AVO 8SX? + Service manual?

[EnyaFox]

So it seems that I have recently picked up an AVO 8SX? On the back, it shows "Model 8X (Panclimatic)" with what I'm guessing is a screened case.

I have found a burnt resistor that only seems to affect the AC side. I am having trouble trying to find a value for replacement. (third photo)

Any help would be appreciated!

[The Philpott]

Can you confirm one end is connected to the transformer and the other to the 10vAC radial contact behind the selector switch?

Dave

[EnyaFox]

Originally Posted by The Philpott

Can you confirm one end is connected to the transformer and the other to the 10vAC radial contact behind the selector switch?

Dave

Yes, it does seem to.

[The Philpott]

I have an early Model 8, it has a winding in this position which i have just desoldered, and it measures 988 ohms. Probably supposed to be 1k..

Dave

[EnyaFox]

I have just found the schematic for the MK3. It does look very similar. It shows being a 9.2K

[The Philpott]

Just looked at Mk III circuit diagram and i agree it shows 9.2k.
(Mk II circuit diagram is different, and shows the 980 ohm resistance i found!)

11k and 56k paralleled gives 9.2k,
10k and 120k paralleled are close enough.

Dave

[EnyaFox]

I think it's a MK3 anyway. The internals look very familiar compared to a MK1,2 and 4

[Superscope]

There are no guarantees, because Avometers always like to
throw curve Balls, but I think you will probably find that your
Model 8sx is actually based on a Mk II design.

You have the Military version of the Model 8sx which you can
tell my the Nato Stock Number engraved along the top of the
Panel. 6625-99-943-1532.

The Military Meters more often than not, kept to slightly older
designs until new versions were fully tested and certified for use
in the field.

The burnt out resister appears to be R7, which on the Mk II
is 980 Ohms. The Mk III is indeed 9.2 K.

I have just looked inside a Military Model 8sx I have here and the
Resistor in question is a Pan-climatic 973 Ohm.

The meter here, is dated May 1964, but without your Serial Number,
we can't be sure if there are significant age related design differences
though.

I can't explain why the resistor is actually 973 Ohms. I have no idea if
the Military Meters were adjusted to a tighter spec, but that value is
clearly marked on the meter I have here.
Sadly, my Camera can't focus in good enough for a useful Photo.



Ian

[EnyaFox]

So the serial number is NO 169948-C-165, which would show is a MKII made in 1965. The resistor that had blown doesn't look like it was panclimatic but, I could be wrong.

[Rileyman]

My Model 8X (A.P.12945A) has 2 resistors in series in that position .
A Welwyn Panclimatic 950 ohms 1 percent , and a small Erie 33ohms 10 percent !
Appears totally unmolested . Serial No. 71988 - C - 358 .

I'll try and send a photo later .

Laurie .

[Rileyman]

He
rewith the photo :-

[EnyaFox]

Unless someone had replaced that same resistor before.

I'm still not reading an AC voltage after replacing the resistor I can't seem to locate a problem somewhere else

[The Philpott]

1. Leaf switches blackened, worn or gone out of adjustment? This has been covered recently so a quick search back through this part of the forum will show some guidance.

2. I was surprised to learn (that on a MODEL 7 at least) that a fault in the transformer can disable ALL the AC ranges, not just the AC Current. I wonder if Model 8 derivatives are similar.

3. Check that the cam rotors on the rear of the rotary switches haven't come loose. Hope it's not this as one of them is inaccessible- the transformer is in the way.

4. Rectifier may be leaky or corroded.

5. Dry joint/s?

Dave

[EnyaFox]

The diode marked "SA1" would it make a difference if that had gone?

[EnyaFox]

Just done some probing, I'm not getting an AC voltage on the input of the rectifier

[The Philpott]

Diode SA1 is not critical to function. There's some question as to whether it's actually needed at all.

No volts at rectifier.....Hmm. COULD be transformer fault/poor connection.

Dave

[EnyaFox]

Sending 240v into the meter, using a digital meter to check voltages. With one probe on neutral going in, I am getting voltage on both AC side of the rectifier. I'm not exactly sure where I should be testing now

[EnyaFox]

I think it could be the rectifier, after disconnecting the AC in I am getting 29V AC from it if thats normal?

[EnyaFox]

Its the rectifier, would you know where I could get a good replacement?

[The Philpott]

Must admit i need to have a think about this, a bit tricky without the meter in front of you.. Ian's better than me at visualising circuit faults from a distance.

The rectifier, if faulty, can be replaced by (four) germanium diodes (eg OA91) but they're temperature sensitive so the leads need grabbing with pointy pliers when soldering, to wick away the heat. (A three handed job unless you've a purpose made heatsink clamp)

dave