| Zombie |
Hello!
Suddenly my Quad 34 preamp looks like a Xmas tree. I think the cause might be that while messing a bit with the AC supply to my TD150 turntable (which is hooked on to the power outlet of the 34) a spike might have destroyed smth.
I have come so far as to conclude that there is no +/- 7,5V feed to the solid state switches.
The +/- 7,5V is created by a 15V zener diode between two resistors (180R and 300R) that are connected to +9,4 and -8,6V rails (that feed all opamps etc.)
However, the resistors take all the voltage drop except for some 0.94V. Have checked the zener - it isn't broke - and I have also replaced it with other 15V zeners I have in my box.
My imagination has run out, need help...:smash:
Schematic for the 34 here |
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| hjelm |
Guessing a short somewhere. I cannot find any electrolytes in the +-7.5V parts which is what i would check first.
Are the IC circuits socketed? |
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| ACD |
You sure got a short if your Zener is ok.
I will suspect the 4001 and 4013's ;) |
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| Zombie |
Hello Jan!
As you see I have a (late) 34 which is way better than the old 44 I had.
Anyway, I have desoldered all 40xx (except the 4066 at the tape and radio/aux), but the Xmas tree still up.
I suspect the transistors at the buttons T5-T12. What do you think? Desolder the whole lot and check for broken ones?
Nice game you did yesterday btw...
Cheers
tom |
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| hjelm |
You have desoldered all, but one, the 40xx and they still shine?
Does any of the controls work(mono,disc,etc..)? |
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| Zombie |
Hello!
Now only the Tape, Aux (CD) and Radio light up.
Got signal out with the CD in the Tape input, but I have also taken out all the opamps
Mvh
t |
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| ACD |
Thanks regarding the game ;)
I once had a similar problem with my 34 after powering it up after some small modifications....
All LED's where on after power up, and I suspected a short or bad connection somewhere in the selector circuit (I didn't meassure the supply), but instead I just let my finger move around the top and bottom of the PCB to see if I could pinpoint the failure.... Not very scientifically I know, but it worked... Suddely the LED's where normal, and I haven't been able to recreate the failure since (this was 1 year ago, and my 34 is in use every day)...
Let me know if you find the "spot" ;) |
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| Zombie |
OK thank you...I'll try that, too
Don't think you will thank me after our teams meet :D
tom |
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| ACD |
I don't care ;)
I only watch motorsports and cycling :D
Tom Kristensen 6 times winner of Le Mans, and Jan Magnusen winner of the GTS class :D
And CSC wins the Tour this year ;) |
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| Zombie |
Hello!
I tried the New Age magic finger method, but no result.
I decided to measure the resistance between the the zener pins. I removed the zener and the two 100uF elcos. AFAIKS it should be an open circuit...I have a reading of 5-6 ohms.
This corresponds to the short circuit you suspected. Now it's just to find where...
Cheers |
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| ACD |
Ok...
A trick that I sometimes uses is to cut the PCB track in strategic places to see if the short disappers ;) |
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| Zombie |
| OK, sounds like a good thing to do. First I try to locate places where the connection between +7,5 and -7,5 is simple...a cap or diode or even a mechanical device...have you the full service manual? I've only got the schematic... |
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| ACD |
| I got the complete service manual ;) |
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| Zombie |
| ...in electronic format, too? |
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| Zombie |
It was the last MAX4066A (one of those 2 I didn't bother to take out) that shorted...I changed it for another and now everything works fine...gee, the last one, typical isn't it?
Thank you for your support and leading me in the right direction!
Hälsningar,
T |
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| ACD |
Always glad to help ;)
Sorry but I only got the service manual in hardcopy, however We may arrange that I send a copy to you by snailmail...
Concact me by email if you are interested ;) |
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