Old Vinyl Record intermittent cracking on Fender Red Knob Twin Troubleshooting!!!!

[jalexquijano]

Well i already swapped the Bias Capacitor 100uf 100V (C202) with an Illinois Brand 100uf 100v electrolytic capacitor and the 1.2k 3 watt (R306) with a 1k2J 3Watt ceramic resistor which is located in the power supply pcb succesfully. However, i am now experiencing an intermitent old vinyl record crackling whenever i play certain notes. I have not changed any preamp or power tubes since i bought this second hand amplifier. However i dont want to spend more money buying tubes as it is not a guarantee that the crackling will cease. Please advise.

I hereby attach the schematic of the amp for ready reference of the parts i have swapped! The bias is now working perfectly as it is now dialing correctly. I guess that cap was worn after 27 years.

[Mick Bailey]

Did you eliminate the speakers? Also, If you thump the amp does it crackle?

Further diagnosis would be to determine whether the fault is in the preamp or power amp section. Does it do this on both channels? You can plug a guitar straight into the return (power amp in) and check whether it still does it. Or run the send (preamp out) to another amp and isolate the preamp.

[jalexquijano]

it does it in both channels. What do you mean by thump?? Hitting the amp with my fist? it crackles intermittenly. I guess if i plug my guitar in the power amp in or plug a guitar cable from power amp in to a small transistor amp and it does not crackle, the problem is in the preamp section. I still have not changed either the 12AX7 preamp tubes which seem to be very old as they are labeled Fender and made in USA nor the preamp electrolytic caps! I have a set of F & T but have ordered some Illinois branded one which fit better there!

[Enzo]

DOnt plug a guitar into it, just power it up ready to play, and yes, whack it on top with your fist. Does that cause a crackle or other response? Turn down the reverb so you don;t get spring noise.

it crackles on certain notes? Like he says, unplug your speakers and run a cable over to some other spaekr cab. Still crackle?

Don't just change out parts because "Gee, maybe it is this."

[jalexquijano]

Just pulled the chassis out, cleaned and tightened all the tube sockets with DEXOXIT pushing the pins together so when the preamp tubes are installed they are kept more firm and tight. Voila crackle is gone!!! Nevertheless i did notice that the 3 position slide switch dptt (part number 026473) found at the bias PCB and which controls the effects loop is not working properly. It does not slide freely and when you dial it on the positions between 1 and 2 or 2 and 3 the amplifier sound becomes brighter. IS this how the amp should sound? Should i treat it with deoxit fader lube or is this a symptom that it needs to be replaced??

[Mick Bailey]

If you move the switch to inbetween positions then there's a possibility of getting unpredictable results. This is not how it's intended to be used. If I press the side of my eyeball I see double, so I don't press the side of my eyeball.

Deoxit may help the switch to move more easily, but I can't recall ever having to replace on of these as they're so simple. It should have three distinct positions and you don't want it loose.

[jalexquijano]

If you move the switch to inbetween positions then there's a possibility of getting unpredictable results. This is not how it's intended to be used. If I press the side of my eyeball I see double, so I don't press the side of my eyeball.

Deoxit may help the switch to move more easily, but I can't recall ever having to replace on of these as they're so simple. It should have three distinct positions and you don't want it loose.

The switch does move to 3 positions and reveals a more treble response when allocated between position 1 and 2 or 2 and 3. Is this normal? Shouldnt the amp sound more brighter. I have not connected an external effects processor to the input loop jacks. Just testing with nothing plug at the rear and just the guitar channel 1 in front. Any opinions?