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Post by Deleted on Dec 28, 2012 15:10:01 GMT -5
I am restoring a Robyn SB-520D. This appears to be bone stock. I just recapped it. I have to take the meters apart to repaint the needles. I repaired the on/off switch by ripping it apart and a simple blast of Dexoit. Functionally, seems good. Audio reports good. Appears to have little of no swing on AM. Maybe some backwards swing on the S meter, but not much. Not sure if that is normal.
Anyway, the only 2 mods I want to do is the clarifier. No problem on that.
Also, I would like to see if I can improve the noise blanker with Schottkey diodes. Would anyone know if this would hel and which diodes I should replace?
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Post by Deleted on Dec 29, 2012 10:14:19 GMT -5
I have narrowed this down to a S/RF meter/ or meter circuit. I have some extra meters. Can these meters be adjusted by that little screw? The S/RF is way off.
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Post by "Doc"Hammer on Dec 29, 2012 10:36:33 GMT -5
If this is a uniden 858 board, VR1 adjusts the S-meter.
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Post by Deleted on Dec 29, 2012 11:23:44 GMT -5
If this is a uniden 858 board, VR1 adjusts the S-meter. No. I meant there is a problem. VR-1 does little or nothing. It is way off. |
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Sandbagger
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Post by Sandbagger on Dec 29, 2012 11:56:40 GMT -5
I am restoring a Robyn SB-520D. This appears to be bone stock. I just recapped it. I have to take the meters apart to repaint the needles. I repaired the on/off switch by ripping it apart and a simple blast of Dexoit. Functionally, seems good. Audio reports good. Appears to have little of no swing on AM. Maybe some backwards swing on the S meter, but not much. Not sure if that is normal. Anyway, the only 2 mods I want to do is the clarifier. No problem on that. Also, I would like to see if I can improve the noise blanker with Schottkey diodes. Would anyone know if this would hel and which diodes I should replace? The NB circuits on the Uniden 858 chassis were not all that great. I'm not sure a diode upgrade would help much. Besides it's a base radio. Not much need for a pulse-type noise blanker, unless your neighbor is running a weed whacker while you're trying to hear some weak signals. |
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Post by Sandbagger on Dec 29, 2012 12:03:35 GMT -5
If this is a uniden 858 board, VR1 adjusts the S-meter. No. I meant there is a problem. VR-1 does little or nothing. It is way off. The mechanical adjustment on the meters is to set the at-rest zero only. If the meter is not sticking and is otherwise mechanically sound, then I would leave that adjustment alone. In this case there is probably something electrical amiss in the meter circuit. |
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Post by Deleted on Dec 29, 2012 22:14:49 GMT -5
Well, the meter was definitely bad. However, 2 other meters so little movement in signal. The meter works fine in RF power mode, but not in S. I checked the voltages at the meter amp, as well as some diodes, and they all jive. Any ideas?
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Post by Sandbagger on Dec 29, 2012 22:23:06 GMT -5
Well, the meter was definitely bad. However, 2 other meters so little movement in signal. The meter works fine in RF power mode, but not in S. I checked the voltages at the meter amp, as well as some diodes, and they all jive. Any ideas? Check C35. It's a 1uF electrolytic. If it gets leaky.... you know what can happen. Also make sure the alignment of all your receiver cans is correct, and the AGC VR2 is adjusted correctly. |
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Post by Deleted on Dec 30, 2012 21:55:20 GMT -5
Well, the meter was definitely bad. However, 2 other meters so little movement in signal. The meter works fine in RF power mode, but not in S. I checked the voltages at the meter amp, as well as some diodes, and they all jive. Any ideas? Check C35. It's a 1uF electrolytic. If it gets leaky.... you know what can happen. Also make sure the alignment of all your receiver cans is correct, and the AGC VR2 is adjusted correctly. Dave, I have aligned this and seems good. It is stock outside of a open clarifier. One thing though. I have the DK set at 4 on AM. However, there is virtually no swing in this radio. Also, I am unable to get more that 8W SSB with the transmitter alignment. If this is normal, let me know and I will just leave it be. I only have cosmetics to do now. |
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Post by Sandbagger on Dec 31, 2012 11:24:45 GMT -5
