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Old 11th Oct 2019, 1:04 pm   #1
WaveyDipole
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Default Heathkit IM-28 VTVM

I have just received a Heathkit IM-28 VTVM from a member. I am very pleased with what I have received. It came in a wooden case and complete with an original probe. The unit itself is in very good and clean condition. No dirt or grime. The inside looks like it had just been manufactured!

Except for the one across the secondary winding which looks like it may have started to leak at the positive terminal, the paper type caps look like new. As is usually recommended, I will be replacing all of them as a precaution.

One thing that puzzles me at the moment is the transformer and I would like to confirm my understanding before applying any power (once the caps have been replaced). The unit has a rather stiff mains cable (it would be a stretch to call it a flex) terminated with a USA style plug. The transformer primary seems to have two windings which are connected in series. The numbers printed on the winding correspond to the list of part in the build instructions (54-2-24) which the schematic and build instructions would appear to suggest can be connected in series for 240v operation, or in parallel for 110/120v operation. Since this example has the two windings connected in series, that would mean that it is configured for 240v use, but if so, then why does it still have a USA plug? I am considering replacing this cable with a proper flex and UK plug.

The total resistance of the two windings in series is about 730 ohms, but there is a disparity between the two windings of about 30 ohms (measured approximately 350Ω and 380Ω). Should I be concerned about that?

The other omission seems to be an internal fuse. Would it be advisable to add one?

My last question has to do with one of the capacitors. It is shown as C1 on the diagram, it is rated as 0.047uF at 1600v. It does appear to be a de-coupling cap so am I OK to use the nearest rated part that I have to hand, which is rated 0.1uF @ 2000v ? With the original having +/-10% tolerance, I can't imagine it being measurement critical, but I'm not sure whether the additional capacitance might have any undesired effect. I don't have 2 to place in series (although I do have two 0.1uF @ 630v). There seems to be no indication of the actual voltages present so I'm not sure whether the two lower voltage ones (possibly allowing for 1260v when in series) would be adequate?
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Last edited by WaveyDipole; 11th Oct 2019 at 1:14 pm.
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Old 11th Oct 2019, 1:35 pm   #2
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Default Re: Heathkit IM-28 VTVM

The two halves of the winding will have different DC resistances, because the inner half of the winding, though containing the same number of turns, has a smaller diameter and therefore a shorter length of wire than the outer half.
Don't worry, this is normal!
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Old 11th Oct 2019, 2:28 pm   #3
WaveyDipole
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Default Re: Heathkit IM-28 VTVM

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Originally Posted by m0cemdave View Post
The two halves of the winding will have different DC resistances, because the inner half of the winding, though containing the same number of turns, has a smaller diameter and therefore a shorter length of wire than the outer half.
Don't worry, this is normal!
Yes, I hadn't considered that. The DC resistance for the shorter length wire would, naturally be lower. I don't know enough about transformers to ascertain how much variance would be expected, but 30Ω represents a variance of approximately 7-8% which does not seem too unreasonable. That would seem to answer one question so thank you for that.
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Old 11th Oct 2019, 2:39 pm   #4
ColinTheAmpMan1
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Default Re: Heathkit IM-28 VTVM

Check the front panel of the VTVM. If the model-number ends in a "U", then it is a UK model, but having said that, it looks to me as if the mains transformer can be connected for 120 or 240 VAC by means of a selector, which looks, from the schematic, to incorporate a mains fuse-holder. Another way of telling if Heathkit items were produced in the UK will be that the meter will either show "Pullin" at the very top for UK models and also show "DAYSTROM Ltd - GLOUCESTER ENGLAND". If I remember correctly, Heathkit used Simpson meters on US-produced kit and the address on the meter was "BENTON HARBOR MICH."

Unfortunately, I do not have any other documentation on the IM-28 VTVM and I know it is difficult to find free documentation on the web, but many of these Heathkit VTVMs had similar circuitry and only really differed in cosmetics. The V-7A VTVM uses a 0.01 uF, 1600 V capacitor in the equivalent position to your C1, so I assume that the voltage is probably more important than the capacitance value. Incidentally, be careful of putting too high a voltage on the double-diode V1 (6AL5). It is a bit delicate, but if you do kill it, I have a load of EB91 equivalents that I bought on eBay when I blew mine.

