have to find equivalent diode to a preamp MA2C029WA

[bonesbiggis]

im reparing a technics su-c2000
the preamp produces low volume that is a bit crunchy

2.36V on base of Q10/Q12 on right channel
same problem on left channel.

I have taken some volt measurements. see attached documents .
have printed voltage that I got with the multimeter on the schematic

i think the two diodes are broken MA29WATA they are also called MA2C029WA can i find similar some where ??

 

[basreflex]

The diode looks like a series connection of 2 Si diodes in one package. That is why each diode reads about 1.2V, and there should be 2.4V on the bases of the 2 cascode transistors. the absolute value of that voltage is not very critical I presume it needs to be above 2.2V and below 3V. So you can replace them with 4 1N4148 in series. the readout or print of 0.7V and 1.5V does not make sense to me. that would mean the cascode is turned off.

 

[bonesbiggis]

so you can replace the diodes straight off with 1N4148?

according to the main schematic its 0.7V and 1.5V

 

[rayma]

Don't shotgun, test the diodes with a DVM. They will be clearly either good or bad.

 

[basreflex]

the technics schematic is erratic. most likely the 0.7 and 1.5V are interchanged.

 

[bonesbiggis]

its true

 

[bonesbiggis]

the diodes are fine reads exactly the same as the other in another circuit

 

[basreflex]

maybe the selector is set to MM while you have a MC cartridge.

 

[bonesbiggis]

maby some of the mos-fet is leaky got 120mv at source

 

[rayma]

All the sources are connected together.

 

[bonesbiggis]

But there should not be that high voltage there?

 

[rayma]

If both channels have the same problem, odds are the power supply (or other circuits common to both channels) is at fault.
Very unlikely to have identical problems develop in both audio channels.

Jumping to conclusions only wastes time, and damages parts and boards, due to replacing good parts for no reason.
First you have to understand how the circuit normally works in detail. Then you can deduce how it will behave due to
various types of failures. Then you select the failure that matches the observed behavior, and see if that failure actually happened.

 

[bonesbiggis]

i have measure around the circuits with voltage meter and everything seams to end up the voltages are exactly the same as the service manual i have bean cheeking every where except from front panel circuit .

and i found 6 places where the voltages did not end up with the manual.

see attached document i have printed the voltage that is wrong with red color!

Can anyone figure out where the over voltages are coming from?

 

[basreflex]

all these voltages are correct. the small positive voltage indicates that the fets are running at below Idss, and that is the correct operating range. The spread indicates the spread in Vgs. As indicated earlier the voltage indication in the service manual of 0.7V is erratic.

 

[bonesbiggis]

when listening to music through the mobile it sounds as if the sound is muffled. I can't figure out what sauces this the volume knob seam's to work fine.

 

[basreflex]

you may be listening through the RIAA corrected amplifier input, that sounds muffled .. try the line / AUX input.

 

[bonesbiggis]

This is the setting I have

 

[basreflex]

the direct button bypasses the tone control, one stage less to worry about. I presume you set it to aux ?

 

[bonesbiggis]

direct button don´t make any different on the sound i have it set to aux i have tried CD to