Cobra 138XLR

[unit435]

I found a supplier of electronic components close by. They have the electrolytic 2.2uf cap in stock and I will try to pick it up today or tomorrow.

I read an article specifically about that cap replacement. The article insisted to replace that cap with another tantalum cap.

I suspect the biggest difference is the availability and the life expectancy but is there any other reason that the tantalum should be used ?

 

[Smokinone]

I found a supplier of electronic components close by. They have the electrolytic 2.2uf cap in stock and I will try to pick it up today or tomorrow.

I read an article specifically about that cap replacement. The article insisted to replace that cap with another tantalum cap.

I suspect the biggest difference is the availability and the life expectancy but is there any other reason that the tantalum should be used ?

I'm no expert, but I believe that the tants are lower leakage than an electrolytic. I was advised while working on another project that a UKL series (low leakage) electrolytic could be used to replace a tantalum type. I think there are also size constraints to be considered.

 

[tramd]

Tantalums have the advantage of operating better than electrolytics where certain frequencies are involved. They are also longer lasting and more stable but more expensive. Would be best to replace it with one of a higher voltage rating such as 35 volts. May last longer, but hey 44 years ain't bad if it's the original one.

 

[unit435]

Thanks. I guess for now I will just go with the elecrolytic as it is in stock close by and I need to get that in so LC can continue his guidance to fix the main problem which is no power on transmit and no receive. I may get that kit from KM and have him include the tantalum cap but first I will need to get this radio working again.

 

[unit435]

Possible Dead xtal

If the crystal went dead, would that cause no power output on transmit ?

 

[groundwire]

If the crystal went dead, would that cause no power output on transmit ?

Yes. That was the issue with mine. No tx or rx, mixing xtal was bad. Put a new one in and bam! It came to life

 

[guitar_199]

Yes definitely. If that 10.24 Mhz reference oscillator is dead... the whole shooting match is dead. If you have the schematic on this.. you may want to find the PLL's lock signal...and see if the PLL thinks it is locked. If it is, you have gone a long way. If not, that needs to be fixed.

 

[loosecannon]

Unit435,

When i posted the original tutorial i had a feeling that IC6 might have gone bad on you.
seems like that is not the case, which means it is entirely possible that your problem is not coming from a missing voltage, and could well be a missing signal, which means that you might need a frequency counter in order to find the issue.

you will need one that has a BNC connector on the front, not the type with SO-239's on it.

If you get time, you want to start checking the voltages on all the pins of the different IC chips, and reference them to the service manual.

this can help find problems.
LC

 

[unit435]

Unit435,

When i posted the original tutorial i had a feeling that IC6 might have gone bad on you.
seems like that is not the case, which means it is entirely possible that your problem is not coming from a missing voltage, and could well be a missing signal, which means that you might need a frequency counter in order to find the issue.

you will need one that has a BNC connector on the front, not the type with SO-239's on it.

If you get time, you want to start checking the voltages on all the pins of the different IC chips, and reference them to the service manual.

this can help find problems.
LC

Unfortunately, my feq counter from long ago was loaned and never returned so I guess I will need to look for another.

Was not able to get to the electronics store yesterday for the 2.2uf but will try today. I suspect a new freq counter is a week or so away so anything that could help without the counter is welcomed. I hope that cap fixes the ssb transmit issue as I am more inclined to think that it was something I disturbed while poking around because that was not a problem before and just now began.

Is there any way to determine a bad crystal without the counter?

I just started a new day lets see if I can get the cap and install it. We are out on the road and did tow the car but it may not be available to me today.

Thanks

435

 

[loosecannon]

If this radio sat on a shelf for a long time and is now being put back in to service, there is a good chance that one or more of your old caps has developed issues.

regardless of what we do or don't find, a re-cap is a very good idea if you plan on using this radio for any length of time.

if they haven't caused problems yet, they will at some point, and yours are over 40 years old at this point.

the fix on c179 is a very common repair for this chassis, and most just replace it with an electrolytic.
it will work just fine.

when C179 starts acting up, it makes the relay chatter on SSB and will blow fuses on TX.

check your voltages on big transistors and IC chips and post back with any differences from normal.
LC

 

[unit435]

If this radio sat on a shelf for a long time and is now being put back in to service, there is a good chance that one or more of your old caps has developed issues.

regardless of what we do or don't find, a re-cap is a very good idea if you plan on using this radio for any length of time.

if they haven't caused problems yet, they will at some point, and yours are over 40 years old at this point.

the fix on c179 is a very common repair for this chassis, and most just replace it with an electrolytic.
it will work just fine.

when C179 starts acting up, it makes the relay chatter on SSB and will blow fuses on TX.

check your voltages on big transistors and IC chips and post back with any differences from normal.
LC

I did get to the electronics store yesterday but they close at 12pm so another day passes without the cap. Cannot make it today so it will be either tomorrow or Friday.

That cap failure must have been a coincidence.

Do you have any suggestion for a counter and probes ?

I would prefer to get the radio up and running before I replace all of the caps as that will be a big job for me at this time so hopefully I can get to the root of the main problem then get the kit from KM and replace the caps.

If all this works out, another modification I would like to do is increase the clarifier slide ability. I have not looked into what I could do to increase that. It currently is just unlocked by cutting the etch and running a jumper. I do remember the freq change was higher to one side than the other. Any suggestions would be welcome.

The other issue I will need to work on is the antenna system in the motorhome. The motorhome currently has a single antenna mounted on the driver's side with no ground plane to speak of. I use a cobra 29 for now which is a borderline match on 19 and has terrible receive qualities and I am sure the transmit qualities are just as bad. I assume a dual antenna system may cure my problem but I am not good with antenna design. The motorhome is built with steel ribbed foam panels throughout. The current antenna is fastened to one of the side panel steel ribs. Other than that, no ground plane reference exists except for the extra coax coiled up inside at my feet.

 

[unit435]

I will be picking up the cap on Friday if everything goes as planned. The electronics store does not seem to have any freq counter options based on the website but I will know better when I get there on Friday

 

[Holydvr]

https://www.ebay.com/itm/3122799101...3D%3D|ampid:PL_CLK|clp:2332490&epid=710122270

 

[Holydvr]

That freq counter is just an example. I have had decent luck with that type, but keep in mind they have different models that cover different freq ranges if my memory serves me.

 

[Holydvr]

Of course you will need a probe to use it the way you need to.