Check C35. It's a 1uF electrolytic. If it gets leaky.... you know what can happen. Also make sure the alignment of all your receiver cans is correct, and the AGC VR2 is adjusted correctly. Dave, I have aligned this and seems good. It is stock outside of a open clarifier. One thing though. I have the DK set at 4 on AM. However, there is virtually no swing in this radio. Also, I am unable to get more that 8W SSB with the transmitter alignment. If this is normal, let me know and I will just leave it be. I only have cosmetics to do now. Well, you seem to be jumping around from one problem to the next. I guess that radio has a bunch..... Let me know if C35 fixes your S-meter, or if you find something else amiss.... As for your transmit situation, 8W on SSB is definitely NOT normal. That chassis should hit every bit of 20+ watts PEP with the ALC cranked open. If it's only peaking at 8 watts, first make sure the ALC isn't set too tightly. If the ALC isn't the issue, check the bias voltages (current). If those check out ok, then I would suspect a weak final. Early versions of this chassis had issues with finals blowing due to overbias, and there were factory mods to change a couple of resistor values in the final bias circuit, to keep things sane. Later versions of the chassis had the problem corrected, but it's still something to check. If you check CBtricks under the Cobra 139 XLR, there is a list of factory service bulletins. The bias mod should be in one of them. |
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Post by Deleted on Dec 31, 2012 16:46:08 GMT -5
Dave, I have aligned this and seems good. It is stock outside of a open clarifier. One thing though. I have the DK set at 4 on AM. However, there is virtually no swing in this radio. Also, I am unable to get more that 8W SSB with the transmitter alignment. If this is normal, let me know and I will just leave it be. I only have cosmetics to do now. Well, you seem to be jumping around from one problem to the next. I guess that radio has a bunch..... Let me know if C35 fixes your S-meter, or if you find something else amiss.... As for your transmit situation, 8W on SSB is definitely NOT normal. That chassis should hit every bit of 20+ watts PEP with the ALC cranked open. If it's only peaking at 8 watts, first make sure the ALC isn't set too tightly. If the ALC isn't the issue, check the bias voltages (current). If those check out ok, then I would suspect a weak final. Early versions of this chassis had issues with finals blowing due to overbias, and there were factory mods to change a couple of resistor values in the final bias circuit, to keep things sane. Later versions of the chassis had the problem corrected, but it's still something to check. If you check CBtricks under the Cobra 139 XLR, there is a list of factory service bulletins. The bias mod should be in one of them. C35 is new. I have the meter problem sortted out. I replaced the final and driver and still get the 8W on SSB. The driver boas seemed high, and I lowered it to about 40. Bulletin said 15. It appears that most of the bulletins were already implemented. Most everything seems OK, with a differing value resistor or so. The problem is I do not know if the Robyn fixes are newer than the Cobra Bulletins. |
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Post by Sandbagger on Dec 31, 2012 18:33:06 GMT -5
Well, you seem to be jumping around from one problem to the next. I guess that radio has a bunch..... Let me know if C35 fixes your S-meter, or if you find something else amiss.... As for your transmit situation, 8W on SSB is definitely NOT normal. That chassis should hit every bit of 20+ watts PEP with the ALC cranked open. If it's only peaking at 8 watts, first make sure the ALC isn't set too tightly. If the ALC isn't the issue, check the bias voltages (current). If those check out ok, then I would suspect a weak final. Early versions of this chassis had issues with finals blowing due to overbias, and there were factory mods to change a couple of resistor values in the final bias circuit, to keep things sane. Later versions of the chassis had the problem corrected, but it's still something to check. If you check CBtricks under the Cobra 139 XLR, there is a list of factory service bulletins. The bias mod should be in one of them. C35 is new. I have the meter problem sortted out. I replaced the final and driver and still get the 8W on SSB. The driver boas seemed high, and I lowered it to about 40. Bulletin said 15. It appears that most of the bulletins were already implemented. Most everything seems OK, with a differing value resistor or so. The problem is I do not know if the Robyn fixes are newer than the Cobra Bulletins. The Cobra bulletins were written in 1977 and cover the earliest production runs of that chassis. If the resistors in the bias circuit are already the same values as what they call out in the modifications, then it is safe to assume that your radio has a later production chassis and the mods were integrated at the factory. I assume you've verified that the ALC is not set too low. If all you can get out of it is 8W PEP on SSB, then it's a sure bet that your peak power on AM will be similarly limited and it will be impossible to get 100% modulation with a 4 watt carrier, and the positive peak will flat top at 8 watts as seen on a scope. If that is not the case and you are able to get 100% (16 watts PEP), then it's likely you have a problem in the SSB section. Make sure you are getting the full 12+V on the collectors of the driver and final in SSB. The ALC adjustment in that chassis is a trimmer cap.When those get old, they sometimes crack and you lose the ability to adjust them. Make sure that's not the case. |
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Post by Deleted on Dec 31, 2012 20:08:57 GMT -5
I do not have any AM swing. In fact, there is a slight hint of backwards swing. I was able to adjust 100% modulation on the scope. I will check the ALC. Also, I am hoping this was not clipped. What are the usually clipped limiters in this radio?
