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Post by Deleted on Jul 18, 2008 20:59:29 GMT -5
I am restoring one and the guy had extra channels installed on the Delta Tune switch. I want to un-do this. If anyone has one, can they take a pitcure or give me the wiring to that switch?
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Post by Deleted on Jul 18, 2008 22:10:32 GMT -5
Crapper ! The ALC cap C96 was clipped. Add a 3.3uf cap to the list.
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Post by Deleted on Jul 19, 2008 17:04:44 GMT -5
Weird. I looked at the schematic, and the Delta Tune switch only has one connection. But it is a 3 position switch, which makes it only working in 2 positions. Not that it matters as the previous guy had used it for th extra channels. But is there any reason for that?
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Post by Sandbagger on Jul 19, 2008 17:18:10 GMT -5
Weird. I looked at the schematic, and the Delta Tune switch only has one connection. But it is a 3 position switch, which makes it only working in 2 positions. Not that it matters as the previous guy had used it for th extra channels. But is there any reason for that? Many Delta Tune circuits are designed so that there is fixed capacitors switched in to drop the frequency. In the (+) position, there is no extra cap, so no connection is needed. In the middle and (-) positions, there are caps switched in to drop the freq. proportionately lower.
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Post by mrgumby OT21 on Jul 21, 2008 19:39:19 GMT -5
I have a midland 77-882 with the o2a chip it's a cibernetics chassis it might be the same one let me know what you think ..maybe I can help you out
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Post by Tombstone (R.I.P.) on Jul 22, 2008 0:14:22 GMT -5
What "extra" channels does the co-pilot have? Chances are that the delta tune wires were removed from the switch and the radio is always on center slot, so to speak, anyway. If those extras are frequencies that you would use, why not leave it the way it is? I know what you mean though, if the delta tune was restored to original specs you can fine tune stations that are slightly off frequency and any extras you want can then be put on a separate switch, but then you're drilling holes for switches unless you use a separate box for your extras but then you have long leads connected and crystals usually don't like that.
Tombstone
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Post by Deleted on Jul 22, 2008 8:54:27 GMT -5
Upon further review it uses more than the Delta Tune switch. But, I might leave it as is. I have the unit ripped apart. First, all the lamps were out, so I replaced all the lamps (meters, RX, TX lamps), with LEDs. The thing was fair to poor condition, until I clean it with Krud Kutter, which essentially just melted away the nicotine stains. What I realized after the nicotine came off, was that the unit is in very good condition, with minimal scratches. In fact, I was going to paint the case, but after the cleaning, there isn't a scratch on it. It was just plain dirty and stained. I swear, I wish I had taken before pictures. This radio has little monitary value. But I love working on them because I learn from them, and they are a piece of Americana from the 1970's. I wish more people used old school more than that new crap. I picked up this radio, along with a Lafayette 25A, 25B, HE-20, for a total of $20. I think the HE-20 is the only one I might not be able to do anything with, as the face has damage.
I will follow up with the extra channels. The radio needs some cap replacements, as there is a hum that eventually fades as the radio warms. You can only here is with the volume turned down. Oh, the mod limiter was clipped. Do I have to buy a cap and put it back in.
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Post by Tombstone (R.I.P.) on Jul 22, 2008 10:08:09 GMT -5
If you don't have the capacitor you'll need to but one, if it's just got one lead clipped maybe you can solder it back into the circuit, there's really no need to clip anything in the limiters unless that's the only way to get full modulation but in that case a power mic should work.
Tombstone
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Post by Deleted on Jul 22, 2008 11:19:14 GMT -5
It's completely gone. The only thing that confuses me is cutting a cap. It is a coupling cap into the ALC transistor, and is supposed to pass AC while blocking DC. Well, if you cut the cap, don't you cut the signal to the base of the transistor? I don't think he jumpered it, otherwise you wouldn't need to clip.
