Cobra 2000 resurrection

[ShakyMcQuiver]

I've been going through my Cobra 2000 I picked up several years ago and I think I have all the prior 'Golden screwdriver' crap removed. Overall the butchery wasn't that bad. I've seen far worse.

Most of the carnage (so far) was in the channel mod. They used the front switches (which I don't care for). I prefer a simple 3-position like an export radio. I build my own 'Expo' kits using relays. They seem to be more stable and no rotary style switches to hang or wear out on you. More on this later...

Anyway...

I was going through the capacitors to see if any need replaced and I noticed the power supply has 2 large capacitors instead of a normal single one. Any idea why someone would do this? Is there an advantage? Extra filtering perhaps? If so - then why a smaller cap? The SAMS/Schematic calls for a single 4700µF 35V cap. My radio has (2) 2200µF 25V caps.

Everything seems to work OK, but should I consider changing that back to a single cap (say a 4700µF 50V)? I'm all for making things better, but this 'mod' or whatever it is seems a little fishy?

I'm also thinking re-capping most if not all of the electrolytics. I've done single replacements here and there, but not an entire radio. From what I've read, it might not be a bad idea as the caps do degrade. I prefer Nichicon and Rubycon from Mouser or Digikey.

Thoughts? Mods? Tweaks?

And just an FYI: This radio's serial number is 93002902 and it's stamped "Taiwan". So I'm not sure what year it was made. I read somewhere that the Taiwan models were only made up to 1988.

I only paid $200 for the whole smash, so I think I got a diamond in the rough. Came with the original speaker and a Silver Eagle mic, (which looked like someone used it for an axe murder) - not sure if it can be saved. That will be a separate post when I get to it. Luckily I have another good working Eagle.

 

[Klondike Mike]

You didn't get rid of all the 'Golden screwdriver' crap. I would recommend that this capacitor setup be removed as well. This present setup yields an equivalent capacitance of 4400uf @ 25v. Whomever did this just grabbed whatever they had lying around to get it out the door.

The output of the diode bridge is putting out 18-20 volts, below the rated voltage (25v) of the capacitors but with little headroom, hence the 35v specified. Anything greater than 4700uf/35v is good.

 

[nomadradio]

Somebody thought the transmit power was being limited by the size of the filter cap, maybe? I have seen this done to radios that were dimming the lights on modulation peaks. In this model, the most-usual cause for that is C302, a 10uf cap wired across the output of the voltage regulator. We learned to just swap out that cap in every Cobra 2000 that we repair. More than one radio came in after the big filter cap, the rectifier bridge and the big regulator transistor all got changed, but with the same "dimming" symptom still present. Hence the habit of just changing it out. Cheap insurance.

73

 

[ShakyMcQuiver]

Thanks for your replies and good eyes! With some deeper diving, I've noticed a few more things that needs to be un-screwdrivered. I thought I had them all, but nope! I'll tackle them one at a time, so apologies in advance for pestering everyone. Many of you are light-years ahead of me on these old classics and I appreciate the help.

A few of the mods I'll keep, but some (like this power supply mess) needs to be fixed. I'm going through the electrolytics and prepping an order with Mouser, so I'll grab the proper 4700µf.

Another thought: What types of capacitors have you been using? They list basically 3 main type: General purpose, Audio and Low impedance. Just curious what you prefer. The General purpose will work fine, but if you can make it better then might as well.

I also prefer to use Nichicon or Rubycon whenever possible. I've used them to repair PC monitors and other minor repairs with great success. And typically I bump up the voltage a bit; (50v - 105c to replace a 35v, 85c etc.). Nothing too wild.

I've never done a full-out recapping, so this is new to me. Not sure if it really needs it, but as old as it is - it might not be a bad idea, or at least keep them on hand.

Thanks again!

 

[Woody-202]

In PS cap replacement I usually pick "General", most other "audio", and many times it's "Whatever I can afford".

 

[ShakyMcQuiver]

After opening up the speaker cabinet to find out why it sounded like crap, I found this inside.
I can't quite put my finger on it... but something just doesn't look quite right?

