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Post by husker on Jan 8, 2018 16:28:20 GMT -5
So, I decided to walk away from my Tram project for a bit..Decided to work on a Cobra I didn't even know I had I put all new caps except on the freq counter (that is what I am working on now) and it powered right up, and worked great! Then it started to cut out just a little every now and then. It was putting out right at 4 watts AM and close to 12 on SSB. So the next time I powered it up, started to cut out just a little more, I thought it could be the stock hand mike I got with it, maybe a bad connection etc. Then I noticed the transmit power was a little lower each time I tested it, slowly going to 3.5, then 3 etc. It would stay steady when keyed, it would show less if I would re key the radio. Then the volume seemed to get hung on it, if I played with the RF gain, it seemed to reset the volume. Last night, I powered it up, would only key 2 watts, then the receive light went out, the transmit light came on, volume and all receive disappeared. THEN the freq counter showed the dreaded 92 2000. Now it seems pretty much non functioning. It powers up, shows the 92 2000 on the freq counter and that's about it. Thought maybe IC4 had gone out (8 volt regulator), and I noticed it had been replaced and IC 6 has also been replaced. And there is a diode attached to pin 1 of IC6. I don't see that in the schematics. So am I correct thinking maybe IC4 has gone out? it sounds like a power issue.... HELP (and yes, here I go again lol)
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Sandbagger
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Post by Sandbagger on Jan 8, 2018 18:30:23 GMT -5
So, I decided to walk away from my Tram project for a bit..Decided to work on a Cobra I didn't even know I had I put all new caps except on the freq counter (that is what I am working on now) and it powered right up, and worked great! Then it started to cut out just a little every now and then. It was putting out right at 4 watts AM and close to 12 on SSB. So the next time I powered it up, started to cut out just a little more, I thought it could be the stock hand mike I got with it, maybe a bad connection etc. Then I noticed the transmit power was a little lower each time I tested it, slowly going to 3.5, then 3 etc. It would stay steady when keyed, it would show less if I would re key the radio. Then the volume seemed to get hung on it, if I played with the RF gain, it seemed to reset the volume. Last night, I powered it up, would only key 2 watts, then the receive light went out, the transmit light came on, volume and all receive disappeared. THEN the freq counter showed the dreaded 92 2000. Now it seems pretty much non functioning. It powers up, shows the 92 2000 on the freq counter and that's about it. Thought maybe IC4 had gone out (8 volt regulator), and I noticed it had been replaced and IC 6 has also been replaced. And there is a diode attached to pin 1 of IC6. I don't see that in the schematics. So am I correct thinking maybe IC4 has gone out? it sounds like a power issue.... HELP (and yes, here I go again lol) What is it with you and basket cases? As always, it's tough to remotely troubleshoot. But I would agree that IC4 deserves a closer look. A good place to start. Hopefully you replaced at least all the 10V electrolytics. They will go bad. It's only a matter of when.
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Post by husker on Jan 8, 2018 18:39:27 GMT -5
Well...lol....I think it was my "I can fix this" stage Yes, every electrolytic except for the freq counter has been changed. The diode and the fact IC4 and IC6 have been changed worries me
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Post by BBB on Jan 8, 2018 20:31:42 GMT -5
I'm assuming you checked the operating voltage of the internal 12VDC power supply for proper output voltage and lack of AC ripple?
Have you tried an external 12VDC power supply?
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Sandbagger
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Post by Sandbagger on Jan 8, 2018 21:19:37 GMT -5
Well...lol....I think it was my "I can fix this" stage Yes, every electrolytic except for the freq counter has been changed. The diode and the fact IC4 and IC6 have been changed worries me IC6 is the audio amp. If your receive audio was working, then that IC is probably ok. Pin 1 is main 12V supply. Where does the other side of the diode go? If it's to ground, I'd say that someone added another polarity protection diode. Why is probably a good question. IC4 is not only the 8V regulator, but it controls TX/RX switching. So if the RX and TX seem out of whack, that's a good place to start looking. Pin 2 is 12V input. Pin 1 is 8V constant output. Pin 8 is TX 8V, Pin 6 is RX 8V. Pin 5 is the TX/RX switch. It is pulled low for TX. Another thing to check is PLL lock. If all the voltages on IC4 check out, then check that the PLL is locked. Pin 6 of the PLL (IC1) will be at ~8V if everything is ok. But if it pulls low (<1V) then it is unlocked. An unlock condition will mute the receive audio and will prevent (via TR35) the radio from going into TX mode. Same thing happens when the channel selector is moved between channels. That should give you a start.......
