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Old 18th Dec 2009, 3:02 pm   #1
oldticktock
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Default Newb Stumped on a Derwent

Hi Board,

Stuck in doors with lots of snow up here on the Pennines so decided to have a go at an unknown 4 valve 1930's Derwent.

It has a round aircraft type dial and says "Puts a Girdle round about the Earth" on the dial. Re-capped and rewired where necessary but now I've got a weird scenario and I just don't know why.

When I turn the set on I hear a music station then it slowly fades out to nothing, if I turn off then on again, It does the same again. The wave change makes a healthy pop when turned.

I'm very new to this hobby and have some old 40's/50's books which assume a certain level of knowledge. I have checked some voltages and the speakers pops and crackles when checking is done. All valves glow. Any ideas, no service sheet not much experience makes for a glum time.

Valves are 80S (rectifier), 42(errrr !!), 6A7 (top cap goes to tuning gang), 75 ( I think this is the output valve as its top cap goes to the output transformer) 78 (is this the AF valve? its top cap goes into a silver can)


All the best OldTickTock
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Old 18th Dec 2009, 3:26 pm   #2
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Default Re: Newb Stumped on a Derwent

Start by measuring the HT in the working then the fault condition. If this is OK, when the set goes faulty, put a signal in at either the gram sockets or the volume control. If this is OK, work back, if not, work forward.

Try cleaning the wavechange switch and the valve pins and sockets. There are all sorts of strange faults here, all caused by muck.

When it goes quiet, try adjusting the tuning - it may be going off tune. Has happened to me once.

Try and find out where the fault is and we can take it from there. Also take readings off all the valves in the fault condition too.

Do you have a signal generator? If yes, try putting the IF in and see if this comes out. Another idea is to get another radio and try and pick up the local oscillator. What is the model of the set - the circuit/manual may be on top there.

Cheers,

Steve P.
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Old 18th Dec 2009, 3:40 pm   #3
Tim
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Default Re: Newb Stumped on a Derwent

The 42 is the output valve. 6A7 mixer, and 75 probably the RF amp.
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Old 18th Dec 2009, 5:02 pm   #4
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Default Re: Newb Stumped on a Derwent

From your description and the valve types, I'm guessing the 75 is the audio amp. A screwdriver placed on the top of this should yield a healthy "buzz" to show that the audio is fine. If this test still works when the signal fades, then the fault lies somewhere in the 78/6A7 HF stages.
You should get some sort of response from touching the top caps of the other valves. Again, if this is missing when the fault occurs, then you may be able to tie it down to one or the other, although a failure in the 78 IF stage will silence any preceding 6a7 response also !
As has been suggested, compare voltage readings on the 6a7/78 valves before and after the fault develops.

Andy
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Old 18th Dec 2009, 5:27 pm   #5
oldticktock
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Default Re: Newb Stumped on a Derwent

Hi started to clean pins on valves as suggested, Just removed the 6A7 and when roatated end on end I can hear, please excuse the next description "tinkles!" is that normal,filaments rattling or is it the sign of my problem I do have a AVO 2 panel tester guess I will have to try it on that.

Just thought if it was broken then it would not be the type of problem i have it would be dead no sound at all
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Old 18th Dec 2009, 5:34 pm   #6
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Default Re: Newb Stumped on a Derwent

Valves of this era do sometimes make mechanical noises like this when shaken. It doesn't necessarily indicate that anything is wrong.

Paul
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Old 19th Dec 2009, 4:06 pm   #7
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Default Re: Newb Stumped on a Derwent

Hi did i read it correctly the top cap of the 75 (g ,1) is going to the output transformer, this cannot be correct. ?
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Old 19th Dec 2009, 4:31 pm   #8
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Default Re: Newb Stumped on a Derwent

No that's me being a plank I traced the wire this time correctly and it goes to the side of the on/off/vol switch. So wish I had a trader sheet, this is too hard for my experience level without one.
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Old 19th Dec 2009, 5:31 pm   #9
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Default Re: Newb Stumped on a Derwent

Don't worry, we all do things like that. There is a lot of difference between a newbie and a plank old lad.

Take readings before and after the fault occurs like we said and we'll take it from there.

