Broken Citation 12-First part of autopsy....

[lasse]

Well, today, I have installed the new transistor in stead of the old pre-driver, and guess??

The unit still displays a number of odd voltages in the wrong places, for example 41V on the speaker terminals

One way of dealing with this (because I NEVER give up) would be to take away all transistors, measure and check all the passive components and then replace all transistors with new ones.

Has anyone seen a list of accessible replacement transistors for H/K Citation 12?


Regards//lasse
Stockholm, Sweden

[lasse]

I'm not sure where you are measuring the voltages, but from my understanding of your diagram, it looks like R736 is open. You can't have 37V across a .27ohm resistor without smoke. It's very common for one of the output transistors to short and the resulting current flow causes one or both of the emitter resistors to open up. Because of the high amount of current during a failure, it's often standard procedure to take a "shot-gun" approach by replacing both outputs and drivers along with the emitter resistors. Also, I would figure out what resistance you need to set the bias current correctly and replace that bias adjust potentiometer with a fixed resistor. If the bias pot is "noisy" it can cause excessive current to flow, resulting in another failure.

Hi and thanks for the feedback.

Of course, I should have told you all. The voltage figures are from my diod-testing features in my multimeters.

I think that abt 0.6V in the right direction and 0V in the other indicates that the transistor CAN be healthy.

I have measured the R736 you mention and it is intact on the right value but any advice is welcome. Someone described a broken amp as something resembling a war-zone so a lot of things can be broken.

Best regards//lasse

[neildavis]

I'm not sure where you are measuring the voltages, but from my understanding of your diagram, it looks like R736 is open. You can't have 37V across a .27ohm resistor without smoke. It's very common for one of the output transistors to short and the resulting current flow causes one or both of the emitter resistors to open up. Because of the high amount of current during a failure, it's often standard procedure to take a "shot-gun" approach by replacing both outputs and drivers along with the emitter resistors. Also, I would figure out what resistance you need to set the bias current correctly and replace that bias adjust potentiometer with a fixed resistor. If the bias pot is "noisy" it can cause excessive current to flow, resulting in another failure.

[lasse]

...and now, I´ve ordered a pair of MJE15034 transistors.

Let´s see if that cures the unit.

Regards//lasse

[lasse]

And here comes the second part....

I have now tested all the other transistors in the faulty channel, compared the results with values on the "good side" and the conclusion is that it is the pre-driver that is the trouble.

Now, the question is to find a good replacement to the RCA 40408

Will be back with more.....

Best regards//lasse

[lasse]

Hi,

The history is here:

http://techtalk.parts-express.com/forum/tech-talk-forum/1325517-reparing-my-citation-12

and the Crime-scene is in the photo:

Now, today, I´ve come around to start looking for the broken parts and started with the more obvious transistors
(Soldered free from the board)

Pre-driver Q704: 0,47V in both directions B-C and B-E
Driver Q706: 0,6 V B-C and B-E, nothing the other way/polarity
Bias Q708: 0,6 V B-C and B-E, nothing the other way/polarity

What do you think? Can a malfunctioning pre-driver cause all that trouble in the whole amplifier?

Best regards//lasse