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Post by husker on Jan 13, 2018 18:23:25 GMT -5
So what do you think my next step should be? I don’t have a dummy load, I use my antenna. Was that voltage on pin1 acceptable? Pin 1 voltage is ok, depending on what channel you are on. It will rise the higher you go. I would be more concerned about what happens on pin 6 when the radio flakes out. ok, so I power it up, the volume stays "hung" if you will at about 1/2 up. I can't turn it down, but I can make it a little louder if I turn it up. At the moment pin 6 is 8.1vdc
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Post by husker on Jan 13, 2018 15:22:12 GMT -5
Ok, I will check it when I get home. Need to run to Costco
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Post by husker on Jan 13, 2018 14:03:01 GMT -5
So what do you think my next step should be? I don’t have a dummy load, I use my antenna. Was that voltage on pin1 acceptable?
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Post by husker on Jan 13, 2018 13:14:13 GMT -5
Yup, that was one of the caps that was replaced with the kit, I reheated the connection to make sure I didn't have a cold joint. Still the same issue, the volume is there, just real soft, if I turn it all the way up it gets a bit louder. But if I turn it all the way down it stays the same low level. I only have 100uf caps, I can replace that one with a 100 to see if that makes a difference. At this point, can you key the transmitter? The reason I ask is that pesky unlock circuit. C232 by way of TR53 will pull the audio nearly to ground and pretty much mute it if there is an unlock condition. It will also disable the ability to go into TX mode by way of TR35. There are two legitimate ways that the unlock circuit can be triggered. The first is if the channel selector is moved off of a channel detent (you may notice that the audio mutes as you spin the dial). The second is if the PLL becomes unlocked. Measure the voltage on pin 6 of IC1(MB8734). It should be at or close to 8V. If it's low, like <1V there is a problem somewhere in the PLL circuit. I've seen older 2000's with dirty channel selectors that if you simply bump them, the audio will mute momentarily. You might try jiggling the selector to see if that makes any difference. You can also try removing C232 and see if the audio returns. If it does and there seems to be no other indications of unlocking, there may be a problem with either the channel selector, TR53 or TR47. If it turns out that the selector is dirty, you can simply cut the trace that goes from the selector to the unlock circuit. It's really not that important. wow, ok...so I powered it up this morning, works fine. Typically takes a while at first to trigger the event. I jiggled the channel selector and it absolutely mimicked the issue. Can I clean it, or should I replace it? Not saying that is the issue, but I am learning one step at a time. Take to each variable so to speak. I dom't know yet if that is the issue until the problem happens. One strange thing, power on ch1 is 2.5 watts output, on channel 40 isnt's almost 3.5. My SWR's are under .5 across the board (using an SWR meter) what would cause the power difference? Ok, when the issue happens Pin1 of IC1 is 3.4vdc. (pin 1 is the pin by the little dot correct? Just making sure)
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Post by husker on Jan 13, 2018 11:54:49 GMT -5
Thanks for the reply! Yes, it keys fine, power is set to 2.5 watts and swings to almost 5. And I did order that part to replace the cap that’s the wrong value. Not sure why they would replace a disk cap with an electrolytic cap. This is the third “golden screwdriver” item I have found. Still not sure about the diode off pin 10 of IC6. I will try the channel selector to see if it’s dirty. It does seem like it is locked , the volume is there, just muted.
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Post by husker on Jan 13, 2018 1:45:32 GMT -5
No joy, replaced the cap as well, just to be safe. I did find a 220uf. Still weak audio
But remember I said there had been someone in this radio, c132 should be a .0047 50v cap, someone put in a 2.2uf 50v cap. Any clue why???
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Post by husker on Jan 13, 2018 1:14:15 GMT -5
I would have a look at C186. The original part would have been rated at 10 Volts. That alone makes it a "regular suspect" when receiver audio alone fails, leaving the S-meter still busy, and has no effect on transmit at all. Original value is 220 uf. Anything from 100 to 330 would likely work the same. 73 Yup, that was one of the caps that was replaced with the kit, I reheated the connection to make sure I didn't have a cold joint. Still the same issue, the volume is there, just real soft, if I turn it all the way up it gets a bit louder. But if I turn it all the way down it stays the same low level. I only have 100uf caps, I can replace that one with a 100 to see if that makes a difference.
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Post by husker on Jan 13, 2018 0:22:28 GMT -5
I was thinking since the S meter is moving, but no audio, I would check IC6. The voltages were strange .. Pin1 12.95vdc Pin2 6.86vdc (check c180?) Pin3 0 Pin4 0 Pin5 1.18vdc(check c179?) Pin6 1.18vdc Pin7 0 Pin8 0 Pin9 6.6vdc Pin10 13.71vdc
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Post by husker on Jan 12, 2018 23:25:01 GMT -5
when the issue happens now to the receive volume, the IC4 voltages are exactly where they should be. looks like the issue is further down the road. The s-meter moves, just no volume
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Post by husker on Jan 12, 2018 22:46:49 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. Guess what, ran great last night, turned it on tonight and the issue started to pop up again. I am very confused..
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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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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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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 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 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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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 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 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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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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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 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 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 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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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 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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Post by husker on Dec 23, 2017 1:43:45 GMT -5
The high reading at pin 6 of V301 could be from R307 getting old and the resistance value dropping with age. We routinely replace it with a 2-Watt ceramic-body resistor without bothering to check it first. But this reading is upstream from where the SSB and the AM signals branch off into separate directions. Since it's not holding back the SSB receive side, it's not the culprit for poor AM sensitivity. Might check pin 1 of V302. If it's way below 130 Volts, suspect R310, maybe R311, maybe both. This is the first AM-only voltage reading downstream from the AM/SSB split. 73 Both r310 and r311 check out good. V302 pin 1 was 121vdc. The no S-meter is also related correct?
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Post by husker on Dec 22, 2017 23:52:16 GMT -5
Check pin 6 of V400, the 6BA6 tube for at least 85 Volts DC. If it's much higher than 100, that's wrong. If it's way low, find R402, the 47k resistor that feeds pin 6. The other end of it should show about 240 Volts DC. If it does, R402 is probably bad. If not, R403, a 2.2k resistor that feeds into it could be bad. Making sure you have a proper peak in each slug of T302, T400 and T401 is important. The single slug in L400 matters, too. But clearly, the problem is in an AM-only circuit downstream from where the AM and SSB receiver signals branch off separately. 73 I just checked the resistors you listed real quick, both check good. I will test voltages on the tube. Ok, pin 6 is 102vdc. The other side of R402 is 245vdc but v301 pin 6 is 182vdc? seems rather high
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Post by husker on Dec 22, 2017 22:44:30 GMT -5
SSB receive is fine, AM receive is very weak, I have to turn the volume up almost all the way. And the S meter is great on SSB, no movement on AM
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Post by husker on Dec 22, 2017 16:18:22 GMT -5
Yes it is negative. And yes, great receive audio on SSB.
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Post by husker on Dec 22, 2017 15:43:13 GMT -5
The final audio tube? It is set at 35v at pin 5. According to what I have read, that's correct. Agreed? lol... I have also replaced r322 to make sure it wasn't the issue on receive. It was older. not factory, but older so I replaced it as well.
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