Something's afoot Watson! <sarc>.

(I have no idea what the original Cobra 2K speaker guts look like. I've never peeked at one. I'm going to take a wild guess here and say Cobra didn't put a speaker within a speaker?).

I have some extra amplified computer speakers I might experiment with. Maybe take the board out and put inside the cabinet. No harm no foul at this point as this speaker is no longer original. Oh well as they say....

 

[TM86]

The original was just a speaker at the front of the cabinet and open space behind it. I've seen people pack the cabinet with acoustical foam to improve the sound. Don't know if it worked.

 

[ShakyMcQuiver]

The project marches on...

I received my Mouser order today. Big bag of spankin' new capacitors and a few other trinkets.
Most of the caps are Nichicon and a few Rubycon 105c, 35V or 50V.

Any tips to share before I dive into the re-capping?
The radio seems to work good but she's almost as old as I am and from what I've been reading - it's a good idea to R&R those caps. It certainly won't hurt.

I was also reading an interesting old post on here about one of the caps put in backwards. Turns out this particular cap (and some others) WERE purposely installed that way from the factory. I never knew they did this!

Good info. and I'm glad I found the post. Of course I always take close-up pics to make sure I put stuff back in the way it came out.


S.

 

[TM86]

What I do as the first step in recapping is to mark all the electrolytics with a sharpie. Just a small mark on one side of the top of the cap. Marking all caps in the same place to show orientation. I usually use the front of the radio as a reference point.

That way if there's any question of how a cap was originally installed it's pretty easy to figure out. Just line the mark up with where it would have been originally, install new cap the same way.

It also gives a visual indication of whether or not I've replaced any given cap. Usually the color of the plastic jacket on the cap is enough, but there's been times when the old part and the new part were the same color.

Just don't throw the old cap in the trash until the new one is soldered so you don't have to dig through the trash because you were interrupted and forgot which way it should go.

Oh, and use isopropyl alcohol to clean the pads between removing the old part and putting the new one in. Especially on the larger caps.

 

[ShakyMcQuiver]

Thanks again for the great tips!

Master Jedi techs: Is there anything else that can be done to bulletproof this radio? The power supply for example? Anything to be done there (besides replacing those wrong 2200 caps)?

I noticed the original 2SD188 (TR401) transistor was replaced with a 2SD717. It's not listed as a cross replacement; however. Not sure why. It also looks like the brige rectifier was messed with and/or replaced. I haven't confirmed this yet.

It all works. Just want to make it as strong as possible and/or update things while I'm ordering parts.

Thanks again... and I apologize again for more questions than a 2-year old! Messing with these old radios is a lot of fun and relaxing for me. A lot like my LONG ago Jr. high school days and model rocketry. I still mess with them with my Grandson from time to time. Old habits die hard!

 

[nomadradio]

It's more a matter of evolution. Metal transistors cost more than plastic. But in 1978 when that model was designed, metal was your one choice, mostly.

Plastic is cheaper. That, and the 2SD717 has a higher current rating than the metal transistor. Don't know when Uniden changed the radio's production from the metal to the plastic transistor. Early 80's, my guess.

More of an upgrade than a substitution.

73

 

[ShakyMcQuiver]

Thanks for the info. I also figured the 717 was an upgrade of some sort too. I'll leave it alone.

I was getting ready to start my recap today... MY BAD... I though the power supply had (2) 2200uf caps. I was incorrect! There IS a 4700uf cap in the original position (it was upside down so I couldn't see the writing on it). The other cap is a 2200uf no-polarized.

I assumed both caps were 2200 as they physically looked the same. I didn't want to take anything apart until I had my new caps, etc.

So when I removed that clamp and rolled the caps over... I could see the writing "4700µf 85°. The other is 2200µf non-polarized.

So why add a non-polarized to it? Filtering of some sort?

Here's a close up pic. The one on the top is the 4700µF and the one on the right is the 2200µF (NP).