Incidentally, can't the member who sold you this VTVM provide you with any other info, like why it is wired for 250 VAC, but still has a US plug?

Good Luck, Colin.
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Old 11th Oct 2019, 3:45 pm   #5
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Default Re: Heathkit IM-28 VTVM

The meter is marked "BENTON HARBOR MICH." and there is no "U", just "Heathkit IM-28", so it would seem to be the US model. I have already PM'ed the member regarding the USA plug. I also noticed that the schematic shows a mains selector and fuse, but this example does not seem to have either. The mains cable is soldered directly on to a tagboard which connects the transformer and elsewhere. The black connector that joins the two windings in series is evidently not original so its seems to have been adapted at some point.

Thanks for the heads up on the 6AL5 and you mention of the EB91 as an alternative. Both the 6AL5 and the 12AU7A are genuine Mullard and look in pristine condition. The only thing I have been able to determine for now is that the heaters look OK. In any case, I will be very careful, although it is good to have a backup option of the EB91 just in case!

BTW, the seller was aware of the US heritage of the instrument because of the Nema5 plug, but was also unsure as to whether it would run on 240v.

Last edited by WaveyDipole; 11th Oct 2019 at 4:14 pm.
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Old 11th Oct 2019, 4:34 pm   #6
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Default Re: Heathkit IM-28 VTVM

For the sake of completeness, the double-diode originally in my V-7AU was a Brimar EB91 6AL5, made in England. The EB91s that I have are Mullard M8212 "made in Great Britain" and also CV4025, so presumable military-grade? I have another nine of them.
It seems like you have a US-specific IM-28. I think if I were you, I would power it up using a Variac (if you have one, of course!) set initially to 120 VAC and check that the heaters are glowing. If all seems good and the heater-voltage is 6.3 VAC, then it is probably set up for US mains. If the heaters are dim, then slowly crank up the voltage on the Variac and if the heaters start to glow like bulbs, take the voltage back down. Otherwise, carry on to 240 VAC and (hopefully) the instrument will work fine. Once that has been ascertained, change the US plug and mains lead for a decent UK one and an appropriate mains fuse. Heathkit were notorious for their bad mains wiring, just using a knot in the mains-lead to retain it from being accidentally pulled out and not having an earth connection at all. It looks like you have an earth-connection, but if there is a knot, replace it with a proper cable retainer. Much safer!
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Old 11th Oct 2019, 5:59 pm   #7
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Default Re: Heathkit IM-28 VTVM

Colin, that sounds like a sound plan. I will try 120v first, but unfortunately I don't have a variac, but I can certainly use a 40w bulb to drop the current. There is a back round cable retainer of the very common type, but this flat rigid 3-core cable looks rather awkward in it. I will be replacing this with a piece of round flex. If it runs off the UK mains then I will use a UK standard plug. If not, then I have a step down transformer that I can use with it.

What you say about C1 makes sense, but can I clarify. You say that V-7A VTVM uses a 0.01 uF, 1600 V capacitor? This IM-28 uses a 0.047uF (47nF), my proposed replacement is 0.1uF (100nF). Can I just confirm that you did mean 0.01uF and not 0.1uF? If its 0.1uF, then it seems worth a try, but if its 0.01uF then I'm not so sure?
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Old 11th Oct 2019, 7:29 pm   #8
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Default Re: Heathkit IM-28 VTVM

Just as confirmation, this from a Heathkit parts list of transformers extracted from the pdf available at:-
https://w5rkl.com/wp-content/uploads...ifications.pdf
54-2-24 Power Transformer (P: 115/230V; S: 130V@10mA, 6.3V@800mA)
Used on IM-28, IM-38, IM-5228, IT-7400
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Old 11th Oct 2019, 7:41 pm   #9
ColinTheAmpMan1
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by WaveyDipole View Post
Colin, that sounds like a sound plan. I will try 120v first, but unfortunately I don't have a variac, but I can certainly use a 40w bulb to drop the current. There is a back round cable retainer of the very common type, but this flat rigid 3-core cable looks rather awkward in it. I will be replacing this with a piece of round flex. If it runs off the UK mains then I will use a UK standard plug. If not, then I have a step down transformer that I can use with it.

