Fender Frontman 65r underlying distortion

[ferdcoz]

Hi all,

This is my first post in this group after a while reading a lot of threads with very interesting information.

The aim of this thread is talk about an issue which is running in a Fender Frontman 65r. The symptom is "underlying" distortion in the clean channel.

I've already fixed several Frontman's with distortion issues, but those were related to the channel controlling JFETs (Q1 and Q4). And also others with power amp issues which caused distorted signal at the output in any volume setting.

But this one it's nothing like that. When it's on clean channel, in relatively low volume (1-3) and I simultaneously turn the drive and volume knob (drive channel) there's an undesired distortion at output. And it's not from the power amp stage, since the test input sine wave it's clean as a whistle when the clean volume knob is up to "3". And of course, in terms of volume, the clean signal is louder than the distorted and you cannot hear it. But in low clean volume there's a mixture of clean and distorted. Nasty

I suspected from the very begging of Q1 and Q4, those controlling the channel switching. But the problem it' not there. I get a clean output until Q6 and Q7 bases. The distorted signal appears from Q12 and Q13 onwards.

I'm quite confused why I'm having this underlying distortion when the power amp it's working as expected. Getting a clean sine wave until 8 or 9 volume (At 10 of course the power amp stage is common to do this).

I'll submit a few scope screenshots later so you can have a better picture of this issue. But I'll like to know if any of you guys experienced a similar issue like this one before.

The schematic to follow the component reference is the following: fender_fm_65r.pdf

Thanks for reading

[nickb]

Please post a scope image of the output when the distortion is happening and visible. Q12 and Q13 are not in the signal path. They are there for output current limit protection.

Listen to the output from the preamp out J3 on a second amplifier and see if the noise is there.

When in clean the drive knob should not do anything. That strongly suggests that you have leakage from the drive channel. Have you checked the voltage on the gate of Q4? Try shorting out D9 to see if the problem goes away.