My Frontman 25R experience

[thaynes]

This is my first post on this site, but I have done a lot of research here on the Fender Frontman 25R. This amp was a recent purchase. This amp is from 2003 and is a Series II amp. This is the blackface-style cabinet with the "wedgie" chassis, so it does not have the speaker out.

This amp was noisy, and I wanted an external speaker jack. The speaker jack was easy to install, but an unshielded speaker jack picks up the EMI from the transformer making the amp useless. I tried a shielded jack, but was never able to get it to work properly, so I gave up on this idea.

These amps are infamous for noise issues, but I did have some success here.

- Used Deoxit on all of the pots. This helped with some of the noise. I also found that the chassis is a common ground for everything. During some testing, I noticed that the pots are extremely noisy if the washer are not fully tightened. The washer on the pot shaft must be a better ground than the ground lug on the PCB.

- There was still a higher noise floor than I would expect so I went down the rabbit hole. The noise sounds like the filter caps. Caps themselves looked good. So, I tested these with ESR meter with the caps in circuit and then removed the caps and repeated the ESR test. This showed that the caps were good. I then tested the caps with a multi-meter and they again they tested good. Soldered them back in. I was shocked to find that when I put the amp back together that the noise level was reduced. My assumption is that the act of soldering the caps fixed this problem. I guess that I could've just re-flowed the solder. I did not do this as an independent test as the solder looked good.

Thanks to the people on this forum for the information in previous posts!

I used these previous posts and there may have been others.
https://www.ssguitar.com/index.php?topic=1527.0
https://www.ssguitar.com/index.php?topic=5516.0
https://www.ssguitar.com/index.php?topic=3445.0


thanks,

Tom.

[g1]

What do you mean by 'shielded jack'?  You should use a plastic type insulated jack so the bushing does not have any continuity with the chassis.
There shouldn't be any more problem with EMI than the existing speaker wires would have.  It may have to mount where the speaker wires route out of the chassis.
Not sure if the amp is capable of driving an additional speaker without disconnecting the internal speaker.

[thaynes]

I started with this jack from switchcraft

https://www.switchcraft.com/1-4-mono-2-conductor-jack-w-nut-and-washer-tip-is-shunt/12a/

installation method is splice it into the speaker wires. I drilled a hole in the back of chassis to mount it. The shunt jack favors the internal speaker unless an external cab is plugged in. This worked as expected, but there was a lot of external noise. It could've been the ground connection as the jack is not isolated. I disconnected the jack from the chassis to provide an air gap with only the speaker wires as a path to ground. There was no change in the noise level. Therefore, EMI.

I next installed a shielded shunt jack.

https://www.switchcraft.com/1-4-shielded-jack-uses-12a-little-jax-jacks-2-conductor/cn12a/

This solved the noise problem, but it would only work with the internal speaker. Plugging an external cab into this jack effectively muted the amp. Thinking about this now, it may be that the shielding can shorts the signal to ground. I could isolate it from the chassis with a couple of nylon washers and some shrink wrap the threads where it passes through the chassis.


thanks,

Tom.

[Tassieviking]

That is the wrong type of jack to use in that amp.
The ring should not be connected to the chassis, it is not at 0V.
You need a jack with a plastic insulated mounting ring that does not connect the ring to the chassis.

[g1]

Yes, it must be insulated from the chassis. 
The shielded jack you posted must have been wired wrong or defective.  It should have acted the same as the other did when it was grounded to chassis.
Also, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.
An insulated jack mounted where the original speaker wire went through the chassis should not pick up any more noise than the original wires did in the same spot.

[J M Fahey]

Both jacks shown are wrong for thatb use.
To boot they are *wired* wrong.
Also external speaker jack must disconnect internal speaker when used, amp can not drive two speakers at once.
Too busy now but tonight will upload proper jack and wiring.

[J M Fahey]

Also, the noise you experienced must be due to the dress of the wires.  Where you routed it through the back of the chassis must be a sensitive area.

Usng a metallic uninsulated jack bolted to chassis shorts a great part of NFB , increases gain big time and makes "natural" noise unbearable.


Solution is usin g a properly insulated jack, properly wired.

[thaynes]

Both jacks shown are wrong for thatb use.
To boot they are *wired* wrong.
Also external speaker jack must disconnect internal speaker when used, amp can not drive two speakers at once.
Too busy now but tonight will upload proper jack and wiring.

both jacks are supposed to disconnect the internal speaker when a 1/4" plug is inserted into the jack. I never intended to drive more than one speaker. I agree that neither switch is insulated.  Nor will I argue that I may have wired these incorrectly. I look forward to seeing your solution!


thanks,

Tom.

[The Dude]

FWIW: Switchcraft makes isolation washers for their jacks.

https://www.tubedepot.com/products/isolation-washers-for-1-4-input-jack-flat-shoulder?srsltid=AfmBOopLRJix5gkjMcz9KdZDMnK0ud8XFPIuZlIIjxTb4ncYBa4kMdX6

[thaynes]

Thanks Dude...this may be an option for me rather than piecing something together at the local hardware store!


thanks,

Tom.