A couple things Re: Cobra 148

[igmo]

I’m in need of help regarding service work I’m trying to perform on (3) 148GTL’s. All three are the older models with the 8 pin IC4, side access mic 8 pin mic plug, and MB8877 chip. I think you get the idea. All 3 have been modified and hacked-up pretty good.

Anyway, I ordered the Sams pub for it and was pleasantly surprised they are still in business however, I’m discovering maddening discrepancies. First, I can’t read the schematics as my eyes are almost done. Secondly, my radios all have a VR7 which doesn’t appear anywhere on the schematics or BOM (bill of materials). Sams also neglected to include a list of resistors and values. Etc.

Here’s my itch. In trying to determine why there’s no am modulation. I found no 6.83v available from a #3 branch originating from a 8.14v leg from IC4. It does read just over 3v now in receive AM mode. One schematic shows exactly what I’m describing but then another schematic shows a different configuration at IC4…plus I can’t read but so much then I have to stop. Fuzzy print, misleading schematics and faulty eyes makes an old man whine.

Question: Is there a clean and readable schematic w/voltages available in large 11x17 format for this older version?? By the way, it wouldn’t do much of anything when I first popped it open. It’s slowly returning to life but that damn 6.83v leg has me stumped. I can get it if I had the proper docs.

Thanks es 73

 

[1iwilly]

here this might help you it's better because you can zoom in as much as you need on your computer while working on the radio hope this helps

 

[Smokinone]

You say it's coming back to life the longer it's hooked up? I would say the capacitors would need replaced. I recently worked on an old Penneys radio and it did basically the same thing. Dead until it sat idle with the power on for a few hours as the capacitors started to come back to life after sitting many many years in a garage. I did change out all the electrolytic type capacitors and it's works great. I had no shorted caps, but there were quite a few that were way out of specification.
Just my experience.

 

[999]

I've found this to be the best resource for 148's. I know it's for the 2000, but they are basically the same radios inside.
This has MUCH more information than you will find specifically for the 148 and should get you though your issues.

Wayback Machine

 

[igmo]

here this might help you it's better because you can zoom in as much as you need on your computer while working on the radio hope this helps

Thank you!!

You say it's coming back to life the longer it's hooked up? I would say the capacitors would need replaced. I recently worked on an old Penneys radio and it did basically the same thing. Dead until it sat idle with the power on for a few hours as the capacitors started to come back to life after sitting many many years in a garage. I did change out all the electrolytic type capacitors and it's works great. I had no shorted caps, but there were quite a few that were way out of specification.
Just my experience.

When I said it was coming back to life I really meant to say I was making progress with it. Started with nothing lit, to no output, then to wrong voltage readings to no modulation. Getting CT2 to cooperate was fun.

 

[1iwilly]

Thank you!!

When I said it was coming back to life I really meant to say I was making progress with it. Started with nothing lit, to no output, then to wrong voltage readings to no modulation. Getting CT2 to cooperate was fun.

yest ct2 is touchy

 

[nomadradio]

Years ago when we quit using a counter for setting crystal trimcaps, listening to the signal on service monitor revealed the same scritchy noises you get turning the knob on a scratchy volume control. Not all trimcaps are created equal, but the ones with the color plastic housing benefit big time from a drop of control cleaner, and a good dozen or so spins back and forth. Just doing this until the tone changes smoothly in the audio of a sideband receiver set for the 7.8 MHz carrier will smooth out this adjustment most of the time. Sometimes I only have to twist it a couple/three times. No point trying to accurately set a dirty/noisy trimmer adjustment.

73

 

[igmo]

Years ago when we quit using a counter for setting crystal trimcaps, listening to the signal on service monitor revealed the same scritchy noises you get turning the knob on a scratchy volume control. Not all trimcaps are created equal, but the ones with the color plastic housing benefit big time from a drop of control cleaner, and a good dozen or so spins back and forth. Just doing this until the tone changes smoothly in the audio of a sideband receiver set for the 7.8 MHz carrier will smooth out this adjustment most of the time. Sometimes I only have to twist it a couple/three times. No point trying to accurately set a dirty/noisy trimmer adjustment.

73

 

[igmo]

Another great tip, thanks! Adjusting CT2 to the 7.8015+5hz is one of my issues. I can only get it to 7.796. So, now I'll go for the cleaner.