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Old 8th May 2022, 3:34 pm   #21
WaveyDipole
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Default Re: AVO Test Bridge

I just downloaded both of the files and can't open either one of them? Just get an error? Have they got corrupted somehow? Tried to combine them manually, but it still will not open as a zip file.

UPDATE: Ok, I have it! Thank you. Had to boot into Windows and download and install WinRAR. It will not open with another ZIP file archiver.

Last edited by WaveyDipole; 8th May 2022 at 3:52 pm.
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Old 8th May 2022, 6:39 pm   #22
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Default Re: AVO Test Bridge

Are the L63 and 6B8G diagrams supposed to be the same?

My unit does not have a diode W2 on the voltmeter board. The valve is not marked with a number unfortunately, but does have its heater on pins 2 and 7 and has more electrodes than would be present in a triode. Those two factors would seem to suggest that its a 6B8G or equivalent. Since there is a comment that W2 appears in the L63 version only, I was wondering whether the function of W2 is performed by one of the sections of the valve on the 6B8G version?

BTW, can I confirm that the rectifier the highlighted long cylindrical item? The photo in the manual states that the rectifier (and valve) has been removed for clarity and this part is missing from the photo but I don't just want to assume. It doesn't appear to test like a diode: - about 180k one way, rising from around 170k to infinity the other, tested in circuit.
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Old 8th May 2022, 8:48 pm   #23
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Default Re: AVO Test Bridge

Hi WaveyDipole,

No, the Schematic for the L63 and 6B8G are not supposed to be the same.
It was an error when compiling the Pages into a single File.

I wrote that Manual Six Months ago, and I know the ATB is a bit of a niche
interest area, but the response was pretty underwhelming, so I never
bothered to correct it.

You are in fact, the first person to ever raise any query on its contents.

So, please find the correct Page 29 attached below.

Regarding W2, this is discussed on Pages 14 and 15.

The L63 being a Triode, does not have spare Diode Capasity to carry out
any additional rectification, hence the requirement for an additional Rectifier ( W2 ).

As you have noticed, the 6B8G ( and the EBF2 ) are both Double Diode Pentodes,
so the spare Diode capasity within the Valve is used instead of an extra
external Diode to rectify the current fed to the movement.

Yes, the long cylindrical item is the Selenium Rectifier W1 as shown on Page 23
at the bottom.

Hope that helps


Ian
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Last edited by Superscope; 8th May 2022 at 8:53 pm.
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Old 8th May 2022, 9:55 pm   #24
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Default Re: AVO Test Bridge

Ian,

I picked up an ATB just for the fun of it at RetroTech today. I've always been curious about measuring RC using a Wein bridge, but never had the opportunity to play with one.

Thank you for the updated diagram and the well written manual. It has been very helpful. So it is as suspected then. Since the L63 does not have a spare diode, an additional selenium diode is used instead.

Regarding W1, I have seen those selenium rectifiers with metal fins before, but not one looking quite like this so just wanted to be sure. As it happens, while scrolling up and down the manual just now, I noticed that you do actually have a picture of it on p23 which I totally missed earlier! I did also find quite a bit of information online about selenium rectifiers online. Apparently due to their relatively high voltage drop they don't test very well with a DMM so my results were probably inconclusive. Some also seem to be of the opinion that for peace of mind these should be automatically replaced as they are prone to failure, and a drop resistor added. Is that what members would recommend?

Hopefully I will have some time tomorrow to bring it up on a variac and do some further testing. If I do decide to replace that diode I will have to figure out what drop resistance to use. Assuming it comes up OK, I should be able to take some measurements. Fortunately, there are no wax paper capacitors inside this unit so they may have already been replaced but I will still be checking everything to make sure parts are within tolerance as the manual suggests.

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Old 9th May 2022, 9:31 am   #25
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Default Re: AVO Test Bridge

I've not had to change my Selenium W1 rectifier yet, it still seems to work fine,
although I'm probably tempting fate by saying that.

