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Old 8th Mar 2020, 9:51 pm   #1
sobell1980
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Default Robert's RMB in for repair

Hello all,

Firstly, apologies for the long absence. I'm still working on various sets but I find I have less and less time these days. This set some of you may remember is the set I cut my teeth on. My very good friend now owns this and is used daily in his home. I've had it back now twice over the years for repair. First time was a bad valve socket pin contact. The second time was an open circuit in one of the valve heaters.
Today my friend dropped the set over saying it didn't work. The trouble is he religiously listens to radio 4 so didn't try and tune into another station or use MW. It was still tuned into R4 on delivery to me.
Firstly, was R4 down this morning?
I've removed the valves as I understand the heater filaments on these valves are very fragile and prone to failure only operating on 1.4 volts.
I have between 7 and 9 ohms on the heaters. However, the DL94 valve heater is measuring 20 ohms! Is this correct for this valve heater?
I've cleaned all the valve sockets and tried the set and R4 burst into life. So possibly R4 was down this morning or I've got a valve base problem . I was concerned about the 20 ohm reading on the DL94?
many thanks
Dave.

Last edited by sobell1980; 8th Mar 2020 at 9:56 pm.
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Old 8th Mar 2020, 10:13 pm   #2
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Default Re: Robert's RMB in for repair

I know the DL94 Heaters were centre tapped unlike the other valves so I would think the 20ohm would be fine.
Steve.
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Old 8th Mar 2020, 10:18 pm   #3
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Default Re: Robert's RMB in for repair

The resistance when cold will differ from that when the filament has current passing through it and is hot.

The filament is rated at 1.4V 0.1Amp. Thus, to pass a current of .1Amp, the resistance would need to be 14 Ohms when hot, so 20 Ohm’s cold doesn’t seem out of the way. In any event, (unlike for example a carbon composition resistor), the resistance of the filament is unlikely to vary during its life. The usual failure mode of battery valves is an open circuit filament.

Or so it seems to me.

Hope that helps a bit.
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Old 8th Mar 2020, 11:37 pm   #4
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Default Re: Robert's RMB in for repair

I think that's back to front somewhere- the cold resistance should be lower than the hot.

However, the DL94 cold 20R is actually 2*10R in series. In parallel this would be 5R which fits in nicely with the values for the other valves which only take half the filament current at 1.4V of the DL94, (50mA vs 100mA).
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Old 9th Mar 2020, 12:08 pm   #5
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Default Re: Robert's RMB in for repair

I have experienced valve holder pin/socket failure on these models and many others in the DK series battery receivers. They tend to decompose and loose their spring contact. They also take on a dull grey appearance. The McMurdo type pins are easily replaced.

It's a guess that it may be the fumes from the batteries, especially the exhausted ones that remain for years in a set aside receiver. This decomposition only appears to take place in battery receivers and can also effect output transformer/I.F. and tuning coils. Worth a check. John.
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Old 15th Mar 2020, 7:37 am   #6
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Default Re: Robert's RMB in for repair

Hi all
I did change all the valve sockets on the set several years ago when I first restored it. This was due to the corroded valve socket terminals that you mention. They were very tarnished and some did crack and break up. I will run through the valve voltages as a start and make a note of those. I will leave it on test for several hours and see if anything is faulting when getting hot. I will let you know.
Many thanks for your replies.

Dave.
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Old 28th Mar 2020, 11:33 am   #7
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Default Re: Robert's RMB in for repair

Today I'm going to leave the set on test most the day. Lots of time for hobbies now. I will feed back yo let you know how it performs. I will also run through the valve voltages.

