I see to pop is immediately you turn the unit back on. This is odd as the HT supply is delayed by the relay fossa is talking about. Either the relay is stuck short or the relay is driven immediately or the HT has not fully discharged o/c bleeder resistors.
Its possible the NE555 does not reset between power cycles. If its a 7555 then the supply may never discharge. C23 should have a small bleeder resistor to take say 5ma to discharge the supply and C27 should have a reverse diode in914/in4148 so it discharges. A multimeter on pin 3 to pin 1 can check the operation of this circuit.
The pop happens immediately like 1 second after pressing on the switch. Yes, the amp still plays perfectly without any imbalanced or noise issues now. It was bought new about 2 years ago and has been great without any issues until about a month ago when the pop started to appear on the left channel.
I roll tubes sometime in the middle of the play session, practicing 5 mins (10 minutes now) interval between off and on. Generally, I will only change tubes before turning on the amp for the first time on any day.
Up to date, I have only upgraded C102 and C103 cap to Jupiter Copper Foil 600V 0.1uf for both channels plus 3 X 8 pins octal sockets and 2 X UX4 sockets. The pop was already there before I did these upgrades about 2 weeks ago. I thought the upgrades might fix the pop issue but didnt. Didnt touch anything else.
Thank you for being patient with me ...lol
OK, I see C23, C27 and NE555. Please advise what and how to proceed with the test.
Could you may add LED + say 2k2 resistor across coil of relay to see when it engages. Hope you have an LED. No other mods. If that works try seeing if the relay is working as described by fossa.
Alternatively, monitor +B1 +B2 of both channels at power up and compare.
By design, +B1 is delayed.
Be very very very careful.
You must use multimeters that are at least 1000V capable to measure the voltages of +B1 and +B2.
Understood...greatly appreciated for your kind concern. I will take max. precaution. I still remember I shorted a 350V cap with a pair of scissors many years ago when I was still a stupid but adventurous kid...LOL
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Hi all, my apology for the late follow up. Things got really busy end of the year as well as it took quite a while for parts to arrive due to the holiday season. After some more researches, I have come across users of this amp having issue with C36 due to heat. I have received the upgraded capacitor and diode, and have done the replacement. At the same time, I have been checking all components and doing intensive testing. Finally, I managed to figure out what is the actual culprit that is causing the pop. It is the PRE-IN input!!!
There have been numerous comments suggesting to always turn on all other components, including the pre-amp, with the amp last. Ironically, this is the reason that causes the pop. To avoid this, I practice switching the input to any except pre-in when turning on the amp and all is fine now... 👍
Again, thank you all for your informative input. Greatly appreciated!!! 👋
There have been numerous comments suggesting to always turn on all other components, including the pre-amp, with the amp last. Ironically, this is the reason that causes the pop. To avoid this, I practice switching the input to any except pre-in when turning on the amp and all is fine now... 👍
Again, thank you all for your informative input. Greatly appreciated!!! 👋