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Post by husker on Jan 9, 2018 10:54:55 GMT -5
I'm assuming you checked the operating voltage of the internal 12VDC power supply for proper output voltage and lack of AC ripple? Have you tried an external 12VDC power supply? I don't have a DC power cord, I guess I would just make one. Good idea! As Sandbagger knows (and is so very patient with me), I am still learning by doing, that being said..what is the best way to test for ripple? To be honest, I am a bit worried about powering this thing back up as it seems the issue gets worse each time power is applied. But with the way it is going, I am pretty sure it is a power related issue.
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Sandbagger
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Post by Sandbagger on Jan 9, 2018 14:43:31 GMT -5
I'm assuming you checked the operating voltage of the internal 12VDC power supply for proper output voltage and lack of AC ripple? Have you tried an external 12VDC power supply? I don't have a DC power cord, I guess I would just make one. Good idea! As Sandbagger knows (and is so very patient with me), I am still learning by doing, that being said..what is the best way to test for ripple? To be honest, I am a bit worried about powering this thing back up as it seems the issue gets worse each time power is applied. But with the way it is going, I am pretty sure it is a power related issue. In lieu of running it on external DC, you can always just meter the internal supply. If it's anywhere near 13V, you're probably ok. And if you replaced the filter cap already, ripple will not be an issue. I'd be more concerned with the 8V regulator/TX-RX switcher.
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Post by husker on Jan 9, 2018 15:21:31 GMT -5
Yup, I replaced the filter cap, I was thinking about disconnecting the 12v PS from the chassis and testing it that way
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Post by Deleted on Jan 9, 2018 22:47:31 GMT -5
The Frequency Counter problem was addressed in a service bulletin. I may have it. I have to look.
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Post by husker on Jan 9, 2018 22:54:41 GMT -5
that would be great, but there is still a power issue. I pulled the fuse to isolate the main brd, I show 13.78vdc at c302. So if I have good power there, is IC4 the next thing I feed for voltage regulation?
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Post by Deleted on Jan 9, 2018 23:04:06 GMT -5
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Post by husker on Jan 9, 2018 23:27:46 GMT -5
thank you!! is there a date on this?
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Post by 2600 on Jan 10, 2018 0:11:12 GMT -5
The "92.200" problem can be caused more than one way. The update that calls for installing the resistors and cap only applies to the first couple of years' production. Later versions of the clock/counter had an updated pc board with those three parts installed at the factory.
My money is on one (maybe two?) of all those electrolytic capacitors installed in reverse. Polarity matters with those. Putting one in backwards places reverse-polarity DC voltage onto the cap. They generally work for a few minutes, get hot and then dead short.
Sound familiar?
Changing one part creates one opportunity for error. Always good to be careful, but replacing three dozen parts gets you three dozen opportunities for just one error.
What are the odds?
Not sure, but they're greater than zero. There's an old principle called "Occam's Razor". Boils down to the simplest answer to a problem being the most likely answer.
Any time I change "just one" part and find the radio has a totally-new quirk when I turn it back on, I look at what I changed and what I may have unintentionally changed. The stray solder glob bridged across two foil traces is a common one.
Also known as the "Law of Unintended Consequences".
Don't know if you smelled any smoke, but some of the electrolytic caps are distributed onto various power-supply sections on the main pc board. They serve to keep the current drain in separate circuits filtered from each other. Many of them have a resistor, maybe 47 or 100 ohms that feed into that capacitor. A very close look with a bright light and magnifier might reveal one of those resistors that looks toasted. Would lead you to the fault that toasted it.
Until the radio is transmitting and receiving again leave the counter/clock module COMPLETELY ALONE!
That part of the Cobra 2000 CAN'T work until the radio is running properly. From the sound of it, yours isn't.
Yet.
73
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Sandbagger
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Post by Sandbagger on Jan 10, 2018 8:10:03 GMT -5
that would be great, but there is still a power issue. I pulled the fuse to isolate the main brd, I show 13.78vdc at c302. So if I have good power there, is IC4 the next thing I feed for voltage regulation? Like 2600 said, leave the freq counter alone until the rest of the radio is operating properly. I would also do as he suggested and make sure all of the parts you replaced went back in correctly. Then it's time to check voltages around IC4, and whether the TX and RX 8V voltages are there when they are supposed to be. If those voltages are not there, or are grossly out of tolerance, the radio won't work.