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Post by Sandbagger on Jul 22, 2008 11:28:20 GMT -5
It's completely gone. The only thing that confuses me is cutting a cap. It is a coupling cap into the ALC transistor, and is supposed to pass AC while blocking DC. Well, if you cut the cap, don't you cut the signal to the base of the transistor? I don't think he jumpered it, otherwise you wouldn't need to clip. That's right, the cap passes A.C. which, in this case, is your modulation. The stronger the modulation, the greater the feedback. The ALC transistor is a variable attenuator, which cuts the level of modulation proportional to the amount of feedback. If you cut the cap, there is no longer any feedback to control the modulation, and it effectively goes to full gain. That's why many people clip the feedback cap. If you want to restore the proper ALC action, you'll have to replace that cap.
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Post by Tombstone (R.I.P.) on Jul 22, 2008 21:10:55 GMT -5
If that cap is meant to pass ac and block dc then it must require a non polarized capacitor, right?
Tombstone
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Post by Sandbagger on Jul 22, 2008 21:26:35 GMT -5
If that cap is meant to pass ac and block dc then it must require a non polarized capacitor, right? Tombstone Not necessarily. There are D.C. voltages present as well (which is why the cap is needed for isolation) Usually the high level modulation side it running between 6 and 12 V depending on the radio, while the ALC circuit is usually running in the 10ths of volts. So a polarized cap will work just fine.
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Post by Deleted on Jul 23, 2008 8:08:35 GMT -5
I ordered the all new caps. Most were pennies. The same cape that went for $5 at Rat Shack is about 30 cents at mouser. The whole radio would cost about $3 in caps, after you add shipping, about $10. I typically order from mouser when I have multiple orders to place, so I save on shipping. The seem to have everything. They even have equivalent replacement heads for D104 for about $4.
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Post by Sandbagger on Aug 4, 2008 21:43:49 GMT -5
OK, I finally cleaned this up, and replaced all the caps including the mod limiter. However, I cannot adjust modulation with RV2. It simply won't move on the score. I put in a 1KHz 6mv signal, and it doesn't adjust. If I increase the signal to about 8mv, it shows 100%, but still won't adjust. Any ideas? When setting the AMC, you have to be able to drive the audio into overmodulation. If your input signal won't drive the radio's output above 100%, you may not see the proper reduction in audio when the pot is set properly. If you raise your audio input level to the point where overmodulation occurs, and you still cannot adjust the AMC for proper 100% max, then there is likely further issues in the AMC circuit. I'd look at the AMC shunt transistor (Q15 and/or Q14) for a clipped leg, as that is the typical way to defeat it.
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Post by Deleted on Aug 6, 2008 20:43:57 GMT -5
Dave,
I had the cap in the wrong spot. My eyes are going. I corrected it, I was able to easily set it afterwards.
However, I ran into a problem. I cracked the L12 slug for the DK. The DK was set about 4.5, and I was turning it down. The thing was brittle, and turned into a powder. Are most of these slugs the same, with just the windings being different? I removed the pieces and it DK's about 1.5 watts. When I have it connected to a stinger, the reports are pretty good. I can leave it like that. But I would rather turn it up or down.
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Post by Sandbagger on Aug 7, 2008 7:19:30 GMT -5
Dave, I had the cap in the wrong spot. My eyes are going. I corrected it, I was able to easily set it afterwards. However, I ran into a problem. I cracked the L12 slug for the DK. The DK was set about 4.5, and I was turning it down. The thing was brittle, and turned into a powder. Are most of these slugs the same, with just the windings being different? I removed the pieces and it DK's about 1.5 watts. When I have it connected to a stinger, the reports are pretty good. I can leave it like that. But I would rather turn it up or down. You really can't use a transformer coupling as a variable power control. Those inductors adjust the impedance matching between the final and the antenna, in much the same way that a tube rig uses the load and plate controls. If you detune the coil too much or in the wrong way, you will also lower peak power out (which will give you horrendous back swing and a distorted waveform), and increase the current draw in the final transistor which could shorten its life. If you are careful, you can chip away at the slug until all the pieces come out, and then replace the slug with one from a junk radio. Align the radio for max peak power out, and if you want to vary the dead key, you can drop the voltage to the driver stage by increasing the value of R21 (and place a fairly high value electrolytic cap across it for added modulation punch), which will drop the dead key power down to your desired level. If you want to make it variable. you can take a power transistor (like a TIP29) and replace R21 with it. The collector lead goes to the "hot" side of R21, the emitter side goes to the driver side of R21, and the base side can be connected to a variable pot connected to 12V through a suitable dropping resistor. Although that's probably more work than you really need, especially if you're restoring it to factory stock condition.