Cheers,

Steve P.
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Old 19th Dec 2009, 6:11 pm   #10
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Default Re: Newb Stumped on a Derwent

Cheers Steve, just taken readings after the fault as the fault occurs so quick

80s= 2 pins read 400, 2 pins read 315
75= 1 pin reads 155 the other 30
42= 1 pin reads 335 the other 305
78= 1 pin reads 75 the other reads 335
6A7= 1 pin reads 335 the other 75

looking at that makes newb wonder about valve (75) readings. All readings taken with AVO model 7

Last edited by oldticktock; 19th Dec 2009 at 6:16 pm. Reason: spelling
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Old 19th Dec 2009, 6:25 pm   #11
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Default Re: Newb Stumped on a Derwent

Go in with a signal at the Volume control. Assuming the control has one end to earth. go to here and the other two pins in turn. One should vary with the control, the other shouldn't. Use music, anything. Does not matter. Make a lead with 0.1uF caps in it (one in series with earth, one in series with live) and use anything that comes to hand. An MP3 player is perfect.

Is there a type or a model number with this set? Then we can look for a manual for you.

Cheers,

Steve P.
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Old 19th Dec 2009, 6:52 pm   #12
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Default Re: Newb Stumped on a Derwent

Hi did that and the same thing happens sound can be heard from the mp3 for about 5-10 seconds before total fade out.

No model number for radio
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Old 19th Dec 2009, 7:00 pm   #13
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Default Re: Newb Stumped on a Derwent

Good. We've established that it's an Audio fault. Just check which valve goes to the O/P transformer again. It should be the 42 judging by my book.

Measure the volts on every pin.

If there is an external speaker connection, try an external speaker.

Cheers.

Steve P
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Old 19th Dec 2009, 7:20 pm   #14
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Default Re: Newb Stumped on a Derwent

Ok the 42 does go to the output transformer
reading only in 5 of the 6 pins
340(dc)
310(dc)
30(dc)
25 (ac) is this correct had to switch avo to ac to read.

tried an external speaker same scenario except more hum than the internal

PS it's started snowing like crazy up here it will be heading southwards during the night prepare yourselves

Last edited by oldticktock; 19th Dec 2009 at 7:26 pm. Reason: update
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Old 19th Dec 2009, 8:04 pm   #15
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Default Re: Newb Stumped on a Derwent

Cathode volts seem ludicrously high for a 42 (im also a little concerned about the HT volts)

Would have expected around 260v on the anode, and 6 -10 on the cathode....
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Old 19th Dec 2009, 9:26 pm   #16
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Default Re: Newb Stumped on a Derwent

Hi check volts on 4 grid 1, this is counting clockwise from one of the thick pins ,pin 1 heater, pin 2 anode, pin3 is grid2 pin 4 is grid 1,pin 5 cathode then pin 6 the other heater pin.this is looking from under the chassis,
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Old 19th Dec 2009, 9:28 pm   #17
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Default Re: Newb Stumped on a Derwent

If the 42's cathode resistor was going high in value, wouldn't that cause the lack of volume, high cathode voltage and contribute to excessive HT as the output valve was basically cut off?
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Old 19th Dec 2009, 10:34 pm   #18
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Default Re: Newb Stumped on a Derwent

Hi guys,

well checked again, and as follows cheers vinrads for the guide to pins.

pin 1, 0 reading nothing (heater)
pin 2, about 25 (anode)
pin 3, 310 (Grid 2, screen grid)
pin 4, 340 (grid 1,control grid)
pin 5, 0 reading nothing (cathode)
pin 6, 30 (dc) (heater)

Guys weird thing hear.... I accidently set the avo 7 to read ac, with the ac dial set to 10 range touch pin 6 with the probe and wow station heard loud albeit a bit a tad distorted this can be trimmed better with the tone. If I change the range up to 100 signal goes.

I'm getting tired and confused and not sure why i'm getting no reading on pins 1 and 5 and why the above would happen, should have stuck to clocks. feel the need for a

is it to do with resistance............. blurt!

Last edited by Station X; 20th Dec 2009 at 1:16 pm. Reason: Electrode names added.
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Old 19th Dec 2009, 10:47 pm   #19
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Default Re: Newb Stumped on a Derwent

Sorry to butt in....Derwent is a popular name but who made/supplied them?



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Old 19th Dec 2009, 10:49 pm   #20
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Default Re: Newb Stumped on a Derwent

hi,that problem might be from an open or missing resistor from the 75 or 42 grid to ground.normaly there is a high resistor(~1mohm)at thse points-check resistance to ground with the set off and the said valves removed.
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