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7603 + voltage rails high

 

Hi everyone, I've got a 7603 with no trace and an odd - to me at least - fault. The - ve rails are just under the - 15 and - 50v specified. The +ve rails on the other hand are at +18, +75 and +179v rather than the 15, 50 and 130v specified.
I'm getting a very light burning smell, but this is faint and it might just be dust - or of course the tail end of an already well fried component.
Any thoughts on where I should investigate to find the cause of the increased voltage, and also whether the voltage going high in this way might have toasted other components?
 

The +50v regulated supply is used in the +5, +15, and +130v supplies. If +50 is excessive, it can cause the problems you describe by overcoming the difference amplifiers, that is Q919 in the +15v regulator, for example. I would start looking in the +50 volt regulator.

Dave

On Mon, Feb 1, 2021 at 02:45 PM, Sparky99 wrote:


Hi everyone, I've got a 7603 with no trace and an odd - to me at least -
fault. The - ve rails are just under the - 15 and - 50v specified. The +ve
rails on the other hand are at +18, +75 and +179v rather than the 15, 50 and
130v specified.
I'm getting a very light burning smell, but this is faint and it might just be
dust - or of course the tail end of an already well fried component.
Any thoughts on where I should investigate to find the cause of the increased
voltage, and also whether the voltage going high in this way might have
toasted other components?
 

The schematic copy I have is very poor, partially un-readable, and I can't make out the bulk voltage value feeding Q869, 872, 874. However 75v seems very high for a 50 volt regulator. Is your line voltage 220/240VAC? If yes, is it possible the line voltage selector is set to 115VAC? What is the bulk supply volts across C809? If it is 55-60v I'm thinking Q869 has a c-e short, or CR868? is leaky or shorted. If Q869 or CR868 are bad, then very possibly Q872 and Q874 are also blown due to excess b-c voltage.

Dave
 

Downloaded a readable copy of the manual from Tekwiki, and indeed the bulk +50 volt supply is shown to be +53v, so a value of 55-60 volts would be reasonable.
If you find Q869 has developed a c-b short, then the +179volts would be applied to the collector of Q860 and Q876 which may have damaged them and Q863 as well.
Before replacing any semis in the +50 volt regulator, I would check them all (in the +50 volt regulator), before prior to powering it up again.

Dave

On Tue, Feb 2, 2021 at 05:03 AM, Dave Hills wrote:


The schematic copy I have is very poor, partially un-readable, and I can't
make out the bulk voltage value feeding Q869, 872, 874. However 75v seems
very high for a 50 volt regulator. Is your line voltage 220/240VAC? If yes,
is it possible the line voltage selector is set to 115VAC? What is the bulk
supply volts across C809? If it is 55-60v I'm thinking Q869 has a c-e short,
or CR868? is leaky or shorted. If Q869 or CR868 are bad, then very possibly
Q872 and Q874 are also blown due to excess b-c voltage.

Dave
 

Hi Dave, thanks for your replies, much appreciated. I'll try out your suggestions this weekend and let you know how I get on.

Julian

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