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Hi There,
i have built the 73 Preamp and have some Problems: 
At general power check after plugging in the Transformer, the Voltage drops to from 24.3V to below 15V and U1 gets hot! (i unplugged the transformer immediatly)

I traced the DC Voltages: 
A-F is fine
G=0v4
H=0v4
I=0v4
J=0v0

Any help ist greatly appreciated

Cheers Max

[JPK]

You have a short circuit somewhere.
On the output transformer connector assembly, check that there in no short between adjacent wires.
Check that no component lead can touch the chassis plate.
Check the position and direction of U1, U2 and Q6.
With the output transformer connected, apply a very brief power on and measure the DC voltage on V+ and on TP2, relative to 0 V.

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Hi JP, 
thx for your help.

V+ to V0 is 15V
TP2 to V0 is 1.5V

i checked thoroughly for shortcircuits and could not see any suspects. 

i remember that i washed the PCB with isopropyl alcohol before testing. although i blew it dry with compressed air, is it possible that the IPA caused a short and klilled a component? 

Thanx again for your help and the making of these beautiful kits!

Cheers Max

[JPK]

TP2 to V0 is 1.5V
It should be -9V. Too much current flowing. There is a short somewhere.

is it possible that the IPA caused a short and klilled a component?
No, not possible.

Check the DC voltage on TP2, relative to 0 V with the output transformer disconnected.
Check the output transformer DC resistances as shown here. Measure with the transformer unplugged then plugged.

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Check the DC voltage on TP2, relative to 0 V with the output transformer disconnected.
Check the output transformer DC resistances as shown here. Measure with the transformer unplugged then plugged.

With Transformer unplugged TP2 to 0V is -9V
The resistances of the transformer are ok. measured while plugged in through the PCB.

But still the voltage drops if i plug in the transformer. Unplugged the input voltage is as expected. really weird
Cheers Max

[JPK]

The issue is very probably somewhere in :
R24, R27, R40 ... R47, Q4 ... Q6, U1, U2. Check value and solder joints.
Also check all the electrolytic caps direction.

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Thank you JP, i think i got it. 
I changed Q4 and Q5 with new ones and the voltage gets stable to 24.1V after a second after pluging in the transformer. 
Also the voltages are like in the troubleshooting guide. So i guess it is working now.
I will do a soundcheck tomorow and report back. 

Thank you very much!!

Cheers
Max

[JPK]

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Success!
I checked and everything is working.
Thanks JP 

On to the next Modules ;-)