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Post by Tombstone (R.I.P.) on Aug 7, 2008 7:20:46 GMT -5
If you apply a little heat with a soldering iron to a slug before trying to turn it and use the right plastic alignment tool. that usually eleminates damaging the slug. If you have a junk radio laying around you could experiment with slugs taken out of that.
Tombstone
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Post by Deleted on Aug 7, 2008 8:39:27 GMT -5
I guess that was one of my questions. Can I stick a slug from a junk radio in that coil (it is a coil, not a transformer). I know the material is the same. But if I can a same size slug from another radio, would it work?
BTW, I have this nice set of GC radio and TV alignment tools, but did not have the proper size for this. I must have about 25 tools in the kit. So I had to resort to an allen wrench, because I couldn't get a tool that size. Gentle and all, I still broke it. I guess I learned my lesson.
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Post by Sandbagger on Aug 7, 2008 12:21:14 GMT -5
I guess that was one of my questions. Can I stick a slug from a junk radio in that coil (it is a coil, not a transformer). I know the material is the same. But if I can a same size slug from another radio, would it work? BTW, I have this nice set of GC radio and TV alignment tools, but did not have the proper size for this. I must have about 25 tools in the kit. So I had to resort to an allen wrench, because I couldn't get a tool that size. Gentle and all, I still broke it. I guess I learned my lesson. If you find a similar slug from another parts radio, chances are good that it will work well enough. There are differences in the permeability mixes in the ferrite that different tuning slugs are made from, which can affect the inductance range (and "Q") of the coil. But in that circuit, it shouldn't be too critical. In any case, it should be better than what you have now.
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Post by Deleted on Aug 7, 2008 20:04:20 GMT -5
This radio was 1Khz off frequency. I aligned the PLL, and now is on the money.
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Post by Deleted on Aug 18, 2008 21:23:51 GMT -5
I finished restoring this radio. Radio reports are horrible, unless I clip the limiter. But now, there is a nasty buzz coming thru the speaker. It appeared after the radio had been one a while. The buzz has is unaffected by volume control, or whether I have an antenna or dummy load. It's still there. Disappears when I pull the mic. All the caps have been replaced. Any ideas?
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Post by Sandbagger on Aug 19, 2008 7:00:37 GMT -5
I finished restoring this radio. Radio reports are horrible, unless I clip the limiter. But now, there is a nasty buzz coming thru the speaker. It appeared after the radio had been one a while. The buzz has is unaffected by volume control, or whether I have an antenna or dummy load. It's still there. Disappears when I pull the mic. All the caps have been replaced. Any ideas? Open mic ground or an incorrectly wired mic seems the most logical first step. You have to be careful that the mic opens the audio lead on receive as it remains "hot" and will affect the receiver's audio if anything is loading down the audio lead. 4 wire Turner base mics and Astatic D-104's short the audio lead to ground on receive and that causes all sorts of bad things to happen on these Cybernet radios. 6 wire mics open the audio lead and are preferred for these radios.
