HELP - LM337 supply problem

[poynton]

Hi All,

I am having a problem fault-finding on the PSU in my Arcam Alpha CD 6.

The section is in the picture below.

The input to the LM337 is correct at 27v.

However, the output is only 1.5v.

I have changed the regulator but it is still the same.

The supply goes to the muting relay circuit and to the -ve supply for the opamps.


Any help fault-finding gratefully received.

Andy

 

[rayma]

A bad part may be drawing too much current. Check D4, D7, C41, C42, and the opamps and their -15V bypass caps.
Did this unit work properly earlier, or did you get it this way?

 

[Elvee]

With an output at 1.5V (very close to 1.25V), C41 would be the prime culprit: try lifting one pin (assuming everything dowstream has been disconnected)

 

[MarcelvdG]

C41 shorted, R57 shorted or R54 open-circuited?

 

[PRR]

You do know that the 337 pin-out is different from 317 pin-out?

(How did I discover that?)

 

[poynton]

Thanks for the input, guys.

It will have to wait 'til after the weekend -- rugby!!

and .. yes, I did know about the pin differences.


Andy

 

[PRR]

I'm glad you knew; but I'm sure someone will come along not-knowing and become enlightened.

I know the physical reason the pinouts differ (most negative major circuit is die-bottom and is soldered to the heat tab). But it baffles almost everybody first-time.