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Oscilloscope 12at7 oscillator help needed

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freya
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Been working my way through another scope from my Taylor collection, the problem is it has no running oscillator hence no trace. The circuit is not found anywhere as yet so its taken me a few days to map and trace it all out a section at a time.
As can be seen its still rough but accurate, the two valves are 12at7`s. Taking some voltages reveals something is not right and I would appreciate some help please. Component values have been checked and all are within 10%, valve base sockets checked.
Injecting a square wave from my function generator into the grid of V5a produces the full width trace as expected. Please no comments regarding symbols or straight lines please :bba

 
Posted : 01/02/2015 5:35 pm
Dr Wobble
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Have you checked the valves? Could it be one triode section very low? Had a similar problem on a sig gen with a 6SN7.

Andy.

Curiously curious

 
Posted : 01/02/2015 7:38 pm
freya
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Yes, they all check perfect. The large voltage on the grid and cathode of V6 seems badly wrong Grid 238 volts Cathode 239 volts. :bbd

 
Posted : 01/02/2015 7:52 pm
PYE625
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Hi, to me I would expect the voltage on v6a cathode to be lower because it looks like its only dropping 3 volts from the anode to cathode.
Should there be a cathode resistor on v6a because i can't make one out on the circuit. However, it looks to be directly coupled to v6b anode and then off to a switch. Where does that go to?

Also v6a looks to be DC coupled to v5b anode so the grid voltage will be fixed, but there is a switch that will move v6a grid to an output from the grid of v5b thats not dc coupled. Then the grid of v6a will float towards ht as there is no grid resistor to ground?

Are all the switches working ok?

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 8:09 pm
PYE625
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Forgetting about the voltages for a moment, I'm probably being totally thick, but I would have thought in order for the circuit to oscillate there should be some feedback from v6b anode back to v5a somehow?

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 8:54 pm
freya
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Hi Andrew,
The V6a does not have a cathode resistor, it was all untouched when I got it, and the only work I have undertaken was to change three of the pins on the input amp valve socket which is all working now.
The switch you refer to is the trace flyback toggle plug on the rear. I am unsure of what the initials stand for F/S or B/M
All the joints on V5 & V6 are still painted with the pink marker, showing nothing has changed since manufacture. Very odd isn't it ? driving me slightly mad :ccb

 
Posted : 01/02/2015 8:55 pm
Refugee
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That circuit looks like there should be a "free run" switch that is not shown in the diagram.

 
Posted : 01/02/2015 9:02 pm
PYE625
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It is rather strange as looking at the circuit you would expect there to be a higher volts drop across v6a.
Perhaps it is normal DC wise and as intended, but I can't see how the circuit could oscillate as there seems not to be any feedback from v6b.
Perhaps I'm being thick and missing something?

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 9:05 pm
freya
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Any suggestion on modification to get it going ?

 
Posted : 01/02/2015 9:08 pm
PYE625
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But you did say that applying an input signal to v5a grid makes the circuit operate, so from that I would expect a signal source from somewhere else perhaps is needed ( eg is there another oscillator section within the scope? )

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 9:14 pm
freya
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Sorry, maybe misleading you, I was just experimenting by applying an external square wave directly to the grid, in essence I suppose driving the output from an external source which proves nothing other than the valve oscillator isn't working.

 
Posted : 01/02/2015 9:27 pm
sideband
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Hi Andrew,
The V6a does not have a cathode resistor,

...so where is the 'return' path? Unless it is relying on the relatively low impedance of V6B when it is conducting? Either way, the cathode of V6A can't be left floating.

*Further thoughts*

It looks like V6B is a sort of 'reactance' valve. The capacitor selected by the timebase switch is charged by the anode voltage of V6B (which is DC coupled to the cathode of V6A). This means that the time-constant is formed by the 47K cathode resistor, the impedance of the valve between anode and cathode and the capacitor selected by the timebase switch. This will (should) produce a pulse (dip) on the cathode of V6A causing it to conduct and a corresponding pulse at the anode. I assume that the pot on the grid of V6B will alter the amount of conduction through the valve and therefore the impedance of the circuit which will alter the frequency by changing the time-constant.....something like that anyway. Not seen that sort of circuit before so it's only an educated guess..... :aaq

 
Posted : 01/02/2015 9:43 pm
PYE625
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I think that v6a and b is an SRPP driver stage, but for the life of me I can't see how it can be an oscillator nor v5 for that matter.
V5 looks like an amplifier stage, not an oscillator.
I'm sure another signal is required from somewhere else to feed into v5a.

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 9:58 pm
freya
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Well thats given me lots to think about, thanks Andrew and Rich. Will ponder more over this tomorrow.

 
Posted : 01/02/2015 10:16 pm
Refugee
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It looks like V6A is switched off by a negative pulse on its grid allowing V6B to discharge the timing capacitor producing the ramp that goes to the output stage.
Does it produce a trace if you apply a signal to the Y input?

 
Posted : 01/02/2015 10:24 pm
PYE625
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No worries, it's an interesting little test on the grey matter in which I am sadly lacking lol !! :aaq

To understand the black art of electronics is to understand witchcraft. Andrew.

 
Posted : 01/02/2015 10:25 pm
freya
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I have to admit my gaze has turned to the only other valve position that could be feasible as an oscillator, it has a Z759 fitted but there is notation of V4 hand pencilled on the chassis (only markings). Not drawn the circuit for it yet and it is possible the valve is incorrect in this position.

More tomorrow.

 
Posted : 01/02/2015 10:58 pm
freya
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Does it produce a trace if you apply a signal to the Y input?

No, if apply a signal in the normal manner the dot expands vertical as expected but nothing will expand the dot into a trace at any time base speed. The dot is movable in the vertical and horizontal planes using the controls.

To help with the circuit I have traced out the vertical section, also I have found the specification from Taylor which explains the circuit so we are not working in the dark so to speak.

 
Posted : 02/02/2015 5:28 pm
Refugee
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Have you located the "triggered/free run" switch yet?
It will need to be set to free run to get a trace. Scopes were always like this to preserve tube life.
There must be a trigger take off connection from the Y channel.

 
Posted : 02/02/2015 5:37 pm
freya
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Another photo of front, it should be running being set to INT TRIG, I also have the newer model 33A (very different inside) but same control layout
second photo (bad focus on phone) is applying external square wave to grid pin 3 plus inputting signal to x input amp.

 
Posted : 02/02/2015 6:01 pm
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