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Post by Sandbagger on Jan 1, 2013 1:06:56 GMT -5
I do not have any AM swing. In fact, there is a slight hint of backwards swing. I was able to adjust 100% modulation on the scope. I will check the ALC. Also, I am hoping this was not clipped. What are the usually clipped limiters in this radio? If you were able to "adjust" for 100% modulation I would assume that the limiter is intact. There should not be any backswing and on an RMS reading meter, either none or slight forward swing at 100% On a scope, if the carrier is set for 2 vertical gradicules, 100% positive will expand to 4 gradicules, and the negative will just touch the zero power point. The wave should remain completely sinusoidal, with no flat tops. If this is all correct, then you are developing 16 watts of peak power, so it remains a mystery why you can get over 8 watts on SSB. |
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Post by Deleted on Jan 1, 2013 10:32:04 GMT -5
The final bias current is way too high and blows my 500 mA fuse in my meter. Also the DK starts at like 4 and gradually fades down. Still troubleshooting.
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Post by Sandbagger on Jan 1, 2013 11:00:01 GMT -5
The final bias current is way to high and blows my 500 mA fuse in my meter. Also the DK starts at like 4 and gradually fades down. Still troubleshooting. You're setting the bias in SSB mode right? With absolutely no signal output, you should get nowhere near 500 ma of current. Something is very amiss there. Make sure the bias pot isn't open (that's common). Also, I remember that I used to change R205 from a 1 ohm to a 10 ohm, so that the pot centered in its range with proper bias, and would not exceed the safe bias range. |
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Post by Deleted on Jan 1, 2013 15:33:47 GMT -5
Rats! Hangover must be impacting my abilities. You were right. I did the measurement in AM. I then did the bias measurement in SSB and was fine. Still back to square one with that swing and SSB power problem.
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Post by Deleted on Jan 1, 2013 17:14:42 GMT -5
OK. I had three 1307 finals. 1 was bad, and the other 3 would not swing. I decided to try a 1969. I now have swing. Low output still on SSB. I am going to need to know the bias current for the 2166 and the 2169 so I can properly set them.
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Post by Sandbagger on Jan 1, 2013 18:21:26 GMT -5
OK. I had three 1307 finals. 1 was bad, and the other 3 would not swing. I decided to try a 1969. I now have swing. Low output still on SSB. I am going to need to know the bias current for the 2166 and the 2169 so I can properly set them. You should probably think less in terms of "swing" and more in terms of peak power. For a test, you should try removing C155 in the ALC circuit, which would effectively defeat it. If your SSB power now jumps up to better than 18 watts, you have a problem in the ALC circuit. There isn't much difference in biasing current. I've seen radios with 1969's in them with bias ratings anywhere from 20 to 60 ma. 2166's seem to be comfortable between 80 and 100ma. |
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Post by Deleted on Jan 1, 2013 19:01:57 GMT -5
Well, I blew it big time. Made big progress. The while repositioning the radio, I appear to short pin 1 and 2 of the mic, and I lost everything. Relay clicks, but TX power at all now. I am done. This thing has pissed me off beyond belief. It is head for the garbage can.
