Need help troubleshooting Weston 981 Tube [Valve] tester

[nzoomed]

I bought a Weston 981 valve tester off Ebay and have been in the process of repairing the unit, the filament selector switch got broken during transit, and ive had to replace the rotary switch.

To cut a long story short, ive replaced the broken switch but im not getting any readings on my meter when testing any tubes.

Im not sure if other things have broken during transit, but the only tube the tester uses (3A4) had popped out of the socket and was rolling round inside.

If anyone can point me into the right direction on where to troubleshoot this unit would be great.

The schematic can be found here on the boat anchor manual archive.
http://bama.edebris.com/manuals/weston/981

[Bazz4CQJ]

Hi,

Re the filament switch; are the valves showing lit filaments? If not, then they don't have voltage to them and you need to get them to light up.

This set clearly measures the Gm of a valve, so the 3A4 needs to be working well and properly seated in the socket; is that lit up?

If the filaments are lit, can you get readings on a diode/rectifier valve under test, for which the Gm circuit is not required?

B

[nzoomed]

Yes, the heaters are glowing up on the valves being tested.

I dont have any spare 3A4 tubes, but I have asked a supplier to look for one last week, because I suspected it cant have been good with it being banged around so much during transit in the aircraft.

The 3A4 heats up, but its not bright because its only a 2V heater.

Good idea, I will try testing some rectifier tubes to see what results i get.

[Diabolical Artificer]

Not familiar with this tester but i'd start with a meter on a valve base and check you have heater 6.3v as Bazz sais and that it has HT etc. Start with a simple triode, if your lucky it's dirty base contact's. A drill of the right size pushed in and out held in your finger's helps.

If no voltage present it's a matter of tracing where the break is, start with the power tfmr and work your way forward.

Good luck, Andy.

Posted same time as the above post. In that case have you got HT and a voltage on the grid?

[nzoomed]

OK, ive just tried some rectifier tubes and im not getting any reading either.

However, I can confirm that at least the meter does work, and without any tubes plugged into the tester for some reason the needle moved right up when i move the anode voltage selector to position "G"

I dont know if this is to be expected or not, but the meter is at least able to get power.

Me thinks that the tubes are not getting any HT supply perhaps?

I will have to measure the anode voltage and see if anything shows on my meter.

[nzoomed]

Im thinking its probably worth replacing the old filter caps, as they are bound to be leaking, but im having trouble finding any 20uf caps, seems most are around 47uf that I can find.
Will it hurt to use larger values? AFAIK, its not a big deal, as more usually is better.

Im not sure what the voltages should even be across these caps when measuring, but they are well under 100v with my meter, the highest one read about 80v and i see these are rated at 250V, so I would have expected the voltage to have been higher?

This schematic is hard to follow as others have complained about on other forums ive been reading also.

Im trying to work out where the HT supply comes from, and what voltage it is supposed to put out, I would expect at least 250-300v would be required to test most tubes?

[Bazz4CQJ]

I wouldn't rush in to this. There are definitely some circuits where more is not better. You could do some tests of the existing components rather than just assume they are bad. The phrase "if it ain't bust, don't fix it", is very sound.

The diagrams of all valve testers are hard to figure; I have an AVO VCM and that's the same. However, it is well worth spending some time trying to figure them out as best you can. I use coloured pens to mark up the various lines.

What are you doing with the bust transformer?

B

[Diabolical Artificer]

I had a quick look at the schematic yesterday and whilst confusing what with all the switching it's pretty straight forward. The mains tfmr T1 is switched to one or more of the multiple taps to get your HT. The 3A4, T2 and the resistor divider etc is to give you your g1 signal (AC approx 1v) to measure gm. Using a high lighter pen to trace what goes where helps here.

So don't go replacing the cap's just yet, you need to know if - 1) you have HT at the valve base and 2) if you have an AC voltage on your grid. With all those switch's I'd make sure you have it set right before opening it up. I'd give all those SW's a good clean too, they're more likely to malfunction than any discreet component. Unfortunately, your probably going to have to read the manual. : ) Most of us don't, right ?

Lastly caps nowadays come in pretty standard value's like resistor's, so 1, 22,33,47,56,68,10. Using a 22u in place of a 20u is fine. It's the voltage rating that is the most important for most application's. It's a lot more complicated than that, but the basic rule is, it's cool to go higher in value by a bit, and cool to go higher in voltage rating, but never lower.

To recap ( pun unintended), check you have HT/AC, if not check you have AC in and out of T1, if yes, check you havn't done something stupid and get the old bright yellow pen out and suss the schematic out.

Andy.

[Bazz4CQJ]

Andy; re the use of highlighter pens to mark circuits. On the one occasion I used fluorescent pens to mark up a circuit, although I could see the markings very clearly, I subsequently found that my Canon scanner could barely detect them. Seemed like a quirky effect, but I've gone back to using standard coloured fibre-tips, which the Canon scans perfectly . Have you any experience of this?

