Loud hum when turning the standby off on my 1990 Fender the twin "red knob" for 20sec

[jalexquijano]

Well i left it on standby for 5 minutes and when flipping to play mode, there was no issue whatsoever. Sorry for the late reply. I thought you only had to leave it on standby for 30 secs and then flip.

[jalexquijano]

I presume turning standby 'off' is the same as putting into 'play' mode. Power on, standby off.



If 30 sec isn't long enough, but 20 sec more is, try letting it warm up with standby 'on' for a whole minute. If it still hums after that, there's something screwy with what ever follows the standby switch.

Schematic herein attached!!

+1 so we can see what's before and what's after the standby switch.

Schematic attached

[eschertron]

I'm glad you checked a longer warm-up time and found that it eliminates the problem.

While all the HT components are after the standby switch, filaments and bias are in front. A potential issue with the bias supply?

[jalexquijano]

I'm glad you checked a longer warm-up time and found that it eliminates the problem.

While all the HT components are after the standby switch, filaments and bias are in front. A potential issue with the bias supply?

Its really strange. Just came home left it on standby for 1 minute, then flip it to play mode and loud hum came out for 15 secs. I start moving the volume 1 pot on the clean channel and it produced some noise during the swipe.

[jalexquijano]

sounds like very slow charging filter caps,if the amp is original i guess its time to replace them,you can try parallel a good 22/47uF one at a time on the preamp/PI ones and hear if it gets better.

THis morning i power the amp on and let it sit for 5 mins. I then took the standby off by flipping it to "play" mode, result = no issue. If i let it warm only for one minute you hear a big hum which starts decreasing until it gets to normal hum level. Is this a preamp or power capacitor issue?? I replaced all of these capacitors 3 months ago. Could one of them be faulty?? How do i verify without removing them? i dont see any leaking whatsover!!

[pdf64]

I think that message #3 offers the most likely explanation of the hum.

To investigate further, I suggest that you use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

[alexradium]

Well i left it on standby for 5 minutes and when flipping to play mode, there was no issue whatsoever. Sorry for the late reply. I thought you only had to leave it on standby for 30 secs and then flip.

one minute is ok for power tubes filaments to go and pull current.
I stress the fact that one or more of the filter cap is slow,with this standby on AC side its normal that caps charge in a few seconds,but hearing hum is more related to PI and preamp side for me,just get a new good cap and put it in parallel with a couple of clip wires,and hear if something changes.

[pdf64]

It should be simple to identify whether the hum is coming from the preamp or from the power tubes / power supplies.
As noted in post #7; remove the LTP phase splitter tube and see what happens to the hum.

https://www.phrases.org.uk/meanings/...-to-water.html

[jalexquijano]

Clever, experienced people have taken the time to respond to your specific query.
How about reading, and acting on, the 1st lines of post #3 and #7?

Well last night i removed the chassis from the cabinet. retensioned all preamp and power tube sockets and spray some deoxit D5 on each socket. Left the amp on standby for 1 minute and then switched to play mode. Result: No cracking sound but still some 60 cycle hum present on the background. What else should i check to eliminate this 60 cycle hum. I want this amp to be as clean as posible. I could also determine the following:

1. Volume 1 potentiometer on clean channel is a Little bit scratchy when swiped.

2. When you raise the volume to 10 you can hear a Radio Station very low in the background.

There must be some leak somewhere.

[pdf64]

.Left the amp on standby for 1 minute and then switched to play mode. Result: No cracking sound but still some 60 cycle hum present on the background. What else should i check to eliminate this 60 cycle hum...

Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.

If it still hums, use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

I wonder if you are viewing the forum in 'threaded' mode, thereby missing messages?
Check your settings - settings, general settings, thread display mode, set it to linear

[g1]

Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.

These amps have a very awkward layout where tube numbers are not called out from left to right.
The attached schematic calls out the tube numbers on page 5 (top view of chassis).
Fortunately, the phase splitter is positioned nearest to the power tubes as is somewhat 'normal'.

[jalexquijano]

Remove the tube in the V4 LTP phase splitter socket and see what happens to the hum.

If it still hums, use a meter and, as per the schematic notes, monitor the bias then balance mV readings over 2 cold starts.
Just leave the standby switch in 'operate' mode (standby is a user convenience rather than being a technical necessity or benefit).
Note how the bias and then balance readings change and then settle down over the first few minutes after powering up from cold.

I wonder if you are viewing the forum in 'threaded' mode, thereby missing messages?
Check your settings - settings, general settings, thread display mode, set it to linear

I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.

[J M Fahey]

I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.

Of course, this is a ***TEST*** to search where the hum comes from !!!!!!!

Let me guess your future answers to our testing questions:

* "Plug your amp into a known good speaker" ...
- "I want to use my own speakers"

* "Unplug your speakers and measure DC resistance"
- "amplifier does not sound without speakers"

* "bend down and listen whether hum comes from speakers or itīs transformer buzzing"
- "I play standing, why would I bend down?"

* "move treble pot from 10 to 0 to check whether it influences hum or not"
- "I always set treble to 7"

and so on and on and on .....

Hope you enjoy Hum

[pdf64]

I did this test yesterday. I removed V4 which is the phase inverter preamp tube and the hum was gone. However the amp is not playable without that tube.

That's important info which has been requested several times, why didn't you disclose it?

How is the hum affected if a guitar (with the volume controls turned down) is plugged into the 'power amp in' socket (not the 'power amp thru' socket)?

How about if the tube in V3 socket is removed?

[jalexquijano]

That's important info which has been requested several times, why didn't you disclose it?

How is the hum affected if a guitar (with the volume controls turned down) is plugged into the 'power amp in' socket (not the 'power amp thru' socket)?

How about if the tube in V3 socket is removed?

V4 (phase inverter) removed, no hum but no sound either

V3 removed, same as above.

Guess the problem is somewhere in the preamp section. Its weird as the 3 preamp filter caps were changed with Illinois Brand new caps.