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Post by Sandbagger on Jan 1, 2013 21:33:34 GMT -5
Well, I blew it big time. Made big progress. The while repositioning the radio, I appear to short pin 1 and 2 of the mic, and I lost everything. Relay clicks, but TX power at all now. I am done. This thing has pissed me off beyond belief. It is head for the garbage can. Send it to me. I can use the parts! ;D |
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Post by Deleted on Jan 2, 2013 9:03:19 GMT -5
I just couldn't do it. I went back to it. I found a bad driver and am back to 4W DK with no peaking and low SSB.
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Post by Sandbagger on Jan 2, 2013 9:21:17 GMT -5
I just couldn't do it. I went back to it. I found a bad driver and am back to 4W DK with no peaking and low SSB. What was your peak SSB power before you "cooked" it? Have you tried disabling the ALC? |
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Post by Deleted on Jan 2, 2013 10:29:51 GMT -5
Disabled the ALC. No change. Checked the mic preamp and the output modulator IC, all voltages check out. My DK is 4W with no swing/peak. I am only putting out a few watts on SSB.
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Post by Sandbagger on Jan 2, 2013 13:16:11 GMT -5
Disabled the ALC. No change. Checked the mic preamp and the output modulator IC, all voltages check out. My DK is 4W with no swing/peak. I am only putting out a few watts on SSB. Before you had this latest setback, didn't you say your AM was starting to swing (higher peak power) higher? I had a Midland radio that had low output, both AM and SSB. I was in a similar situation to what you have now. I replaced the final and driver to test if this was the problem (it wasn't), voltages and biases were all ok, no ALC issues. I was pulling my hair, because I won't accept defeat from a piece of radio gear. But then I got lucky. I happened to notice that something was heating up while I was transmitting. That "something" turned out to be a disk capacitor in the Pi network between the final and antenna jack. Sure enough, replacing the cap cured the problem. So maybe you have something similar in your radio. |
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Post by zman on Jan 18, 2013 15:09:41 GMT -5
If worse comes to worse it will make a decent parts rig!!!! I have a few of them around using radios that had more issues than i cared to fix...  |
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Post by gator7 on Jan 19, 2013 10:40:58 GMT -5
More than one of us are willing to help out with taking it off your hands. LOL. ;D
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Post by rfoster950 on Jun 6, 2019 10:19:18 GMT -5
Hi Rob 2wr310 Cincinnati oh I am restoring 8 sb520d, I thought may be we could help each other out didn't see the date of your post but maybe I can we can share info. I seen you post talking about meters and I learned neet trick from Mike's cb repair that still has two of my radeos if you have any info on Mike please let me know going on year + and little communication. So may be driving to Fairfield Pa to find my radeos. Anyways if you take fingernail polish remover and dropper some on srew that gels the glue they put on there then turn little screw until it's free then that glue will harden back up. Then what I did was take two bulb lights out and put in these little small LEDs found on Ebay for 6 bucks that send you all different colors and they fit perfectly in the rubber boot that the condesent buld came out of see pic. So 73's if no anything let me know. I would like to communicate with Mike and if he is going to get to my radeos I just want to know what brand of caps component he is using so I can order same. There was chatter back in 2016 that he was in some kind of trouble. Very frustrating he has replied two time sating hoping to get on bench this week. Phone number I found once mail box was full. The i lost number yeah scared that my property was sent to wrong person at wrong time. Hope not..
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Post by BBB on Jun 27, 2019 16:08:18 GMT -5
I picked up a bone stock Robyn SB-520D earlier this year and have been using it for a few months now. It's not a big audio swinger but the receive is excellent. I don't know why I really like it?? Maybe knowing it has the good stuff inside and the controls are simplistic yet get the job done. Who knows when it will be retired from the bench but it continues to satisfy.  |
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Post by Sandbagger on Jun 28, 2019 12:12:59 GMT -5
I picked up a bone stock Robyn SB-520D earlier this year and have been using it for a few months now. It's not a big audio swinger but the receive is excellent. I don't know why I really like it?? Maybe knowing it has the good stuff inside and the controls are simplistic yet get the job done. Who knows when it will be retired from the bench but it continues to satisfy. View AttachmentWhy do you like it so much? Maybe for the same reason I like my Console II. No logical reason, there's just something likable about it. |
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