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Old 14th Jul 2013, 9:21 pm   #881
Mike Brett
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Richard
I get 0.8v between emitter and collector, -40 v between base and emitter and -40 v between base and collector.
I tested the pot with my meter and the resistance goes smoothly from 0 ohm to 50 k ohm.
Do you still think its the pot only it's a brand new Vishay one.
Mike
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Old 14th Jul 2013, 9:42 pm   #882
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Default Re: The "Sussex" Homebrew Valve Tester.

Have you measured from each end of the track, it should be 50k
Mike
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Old 15th Jul 2013, 7:11 am   #883
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Default Re: The "Sussex" Homebrew Valve Tester.

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Do you still think its the pot only it's a brand new Vishay one.
I would be inclined to think that a new Vishay pot will be okay. If you disconnect the wire from the wiper, and repeat the measurement, do you then get a steady change in voltage on the wiper as you rotate the pot?

If so, then either TR8 is faulty, or the ECB pins are wrong way around, or there's a wiring fault between the pot and the PCB.

Richard
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Old 15th Jul 2013, 7:42 am   #884
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Default Re: The "Sussex" Homebrew Valve Tester.

40v between base and emitter of any bipolar transistor is too much if it is in the direction of forward bias, the transistor should limit it to under a volt, unless the base connection has gone open. If the 40v is in the reverse bias direction, it is far larger than the absolute max rating of any transistor I know of, and I'd expect the base-emitter junction to be breaking down in avalanche mode. Most bipolars break down (reverse bias on b-e) between 7 and 15 volts. IF the source of the reverse bias limits the current to a low enough value, the device might not be absolutely destroyed, but it will have progressively degraded gain, frequency response and worsened noise.

Wherever this transistor is, it's in trouble.

David
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Old 15th Jul 2013, 9:02 am   #885
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Default Re: The "Sussex" Homebrew Valve Tester.

I was just looking at the current limiters on the anode and screen circuits.

Is there any reason why the anode limits at 95mA? Is there any real danger in using a momentary push-button to increase the limit to, for example, 150mA? The MosFET will handle 4.5A and the transformer should be good for a few more milliamps for a few seconds.

I'm keen to test some 6L6 valves which need about 120mA.

Richard
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Old 15th Jul 2013, 12:10 pm   #886
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi all
I changed the MJE350 TR8 on the board and it all started working as it should, I now get the 0 to - 44 v on the meter. Both of the MJE350 came from the same supplier so I guess I was just unlucky to choose the duff one, unless I damaged the first one soldering it in. Anyway thanks very much for your help guys. I am not out of the woods yet though, as I still have to connect the HT taps and test the rest of it. I see it is recommended to supply power slowly through a variac, as I do not have one of these would it be a good idea to power it up with a light bulb in series . Never tried this before but if it is safer I will give it a go.
Cheers Mike
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Old 15th Jul 2013, 5:43 pm   #887
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Default Re: The "Sussex" Homebrew Valve Tester.

Good news indeed!

I didn't use a variac either, both of mine (one single and one three phase!) are dismantled at the moment!

I did power up the Sussex in stages, however, testing each part as I went. I have used the lightbulb technique in the past, it does help to limit the current in the transformer primary if there's a problem somewhere. I prefer the stage by stage approach however, because I can see that each part of the circuit works before going on to the next stage.

Richard
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Old 15th Jul 2013, 7:04 pm   #888
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Default Re: The "Sussex" Homebrew Valve Tester.

Great news Mike! I used a variac because I happened to have one and it is a good piece of gear for slowly winding things up, watching for anything going astray and hopefully saving smoke and burnt parts. Certainly use a lamp limiter, I would if I did not have a variac in this application. If you have the extra HT fuse fitted then that may help if anything serious is amiss, else go for it! You should be able to see the HT come up and with luck the Mosfet outputs will change as per the voltage switch settings on both Anode and Screen circuits. Richard, wrt the increase in available current, the enemy here is power dissipation especially on the lower voltage settings where the device has a much higher voltage between Drain and Source. I did do a change to my Sussex as detailed in the handbook:-
It was felt that a bit more A1/A2 anode current would be beneficial when testing some valves so R39 was deleted and R35 changed from 27Ω to 11Ω which with R6 gives approximately 120mA. To avoid having three resistors in parallel for the diode current, R38 was deleted and R36 and R37 made 27Ω each.
If you use the tester as per the AVO CT160 instructions then you may not need the higher current but I altered my tester so that I could plot characteristics of my EL34's and KT88's.
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Old 17th Jul 2013, 10:15 am   #889
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Default Re: The "Sussex" Homebrew Valve Tester.

