Rocktron Rampage R80 - No Sound problem

[dmeek]

C6 and C7 are part of the compressor in the overdrive channel.

[paolone]

Hi



Today I have tested all the 7 pots
In the photo you can see the min and max impedance misured between pin A and B and between pin B and C.
Some pot don’t increase the impedance even if you turning it up or down and some pot increase the impedance too low or not untill the max quote, turning it left or right.
Only 3 pots are OK.

I’m gonna change them.

Thanks

Paolo

[phatt]

If you are taking pot measurements while they are still in circuit you will get false readings as other parts are still connected.
You have to remove them from circuit to get accurate readings.
If pots don't make noise when turned then it's likely they are fine.
Phil.

[dmeek]

Sometimes the terminals get loose. Check them after they're removed from the board. If the terminals can be moved side-to-side at all, crimp them or just replace the pot.
I had a Vox amp a while ago where almost all the pots needed to be crimped

[g1]

Sometimes the terminals get loose. Check them after they're removed from the board. If the terminals can be moved side-to-side at all, crimp them or just replace the pot.

What techniques/tools do you use for recrimping pot lugs?

[dmeek]

I posted this photo at mef a few years ago: https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/44114-possible-to-repair-a-loose-potentiometer-leg/page2

I ground out these pliers to use for AC cord strain relief installing/removing. They work very well for that, so when I was looking for a crimper the tool was at hand. I have to be careful not to squeeze to hard. It can easily break the wafer.

[willpirkle]

dmeek — now *that* is awesome. I want one!

[paolone]

Hi
@dmeek
Today i’ll buy an used oscilloscope (a kikusui 538a, yes old but still working...)
Can you tell me how and what i have to test in the amp?
What kind of input i have to use in the amp?

Thanks

Paolo

[paolone]

Ok

Now i have an oscilloscope:
Kikusui 538a

Here is the manual
https://manual.kikusui.co.jp/NUMERIC/538A_E.pdf

[dmeek]

You need a stable tone of some kind. There are many oscillator apps for phones and computers or you could use a keyboard organ sound, just tape one key down.
Plug the tone generator into the input and check its level with the scope. Set the vertical gain for about 1 division amplitude on the scope, then move the probe to
IC6 pin 1. the amplitude should be about 5 divisions. Turn the Clean Gain full up, move the probe to the center terminal of the gain pot. The level should be the same 5 div.
Move the probe to IC6 pin 7, should be more than 5 divisions amplitude.

[paolone]

Hi

I have download NCH Tone Generator (free for 5 hours, only for test the oscilloscope, now i have to find another software...)
https://www.nch.com.au/tonegen/index.html

I have gerated a tone “A” (musical note LA) in 599Hz
and i have connect the ground of the sonda to the ground of the sound cable (channel R) of the output of the computer and the positive of the sonda to the positive of the sound cable (channel R) of the output of the computer.

The software can produce sine, triangle, square wave.

I have posted the photo.

Now that I know that my oscilloscope is OK, i can test my amp.

Thanks

Paolo

[paolone]

Hi

Before to start the test i have change all the 10 “1uf 63v capacitors” in the board.

Let’s start the test:
I have found another easy and free Tone generator directly in the Microsoft store: “PC Revue Tone Generator”
https://www.microsoft.com/en-us/p/tone-generator/9wzdncrdhjqn

I have generated a tone of 30Hz on the frequency of 500Hz. (Photo 2)
I have connected the left channel to the amp and i have started the 4 test that @dmeek  told me:
1- Plug the tone generator into the input and check its level with the scope. Set the vertical gain for about 1 division amplitude on the scope
    OK - Photo 3
2- then move the probe to IC6 pin 1. the amplitude should be about 5 divisions.
    For me is 4 Division, not 5... - Photo 4
3- Turn the Clean Gain full up, move the probe to the center terminal of the gain pot. The level should be the same 5 div.
     For me is always 4 Division, not 5... - Photo 5
4- Move the probe to IC6 pin 7, should be more than 5 divisions amplitude.
     For me is only 2 Division, not 5... - Photo 6

What can be?

Here is the datasheet of IC6, I think is that right JRC 072D:
https://www.mouser.com/datasheet/2/294/njr_njm072_njm072b_njm082_njm082b_e-547738.pdf

Thanks
Paolo

[dmeek]

If the overdrive channel is OK, the problem in the clean channel must be R44 100K, or Q4 2SK30A, or R41 10k
All ICs in your amp are dual op-amps like this

[paolone]

Thanks dmeek,

How can i test them, with the oscilloscope?

Or i have to desoldered them?

Thanks

Paolo

[dmeek]

Try the amplifier with Q4 removed. The clean channel will be always on even when switched to overdrive but you can see if there is a difference in gain.
You can measure the resistors after desoldering one end.