Carlsboro Cobra 1100 repair.

[brymac]

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

[Lucien Nunes]

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?

[brymac]

I've just checked and yes they are - one 7815 and one 7915. Does that blow my test failure out of the water

[brymac]

7915 definitely failed - new one arrives tomorrow!

[Diabolical Artificer]

Progress though I reckon you'e 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.

[brymac]

Thanks Andy - I still don't have a schematic, but the attached is the closest I've got to what is happening at the PSU end - it may help with the diagnostics.....!!

Bryan

[Station X]

If the OP was initially unaware that the two voltage regulators were of different types with different pinouts is it possible he tested them incorrectly?

[vidjoman]

I would suspect that the + & - 15 volt supply is for a preamp section. From the earlier posts it looks like the main supply for the power amplifier is + / - 35 volts.
Without more details it maybe the are 2 or more faults.

[Herald1360]

18Vac will give around 25Vdc on the regulators. In spec, but a bit on the high side for dissipation. Without heatsinks, they'll only be good for about 100mA, with will depend on the thermal R.

[brymac]

Graham - once I realised that the pin out for the voltage regs was different, I tested them again. Results attached - the L7915CV is definitely duff!!

[Herald1360]

If you're only putting 14.1V into a 7915, would you expect to see anything out? They need about a couple of volts headroom to regulate properly. (>16V in)

[brymac]

And yes, the 15V supply appears to be for the preamp, so there may well be another fault....!

[Diabolical Artificer]

so there may well be another fault....!

An OP sparking and smoking indicates more than a duff 7915. As advised see if you can find what the 7915 was attached to.

Also as you say the +15/-15v rails are for the preamp, the main PSU might be ok, but worth checking. With the 7915 removed do a resistance test from it's OP pin pad to gnd.

Andy.

[brymac]

Sorry Andy - don't know what "pin pad to grid" means....!!??

Bryan

[Station X]

It says "gnd" (ground), not "grid".

[brymac]

Sorry - thanks!

But what's an Output pin pad

Bryan

[Herald1360]

A PCB land connected to an output pin?

[Diabolical Artificer]

The output (OP) pin of the 7915 or the regulated -15v rail. You're looking for any damage the duff 7915 may have caused, so before re-fitting a new 7915 you check for faults. If you don't you may re-fit said 7915 and bang, another dead IC (integrated circuit). Hence my suggestion you do a resistance test from the 7915 OP pad (or where it is soldered to the PCB) to ground (gnd). Engineers shorthand, saves bashing the keyboard with sausages.

Andy.

[brymac]

Thanks for your patience Andy! I have the fatal combination of being 70 years old and never having "done" electronics for a living (but I have been successfully building and fixing stuff for nearly 50 years and now run the local repair cafe......!!)

But this is really baffling me….

I have replaced both power transistors, both voltage relulators, the 4 largest capacitors, all 4 diodes in the bridge rectifier and checked numerous components in and out of circuit. I have unplugged the front panel, thus limiting the problem to the AC input from the transformer (which is fine at 36v AC) onto the power board.

But - I am still getting 36V DC out of the speaker sockets!!!!! See photos attached.

Oh - and although I still don't have the schematic for this, you will see that there are no ICs on this unit! It predates that!!!

What have I missed??

[Station X]

Voltage regulators are Intregrated Circuits.

Are there capacitors in series with the speaker outputs?