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Post by husker on Jan 10, 2018 9:34:24 GMT -5
that would be great, but there is still a power issue. I pulled the fuse to isolate the main brd, I show 13.78vdc at c302. So if I have good power there, is IC4 the next thing I feed for voltage regulation? Like 2600 said, leave the freq counter alone until the rest of the radio is operating properly. I would also do as he suggested and make sure all of the parts you replaced went back in correctly. Then it's time to check voltages around IC4, and whether the TX and RX 8V voltages are there when they are supposed to be. If those voltages are not there, or are grossly out of tolerance, the radio won't work. Agreed, after making sure that the power section was indeed putting out the required 13.8 vdc, it's time to move to IC4. I did some "quick" testing as it was past midnight and I do turn into Cinderella after 12 But I was checking and I could only pull 4-5 vdc off pin 6 of IC4. I am going to check pin2 for 13.8, and pin 6 again. I did go ahead and order a new MB3756 from Mr. Barkett just to be safe. Funny thing was when I did plug it back in to do a couple quick voltage tests, everything including the Freq counter worked perfectly. Oh and I did check every single Cap last night like 2600 suggested. They are all in correctly unless either a cap has the wrong label or the + is wrong
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Sandbagger
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Post by Sandbagger on Jan 10, 2018 12:39:14 GMT -5
Like 2600 said, leave the freq counter alone until the rest of the radio is operating properly. I would also do as he suggested and make sure all of the parts you replaced went back in correctly. Then it's time to check voltages around IC4, and whether the TX and RX 8V voltages are there when they are supposed to be. If those voltages are not there, or are grossly out of tolerance, the radio won't work. Agreed, after making sure that the power section was indeed putting out the required 13.8 vdc, it's time to move to IC4. I did some "quick" testing as it was past midnight and I do turn into Cinderella after 12 But I was checking and I could only pull 4-5 vdc off pin 6 of IC4. I am going to check pin2 for 13.8, and pin 6 again. I did go ahead and order a new MB3756 from Mr. Barkett just to be safe. Funny thing was when I did plug it back in to do a couple quick voltage tests, everything including the Freq counter worked perfectly. Oh and I did check every single Cap last night like 2600 suggested. They are all in correctly unless either a cap has the wrong label or the + is wrong Check pin 1 and pin 6 of IC4. Both should have 8V on them in the RX mode. Pin 6 should go low and pin 8 should show 8V in TX mode. Pin 1 should have 8V all the time. Lucky you could find a replacement chip. They are getting harder to find.
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Post by BBB on Jan 10, 2018 13:28:49 GMT -5
I don't have a DC power cord, I guess I would just make one. Good idea! As Sandbagger knows (and is so very patient with me), I am still learning by doing, that being said..what is the best way to test for ripple? To be honest, I am a bit worried about powering this thing back up as it seems the issue gets worse each time power is applied. But with the way it is going, I am pretty sure it is a power related issue. You can check for AC ripple on the internal power supply output feed with your digital or analog voltmeter set to the AC Voltage/ 20 volt scale setting. Generally, anything over 1 volt should be suspect that something is wonky in the internal power supply. Less is better. Too much ripple could point to the voltage regulator, filter caps or bridge rectifier diode(s) going south. Have you tried the **drop test meaning with the radio on and functioning properly lift up a side 1/2"-1" and let it fall? Does the radio fail again? This can be done with the radio on and in "failed" mode as well to see if the radio will work properly. Either of these situations would point to a loose solder joint, wire connection or dirty relay contact. **Not recommended for tube radio equipment, haha. Odd that the radio worked fine after you turned it on again. Intermittent failures are harder to nail down. Sometimes using a heat gun along with freeze spray can help find these type of failures if due to thermal expansion/ contraction issues. FYI, the Cobra 2k takes a standard (3) pin mobile CB radio power cable found with most mobile CB radios. A typical 3-5 amp external 12VDC power supply will power the radio. There is a switch on the back panel of the 2K that toggles between internal power supply (AC) and the external (DC) power supply option. The pins you can see on in connector are labeled on the rear panel to a degree. The two pins grouped together on the left are the (-) or negative and the pin by itself on the right is the (+) positive. You can use some small insulated alligator clip leads on the pins, but be careful not to short the (-) & (+) pins together.