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Post by Deleted on Aug 19, 2008 7:49:23 GMT -5
I tried 2 mics. A D104 handheld that is 6 wire, and a old Turner +2 handheld. Same with each. I will look for the internal wiring on the radio. Also, can you take a look at the mod limiter here: www.cbtricks.com/radios/rca/14t260_70_14t303_4_5/graphics/14t260_70_14t303_4_5_pg33_34.pdfThe limiter is C96, a 3.3uf electrolytic. can I change it to a value of something else? I am getting terrible audio reports with it in, and somewhat better with it removed, but still no good. The scope is showing dead on 100%. When I adjust higher, it goes from crappy to less crappy. The d**n waveform looks so nice, too. I can't go much higher as the waveform distorts.
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Post by Tombstone (R.I.P.) on Aug 19, 2008 8:56:33 GMT -5
Did you replace the filter cap in the power supply too? If not then that might be the buzzing problem. If you replaced that cap then there might still be rectifier problems. Will that radio receive without the mic plugged into it? If not you can try jumpering across the receive pins on the radio's mic connector and see if you still get the buzz, if it's still there then I'd say that there's another problem in the power supply like a shorted rectifier or bad diode. I'm not sure how they have that power supply set up and I don't know if a shorted rectifier would blow the fuse or not. Just a guess, hope you find the problem.
Tombstone
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Post by Deleted on Aug 19, 2008 9:14:55 GMT -5
All the electrolytics were replaced. I usually replace the power supply caps first, and work my way down, from biggest to smallest. I tried 2 mics. I think I might have found something. The audio chip is not perfectly against the heatsink, which changes the pitch as a push it. I'll try that. That explains the noise after it's been on a while.
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Post by Deleted on Aug 19, 2008 15:47:48 GMT -5
Good call, Tombstone. I bypassed the power supply, and used an external. Buzz disppeared. I rechecked about 2 more times. Same result. Both caps were already replaced. I then replaced the diodes with some scrap BA (Big ass!) diodes from another power supply. These diodes were much bigger, so they should be plenty. Noticeable improvement.
BTW, you might ask, "Why am I working on a radio like the RCA Co-Pilot?" Well, this thing was filthy and dead and buried. It's not a Browning, Tram, etc. But it's part of Americana none the less, and I am trying to keep that alive. I took this from poor to excellent condition. Now, I want it to be my bench CB so I can talk and listen while I am restoring others.
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Post by Tombstone (R.I.P.) on Aug 19, 2008 18:49:52 GMT -5
Great! Glad that you isolated and repaired the problem. I don't blame you for restoring your Co Pilot, I bought a new 23 channel Co Pilot mobile back in the 1970's and it was a great radio.
Tombstone
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Post by Deleted on Aug 19, 2008 21:11:00 GMT -5
Stone, I had the radio on all night. The difference was simply amazing. No buzz, and no complaints. I didn't know the diodes could crap out like that. I figured they would short and kill the power supply. Never did I think they could cause that.
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Post by Sandbagger on Aug 19, 2008 21:20:57 GMT -5
Stone, I had the radio on all night. The difference was simply amazing. No buzz, and no complaints. I didn't know the diodes could crap out like that. I figured they would short and kill the power supply. Never did I think they could cause that. I would tend to agree with you. I figured a diode was a device that was either good, open, or shorted. But then I was restoring my Cobra 139 and I had a problem with the built-in power supply not holding voltage under full load. The regulators were good, but I noticed that the raw D.C. was dropping below 15V, which I knew was not enough to maintain good regulation. The filter cap was then replaced and it didn't really help. The diodes tested good with the standard diode check with a VOM. But just for grins and giggles I changed them, and boy what a difference. The raw D.C. then stayed above 17V and the regulator held the voltage at 13.8 V. So you never know what can happen to a 25 year old radio and the parts in it. The lesson I am now learning is that none of the parts are immune from failure or drift.
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Post by Deleted on Aug 19, 2008 21:29:07 GMT -5
I learned my lesson. I took these large diodes from an old Cobra 93 LTD. They were pretty large, so are usually high current. I am wondering if I should put the limiter back in. I am sick of removing it and installing it.
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