I hear they are very smelly and acrid when they fail.
If it needed replacing, I would always make the effort to re-stuff a component
like this, to preserve as much of the historical accuracy as possible.

Regarding W2, although not relevant to your ATB, it's actually not Selenium,
its a Copper Oxide Westector Diode, briefly discussed earlier on this Thread on
Post #11.

Anyway, would be good to see how you get on bringing the ATB back to life.


Ian
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Old 9th May 2022, 1:35 pm   #26
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Default Re: AVO Test Bridge

I was thinking to start a new thread, although was thinking that two might be confusing, but events have overtaken me.

I brought it up slowly on the variac this morning. The valve heater glowed and nothing was getting hot or smoking at full voltage. So far so good. I tried measuring a 47nF capacitor and 1k resistor, but the readings did not make much sense. It seem to go to a minimum at the extreme end of the main dial range and at max at the other. Tried all ranges. The power factor setting had no effect. With the 1k resistor connected, the neon bulb lit up so I know that at least that works.

I was concerned that the DC voltage is much higher than stated. I was reading 260Vdc with the mains supply reading 217Vac. The 50Vac tap read around 60Vac and there was 40Vac across R6 but my main concern was the 260Vdc.

I then measured a few components, noted down the readings and plugged it back in again to take some further voltage readings, but unfortunately there was no life at all. On checking further it looks like the transformer primary has gone open. No noise, no smoke, just suddenly on power up there was no life on the secondary side. The switch and fuse are OK, line voltage present at the input to the switch. Continuity across the switch is fine.

Its a real shame as I was looking forward to experimenting with it :-(
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Old 9th May 2022, 2:36 pm   #27
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UPDATE: seems that my pronouncement of death may have been a little premature! I removed the two screws securing the Expansion board because I wanted to gain better access to the contacts so that I could trace the the wires. I wanted to be sure that hadn't missed anything. As I lifted the board, a wire came away from underneath one of the tags. This version of the board has two rows of tags. The outer edge contains 4 tags connecting the transformer primary, allowing the configuration of input voltage. The outer two connect to the transformer and the inner pair join the two windings in series for 240Vac input. It was one of those that had come away.

I had just completed re-flowing a joint that connects R10 to the positive rail on one of the tags on the opposite side of the board and I guess that must have disturbed the transformer wire enough to loose contact in the process.

This wire that came away has now been re-soldered to the tag and the secondary side has power again! The meter seems to be working in a somewhat sensible fashion but while it is possible to get a definite minimum point when measuring components and the readings seem to fall in the right ball park, the results are too vague to reliably determine a an accurate value. There also seems to be no way to zero the meter when set to 'V' (voltmeter) mode. Both R11 and R9 are high (6.8meg and 3.7meg) so well out of tolerance. R8 and R10 are also high (105k and 110k respectively), but just about borderline in spec. I haven't yet checked the capacitors or the Standards board, but at least some functions seem to be working to a fashion so I am happy about that.

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Old 10th May 2022, 11:52 am   #28
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Default Re: AVO Test Bridge

The parts on the standards board have been checked and are very close to their nominal value with the exception of the C1/C8 combo which is about 10% off the mark (110.9pF) if my DE-5000 LCR meter readings are correct. C8 will need to be adjusted to bring this down to 100pF.

I checked a variedty of components on all ranges. Not surprisingly, the readings on the C1 range were somewhat adrift, but the readings on all the other ranges were encouraging. One rather odd problem is that I couldn't read a 220nF capacitor on the C3 range (100nF to 50µF) even when bringing in the Power Factor which was essential when measuring electrolytics. It could, however, be read on the C2 (1nF - 100nF) range so a bit of an anomaly there.