What you say about C1 makes sense, but can I clarify. You say that V-7A VTVM uses a 0.01 uF, 1600 V capacitor? This IM-28 uses a 0.047uF (47nF), my proposed replacement is 0.1uF (100nF). Can I just confirm that you did mean 0.01uF and not 0.1uF? If its 0.1uF, then it seems worth a try, but if its 0.01uF then I'm not so sure?
Ok, so if you don't have a Variac but do have a step-down transformer, why not try powering it up with the transformer first? if the heaters look to be glowing right and the heater voltage is 6.3 VAC, then it is a 120 V model and no harm has been done.

Yes, the equivalent capacitor in the V-7A is 0.01uF at 1600 V. If you just want to see if the VTVM works, and you aren't going to put a signal above a few volts into it, then I don't think you need to bother about the capacitance value too much and that a cap with a couple of hundred volts rating will do. I was hoping that I might find a suitable replacement to give you in my collection, but regrettably not. You might be able to find a suitable capacitor from a UK seller. Bitsbox has 10nF, 3kV for 0.28 (https://www.bitsbox.co.uk/index.php?...oducts_id=3262), but Dave Cantelon at Just Radios has tubular axial metallised polyproylene caps 0.01uF at 1600 v for $1.49 (https://www.justradios.com/cart.html). His minimum order is $20 plus shipping, but if you take a look at his site, he has lots of really interesting stuff - regrettably he is located in Canada, so shipping-times are a little long.
Colin.
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Old 11th Oct 2019, 8:19 pm   #10
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Default Re: Heathkit IM-28 VTVM

Oops, I have just realised that I was looking for 0.01 uF instead of 0.047 uF. Bitsbox doesn't have anything like that, but Dave Cantelon has 0.05 uF, 1600 V tubular axial metallised polypropylene caps at $2.49. They should do the job fine.
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Old 11th Oct 2019, 9:59 pm   #11
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by buggies View Post
Just as confirmation, this from a Heathkit parts list of transformers extracted from the pdf available at:-
https://w5rkl.com/wp-content/uploads...ifications.pdf
54-2-24 Power Transformer (P: 115/230V; S: 130V@10mA, 6.3V@800mA)
Used on IM-28, IM-38, IM-5228, IT-7400
Thank you for that information which confirms that the transformer is rated for UK mains operation. I have saved a copy of that document for future reference.

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Yes, the equivalent capacitor in the V-7A is 0.01uF at 1600 V. If you just want to see if the VTVM works, and you aren't going to put a signal above a few volts into it, then I don't think you need to bother about the capacitance value too much and that a cap with a couple of hundred volts rating will do. I was hoping that I might find a suitable replacement to give you in my collection, but regrettably not.
Thank you for having a look. I had another look and found a ceramic cap 103 (10n) @ 2kv. While a ceramic cap is not ideal due to its microphonic properties and thermal characteristics which might cause problems, it was sufficient for power on test purposes. I have, of course, replaced all of the other paper caps as well.

Quote:
Ok, so if you don't have a Variac but do have a step-down transformer, why not try powering it up with the transformer first? if the heaters look to be glowing right and the heater voltage is 6.3 VAC, then it is a 120 V model and no harm has been done.
I has enough time to do a quick power on test at 120v. The voltage measured across the DC smoothing cap was about 80v (well short of 120v) and across the heater the AVO read about 3vAC, so half of what it should be. Strangely the valves did seem to operate as I could get a DC reading on the scale, but the reading was only about half of what it should have been. All (including the data from buggies) would seem to indicate that this unit is indeed configured for 240v. I will do some further testing, including at UK mains voltage tomorrow.

I should have looked at the voltage range of the meter. The highest range is 1500v, so C1 no doubt needs to be rated to carry at least that voltage. I will have a look around for a suitable cap. Thanks.
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Old 12th Oct 2019, 3:35 pm   #12
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Default Re: Heathkit IM-28 VTVM

I tested it on 240v today, first with a lamp limiter, and having been satisfied with that, then directly on the mains. I got approximately the correct voltage reading at the heater (6.8vAC), slightly high but in the right ball park. The DC voltage across the smoothing cap does seem rather high at 167v, but I had no indication that anything was being stressed. The valve heaters glowed red at about the brightness they should do.

