check S9 for good contacts.

Hi
Thanks for the info I will try it out.
Ian

jdwn405 · 24th Mar 2016, 6:48 pm

Quote:

Originally Posted by turretslug

R41, feeding the screen grid rail from the HT supply, is indeed three parallel 23k resistors, noted as 7670ohms on the circuit, the lower section of the screen grid potentiometer being R42 consisting of two parallel 7k5 resistors and noted as 3750ohms

Hi
I have isolated the screen grid potentiometer network and now find the readings to be close to spec 7k3 for the three parallel 23K resistors and 4k7 for the two 7k5 ones.
I am beginning to think that the 225V supply feeding this network is low. The drop over the choke is only 9V. From the circuit I should be getting practically the full output from the U12 rectifier. Can anyone tell me in fact what voltage I should expect from the rectifier?
Ian

23rd Mar 2016, 7:35 pm	#1
jdwn405 Diode Join Date: Mar 2016 Location: Clackmannanshire, Scotland, UK. Posts: 8	HMV 900 Pre-War Mirror Lid TV - Radio Fault Hello I wonder if anyone can help me with a problem with the radio of my HMV900 pre-war mirror lid TV. I had not switched the set on for some time so I thought I would start with the radio section It powers up no problem HT line is 315V. The only response I was able to get was one fading in and out radio station on Short Wave and on Long Wave an audible tone which increased when turning to the lower end of the scale. Nothing at all on Medium Wave I thought the X41 mixer/oscillator might be the problem and changed it but with no improvement. Here are the current voltages on the RF amp VMP4G and the mixer/oscillator X41 valves VMP4G Grid pin 2 - 0.72V Supressor grid pin 3 +0.11V Cathode pin 6 +0.11V Screen grid pin 7 + 8V Anode (TC) +244V X41 Osc Anode pin1 +100V Osc mixer grid pin 2 + 3V Screen pin3 +40V Cathode pin 6 +1.53V Anode pin 7 +247V Control Grid (TC) 0V I can't locate my circuit of the Radio chassis at the moment. Not sure if the diagram has the valve voltages but if anyone with an knowledge of this chassis could look over the above voltages and let me know if they are what they should be. The only thing I thought the screen voltage on the VMP4G at 8V was low as the tech spec for the valve says 100V is recommended. I am a bit of a disadvantage because I can't find the circuit but both these valves seem to have the same grid supply feed. They are fed from a 215V line which is dropped to 73V by 3 large resistors in parallel giving an overall measured 22K resistance This 73V line then supplies the X41 screen pin 3 with 40V via a resistor which measures 22K The screen of the VMP4G is fed via a resistor which measures 330K to give the 8V. Any advice/suggestions appreciated Ian

23rd Mar 2016, 8:16 pm	#2
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Without a circuit, the VMP4G screen grid does not look right. Is it supposed to be a 330K or is that the measured resistance?

23rd Mar 2016, 8:47 pm	#3
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Here's the circuit for the 900 radio section. I agree with the comments that the HF amp screen grid voltage is too low! Attached Thumbnails

24th Mar 2016, 9:05 am	#6
peter_scott Dekatron Join Date: May 2003 Location: Edinburgh, UK. Posts: 3,274	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Hi Ian, You can download the Omnibus service manual from Jon's site: http://www.thevalvepage.com/tvmanu/hmv/hmv.htm Scroll to the bottom of the page. Peter __________________ http://www.nostalgiatech.co.uk https://www.youtube.com/c/PeterScott/videos

24th Mar 2016, 9:41 am	#7
jdwn405 Diode Join Date: Mar 2016 Location: Clackmannanshire, Scotland, UK. Posts: 8	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Hi Peter Thanks for that valuable info The manual does not seem to be available for direct download from the website anymore but I have sent Jon an email request Ian

24th Mar 2016, 12:34 pm	#8
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault You have a PM, Ian. The omnibus data is in my dropbox and I've sent you a link.

24th Mar 2016, 1:27 pm	#9
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,398	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Just as an aside, a couple of rather nice and rather similar circuit pre-war radio chassis from this stable (Marconi 559 and HMV499) have been on the bench recently and the 1600ohm loudspeaker magnetising coil does indeed lose around 100V-worth of HT. I'm struck by the general similarity of circuit approach (apart from the TV sound part of the bandswitching, obviously)- perhaps design effort in common. The RF valve does seem to be almost muted by that low screen voltage, judging by the very low cathode volts- could the bypass cap be leaky with that high feed resistor giving low screen volts?

24th Mar 2016, 1:38 pm	#10
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault With a circuit, I am not so sure it is wrong as the screen grid voltage is changed according to the selected waveband to alter the RF amp gain. Usual trick to test the aerial and RF amp circuit would be to attach a wire aerial to the mixer grid (TC). The other odd voltage is the X41 oscillator grid which should -ve wrt the cathode. One of the problems with measurement here is that the meter capacitance may be enough to kill the oscillator so detecting the oscillator using another radio is the best check.

24th Mar 2016, 1:44 pm	#11
hilitevr Pentode Join Date: Jul 2006 Location: Sidcup, Greater London, UK. Posts: 122	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault Hi, Ian There appears to be some confusion re the screen grid feed to the VMP4G. What you see as three resistors in parallel is in fact a represntation of three positions of switch S9, part of wavechange switch. Try rotating W/C switch to see if screen grid voltage alters, if not check S9 for good contacts. Ht switching on yaxley switches was never a good idea. Hope this helps __________________ Regards Trevor BVWS member

24th Mar 2016, 3:11 pm	#12
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,398	Re: HMV 900 Pre-War Mirror Lid TV - Radio Fault R41, feeding the screen grid rail from the HT supply, is indeed three parallel 23k resistors, noted as 7670ohms on the circuit, the lower section of the screen grid potentiometer being R42 consisting of two parallel 7k5 resistors and noted as 3750ohms. Re. post 1, they ought to measure less than 22k in situ.