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Post by husker on Jan 10, 2018 23:43:02 GMT -5
ok, so I know I wasn't supposed to touch the Freq counter, but had already re-capped it when I read 2600's post. So I didn't change anything besides that. I checked voltages on IC4, but I was having a big issue getting steady readings ( most likely me). I did notice everything was working mush to my confusion. So I put the inside covers back on and it's been running for 30 minutes flawlessly.............I am SO confused....lol
Spoke to soon.....after 30ish minutes I started to lose audio again. Everything else appears to be working at this point.
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Sandbagger
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Post by Sandbagger on Jan 11, 2018 8:37:29 GMT -5
ok, so I know I wasn't supposed to touch the Freq counter, but had already re-capped it when I read 2600's post. So I didn't change anything besides that. I checked voltages on IC4, but I was having a big issue getting steady readings ( most likely me). I did notice everything was working mush to my confusion. So I put the inside covers back on and it's been running for 30 minutes flawlessly.............I am SO confused....lol Spoke to soon.....after 30ish minutes I started to lose audio again. Everything else appears to be working at this point. I have seen IC4 fail in such a way as to output 11V. This usually causes a warble on SSB, as the first sign. But it can also heat up the PLL causing all sorts of bad things. It sounds like your radio is either suffering from some sort of intermittent connection (bad solder joint etc.), or thermally intermittent part (IC4?), or might be getting too high of a voltage out of IC4 causing the PLL to have lock issues. Remember, the unlock circuit of that radio will mute (or nearly so) the receive audio when it unlocks as well as disallowing the TX mode. You need to check the voltage coming out of IC4 pin 1 especially to make sure it's 8V.
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Post by husker on Jan 11, 2018 11:08:11 GMT -5
I try, but I can't seem to get a straight voltage out of any IC4 pins. They seem to oscillate an awful lot. I can use the chassis for the ground to test them correct?
Also, I only get 2 watts am output.
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Post by husker on Jan 11, 2018 13:15:03 GMT -5
Ok, After reading 2600's post in a different forum about a floating ground, it made complete sense why I was seeing issues trying to measure voltages.
Using the neg side of the big filter cap as ground. 13.78vdc at the fuse (making sure I am getting correct voltage to IC4) and see below for IC4. This is a "cold" just started IC4 and PS. When I have more time I will let it warm up for 30 mins and check again pin 1 8.3vdc pin 2 13 vdc pin 3 8.3vdc pin 4 33mvdc pin 5 11.5 vdc pin 6 8.35dc pin 7 70mvdc pin 8 244 mvdc
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Post by 2600 on Jan 11, 2018 16:23:15 GMT -5
Has C186 been changed? It's a 220uf cap just inboard from the audio chip.
When a factory-original fails, it's usually all at once, dead short. Shuts down receiver audio only, with the S-meter still kicking normally on channel noises, and transmit still fully okay AM and SSB.
Just a hunch.
The voltage readings above on IC4 are all perfect. But if the radio was running okay when you made those measurements, this won't tell you what they read when the radio goes south. Might read the same, might not.
The only measurements that can point you to the fault have to be made while it's acting wrong.
73
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Sandbagger
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Post by Sandbagger on Jan 11, 2018 16:31:34 GMT -5
Ok, After reading 2600's post in a different forum about a floating ground, it made complete sense why I was seeing issues trying to measure voltages. Using the neg side of the big filter cap as ground. 13.78vdc at the fuse (making sure I am getting correct voltage to IC4) and see below for IC4. This is a "cold" just started IC4 and PS. When I have more time I will let it warm up for 30 mins and check again pin 1 8.3vdc pin 2 13 vdc pin 3 8.3vdc pin 4 33mvdc pin 5 11.5 vdc pin 6 8.35dc pin 7 70mvdc pin 8 244 mvdc As you found out (the hard way), you can't use the chassis as a reliable DC ground. Your voltage readings look good. Recheck them again if/when it decides to fail and see which, if any, change. Don't worry about pin 7 (it's not connected to anything), and pin 4 (ground). Pin 8 should go to 8v when you key the mic. Pin 5 is the control pin and that should drop voltage to near 0V when the mic is keyed.
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Post by husker on Jan 11, 2018 18:25:40 GMT -5
I found my smoking gun.....here are IC4's pin voltages after it warms up 10ish mins.