I followed the calibration procedure and found that DC+ and DC- zero balance nicely and the voltage readings seem quite accurate on all scales. I used a known DC power source rather than a battery. The AC balance was also readily set and I could achieve equilibrium between the AC, DC- and DC+ zero settings, but the AC calibration control just barely allows me to set the correct AC voltage with the wiper at far left. I used a transformer rather than direct mains as per the instructions, and measured the output from this using a DMM and then tried matching that with the VTVM. It is by no means exact but is safer than prodding the mains directly.

The calibration controls seem a bit finicky and do seem to drift, although I might have allowed a longer warm-up time before making adjustments. The Ohms Adj control also seems a little temperamental about the infinity mark although I haven't squirted anything into the pots yet to clean them.

Both DC and AC voltage readings seem to compare well with the DMM, but the resistance readings are a little inaccurate (e.g. 4.7k reads 5.5k). This might be down to the fact that the pointer does not seem to fall back to zero when the probe is touched to the common lead after setting infinity. Naturally there would be some lead resistance of an ohm or two registering at x1, but at x1k it ought to be zero, yet it fell a little short of that by a minor mark or two. The battery reads over 1.5v on my DMM so should be good. I also noted the position of the switch on the probe and made sure that is was set to AC/Ohms for Ohms measurements.

The mains cable has now been replaced with a proper flex. I found a barrel type fuse holder, but installing it would involve cutting a hole in the case, which I didn't want to do. Installing an IEC connector might have been another option were it not for the fact that it can't be easily accommodated on the rear plate, and it would be impractical to mount it on the rear outer case, so I will leave it as it is. Once the unit is re-assembled it will be fitted with a fused UK mains plug anyway.

I guess the next step is to check resistors particularly in the AC and Ohms paths. A suitable capacitor is on order for C1 and will be fitted when it arrives.
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Old 12th Oct 2019, 4:11 pm   #13
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by WaveyDipole View Post
The meter is marked "BENTON HARBOR MICH." and there is no "U", just "Heathkit IM-28", so it would seem to be the US model. I have already PM'ed the member regarding the USA plug. I also noticed that the schematic shows a mains selector and fuse, but this example does not seem to have either. The mains cable is soldered directly on to a tagboard which connects the transformer and elsewhere. The black connector that joins the two windings in series is evidently not original so its seems to have been adapted at some point.

Thanks for the heads up on the 6AL5 and you mention of the EB91 as an alternative. Both the 6AL5 and the 12AU7A are genuine Mullard and look in pristine condition. The only thing I have been able to determine for now is that the heaters look OK. In any case, I will be very careful, although it is good to have a backup option of the EB91 just in case!

BTW, the seller was aware of the US heritage of the instrument because of the Nema5 plug, but was also unsure as to whether it would run on 240v.
When I first saw the transformer primary connection. I knew that it was connected for 240 volts.
The later Heath offerings were all designed for export using dual primary transformers. I think their reasoning was to ship everything out of the US instead of locations out of the country. Around the same time, they opened retail establishments in the larger US cities. Before that, everything was mail-order.
Dave, USradcoll1, trying to remember!
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Old 12th Oct 2019, 4:27 pm   #14
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Default Re: Heathkit IM-28 VTVM

Apart from the schematic that you provided in post #1, do you have any other documentation? I ask because I could probably send you a pdf of the manual/assembly guide for the V-7AU that I have. As I said, these Heathkit VTVMs use largely the same circuit, with only minor cosmetic and/or circuit differences, so it might be helpful for set-up/calibration.
Colin.
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Old Yesterday, 2:43 pm   #15
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by usradcoll1 View Post
When I first saw the transformer primary connection. I knew that it was connected for 240 volts.
The later Heath offerings were all designed for export using dual primary transformers. I think their reasoning was to ship everything out of the US instead of locations out of the country. Around the same time, they opened retail establishments in the larger US cities. Before that, everything was mail-order.
Dave, USradcoll1, trying to remember!
Thanks for that observation. It would make sense to ship outside the USA with the option for both mains supply voltages. I wonder whether this was a project that someone never really made a start on except for making a change in the transformer wiring? I suppose someone might have adapted a Nema5 socket or bank of such for UK mains use, but the Nema5 plug is marked 15A 125V so although that might work, it doesn't sound like it would be suitable for UK mains use? Whatever the case, clearly you are correct and the unit does operate on 240V, just for some reason still had a USA 120V mains cord.