B

[nzoomed]

OK, that's good to know about the caps.

I have not had much time to look at it, but hope to check the voltages later today.

Bazz4CQJ, the winding that broke on the transformer was for the heater supply to the 3A4.

I was able to thankfully re-solder the broken wire back to the transformer with a bit of perseverance and got it to work, so everything there is OK!

I also replaced the rotary switch and got that all working.

I will pull the scope out too and scope the grid output.

My suspicion is that the 3A4 may have suffered damage from transit as it was loose in the bottom of the case rolling around with bent pins.

[Diabolical Artificer]

What meter is your DMM? A half decent one will measure 1v AC 5khz so no scope needed, you should have a few volts of negative DC on g1 too. This bit (see pic) in your manual gives you a simple schematic to follow.

Andy.

[nzoomed]

I confess that my multimeter is a cheap chinese meter, but has done the job for me for some time. Im am wanting to get a fluke 77 series meter very soon however!

I dont know how accurate it is when measuring anything under 1V, AC in particular, so the scope would be useful.

Anyway, I cant measure any voltage on the HT supply at all on the tube socket.

I was also going to ask if i should expect to get a reading all the time, or only when I flick the switch to run a Gm test?

I was reading another thread on antique radios dot com and someone had the same issue as me, and replaced the meter sensitivity pot to get theirs working, but I dont know if thats my problem (yet).
I will check the grid voltages etc, but it wont surprise me if its a bad contact on a switch, (there are many of them!) might have to spend a bit of time cleaning them all.

[nzoomed]

OK, well im having a bit of trouble tracing through the circuit.

If only it said what colours the wires were!

Some things don't add up, as the selenium rectifier is actually two soldered in series, but the schematic does not show this.

It appears that it's only using half wave rectification in this tester also.

I'm not getting any reading when doing line check, and one of the electrolytic caps is reading 200v across, but the others are not showing much.

With an 807 tube I'm getting about 40v negative bias when measuring this, but no plate voltage, tracing this back is the hard bit.

Doing a line check also does nothing, I believe this should make the needle move on the meter, but I'm getting nothing.

[Diabolical Artificer]

I'd replace the selenium rectifier's ASAP with a modern silicon bridge or 1N1007's. If they arn't duff now they might soon be. 200v on a cap sounds promising. -40 on an 807 sounds a bit much, that's right on cutoff. Still, you have some power, that's good.

These thing's can be a pain to work on but with the power off and some time spent with a continuity tester you'll crack it. Check all the diodes are ok with your tester, esp CR1 & 2.

First though start at the beginning, IE L1 your mains power transformer, have you got the voltages present? Just a few of them will do like the 100, 115 and 117v . The 117v feeds L2 to power your 3A4 oscillator, the 100v your HT. If all ok have you got DC at the line check SW? Check the G1, P1 K1 switches. The top caps are unswitched so are a good place to look for your HT. Sw's 1A to 4 looks like the HT switch. Also is the meter ok? These are just a few random thought's, but. Once you start studying the schematic and working on the thing, it should start to make sense.

Can you post some pics of the innard's, the caps, tfmr etc, these might help.

Onwards and upwards, Andy.

[nzoomed]

Attached is a photo of the chassis.

The parts list on the manual does not match up with what is shown on the schematic.
For example, there are only two selenium rectifiers in this unit (both in series, SR1 and 2 or CR1 and 2)
Have no idea on what CR5 and 6 are supposed to be, as they are simply not there.

The germanium rectifiers are there listed as CR3 and CR4.
They can all be seen in the photo.

Im not sure what the yellow electrolytic caps true value is supposed to be, they simply have 1170 stamped on them and thats all.

I am measuring voltages off the secondary, but they all vary and i cant find the 177v winding so far. There are so many combinations, Im still trying to work out there the top and bottom of the winding is.

[Diabolical Artificer]

T1 sound's ok in that case. Those two orange jobbies are more than likely CR1/2 as you say. At one end connected to these the schematic shows three resistors, R5/6/7 across these are C3. You should have a DC voltage across this/them. You still havn't said if you have HT on the valve under test or anywhere.

Once you have found a high DC voltage, over 100v say, we know T1 is ok as are CR1/2. If they're broke - no HT. They could do with replacing anyway, the big electrolytics look very old too, but first things first.

What you have is a puzzle which you need to solve using the schematic, parts list and a big pad of paper. Where is X, Y and Z on the actual tester. Once you know where R1 is, your off, then it starts making sense. BTW looks like someone has replaced R27, that big white resistor top right.

Lastly, it may not look like all the bits are there, makers of stuff hide components, change their minds and leave bits off just to make things interesting.

A.

[FrankB]

If you replace the selenium rectifiers and germanium, the B+ will INCREASE.
You will need to add a dropping resistor.

Also replacing the 'lytics is a good idea. 22 mfd are pretty common, and should replace the 20's nicely.