The "Sussex" Homebrew Valve Tester.

[Mike Brett]

In answer to Richard.
The 44 VDC is on the two rear coil pins of RL2, I don't think it is a negative voltage. It is also on pins 14 and 11. I assume this must come from the AC pin as this is the only connection to pin 14. In turn the AC pin is connected to D8 and D9 which is the 35Voutput from the bias supply.
I have not seen any output on the bias meter as yet.
Mike

[Top Cap]

Yes a Negative voltage should be available at the main board pin U to supply RL2. RL1 is fed a Negative voltage from pin V. Pin AC which connects to the other side of the coil for RL2 should be at 0V Ground. It is clearly shown on the circuit - Pin AC is connected with pins M, AD and AG - all at 0V Ground. At position 1 (HTR CONT) a 0V Ground from pin AC is routed through contacts on pins 14 and 11 of RL2 (now energised) to the Heater Switch Board pin C5. If you used a connector(s) for board interfacing (I used pin 33 of a 50 way cannon D) the 0V then goes through the valve heater under test (you can use a wire link) back to the Heater Switch Board C7 via the fore mentioned connector (I used pin 30). The returning 0V now goes through RL2 contacts (pins 21 and 24) to the Heater Switch Board pin C3 and the fore mentioned connector (I used pin 31) to the Anode of LED 4. The cathode of this LED is connected to -40V via the 10k resistor R28 so the LED should light. I can confirm that in the position of the electrode switch i.e. HTR CONT, it should be possible to see the Grid voltage on the grid meter or alternatively on the two 4mm test sockets GRID w.r.t Cathode. Hope this helps, Les

[Top Cap]

The Heater Switch Board showing Pin 14 of RL2 going to pin AC (0V)

Did you cut the track on this board linking RL2 pin 14 to C4? You need this part of the board open circuit, the mods were required to prevent shorting of the 40V supply through a valve with a shorted cathode/heater.

[Mike Brett]

Here is my finished Sussex. Still trying hard to get it working though, so still could end up as a very expensive £380 door stop.
Mike

[Mike Brett]

I tried removing the wires from AJ20 and AK21, but it made no difference, I still had the 40 v on the heaters.As I said before if I remove the 35v from the board all the heater voltages are spot on. If the gride transistor is TR8 I measured 40 v between the emitter and the collector and 0 v between the emitter and base, this did not change when operating the bias control. These results where the same with wires AJ20 and AK21 removed.
It would seem to me that the first important component in the bias control is TR8. Would it be a good idea to replace this to see if it was broken. Not sure if this could possible cause the 40 v effect on the heater pins though. I keep checking for faults or shorts in the wiring but have still no found any.
Mike

[Richard]

Originally Posted by Mike Brett

Here is my finished Sussex.

Nice job

[Top Cap]

Hi Mike, by all means lift TR8 from the circuit then power up. Check the Base Pad gives you zero to -40 something volts as you turn it. That should, if correct, give you a variable voltage unless the transistor is short circuit. TR8 has an easy life really as the current is only flowing through R24 (10k) and apart from the initial charging current for C9 (only 1uF) it just provides a very small grid current. The harder job of energising relays is of course done direct from the bridge rectifier output. R26 and C10 provide additional ripple rejection for the Base bias control VR3.

AJ and AK have nothing to do with the -40V supply, it is the Anode current control circuit.

[Mike Brett]

Slight bit of progress. When the Function switch is in position 1-2 or 3 I get all the correct voltages on the valve heaters. It only jumps to 42 volts when the Function switch is in the 0 position. Still no voltage on the bias meter as yet. Just checking all the wiring especially around RL2 .Regarding RL2 at present when the Function switch is in position 0 there is -40v on pins 12- 22- 21, -15v on pin 24 and 0v on pin 11 and 14. When the Function switch is in position 1-2 or 3 there is -42v on pins 11-12-22, -15v on pins 21-24 and 0 v on pin 14.
Mike

[Top Cap]

The Heater rail WILL rise to -40 odd volts when the Function switch is in 0 position. This is the Heater Continuity supply and will disappear when RL2 de-energises in the other switch positions. You can remove the cathode wire from the HEATER CONTINUITY LED to prove this or be brave and quickly short out the heater lines with the Function switch in the 0 position, you should see the HEATER CONTINUITY LED illuminate.

