Technics SU-V707 Repair - diode help!

Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Hello all! I am sorry if this is in the wrong section, if it is please let me know :)

I'm repairing a Technics SU-V707 Receiver, the pre amp stage had a few components blow, and of those I've found replacements for all of them but two diodes. Technics is a Panasonic brand, so the diodes are Panasonic models.

One is a 20A90 Geranium diode, which I had a hard time finding specs on, however I think I have found a suitable Schottky replacement - here.

And the other is a MA2 Series MA2B027WA diode - here. It is some kind of variable resistor diode, which I cannot find a replacement for. I was curious if I could use a TVS diode in its place?

Here is a snap of the circuit they are implemented in - here.
Diodes D316 and D326 are the diodes of quesion.

Any help or suggestions would very greatly appreciated!
Thank you all for your time.
 

R!f@@

Joined Apr 2, 2009
9,734
Hey Welcome
The germanium diode can be replaced with OA91.
As for the MA2. this is something I have been fighting with.

As far as I have checked this diode Vf drop is 1.1V and this voltage is a must for proper biasing for the Voltage drive Stage. If you want the THD to be below 0.04% or so. What I did is I used two 1N914/916/4148 combination's in series and checked for the voltage drop using a supply and a resistor.. You might need to try a couple of them to get the exact 1.1V .

If you have checked the forum you would find that there couple of threads concerning those diodes from the infamous Technics
 
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Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Thank you for the quick reply!
I will check the forum some more and see what I can come up with. As for the 1N914/916/4148 combo's in series, what supply voltage are you using?
 

R!f@@

Joined Apr 2, 2009
9,734
Low voltage....resistance combination should be as per data sheet forward current specs.
With those If value Vf should be 1.1VDC
 

Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Alright, I've done some research.

I removed one of the good MA2 diodes and tested with 5 Volts DC and adjusted a POT for exactly 3 mA as per the datasheet. The voltage drop was indeed exactly 1.1 vdc. I then took two 1N4148's that I had laying around and hooked them in series in the same circuit. The voltage drop was also, to my surprise, exactly 1.1 vdc also.

So should the two 1N4148 diodes in series suffice as a replacement MA2? Or am I completely neglecting some factor..
 

R!f@@

Joined Apr 2, 2009
9,734
It's what I tried. To my surprise it works for me. Try it but do not over drive the amp.
You should be checking the voltage drive biasing from time to time to see how it holds for temperature variation.

The Panasonic diode has very good temp tolerance. I am thinking of using a TL431 as the 1.1V bias as a replacement for the diode. Not sure how yet but it is one thing that I am doing as time permits.

I have a job that requires this diode and need to find a permanent replacement
 

Thread Starter

RidDleZ

Joined Apr 2, 2011
11
I will try it and let you know how it works. I will be ordering the rest of the parts I need from mouser later, so I'll let you know once I get it all back together again. As for the TL431 I may want to mess around with that and see if I can't figure something out as well. A permanent solution would be great!

Thank you for your help, I'll be getting back with you shortly!
 

Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Hello again!
I've gotten all the parts in, and tried firing it up. Right after I push the button I see a nice firework show. Resistors R358, R364 and Thermistor TH302 burned instantly. I then investigated the circuit and found Diode D320 was now bad.. Any ideas anyone? I'll investigate it more a little later and post up.
 

R!f@@

Joined Apr 2, 2009
9,734
Unfortunately ...I forgot to say not to fire up before preliminary checks....Sorry u have leaned it the hard way.

I have done a lot of fixing in technics here and yuo should follow my lead to avoid further smokes.

First off all u shud remove the Current drive circuitry completely.
And go on fixing the voltage drive.

After u are sure about the voltage stage then u shud reconnect the current drive.
But before all tht, you have to make sure the current amp has not shorted, good news is they are pretty tough suckers.

Your voltage preamp IC could be ur main culprit.
Every time the voltage stage fails ( which is the main failure i have came across) the diodes, current mirrors together with few diodes and the driver goes up in smoke.

So post what have u done. I think u need me to play a major role in helping u.
I have to guide u very intensively on this. are u game enuf? :D

{ed}
to make things worse, u have a transistor Output stage. I had always dealt with Hybrid types.
But it's not tht I have not fixed this type of other brands.
Can u send me the Schematics. My collection is missing that model.
May retched has it in the CD tht I bought. Have to ask him.
 
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Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Oh yes, I'm game.
I've dabbled in fixing plasma tv's and such, and am always up for learning something new. Also, I very much appreciate your help in this!

I could not find the exact transistors, so I found ones with the same characteristics. - I hope this is not a no no, I've had luck doing this before.
The parts I've replaced are:

Power Amp:
Q325,326 -Replaced with NPN FJA4313
Q327,328 -Replaced with PNP FJA4213

Pre Drive:
Q318 -Replaced with NPN 2SC2705; was 2SC2631

Drive:
Q322 -Replaced with NPN 2SC5171; was 2SC2592
Q324 -Replaced with PNP 2SA1930; was 2SA1112

Diodes D316,322 -Replaced with 20A91 and D326 -the the series fix
Thermistor TH302 and resistor R358.

Q318 and R358 were visually burnt.
How would you like me to send you the schematic?
 

R!f@@

Joined Apr 2, 2009
9,734
I am downloading a schema for this right now.
After that I 'l check ur model

dang!!! I'm having problem downloading.....it just sits there.....

It seems FSM does not have ur model. Can u email me the schema
 
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R!f@@

Joined Apr 2, 2009
9,734
checking........

{ED}
Got ur mail..but the site gives me server reset eror.
Try attaching the schema to my hot mail. it is the best option
 
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Thread Starter

RidDleZ

Joined Apr 2, 2011
11
Please forgive my ignorance, but I cannot find your hotmail address. It does not show me, and I cannot find how to add an attachment.. lol.
 
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