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Old 10th Aug 2022, 5:52 pm   #21
EnyaFox
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Default Re: AVO 8SX? + Service manual?

I will try different diodes and rectifiers that could possibly work. I will keep this thread updated on what I come across with! I did try use the rectifier from an old model 40 that needs a full repair and the needle did move.
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Old 10th Aug 2022, 6:13 pm   #22
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Default Re: AVO 8SX? + Service manual?

With a Burned out Resistor on the AC Range, I guess a damaged
Rectifier was the next most likely candidate for failure.

The Transformers can fail, but it seems very Rare that this happens.

The Rectifier in the 8sx should be an encapsulated Full Wave
Rectifier. i.e. It's had the Pan-climatic treatment.

So, basically it can't really be repaired.
Although, don't tempt me!

How did you test the Rectifier?
Has it been changed out for individual Diodes ?

This subject has come up a number of times, and the General
consensus is as Dave says, to use Germanium OA91's as replacements.

Although, I found these difficult to find at sensible prices.

What I would do first, is throw in a Silicon Signal Diode like
an IN4148 just to test that the rectifier is indeed faulty.
Ubiquitous and cheap and you probably have some already!

I actually used IN4148's on a Model 5 restoration, and the AC
ranges appeared to function Ok, so I left them in.
Although I need to re-visit that meter to tidy up a few bits.

It's in my plan to test out various different modern Diodes for Avometer
repairs, but I haven't got round to it yet, so I can only go by previous
recommendations.


Ian
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Old 10th Aug 2022, 6:40 pm   #23
EnyaFox
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Default Re: AVO 8SX? + Service manual?

I first removed the AC side from the rectifier and got a voltage so I tested with a rectifier that was in a model 40 and the needle had moved to 200v on a 240v input. I did work my way down the voltage steps.
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Old 10th Aug 2022, 7:08 pm   #24
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Default Re: AVO 8SX? + Service manual?

To be honest, a Model 40 rectifier should of worked Ok.
Unless of course, it is also faulty. i.e. Leaky

You may have some other problem as well, related or not
to the burned out Resistor.

As I think Dave mentioned, you might have some tarnished
Switch contacts as well, creating some High Resistance joints
in the AC path.


Ian
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Old 10th Aug 2022, 7:44 pm   #25
EnyaFox
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Default Re: AVO 8SX? + Service manual?

So I have made one out of 4 1n4007 diodes. I'm getting the same result. I will try play around with it a bit more. I'll try get to some contacts to clean them etc even if they look clean.
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Old 10th Aug 2022, 9:57 pm   #26
The Philpott
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Default Re: AVO 8SX? + Service manual?

Don't use anything too abrasive, contact cleaner on a piece of card should be adequate.
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Old 11th Aug 2022, 10:41 am   #27
EnyaFox
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Default Re: AVO 8SX? + Service manual?

Is there an easier way to get to everything without too much dismantling?
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Old 11th Aug 2022, 10:50 am   #28
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Default Re: AVO 8SX? + Service manual?

In short, no!

Visibility is improved by sliding white paper behind the switch stack in question, using magnification (and omnidirectional daylight rather than a lamp if possible)
My x10 loupe seems the best compromise but if you can't get close enough to focus you have to revert to x5 or x7.

I think at least half of my avometers arrived with a leaf switch problem of some sort- sometimes this was what put them on the market, sometimes it's just incidental.

I usually end up using long tweezers with a fragment of card cut from a loo roll tube (this cardboard is a bit furry but it makes a good cleaner if soaked in isopropyl alcohol)
The rotary switches can be operated to trap the card between the switches you want to clean with distorting them. Occasionally a leaf needs to be bent marginally to ensure that a switch closes- the manual quotes 'duck-bill pliers' to do this, but i make do with snipe nose or obstruction snipe-nose pliers.

Dave
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Old 11th Aug 2022, 1:12 pm   #29
EnyaFox
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Default Re: AVO 8SX? + Service manual?

Nothing is going well with this meter anymore. I might just have to put it up and come back to it another time
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Old 11th Aug 2022, 4:27 pm   #30
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Default Re: AVO 8SX? + Service manual?

Absolutely. Also when it's cooler it will seem less recalcitrant. It's amazing what a sweat you can get into just from peering and pondering.

D
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Old 28th Sep 2022, 7:24 pm   #31
EnyaFox
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Default Re: AVO 8SX? + Service manual?

I'm back again! I have obtained another AVO 8 MK2 and measured all the resistors with the schematic. The 8K resistor that joined the AC negative had gone so I had replaced that. A problem I have now run into is that its a volt lower on the lower voltages and I'd say 40% lower on a higher voltage (Used a variac and a digital multimeter)
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Old 28th Sep 2022, 7:28 pm   #32
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Default Re: AVO 8SX? + Service manual?

Deja vu- first suspect to check is those dastardly leaf switches. If the current is taking a longer route through the meter than it should, the reading will be significantly low. A drop of 30 or 40% sounds very familiar to me!

Dave
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Old 28th Sep 2022, 8:14 pm   #33
EnyaFox
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Default Re: AVO 8SX? + Service manual?

The leaf switches seem to be good. I have been looking at the switch cam for the DC side. There is only one lump to push the leaf switches. I honestly can't figure out between resistance and AC. Resistance says closed on all 3 and AC says open on all 3 on the schematic but it doesn't seem to add up looking at the meter
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Old 28th Sep 2022, 8:24 pm   #34
EnyaFox
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Default Re: AVO 8SX? + Service manual?

I can see whats gone wrong
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Old 28th Sep 2022, 8:32 pm   #35
EnyaFox
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Default Re: AVO 8SX? + Service manual?

Alright, 10V AC input. Now shot up to 30V on 100v range
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