[mole42uk]

I have just now finished commissioning my Sussex. It seems to work as expected although I do have some difficulty getting text-book readings from the valves I've tried so far - a clutch of EF86, a few ECC83 and a new 6L6 which failed because it needs 100mA anode current. It's good to know that the current limiter works!

I did have a few hours of frustration this morning because the Gm meter didn't display anything except 0.00 even though the Ia meter was showing an acceptable anode current.

The screened cable connecting the 100mV AC signal and the bias to the grid had a core-to-screen short somewhere in the loop. I was wondering why I was having trouble setting the 100mV.... A new loop of screened cable to all nine switches, tested before installation, fixed that one and the Gm meter is now showing something believable.

One idea - has anyone got a calibrated AVO to check a few valves from me to the correct test conditions, so that I have some valves to calibrate my Sussex with? I would be willing to lend them out to other Sussex constructors for the same purpose.

Richard

[Top Cap]

Hi Mike, I have been looking so hard to find anything that may cause failure of your variable bias supply. I note that the board connections for TR8 appear to be wrong, after all this time!
Anyway, I would imagine you would have realised the transistor just drops in-line but I have attached how I think the board layout should be for TR8.

I hope I have it right now, perhaps you could confirm?

Cheers
Les

[mole42uk]

Originally Posted by Top Cap

I note that the board connections for TR8 appear to be wrong, after all this time!

I think you are correct Les, I remember checking that with the transistor pinout when I assembled the board. I had to check the MosFETs pretty carefully as well because I mounted them off-board.

Richard

[Top Cap]

Hi Richard - Yes the dangers of cut and paste - especially after midnight and a few cans too!

[Mike Brett]

Les
Thanks for the info about the -40v on the Function switch zero setting, it would seem at least I have got the heater section sorted.
I had already seen somewhere that TR8 should read E C B, this is the correct sequence on the pins of the MJE 350.
I have printed out an enlarged schematic diagram and am tracing every single wire in my Sussex ,any discrepancy and I will hopefully find it.
Cheers Mike

[Top Cap]

Here to help Mike, the only thing that could load down the bias voltage is a fault in the wiring to the voltmeter (output from board T) as everything else is current limited by 10 k resistors R24 and R23. However, it is possible that you have D7 wrong way round or even C9 1uF. If the voltage on the pot is swinging from zero to -40 odd volts then TR8 should be passing this change to its Emitter.

[Mike Brett]

Hi Les
I have checked the wiring from T 45 ,emitter to the voltmeter and also N 46 to the voltmeter, and D7 and C9 are orientated correctly.Regarding your comment ,"If voltage is swinging from zero to -40 volt on the pot." there is no voltage on the pot,the meter remains at zero.
Mike

[mole42uk]

So, have you got the -40v across C11 and C10?

If you cannot get the slider of the pot VR3 to adjust between 0 and -40v then you won't get any output from the pass transistor TR8.

Can you check the voltage across the 8v2 zener - ZD10? That should be about -8.2v and if that is present at least you can verify the -40v supply is working.

Richard

[Top Cap]

Hi Mike, the -45V supply must be there as the relays are operating correctly. The pot is fed via R26 (470 Ohms), additionally smoothed by C10 (100uF) then fed out of the board on pin Y and (on my unit) pin 49 of the interface connector. This should connect to the pot contact that sees the wiper at maximum rotation i.e. clockwise. The pots earth return from its other outer tag goes to 0V via pin 48 of the interface connector and board pin W.
The pot is a simple variable voltage divider whose wiper should go from zero to approximately -40V, this being fed to the base of TR8 via the resistor R25 (1k). I have checked over the main board circuit and it seems OK, with W, X and Y going to the right points.

[Mike Brett]

Right I have -44v across C11 and C10.
There is -8.0 v across zener ZD10.
Pin 48 on the board goes to the back of the pot.
Pin 49 goes to the connection marked CW on the pot, and pin 50 goes to the wiper, middle pin on the pot.
There is -43v across the pot on the end pins but nothing registers on the middle pin until almost to the end of its travel and then I get about 1.7 v.
Mike

[mole42uk]

Okay, so you must have a faulty pot.

When the wiper gets to 1.7v, what output do you get from the transistor?

Richard