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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old Yesterday, 12:26 pm   #21
brymac
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Default Re: Carlsboro Cobra 1100 repair.

I think I've cracked it!

Having pulled and tested all the large semiconductors, I pulled and tested the two L7915CV voltage regulators. One was outputting 14 volts when fed 14V DC (yes, I checked the data sheet first) the other half a volt...

See photos.

New ones arriving tomorrow, together with two new o/p transistors (which I may not need...)

I'll keep you posted...!!

Bryan
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Old Yesterday, 1:28 pm   #22
Lucien Nunes
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Default Re: Carlsboro Cobra 1100 repair.

Quote:
the two L7915CV voltage regulators
I don't have the circuit but it seems highly unusual to have two negative regulators. Are they not a pair of 7815 / 7915 for +/-15V rails?
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Old Yesterday, 1:42 pm   #23
brymac
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Default Re: Carlsboro Cobra 1100 repair.

I've just checked and yes they are - one 7815 and one 7915. Does that blow my test failure out of the water
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Old Yesterday, 10:18 pm   #24
brymac
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Default Re: Carlsboro Cobra 1100 repair.

7915 definitely failed - new one arrives tomorrow!
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Old Today, 5:30 am   #25
Diabolical Artificer
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Default Re: Carlsboro Cobra 1100 repair.

Progress though I reckon your not quite out of the woods yet. So, the 7915 is dead. The pinout on these is different to the 7815. The 7815 goes IP, GND, OP, the 7915 goes GND, IP, OP. They often fail but what damage has it caused downstream. As we have no schematic to guide us I'd start buzzing the PCB out. That means one probe on the -15v 7915 OP or output pin then the other probe to all PNP type small signal Q's. When you get a beep or a reading check the tranny, might be toast.

What is the voltage going into the 7915? It should be only a few volts, which you wouldn't think would cause much trouble but the designers/makers might have been silly and it could be a lot more. There might be a dropping resistor somewhere that's failed dropping 35v say (or 50v! I've seen 80v rails and a dropping R and a poor LM78/79 at the end) down prior to the 7915, worth a look and check.

Andy.
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