Dave
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Old 29th Mar 2020, 4:39 pm   #8
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Default Re: Robert's RMB in for repair

Hello,
I've spent some time on the set today. I had it on yesterday for over 5 hrs. Seemed ok, I'm not used to these sets and the volume seemed loud but nothing like my other valve sets. From cold it bursts into life in seconds so I guess the valve heaters are good.
I've disconnected each valve in turn to take my valve measurements so I didn't have to dismantle the set and the very fragile frame aerial.
So, perhaps my readings are not textbook as the circuit to each valve was not under load when tested. Here are my findings,

V1 Anode 81.00 volts DC- should be 85 volts DC
V1 Screen 82.00 volts DC- should be 44 volts DC
V1 heater 1.50 volts DC

V2 Anode 79.00 volts DC - should be 85 volts DC
V2 Screen 28.00 volts DC - should be 28 volts DC
V2 Heater 1.5 volts DC

V3 Anode 75.00 volts DC - should be 11.00 volts DC
V3 Screen 30.00 volts DC - should be 3 volts DC
V3 heater 1.5 volts DC

V4 Anode 128 volts DC - should be 81 volts DC
V4 Screen 128 volts DC - should be 85 volts DC
V4 Heater 1.7 volts DC

The service data also states that the quoted figures should be approx 10% higher when running on mains and not the battery.

What do the experts think?

Dave.
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Old 29th Mar 2020, 4:52 pm   #9
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Default Re: Robert's RMB in for repair

Quote:
Originally Posted by sobell1980 View Post
Hello,
I've disconnected each valve in turn to take my valve measurements so I didn't have to dismantle the set and the very fragile frame aerial.
So, perhaps my readings are not textbook as the circuit to each valve was not under load when tested.

Dave.
I haven't looked at the circuit, but I'd have thought the readings will be screwed on account of no anode or screen current being drawn. There'll be no volt drop across any screen dropper resistors for a start, so the readings will be high.
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Old 29th Mar 2020, 4:55 pm   #10
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Default Re: Robert's RMB in for repair

Ditto.

Lawrence.
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Old 29th Mar 2020, 4:57 pm   #11
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Default Re: Robert's RMB in for repair

I thought as much chaps. Circuit isn't under load. Leave it with me. I will get back to you with some sensible readings.
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Old 29th Mar 2020, 5:08 pm   #12
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Default Re: Robert's RMB in for repair

Quote:
Originally Posted by sobell1980 View Post
I thought as much chaps. Circuit isn't under load. Leave it with me. I will get back to you with some sensible readings.
Also note that the voltages as measured in the Trader sheet were measured with an Avo 7 meter, that meter has, for loading purpose calculations, an Ohms Per Volt of 500 Ohms.....Eg on the 400 volt range its input resistance as presented to the circuit would be 200k.

You should take that into account if using a DMM or a 20k OPV analog meter such as an Avo 8 etc.

Lawrence.
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Old 29th Mar 2020, 5:10 pm   #13
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Default Re: Robert's RMB in for repair

Yup! I've been caught by that one and tried to find non-existent faults.
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Old 30th Mar 2020, 8:49 am   #14
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Default Re: Robert's RMB in for repair

Ok, here are my valve readings with all valves in circuit. Set to MW and tuned to the lowest wavelength.
V1 ANODE - 88V - SHOULD BE 85V
V1 SCREEN 39V - SHOULD BE 44V
V1 HEATER 1.36V

V2 ANODE 88V - SHOULD BE 85V
V2 SCREEN 38V - SHOULD BE 28V
V2 HEATER 1.37V

V3 ANODE 69V - SHOULD BE 11V
V3 SCREEN 22V - SHOULD BE 4V
V3 HEATER 1.37V

V4 ANODE 86V - SHOULD BE 81V
V4 SCREEN 87V - SHOULD BE 85V
V4 HEATER 1.37V

much more sensible readings.
The ones I would like to query are, V2 screen seems slightly high.
V3 anode is high but the quoted figure of 11v seems very low to me?
Same with V3 screen as for its anode.
Service data quoted 4V.

The rest of the figures are about there I'd say.

