I cannot seem to set the center voltage on my Quad 303, started with replacing R130 as was open, I've changed the adjustment pot but to no avail I still cannot get the voltage to go under 40vdc ( should be 33.5vdc)
Any ideas on where to look next?
Any ideas on where to look next?
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Check the input capacitor. If it goes leaky and is connected to a source, it pulls down the base of Tr100, which pulls the output voltage up. Otherwise check Tr100 itself, and the 300uF decoupling capacitor on its base.
Apply the tests mentioned in the service manual about grounding the bases of Tr101/2 to ground. At this stage I would be suspecting Tr102.
Early plastic encapsulated transistors are not the most reliable as the decades pass. If they do not fail totally their behaviour can become noisy or the gain can go way down. Known euphemistically as "epoxy rot".
Having said that, fingers crossed my 303 soldiers on. The only things I did was replace the input cap on each board, and reverse the chassis electrolytics so the vent was pointing up rather than down. (Quad will sell you a kit of bits to do this, along with lengths of the correct colour wires and a tagstrip)
Having said that, fingers crossed my 303 soldiers on. The only things I did was replace the input cap on each board, and reverse the chassis electrolytics so the vent was pointing up rather than down. (Quad will sell you a kit of bits to do this, along with lengths of the correct colour wires and a tagstrip)
I'll have a read r.e the service manual instructions,
In the meantime
Tr102 replaced and no change
supply voltage correct and does adjust
I'll take R112 39k off board and test but does measure OK in circuit
In the meantime
Tr102 replaced and no change
supply voltage correct and does adjust
I'll take R112 39k off board and test but does measure OK in circuit
Another resistor that could cause this is R113 although that would also change the gain making that channel a little louder if it had gone high.
High value resistors and particularly those that see a highish voltage across them are always favourite suspects.
High value resistors and particularly those that see a highish voltage across them are always favourite suspects.
Ok so measuring voltages between Point 9 (ground) and Point 5 (output)
Tr102 with base grounded 67Vdc which is rail voltage which I'm assuming correct
Tr101 with base grounded 1.5vdc which according to service manual is normal
what does this mean? Which part of the circuit is this saying is ok?
Hi Chris,
Many high value resistors drift up and close to open. Check R111 (2M2) and the voltage at that test point (~8.5 VDC). TR102 is obviously not conducting hard enough, so maybe even MR100 or MR101 has gone funny with a lower drop. TR100 or TR101 a bit high on conducting would do it as well.
Never liked circuits like this as they can come down with really weird faults.
Many high value resistors drift up and close to open. Check R111 (2M2) and the voltage at that test point (~8.5 VDC). TR102 is obviously not conducting hard enough, so maybe even MR100 or MR101 has gone funny with a lower drop. TR100 or TR101 a bit high on conducting would do it as well.
Never liked circuits like this as they can come down with really weird faults.
Hi Chris,
Okay, that's one channel How does the other measure?
Something is dropping the voltage there, compare the R111 voltages on both sides between channels. A leaky C104 (12uF) can cause this as well. Pull one lead of C104 to check DC conditions.
Okay, that's one channel How does the other measure?
Something is dropping the voltage there, compare the R111 voltages on both sides between channels. A leaky C104 (12uF) can cause this as well. Pull one lead of C104 to check DC conditions.
Are there any other clues besides just the midpoint that will not set?
Does the amp play normally?
Can you alter and set the bias current correctly?
Have any semiconductors in the output stage been replaced? Thinking of possibilities this could be oscillating and not a true DC fault.
Does the amp play normally?
Can you alter and set the bias current correctly?
Have any semiconductors in the output stage been replaced? Thinking of possibilities this could be oscillating and not a true DC fault.