On the low voltage board for the DC 2v I have 2.37v which I assume is acceptable as this is under no load conditions. But for the DC 1.4v I have 2.23v , do you think this is just a bit to high and needs to come down a little.
Mike
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Old 19th Jul 2013, 8:55 pm   #890
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Mike, yes by all means add more diodes to the circuit as already given as options. To be honest, the existing circuit was a quick knock up to save having to apply an external supply for these battery valves. As others have realised, it probably would be just as well to add dry batteries to achieve this function or a custom PSU unit to be applied to the EXT sockets. Since having to sort out the -45V shorting problem I realised that the -45V current limited voltage from the cathode leakage LED may upset the working of the tester when battery valves are being tested. The only valve I have here is a DL93 with centre tapped heater, not the best to use for evaluation purposes. I still think the circuit needs to remove the cathode leakage circuit when battery valves are tested yet it is still desirable when EXT heater supply is selected provided you are not powering battery valves or other directly heated types. I would be interested to see what happens if the Cathode Heater LED is connected to 22 instead of pin 12 of RL2. Perhaps a better idea is to provide another isolation Relay so that the heaters are totally isolated from everything else, except heater supplies, when the battery valve positions are selected. Alternatively a small toggle switch for isolating the Cathode Leakage LED at these settings may be a quicker and cheaper alternative.
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Old 19th Jul 2013, 9:54 pm   #891
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Les
Yes I was thinking along the lines of another isolation relay to protect the battery valves, I grant you a switch would be quicker but if one was to forget to operate it!.
I had fitted an extra digital meter to my Sussex to show heater voltage but when I was having trouble with my high heater voltages this blew the meter and I had to order another one from e-Bay. At present I am leaving this meter isolated until I can figure out the best way to wire it.
Mike
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Old 20th Jul 2013, 9:15 am   #892
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Mike, when your heater meter blew, was it on a common supply with one of the other meters? This could certainly cause its demise.
You should be able to add another meter supply winding with 19 turns of thin insulated wire in the small gap between winding and core.

Ed
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Old 20th Jul 2013, 2:15 pm   #893
Mike Brett
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Ed
No it was on a separate small transformer that I had to hand. Actually after a short while the digits on the meter did come back on, but the meter back light did not so it still needs changing.
Mike
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Old 20th Jul 2013, 4:40 pm   #894
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi Les
I am just doing the final tests on my Sussex and the voltages I am getting on my multi-meter do not match the dial markings.
On my front Anode and Screen dials I have these values.
0v--50v--75v--90v--100v--120v--150v--175v--200v--225v--250v--300v.
I am actually getting these.
0v--60v--91v-110v--122v--154v--185v--217v--250v--280v--312v--312v.
I seem to have missed out the 75v and got two 300v, Ugh.
Mike
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Old 20th Jul 2013, 5:56 pm   #895
Mike Brett
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Default Re: The "Sussex" Homebrew Valve Tester.

Les
I think I can see where the problem lies. I have followed your physical wiring diagram on page 18 and the connections from the board numbers 34 and 36 both go to the anode and screen switches pins 11. Should these not go to pins 12 on both switches otherwise the resistors in between pins 11 and 12 are not being used. This would explain me getting 300v on pins 11 and 12 instead of 250v and 300v.
Mike
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Old 20th Jul 2013, 6:03 pm   #896
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
Originally Posted by Mike Brett View Post
I am just doing the final tests on my Sussex and the voltages I am getting on my multi-meter do not match the dial markings.
Mine isn't absolutely precise, but it's a bit closer than that.

The resistor chain on the voltage switch is a bit of a challenge but it might be worth checking that. Most of them are paralleled 150k resistors, but there are a few 15k ones in series. I used some precision resistors from my junk box: here's a picture.....
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Old 21st Jul 2013, 1:06 pm   #897
Mike Brett
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi all
I now have the correct voltages on my screen and anode switches. Most of you will already know this but for those just starting on the Sussex I can confirm that the wires from board pins 34 and 36 go to position 12 on the screen and anode switches, and NOT as shown on the wiring diagram going to position 11.
Mike
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Old 21st Jul 2013, 2:54 pm   #898
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Default Re: The "Sussex" Homebrew Valve Tester.

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One idea - has anyone got a calibrated AVO to check a few valves from me to the correct test conditions
To follow-up that, David Simpson kindly sent me a 'standardised' valve, complete with a chart of the test from his CT160 tester.

I put this valve through its paces on my Sussex, and produced exactly the same curve as he has, although the actual numbers were slightly different, probably due to my heater voltage being a little higher than text-book.

The Gm meter reads accurately the anode current slope.

I also have a used Mullard ECC85 which has been labelled with the tested mA/V figures, so I'll check that next.

Richard
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Old 21st Jul 2013, 3:34 pm   #899
Mike Brett
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Default Re: The "Sussex" Homebrew Valve Tester.

Hi all
Just got to the last test on my Sussex, testing for oscillations across the Grid and Cathode terminals using an R.M.S meter.Of course you know whats coming now, I have not got one of those meters folks. Is it possible to test this with perhaps an oscilloscope.
Mike
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Old 21st Jul 2013, 3:55 pm   #900
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Default Re: The "Sussex" Homebrew Valve Tester.

Quote:
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Is it possible to test this with perhaps an oscilloscope.
Of course. Just connect the scope to the grid, switch it to AC and check the oscillator is running. I don't remember the frequency, but it's a lot higher than 50Hz!! You'll be able to adjust the oscillator for the best output, then adjust the other pot for a 280mV peak-to-peak signal on the grid. That does it.

Richard
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