again troubles with Fender Sidekick-35 Reverb

[Gokarna]

Hello and a happy new year to you all !!!

Again I'm working on my customised Fender Sidekick 35 Reverb and still have some problems with it:

1. I always wondered, that the sound "distorts" very fast, when the Volume Poti is turned on, clean sound is only possible when its position is somewhere between 0 -1.
After reading all the comments at harmony-central.com, I realise, that this isn't normal for these small MIJ Fender transistor amps.
Once, I also had also an Sidekick-30 amplifier, that was nearly impossible to distort.

I haven't taken a look on the pcb and it's parts or measured something, my guess is that the volume-poti might be bad or have a bad solder joint, only switching between full DC-resistance and zero ohms.

Does anyone here has an idea what might be the problem ?


2. Another thing is the reverb circuit.

The before mentioned Sidekick-30 amp Reverb had a much longer and nices reverb sound than my Sidekick-35 Reverb.
I know, that the accutronics reverb-tanks are not the same in the 2 amps, the Sidekick-30 has a reverb-tank with "longer" reverb specs.

Will it help to change the reverb tank to the specs of the Sidekick-30 Reverb amp or is this also a question of the different circuit of this 2 amps ?

I'm looking forward for your answers



Sincerely
Gokarnal / Frankfurt-area (Germany)

[Gokarna]

Hi there,

No ideas, what might be the problem ?

Best regards,

Gokarna

[Enzo]

Rather vague at this point. I'd check the gain pot to make sure it isn't open near one end. You already suspected that, so... check it out. I'd also be looking for any unwanted DC offset in any of the op amps. You may be running out of headroom instantly because the signal is pushed off to the side.

[Gokarna]

Hello again,

still not further in progress, I've checked every part from the plug-in jacks to R11 in the circuit and also the second opamp in the circuit (sorry, I only have a printed schematic, but not as file...)
I replaced the first opamp by an TL072 and also the first cap (C1).
Now it's a only very, very little bit better.

So I concentrated on the wiring from the 2 input jacks to the pcb and realised, that the wiring is differnt here.
I drawed the little excerpt of the schematic below:




Could this be the problem ?

The white and the orange wire is clear to me, but I have my difficulties with the black (ground-)wire from the pcp and the "automatic" ground of both input jacks.

Can someone help me(again) with this ?


Best regards and a happy sunday !

Gokarna

[ambiophonics]

It looks like both schematics accomplish the same thing - just drawn differently. Did you check the volume pot and DC offset as Enzo suggested? I would start there and confirm the voltages that run your opamps are present and clean, then move on to tracing signal through the amp and find out where it becomes distorted.

[Gokarna]

Hi ambiophonics,

It looks like both schematics accomplish the same thing - just drawn differently. Did you check the volume pot and DC offset as Enzo suggested? I would start there and confirm the voltages that run your opamps are present and clean, then move on to tracing signal through the amp and find out where it becomes distorted.

.

Yes, I checked every part around between input jacks and 2nd opamp including resoldering the Volume- and Gain-poti. Everything okay.

Perhaps I have a translation problem: What exactly is "DC - offset" and how do I check it ?

Thanks again for any answer,

Gokarna

[ambiophonics]

Let's start by checking our voltages at the power supply - looks like you should have +/- 14, 16, and 32 volts DC coming off the power supply. I would also suggest checking signal at the preamp output jack as well as injecting signal into the power amp input, that will help determine if the distortion is happening in the preamp or poweramp section. You could also plug some headphones in just to confirm the speaker isn't the issue.

[Enzo]

Op amp ICs like the TL072 are dual section ICs. The output pins of the two sections are pinbs 1 and 7. They should all sit at about zero volts. If you find some number of DC volts there, it is said to be "offset" from zero.

[Gokarna]

Hello ambiophonics, Hello Enzo

that's it !!!!

I have 12,86 V DC on Pin 7 of IC-3, all other opamps show no (less 5mV) voltage on Pin 1 and 7.

According to the schematic, that's the opamp on the output side of the reverb.
The parts close around it are the caps C35 - C37 and the resistors R65 - 67.

What have I to do or check next ?

Best regards,

Gokarna

[ambiophonics]

I would carefully remove IC3 and measure the voltages on the board again with it removed. Most likely IC3 itself developed an internal short and needs to be replaced. Have we confirmed other voltages at the power supply, and confirmed the poweramp section is clean with signal inserted at the poweramp input jack?

[Gokarna]

Looks as if it works fine, now !

Hi again,

I would carefully remove IC3 and measure the voltages on the board again with it removed. Most likely IC3 itself developed an internal short and needs to be replaced. Have we confirmed other voltages at the power supply, and confirmed the poweramp section is clean with signal inserted at the poweramp input jack?

I checked that:

Voltage is like that (I hope you have the same schematic as me):

A) 30.1 V DC B) - 30,6 V DC
C) 15,6 V DC D) - 15,8 V DC
F) 13,7 V DC E) - 13,8 V DC

Poweramp is clean. I inserted the guitar directly into the main-In Input and also the preamp (with guitar) from another amplifier. Both times total undistorted signal.

Headphones into the Preamp-Output or preamp (of this amp) into the poweramp of an other amplifier: Distorted signal.


I have checked the 3 resistors close to the mentioned opamp, they're all okay.

Just removed the old Mitsubishi M5128-opamp and put in a chipsocket with an Texas Instrument RC4558P-opamp.

Plugged into the wall and...

No DC on Pin 7 of the new IC !

Thank you very much a lot !!!

I hope, I can close this thread soon, I will write again after a test with guitar and speaker.

Best regards,


Gokarna