Cobra 2000 clock/counter

[El Kabong]

Hello gent's,

The clock/counter on my 2000 is dead. No workey no mo. Toast. History. Joined the choir invisible. Its a goner.

So now I have to either fix or replace it. It's easy to remove and replace so thats not an issue. The issue is price, and trust. I refuse to pay more than the radio cost for one transistor replacement. Buying another "working" clock/counter module from a junked 2000 requires a lot of trust that eBay has worn away from me lately. And again the price.

Since this is a fairly common issue I wonder what others have done to solve it? No, there are no good or even bad techs near here.

Thanks all.

 

[Robb]

Bought a used 'good' (ha!) freq counter module from Ebay, and it wasn't in good repair as claimed.
Fixing these counters has been hit or miss with me; YMMV.

It is a double sided board with poor quality traces. Solder doesn't want to stick, unless you scuff the copper really well. Time consuming. Little op amp in the front end of these units can go bad and the only supplier is in China and sells for top dollar. The Plessey SL-1611C. Caps can also be bad too. Bad solder joints are a real possibility due to the poor copper traces on both sides of the board.

Also, the counter might actually be working, but someone has boned the clarifier mod up - so it won't get a proper signal. Put the clarifier back to stock, and the counter works. Going thru that very scenario right now, and forces me to go thru the entire front end wiring since I want to unlock the clarifier.

 

[El Kabong]

I see there's a seller on eBay now with those opamps asking $35! When they go bad does that turn the whole display off? Clock and counter?

 

[ExitThirteen]

If the display is completely dark, it's likely the +5V regulator or, the 5.7V 1/2W zener diode. The zener diode in particular is a high failure component, and should be replaced with a 5.6V or 5.7V 1W zener diode. But either part can cause the "blackout" issue you're experiencing.

(Edit) I found the SL1611C IF amplifier IC, qty. 3 for $14.00. Shipping is a touch high at $8.50, but still well worth it. I use these in my counter repairs as needed. There's also a Service bulletin from Cobra that addresses the failures of this particular IC, and provides a permanent fix. It's Technical Bulletin #1232.


~Cheers~

 

[nomadradio]

I have repaired more than a few of these over the years. Didn't see them start to break down all that often until the first 1978 radios got to be 15 or 20 years old. A totally-dark display just says "power supply". In particular, the 5-Volt regulator already mentioned.

There is no "always goes bad" chip. I suspect there was one more more "bad batches" of one chip or another. But too many of them are rare. None of them are made any more. Doesn't make repair impossible, only raises the ante.

Like a car that won't start. Might just be overdue for a tuneup. Might have a blown head gasket.

Years ago the average TV-repair shop did not repair the tuner section of a TV. This was typically handled by a specialist with all the high-end instrumentation, parts and skills needed for alignment. The big outfits would exchange a rebuilt for your old tuner.

Makes me wonder why nobody has tooled up to do this with the Cobra 2000 clock/counter. Won't catch me trying to specialize this way.

73

 

[El Kabong]

You wouldn't have a part number for that +5V regulator so I can try a simple repair for myself?

If the display is completely dark, it's likely the +5V regulator or, the 5.7V 1/2W zener diode. The zener diode in particular is a high failure component, and should be replaced with a 5.6V or 5.7V 1W zener diode. But either part can cause the "blackout" issue you're experiencing.

(Edit) I found the SL1611C IF amplifier IC, qty. 3 for $14.00. Shipping is a touch high at $8.50, but still well worth it. I use these in my counter repairs as needed. There's also a Service bulletin from Cobra that addresses the failures of this particular IC, and provides a permanent fix. It's Technical Bulletin #1232. View attachment 41640


~Cheers~

If the display is completely dark, it's likely the +5V regulator or, the 5.7V 1/2W zener diode. The zener diode in particular is a high failure component, and should be replaced with a 5.6V or 5.7V 1W zener diode. But either part can cause the "blackout" issue you're experiencing.

(Edit) I found the SL1611C IF amplifier IC, qty. 3 for $14.00. Shipping is a touch high at $8.50, but still well worth it. I use these in my counter repairs as needed. There's also a Service bulletin from Cobra that addresses the failures of this particular IC, and provides a permanent fix. It's Technical Bulletin #1232. View attachment 41640


~Cheers~

 

[ExitThirteen]

Oops, correction, the "regulator" I talked about is actually an SCR. These are the two problem areas as outlined in the picture. You can get the SCR on Ebay still, and the diode can be either a 5.6V 1W, or a 5.7V 1W zener.




Hope this helps.


~Cheers~

 

[El Kabong]

And a picture too. Thank you kind sir! 5P4M P39 same as 5P4M P1Z? Either more durable?

 

[nomadradio]

Well, for one thing I misspoke. The clock runs from 8 Volts, regulated by TR522 and TR521. They both depend on the 5.6 Volt zener diode D528 that controls TR519, the actual 5-Volt regulator.

This circuit contains a current limiter, TR515. A shorted electrolytic cap will cause it to limit the fault current, but TR522 will get really hot.

One accurate measurement is worth a thousand expert opinions. The DC voltage at the emitter lead of TR515 would be the best place to start.

73

 

[El Kabong]

Thanks.

 

[El Kabong]

Update: It wasn't the zener. Waiting for the transistor.