Many thanks

Dave
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Old 30th Mar 2020, 8:52 am   #15
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Default Re: Robert's RMB in for repair

Where screen grid voltages are wrong check the screen dropper resistor (may have changed value) and the capacitor from screen grid to chassis. (may be leaky).
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Old 30th Mar 2020, 10:12 am   #16
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Default Re: Robert's RMB in for repair

Can I just confirm a couple of things,
Does the service data seem correct for valve 3, DAF91? 11v on the anode and 4v on the screen? This seems awfully low?
If you look up the valve data my readings are within the valve limits. Can anyone explain why the quoted figures for v3 are so low?
The figures quoted are when the set was being operated off a battery. Please see valve data sheets comparable to my readings for valves 2 and 3.
I appreciate what you are saying about anode and screen grid resistors but in my experience they normally go high in value and not low.

Dave.
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Old 30th Mar 2020, 10:51 am   #17
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Default Re: Robert's RMB in for repair

The service sheet quotes 11V and 0.1mA for V3 anode. The anode resistor is 560k, so the volt drop across it will be 56V. 90V HT - 56V equals 34V. Measuring the anode voltage with an AVO model 7, as quoted in the service sheet, will pull more current through the anode resistor dropping the voltage still further.

Check the value of the anode resistor and measure the voltage across it.

I'll let you work out the volt drop across the screen dropper resistor
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Old 30th Mar 2020, 10:53 am   #18
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Default Re: Robert's RMB in for repair

Quote:
Originally Posted by sobell1980 View Post
Can I just confirm a couple of things,
Does the service data seem correct for valve 3, DAF91? 11v on the anode and 4v on the screen? This seems awfully low?
If you look up the valve data my readings are within the valve limits. Can anyone explain why the quoted figures for v3 are so low?
The figures quoted are when the set was being operated off a battery. Please see valve data sheets comparable to my readings for valves 2 and 3.
I appreciate what you are saying about anode and screen grid resistors but in my experience they normally go high in value and not low.

Dave.
First, see my previous post, post#12....

Trader says voltages measured with an Avo 7 on the 400 volt range. You should now know that on the 400 volt range that the resistance between the meter probes with the meter set to measure 400 volts will be 400*500,
that's the meters range setting multiplied by the meters internal meter movement circuit resistance of 500 ohms, that results in a resistance between the meter probes of 200,000 ohms (200k) that means when that meter is connected in circuit it's the same as connecting a 200k resistor in circuit, apply simple potential divider theory and you will now see why the measured voltage in the Trader sheet is so low....Here's a potential divider calculator, bung the numbers in and see what you get, I make it approx. 3.5 volts, which is in the ballpark taking into account meter accuracy, component tolerances, human error etc:

https://www.electronics2000.co.uk/ca...calculator.php

Circuits are comprised of potential dividers.

EDIT: Post crossed.

Lawrence.

Last edited by ms660; 30th Mar 2020 at 11:12 am. Reason: added word
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Old 30th Mar 2020, 10:58 am   #19
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Default Re: Robert's RMB in for repair

The service data figures should specify what meter was used to get the readings. Chances are it would have been one with a lower input resistance than whatever you're using hence the lower indicated readings.

The DAF91 will be running at Ia probably in the 10s of uA region with a megohm or so anode load so any voltage indicated will be dictated more by the resistors than anything the valve is doing!

Edit: We're all saying the same thing more or less succinctly.

I would add that if you're using a cheap DMM it'll probably present a 1meg load (five times the AVO 7); a decent one 10megs.....

As an extra comment, measuring voltage across the anode or screen resistors themselves will be just as " inaccurate" since the electrode current will see a lower resistance (resistor and meter in parallel) and develop less voltage for the meter to see.
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Old 30th Mar 2020, 11:05 am   #20
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Default Re: Robert's RMB in for repair

See post #12.

V3 will be the only valve with a resistive anode load rather than a transformer (IF or output), so will be more affected by the low resistance of the AVO 7 used to generate the voltages on the Trader sheet. Your measured anode voltage is probably fine for a high impedance meter. For screen grid voltages, you need to look at the screen resistor values (possibly zero Ohms for the output valve V4).

Stuart
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