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help to diagnose simple fault please
help to diagnose simple fault please
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Old 18th April 2018, 08:45 AM   #1
Pauld35 is offline Pauld35  New Zealand
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Default help to diagnose simple fault please

hello i have a dunlop wah.

it would only work briefly as the power drained when i disconnected the 9v battery or the 9vdc adapter power supply.

i have connected it to a 5vdc battery power supply and it works fine.

here is the schem. I hope this is enough info for someone to advise me of possible problem,

thanks
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File Type: jpg cry-baby-wah-gcb-95-bias.jpg (119.4 KB, 83 views)

Last edited by Pauld35; 18th April 2018 at 08:46 AM. Reason: typo
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Old 18th April 2018, 08:57 AM   #2
Mooly is offline Mooly  United Kingdom
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help to diagnose simple fault please
Working briefly during power on or power off suggests a classic type of problem which is incorrect biasing.

The first step would be to measure the static DC voltages on all three transistors. So that is nine measurements in total, E,C and B for each. The base voltage may be measured a little innacurately due to the high circuit impedances.

Measure them and report back with the results.
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Old 18th April 2018, 08:59 AM   #3
Mooly is offline Mooly  United Kingdom
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help to diagnose simple fault please
Just to add that the three most important results are the 3v on the emitter of Q0, the 4.9v on the collector of Q1 and the 4.3v on the emitter of Q2
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Old 18th April 2018, 09:10 AM   #4
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Check C6, C5 and D1.
If the incorrect polarity has been applied, D1 will be leaky. It is a 9.3volt Zener. No idea why it was chosen as a new PP3 battery reads more than 9.3volts!
The early peddles used a 1N4001 as protection with no 'clamp'.
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Old 18th April 2018, 06:16 PM   #5
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by JonSnell Electronic View Post
Check C6, C5 and D1.
If the incorrect polarity has been applied, D1 will be leaky. It is a 9.3volt Zener. No idea why it was chosen as a new PP3 battery reads more than 9.3volts!
The early peddles used a 1N4001 as protection with no 'clamp'.
thanks for the help, i will check. the voltages were already on the schematic when i downloaded it.
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Old 18th April 2018, 08:00 PM   #6
Pauld35 is offline Pauld35  New Zealand
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Quote:
Originally Posted by Mooly View Post
Working briefly during power on or power off suggests a classic type of problem which is incorrect biasing.

The first step would be to measure the static DC voltages on all three transistors. So that is nine measurements in total, E,C and B for each. The base voltage may be measured a little innacurately due to the high circuit impedances.

Measure them and report back with the results.
thanks for helping, here are the voltages with just a separate lead plugged in each jack . (no amp or instrument connected)
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Last edited by Pauld35; 18th April 2018 at 08:04 PM. Reason: test conditions
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Old 18th April 2018, 09:10 PM   #7
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by Pauld35 View Post
...here are the voltages...
Interesting, I don't see any obvious signs of a fault. Perhaps someone else will spot something I missed.

-Gnobuddy
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Old 18th April 2018, 09:29 PM   #8
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Q0 has the same potential on the collector as on the battery = no current being drawn through the transistor yet 0.6 ish volts drop BE. I would take a look here first.
Maybe Rin2 is O/C?
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Old 18th April 2018, 09:53 PM   #9
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by JonSnell Electronic View Post
Q0 has the same potential on the collector as on the battery = no current being drawn through the transistor yet 0.6 ish volts drop BE. I would take a look here first.
Maybe Rin2 is O/C?
Q0 has a very small collector resistor, so very slight voltage drop across it. The meter isn't showing that slight drop.

The key point is that the emitter voltage Pauld35 measured (4.01 V) shows that Q0 is operating normally - it is an emitter follower, and the emitter is sitting at quite closer to half the battery voltage, exactly as it should. Emitter current is around 400 uA.

So I don't think there is a problem with Q0...

-Gnobuddy
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Old 19th April 2018, 12:15 AM   #10
Pauld35 is offline Pauld35  New Zealand
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hi,
i will the check readings again plugged into the amp incase its intermittent
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