Pin1 8.36 vdc Pin2 13.70 vdc Pin3 8,36 vdc Pin4 47mvdc Pin5 5.5-4.3 vdc **** When it falls below 4.8 the audio cuts out Pin6 8.36 vdc Pin7 23 mvdc Pin8 200mvdc
So it appears IC4 is indeed bad and dropping voltage on Pin5 below the spec'ed 13.77 vdc, I say this because the input voltage is within spec on Pin 2 at 13.7 vdc and stays there. Well....does it sound like I figured this out? Did I trouble shoot this correctly ( with help of course )?
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Sandbagger
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Post by Sandbagger on Jan 11, 2018 20:30:30 GMT -5
I found my smoking gun.....here are IC4's pin voltages after it warms up 10ish mins. Pin1 8.36 vdc Pin2 13.70 vdc Pin3 8,36 vdc Pin4 47mvdc Pin5 5.5-4.3 vdc **** When it falls below 4.8 the audio cuts out Pin6 8.36 vdc Pin7 23 mvdc Pin8 200mvdc So it appears IC4 is indeed bad and dropping voltage on Pin5 below the spec'ed 13.77 vdc, I say this because the input voltage is within spec on Pin 2 at 13.7 vdc and stays there. Well....does it sound like I figured this out? Did I trouble shoot this correctly ( with help of course )? Whoa! Don't jump to any conclusions yet. Pin 5 is the TX/RX control pin. It is pulled to 13V via R162. Now, there is also a 10V cap, C133 from pin 5 to ground. Now I know you said you replaced every cap, but I would doubly check that part. If it starts to short, it will pull pin 5 lower, and at some point it will go into TX mode.
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Post by husker on Jan 11, 2018 21:00:36 GMT -5
That is exactly what it did when this first happened. It went into tx mode all by its self. The cap looks fine, I will see if I have another of the same or close value to replace it if needed.
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Post by husker on Jan 11, 2018 22:44:11 GMT -5
I found my smoking gun.....here are IC4's pin voltages after it warms up 10ish mins. Pin1 8.36 vdc Pin2 13.70 vdc Pin3 8,36 vdc Pin4 47mvdc Pin5 5.5-4.3 vdc **** When it falls below 4.8 the audio cuts out Pin6 8.36 vdc Pin7 23 mvdc Pin8 200mvdc So it appears IC4 is indeed bad and dropping voltage on Pin5 below the spec'ed 13.77 vdc, I say this because the input voltage is within spec on Pin 2 at 13.7 vdc and stays there. Well....does it sound like I figured this out? Did I trouble shoot this correctly ( with help of course )? Whoa! Don't jump to any conclusions yet. Pin 5 is the TX/RX control pin. It is pulled to 13V via R162. Now, there is also a 10V cap, C133 from pin 5 to ground. Now I know you said you replaced every cap, but I would doubly check that part. If it starts to short, it will pull pin 5 lower, and at some point it will go into TX mode. so the cap and resistor look fine. The cap has the correct polarity as well. But under the brd, the soldered leads from pin 5 to the caps pos lead don't touch. should they? they are on the same trace so I wouldn't think that is an issue. Just looking for anything... if I use a 10uf 25 volt cap for c133, that's workable correct?
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Post by husker on Jan 12, 2018 0:40:33 GMT -5
Well.....Sandbagger is the man of the hour!! I pulled C133 the cap that feeds pin5, I figured I had one, why not, go ahead and replace it..and it works great now. Ran for almost 45 mins without a flaw.
Great lesson, one little 50 cent part an bring a beautiful radio to it's knees!!!!
THANK YOU!!!!!!!
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Sandbagger
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Post by Sandbagger on Jan 12, 2018 9:12:15 GMT -5
Well.....Sandbagger is the man of the hour!! I pulled C133 the cap that feeds pin5, I figured I had one, why not, go ahead and replace it..and it works great now. Ran for almost 45 mins without a flaw. Great lesson, one little 50 cent part an bring a beautiful radio to it's knees!!!! THANK YOU!!!!!!! Hopefully it will stay working for you. But I have to wonder why the cap you already replaced once was bad. You usually don't see this with brand new parts.
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Post by husker on Jan 12, 2018 10:58:57 GMT -5
Well, it was either a bad part, or a cold joint. I have had 1 miss-labeled cap in the past 2 months. never know I guess.
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