Problem putting together my preamp (simple) - please help :) - diyAudio
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Old 31st March 2003, 07:28 PM   #1
elizard is offline elizard  Canada
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Default Problem putting together my preamp (simple) - please help :)

Hey guys
Well, I managed to put together a good part of my preamp. It comes with 2 PCB's, one just for left channel input, the other (bigger) for right channel input, and everything else.

I managed to finish the left one, and do about half of the right one so far. And I decided to test the diodes, make sure I didn't fry anything.
All of the diodes on the main PCB test ok (~ 0.5v one way, nothing the other), but the diodes on the left channel pcb (6 of them) all test 0.125v both ways (when i reverse the multimeter probes). So I thought I fried the diodes, so I desoldered one leg of one of the diodes (the other leg was still soldered to the pcb) and I tested it again, and this time it tested ok (0.5v one way, nothing the other). So I know the diodes are ok.

I tried looking on the board for anywhere where I might have messed up a soldering and accidentally caused a short, but I can't find anything. Can anyone suggest anything else that might be causing this?
The preamp schematics and the manual can be found at http://www.velleman.be/downloads/0/Manual_K8021.pdf
The PCB contains several transistors, reed relays, capacitors, resistors and rca connectors. Could it be that one or more of these are not working properly so that they're causing a short? Or is it most likely just a short while soldering?

PLEASE PLEASE help with this .. I greatly appreciate it
I'm a newb at all this, so my knowledge is very limited.

Thank You in advance!
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Old 31st March 2003, 07:57 PM   #2
Netlist is offline Netlist  Belgium
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Hi, elizard

I didn't have an in-dept look at the schema, but I guess you mean the diode in parallel with the relays?
When everything is well, you should measure about 0.125V over the diodes.
So I think there could be something wrong if you measure 0.5V.
The diodes are in parallel with the relay coil.
This coil will always measure low DC resistance, so if you parallel them with a diode (like it should be, the drawing is correct)
you measure that same low resistance. If you don't, something is wrong.
Measure the resistance of the coils when not in the circuit.

"Diodes are most often used across the coil to provide a path for current when the current path to the relay is interrupted (i.e. switched off, coil no longer energized). This allows the coil field to collapse without the voltage spike that would otherwise be generated. The diode protects switch or relay contacts and other circuits that may be sensitive to voltage spikes."

Quote from http://www.the12volt.com/relays/relays.asp

Does your left channel work?

/Hugo
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Old 31st March 2003, 08:24 PM   #3
elizard is offline elizard  Canada
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hmm, that's weird ..

this is the mutlimeter that i have (the instructions on how to check diodes):
http://support.tandy.com/support_meters/doc6/6269.htm

For checking the diodes on the main pcb, i get the readings that conform to:
.5-0.6 OL silicon good

the readings on the left channel pcb are:
very small very small germanium/silicon shorted

(note: in my manual it says 'very small, approx. 0.1v or less)

also as i said, when i remove the diode from the circuit, it measures 0.5v when i hook up the + to anode, - to cathode, and OL when i reverse the probes .. which is what its supposed to be according to the multiimeter manual
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Old 31st March 2003, 08:29 PM   #4
elizard is offline elizard  Canada
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wait a minute .. i think i just realized something ..

don't i need to actually remove the diode from the circuit itself before measuring in order to determine if its ok or not?

that would make sense .. since the left channel PCB is completed (i.e. all the components are soldered), but the right PCB isn't completed .. so on the right pcb it would be as though i removed one lead of the diode from the board, right?

or does that actually not make any sort of sense whatsoever?
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Old 31st March 2003, 08:35 PM   #5
tiroth is offline tiroth  United States
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You'd really have to try hard to kill a diode from soldering.

Like Netlist said, you are measuring the parallel resistance of the coil, in which case your meter will not return a proper result. Just check orientation and for obvious shorts, and don't worry about checking the diodes in circuit. They are only there for back-EMF protection at any rate.
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