Resistor R11 still needs to be replaced. R9 is a little high but still within the 20% tolerance.
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Old 14th May 2022, 2:56 pm   #29
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I noticed that this unit has a couple of additional parts between each side of R6, the main dial potentiometer, and the two terminals marked with the Earth symbol. They appear to be wirewound potentiometers and measure approximately 1 ohm. Both are set at maximum. I have marked these as VR1 and VR2 on the attached circuit diagram. I am guessing that these might be calibration presets and can be used to fine tune dial position? Or is there some possible other reason for them being there? One ohm either side is not going to give much of an adjustment range?
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Old 14th May 2022, 3:27 pm   #30
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Default Re: AVO Test Bridge

My first thoughts are, that they have probably been added by somebody.
But for what purpose?

The only thing I can think of is to prevent either of the Earth marked Terminals
truly going down to Earth potential if V or L are selected. Maybe a Current Limit.
They at least keep the balance of the Dial intact of course by being equal and either side.

They could of course also be as you suggested, a Dial Calibration.
But Avo never thought it necessary.

Yours is a very Late version of the Bridge, but Peter Wotton's Bridge was slightly
later and did not appear to have these.

But you can't rely on Avo not doing undocumented Mods themselves.
Nothing can be assumed.


Ian

Last edited by Superscope; 14th May 2022 at 3:37 pm.
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Old 14th May 2022, 7:46 pm   #31
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Why indeed. If they were put in place for current limiting purposes, then why not just use fixed 1 ohm (or whatever value is deemed appropriate) resistors, which would probably have been a cheaper part anyway? On the other hand, if they were put in place for calibration purposes, then why are they both set at max value? If AVO didn't deem such a thing to be necessary on other models, earlier and later, then that does tend to suggest they might have been added by someone after the unit left the factory.

I have not seen components of this curious design before. It has no central wiper contact like a potentiometer. It has only two contacts, the wiper being connected to one of them and the lead from the main potentiometer to the other. The "wiper" is fixed but the disc can be rotated. They do seem to be quite old and have no markings. I had to check the value by unsoldering one end and measuring with a DMM.

However, there is one other curious point that I have just observed. It can be seen in the second photograph how the wire from the main potentiometer goes into a grove on one side of the component. The bare end is then wrapped around almost the entire circumference of the component, threaded through a hole in the disk and back out through another for security as can bee seen, finally re-emerging to be wrapped first around the end of the resistive wirewound "track" and then multiple times around the contact point. Had the wire originally been connected directly to the terminal, there would have been insufficient length of it available for winding around the component in such an elaborate manner to make the connection. Of course, someone could also have replaced the original connecting wires with longer ones, but would have had to find some lengths of original looking wire and also gone to the trouble of winding and threading the wire as already described. Either that, or the components already had similar looking wires attached.

Could it have been a prototype? A mod carried out in the day? Or was it carried out later? I guess there is no way to tell. With the exception of the joint I de-soldered and re-made earlier for the purpose of measuring, the solder joints look pretty much untouched. On the other hand there is no denying that the unit had been previously worked on, although this seems to have been some time ago. Most parts including resistors look original, but there were none of the original paper wax capacitors present. An incorrect value of capacitor had been fitted in place of one of them, but the soldering looks very clean and professional. Whoever had worked on it previously evidently had very good soldering skills.

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Old 16th May 2022, 5:16 pm   #32
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I had a bit of a rough tail end of last week and weekend but now that I am feeling a little better I did a little more investigating today. Firstly, I noticed a curious arrangement on the main dial potentiometer. I had noticed the three wiring points on the main potentiometer previously but hadn't appreciated the significance of this. The arrangement effectively bypasses the 1Ω preset between R6 and R5/Point A when the main dial is set to 'V' in order to ensure a direct connection to chassis ground. There is a small 2mm gap separating the last 4mm or so winding. The separated section corresponds to the position of the wiper in the 'V' position. The ground terminal is connected directly to this separated section. The 1 ohm preset connects between what is now the end of the main winding and the ground terminal. I can only speculate, but it has been done very cleanly and does look like a factory arrangement. Curious.

Since in 'V' (voltmeter mode) mode movement pointer is deflected approximately halfway, I thought to check voltages, both AC and DC around the circuit.

