Fender Champion 30 Hum

[IndyJosh69]

Greetings Folks,
I just signed up to this forum for soliciting help with troubleshooting a Fender Champion 30 non DSP version that I purchased used recently. PCB has a 1999 date on it. The seller indicated the amp would turn off after running for 4-5 minutes, but I wasn't able to replicate the issue. The amp runs fine but hums continuously irrespective of the position of the volume control knobs on either channel. Plugging in a guitar showed both channels are functional and switching between the channels was works, as is the reverb, but the hum presists.

To isolate the problem, I desoldered the power amp input AC coupling capacitor (C41), and reflowed all the TDA1514 pins but the amp continues to hum. Feedback resistors checked out ok and the voltage between pins 3 and 4 of the TDA is in the 6-7V. I attempted to mute the power amp by shorting pin 3 to the ground through a 100ohm resistor. This resulted in a reduction in humming while pin 3 was grounded but the hum returns as soon as the 100ohm grounding resistor is removed.

One additional bit of information that may or may not be useful. The 15V zeners 1N4744As appear to have been running hot based on the color of the PCB around them and I replaced both of them but I was surprised to note the voltage across them is 16.2-16.3V and they continue to run hot. The 27V system voltage runs around +/-26.4Vdc instead of 25.9Vdc as suggested by the service manual. I suppose this might be due to lower than expected load on the power supply.

I'm struggling to decide whether the TDA1514 is bad as it hums without a signal but it also appears to be functioning as I can hear sounds through the speaker despite the annoying hum.
Thanks for your help in advance!

[Jon Snell]

The frequency of the hum is important.In the US 120HZ means a main smoothing issue, 60HZ a poor connection on a ground somewhere.
Check that out first.
If the TDA were faulty, you would have no sound or DC on the speaker.Neither of which applies in your case.

FYI; EU/UK/Australia, the hum is 100HZ or 50HZ.

[IndyJosh69]

Hi Jon,

Thanks for your response, and apologies for the tardy response. An unexpected business trip led me out of town for the past week.

I measured the frequency at the speaker terminals using a Fluke 87 III multimeter and it indicated a frequency of around 80kHz, which I subsequently verified with with an oscilloscope. The output signal was approximately a clipped sinusodial waveform. Voltages measured at the output and non inverting input pins suggest the amp was operating at a gain of around 46dB which is the maximum gain per the application note. Test point voltages listed on Fender's amp schematic indicate the amp should be operating at a gain of around 32 instead. The amp and heat sink was also getting very warm, although not uncomfortable to touch. I was hoping to take a temperature measurement but couldn't located my non contact thermometer.
Gently tapping on the passive components around the amp resulted in a large amount of reduction in output volume, and shorting the input pin to the ground using a 22k resistor resulted in further reduction in output volume. A slight amount of residual 60Hz hum remained at the end of the exercise. I suspect the amp may have been operating at the listed max open loop gain and oscillating severely due to loose components in the feedback loop. Consequently, I am planning to replace all the resistors and electrolytic capacitors around the amp before proceeding further. This may sound like overkill but this will also make me reflow all connections around the amp.

While at it, I am also planning to replace the electrolytic caps in the power supply as they're almost 25 years old at this time. I don't have a plan yet for reducing the 15V zener overheating and would like your thoughts.

Apologies for the long post, and thanks for your help. I would have almost certainly junked a good amp without your feedback!

[clintronics]

Hi Jon,

Thanks for your response, and apologies for the tardy response. An unexpected business trip led me out of town for the past week.

I measured the frequency at the speaker terminals using a Fluke 87 III multimeter and it indicated a frequency of around 80kHz, which I subsequently verified with with an oscilloscope. The output signal was approximately a clipped sinusodial waveform. Voltages measured at the output and non inverting input pins suggest the amp was operating at a gain of around 46dB which is the maximum gain per the application note. Test point voltages listed on Fender's amp schematic indicate the amp should be operating at a gain of around 32 instead. The amp and heat sink was also getting very warm, although not uncomfortable to touch. I was hoping to take a temperature measurement but couldn't located my non contact thermometer.
Gently tapping on the passive components around the amp resulted in a large amount of reduction in output volume, and shorting the input pin to the ground using a 22k resistor resulted in further reduction in output volume. A slight amount of residual 60Hz hum remained at the end of the exercise. I suspect the amp may have been operating at the listed max open loop gain and oscillating severely due to loose components in the feedback loop. Consequently, I am planning to replace all the resistors and electrolytic capacitors around the amp before proceeding further. This may sound like overkill but this will also make me reflow all connections around the amp.

While at it, I am also planning to replace the electrolytic caps in the power supply as they're almost 25 years old at this time. I don't have a plan yet for reducing the 15V zener overheating and would like your thoughts.

Apologies for the long post, and thanks for your help. I would have almost certainly junked a good amp without your feedback!

If you can hear it, it isnt 80K

"Humans can detect sounds in a frequency range from about 20 Hz to 20 kHz. (Human infants can actually hear frequencies slightly higher than 20 kHz, but lose some high-frequency sensitivity as they mature; the upper limit in average adults is often closer to 15–17 kHz.)"

[g1]

Did you have a load on the amp when you scoped the 80Khz?
If so, it's possible the amp is putting out full power at inaudible frequency and loading down the power supply. That would increase the 120Hz ripple which would be heard as hum/buzz.
The zeners are supposed to get hot. They regulate voltage by converting it to heat. Eventually the board will brown, but as you checked, the zeners are still ok and doing their job. If you want, you can raise them off the board a bit which will reduce the browning of the board.

[IndyJosh69]

Thanks for your responses, Good to hear the zeners are supposed to run hot and I don't have to worry about them! I had made measurements of the amp output with and without the speaker connected and found the measurement did not change significantly. I must admit, despite my enthusiasm, it isn't easy to sit next to a speaker putting out 20-30 Watts of hum!
I decided to make another set of measurements this morning with the speaker connected after Clintronics' comments. I'm glad to report, I had not lost my mind and my measurement tools were indeed in agreement. The following pictures were taken with the Fluke and the scope connected to the speaker terminals. The scope is using a 10Mhz probe set at x10 and 1V/div, 5us/div on time scale. I estimated the time period to be ~12us which translates to ~83kHz. The fluke is measuring 81kHz, so I do believe the amp is putting out power at frequencies outside the audible range.

[Helmholtz]

Schematic?

[IndyJosh69]

Hello,
It appears I have run into file upload restrictions.



The schematic for this amp is included in the service manual attached to a message within this forum. I'm posting a link to the message as I cannot upload the file linked to the message.

https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/975227-fender-champion-30-no-output#post975227


Thanks!

[clintronics]

ground missing somewhere in output section? the .022UF capacitor on pin 5 is used for stability and prevents oscillation. make sure R70 isnt open.