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Apart from the schematic that you provided in post #1, do you have any other documentation? I ask because I could probably send you a pdf of the manual/assembly guide for the V-7AU that I have.
Colin.
Thank you for the kind offer of the assembly guide for the V-7AU. My apologies for omitting to mention it, but I did find the Heathkit Assembly Manual for the IM-28 online prior to starting work on the unit and I am actually following the calibration instructions that I found in that manual. Once I have replaced the remaining capacitor and checked for, and if found, rectified any further problems such as resistors out of tolerance, I will run the calibration steps again allowing a bit more warm-up time.
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Old Yesterday, 3:08 pm   #16
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Default Re: Heathkit IM-28 VTVM

If I were to hazard a guess, I would say that this IM-28 was originally made in the US and wired for 115 VAC including the US-type mains plug. After all, the change from 115 VAC to 230 VAC is effected by changing the tap on the mains transformer primary, not the more common changing of two primaries from parallel to series. This conjecture would conveniently explain the existence of the US-type mains plug and the slightly "bodged" appearance of the "choc-block" connector, which I am sure will not have been original.

As a small query, what is printed at the very top of the meter, right by the place where the "power" light might be? Does it suggest that the meter is a Simpson?
Colin.
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Old Yesterday, 4:56 pm   #17
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by WaveyDipole View Post
Quote:
Originally Posted by usradcoll1 View Post
When I first saw the transformer primary connection. I knew that it was connected for 240 volts.
The later Heath offerings were all designed for export using dual primary transformers. I think their reasoning was to ship everything out of the US instead of locations out of the country. Around the same time, they opened retail establishments in the larger US cities. Before that, everything was mail-order.
Dave, USradcoll1, trying to remember!
Thanks for that observation. It would make sense to ship outside the USA with the option for both mains supply voltages. I wonder whether this was a project that someone never really made a start on except for making a change in the transformer wiring? I suppose someone might have adapted a Nema5 socket or bank of such for UK mains use, but the Nema5 plug is marked 15A 125V so although that might work, it doesn't sound like it would be suitable for UK mains use? Whatever the case, clearly you are correct and the unit does operate on 240V, just for some reason still had a USA 120V mains cord.

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Apart from the schematic that you provided in post #1, do you have any other documentation? I ask because I could probably send you a pdf of the manual/assembly guide for the V-7AU that I have.
Colin.
Thank you for the kind offer of the assembly guide for the V-7AU. My apologies for omitting to mention it, but I did find the Heathkit Assembly Manual for the IM-28 online prior to starting work on the unit and I am actually following the calibration instructions that I found in that manual. Once I have replaced the remaining capacitor and checked for, and if found, rectified any further problems such as resistors out of tolerance, I will run the calibration steps again allowing a bit more warm-up time.
The 125 volt rating on the plug means that it is the NEMA 125 volt design, not the maximum voltage that it can safely handle. All our 125 volt devices and flex, known as cord sets here are at least tested for 300 volts.
BTW, the schematic shown and the one available here is the UK version, showing a different transformer primary. The US version uses the series-parallel transformer primary.
Dave, USradcoll1
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Old Today, 10:19 am   #18
WaveyDipole
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Default Re: Heathkit IM-28 VTVM

Quote:
Originally Posted by ColinTheAmpMan1 View Post
As a small query, what is printed at the very top of the meter, right by the place where the "power" light might be? Does it suggest that the meter is a Simpson?
Colin.
There is only an indication of R.M.S voltage: p-p=2.8.3 R.M.S in the top left corner, but no indication that is is a Simpson meter. Bottom left states 'THE HEATH COMPANY' and bottom right 'BENTON HARBOR MICH.' .


Quote:
Originally Posted by usradcoll1 View Post
The 125 volt rating on the plug means that it is the NEMA 125 volt design, not the maximum voltage that it can safely handle. All our 125 volt devices and flex, known as cord sets here are at least tested for 300 volts.
BTW, the schematic shown and the one available here is the UK version, showing a different transformer primary. The US version uses the series-parallel transformer primary.
Dave, USradcoll1
Thanks for the clarification on both the NEMA5 testing standards as well as the transformer. I imagined that the cable and plug would be tested to well beyond their rated voltage, but interesting to note nevertheless.

Oh, and greetings to you 'across the pond' as well!
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