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Old 15th June 2004, 09:18 AM   #1
Zombie is offline Zombie  Sweden
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Default Quad 34 like Xmas tree

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...

Schematic for the 34 here
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Old 15th June 2004, 12:06 PM   #2
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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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Old 15th June 2004, 12:32 PM   #3
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You sure got a short if your Zener is ok.
I will suspect the 4001 and 4013's
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Old 15th June 2004, 01:21 PM   #4
Zombie is offline Zombie  Sweden
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Default Hmm...

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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Old 15th June 2004, 02:25 PM   #5
hjelm is offline hjelm  Sweden
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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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Old 15th June 2004, 02:43 PM   #6
Zombie is offline Zombie  Sweden
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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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Old 15th June 2004, 03:30 PM   #7
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Default Zombie

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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Old 15th June 2004, 03:37 PM   #8
Zombie is offline Zombie  Sweden
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OK thank you...I'll try that, too

Don't think you will thank me after our teams meet

tom
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Old 15th June 2004, 03:45 PM   #9
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I don't care
I only watch motorsports and cycling

Tom Kristensen 6 times winner of Le Mans, and Jan Magnusen winner of the GTS class

And CSC wins the Tour this year
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Old 15th June 2004, 04:45 PM   #10
Zombie is offline Zombie  Sweden
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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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