Firstly, I checked the output from the selenium rectifier and see around 30Vac at its output. Since I am measuring with an AVO, this does concern me. I don't see any AC at point TC (grid) or pin 5/R8, however I did find a DC voltage at these points. Approximately 0.25Vdv at TC and 13Vdc at pin 5/R8. With the valve removed, those voltages disappear. This causes me to wonder whether there is likely to be a problem with the valve? Since I do not have a valve tester, I am not in a position to test it. Could there be another explanation?

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Old 16th May 2022, 7:03 pm   #33
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Default Re: AVO Test Bridge

Updated diagram of main potentiometer and preset arrangement.
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Old 19th May 2022, 2:20 pm   #34
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Default Re: AVO Test Bridge

Unfortunately, the replacement valve did not make a difference. I am still getting over half way deflection on the 'V' mode and the problem on the C1 reading.
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Old 19th May 2022, 3:03 pm   #35
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Default Re: AVO Test Bridge

It looks like you have at least Two individual problems going on here.


I'm absolutely not the right person to ask about Valve Amplifiers, but
could it be something to do with R12 ?

This provides a small bias for the Valve as I understand it, so it stands
to reason this might effect the output.
Maybe you could experiment with R12 to see if the V function can be
made to work correctly. Maybe there are high Z connections in the wiring.

Regarding the C1 issue, this has to be some capacitance somewhere it
shouldn't be. The fact that there are Two pots where they are not normally
known to be must raise suspition.
It might be worth shorting out these Pots (and the break) to return the
Bridge to what would be considered a Normal state.



Ian
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Old 19th May 2022, 3:07 pm   #36
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Default Re: AVO Test Bridge

Even if the original wax caps were replaced I would make sure the replacements are good.Les
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Old 19th May 2022, 9:29 pm   #37
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I worked on a version with the 6B8G valve.

I was not happy with the output from the selenium rectifier so I replaced it with an 1N4007 and a series resistor. I had no information on the correct HT voltage so I set it low (110V ?) by using a high value resistor.

It seemed to work OK.
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Old 21st May 2022, 9:00 am   #38
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Thank you for the further suggestions. I will check all of these out when I am feeling better. Unfortunately I am laid up in bed at present.

Shorting out the added presets is got to be worth a try. I am open minded at present as to whether there are two separate problems or just one. Ian, I suspect you might be right.

TowerRadio, that is a good point and definitely worth checking.

Silicon, with regards to the Selenium rectifier, I have been in two minds about replacing this. I tried a silicon diode (1N4005) that I had to hand in series but it made no difference. Nevertheless I have done a calculation for the drop resistor should I decide to go ahead and replace it. I find it interesting that you set the voltage to 110V? The ac output from the secondary is 217V, but the DC voltage after the Selenium diode is 260V. Wouldn’t that seem to be rather excessive for a half wave rectifier? Measurements ar LoZ i.e. made with AVO meter not DMM.
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Old 22nd May 2022, 10:36 pm   #39
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Default Re: AVO Test Bridge

I was overly cautious when I set the HT voltage. It is not meant to be a recommendation to set it low.

I just wanted to point out that the test bridge worked on a low HT voltage.

A half wave rectifier can generate a relatively high voltage when there is no load.
This drops considerably when a significant current is taken.

In theory, the 217 V AC (RMS) could develop 306 V DC across the reservoir capacitor if there is no load.
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Old 23rd May 2022, 8:10 am   #40
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Default Re: AVO Test Bridge

Fair enough. It is interesting though that it CAN work with the HT voltage set to much lower levels. I also spotted in the manual posted by Ian Ian on page 16 under the heading „Leakage Detector” that the capacitor under test with unknown leakage Rx is „placed in series with the neon tube and the high resistance of (R7), and therefore connects Rx between earth and approximately 300V” which would seem to indicate the voltage level that might be expected on the HT rail.

The meter seems to draw only around 1.6mA so not exactly a significant load, but in light of your comment probably enough to drop a few volts, so this is all begining to make sense.
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