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Fender 112 Deluxe Plus

Started by FenderDeluxe112Plus, April 12, 2022, 04:39:01 PM

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SemiConductive

FWIW, I think Fender isolates signal ground from the chassis in all their small SS amps.

joecool85

Quote from: phatt on April 24, 2022, 09:39:20 PMA scope may not help much if it's ground hum.
I'm assuming the amp is working and passing signal except for the hum?

Well as there is no ripple on the supply and voltages are within spec then the most likely issue is one of 2 possibilities.
1/ open ground connection somewhere.
If the star ground node of main supply is not connected back to chassis correctly then it can cause hum issues. (Which is the node between C47 & C48 or Conn CP4)
You can test simply by shorting the main Common back to chassis with a wire. If the hum stops then you found the issue,, if hum increases then you just created a ground loop,, which brings us to,, 2/ Something is grounded that should Not be grounded, which would cause a ground loop hum.
Ground plane issues can be a nightmare to track down.
sometimes in design phase one has to use an alligator clip to find the best ground path before committing to a layout.

Often components like input jacks, FX loops and speaker outputs,  even the PCB need to be *Isolated from chassis*
This will depend on how the circuit common was designed which is hardly ever noted on schematics.

Tiss easy to loose these little isolation washers or forget to reinstall them.

If you acquired this amp with issues then someone may have already worked on it and if small parts have been lost then you have no idea why it hums.

*I would be researching ground path issues long before replacing parts in hope.*

I got caught once, I did not notice an isolation washer dropped off a PCB post and the hum drove me nuts for a few days.
finally found it hiding in the chassis corner. >:(
Phil.

Been there.  I did this on my Dean Markley amp years ago.  Took forever to figure it out!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

FenderDeluxe112Plus

I am reading around 24ohms resistance from cp4 to chassis ground.

joecool85

Quote from: FenderDeluxe112Plus on April 28, 2022, 11:45:12 AMI am reading around 24ohms resistance from cp4 to chassis ground.

What about from CP2 to chassis ground?
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

FenderDeluxe112Plus


g1

#35
R4 and R5 run in parallel from circuit ground to chassis (pot brackets).
As they are each 47 ohms, 24 ohms seems the correct reading from CP4 to chassis.
It's possible there is another connection to chassis somewhere but I don't see anything on the schematic.
You could try shorting across R4 or R5 as a test and see if it affects the hum/buzz. 

FenderDeluxe112Plus

Thanks g1,
Have just checked r4 and r5 with multimeter and the grounding seems good and as expected.

FenderDeluxe112Plus

Checking grounding of some of the electrolytics-I am getting 757 ohm resistance to chassis  on pos lead of c53.

Enzo


phatt

Just turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.

joecool85

Quote from: phatt on April 29, 2022, 02:23:11 AMJust turn the amp on and with a length of wire with alligator clips on the ends and just probe the Known ground points of the pcb and related ground points back to chassis.
**Just make darn sure you don't short power nodes back to ground.**
Use the schematic to verify test points and ground nodes.

If hum increases then you have created a ground loop.
If it reduces hum then you know there is a ground missing somewhere.

BTW the speaker Neg terminal is NOT Ground. This is a current feedback system and spk NEG is lifted from circuit common via that big 10 Watt resistor, R76.

You can visually work out the ground path but often not obvious.
Hence the wire probe can help define the problem.
Maybe post some circuit pictures and we might be able to help more?
Phil.

I'm with Phil on this one.  I actually have an old test lead from a dead multimeter than I connected an alligator clip to for this purpose - works great.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

FenderDeluxe112Plus

Thanks for the help,
Jumping the node at cp4 to chassis ground produces same buzz.  With neg lead of c53 shorted to chassis getting same buzz. I double checked Enzo's suggestion of shorting q7 at r59 but still buzzing.
I just plugged guitar in to check signal is passing through amp to speaker. Input 1 produces very distorted sound on clean channel. Input 2 is less distorted.

Clip of the sound attached...

You cannot view this attachment.

FenderDeluxe112Plus

Hello Phil,
Here are some pics of the wiring from the Transformer and power inlet area of the circuit.

The dodgy(!) looking joints on the board have since been reworked and checked with a mm.

You cannot view this attachment.You cannot view this attachment.You cannot view this attachment.You cannot view this attachment.   

g1

The pot brackets in these amps are used to carry ground connections.  I've circled one example in your photo below.  I would just resolder them all.
Suggest you deal with the obvious audio problems first (ch.1 distorted), maybe you will get lucky and fix the hum while you are at it.


You cannot view this attachment.

phatt

Oh dear, Reset,, that noise is not ground hum/buzz that sounds more like a blown poweramp. :o
I asked if the amp is working and passing signal except for some hum/buzz,, if so you may have a ground issue.

Obviously this is NOT the case.

In your fist post you said you replaced the power transistors but there might still be a problem in the power amp.
My best guess is the opamp at the input of power amp stage might be stuffed.
Check the DC test points on U1. (pin7&4)
Phil.