GIA20G(U65G) low volume noise

[uki]

Hi guys !
Giannini GIA20G
This amp was gifted to me, and wasn't working, it was a bit messy, loose wires and components, I've replaced the power transistor (TIP41C) for TIP41 what I have, all power transistors were busted. Also replaced one of the electrolytic caps, way out of spec.
The amp works, all pot controls work, but the amp have low power/volume, it is a bit noisy ground like noise and sound come out distorted.
So I got to the point where I 'm not sure how to proceed. I appreciate some guidance, thanks in advance.

[PRR]

> not sure how to proceed.

Schematic? Voltages?

[uki]

Thanks PRR !

The schem is attached in the 1st post, where to ready the voltages ?

[Willabe]

The schem is attached in the 1st post, ....

The schematics not in that link.   

[sluckey]

The schem is attached in the 1st post, ....

The schematics not in that link.   

It's in the link directly above the pics.

[Willabe]

Schematic? Voltages?

https://el34world.com/Forum/index.php?action=dlattach;topic=28248.0;attach=95273

[PRR]

Three blind mice? Here it is as a GIF:

[uki]

Voltages from PT and transistors:

PT primary input 115vac , secondary 22vac each side and 44.4vac across the whole winding.

Q1,Q2,Q6 and Q7 are BC549C

Q1 B 0.8vdc  C 6.8vdc   E 0.2vdc

Q2 B 6.2vdc  C 8.4vdc   E 5.8vdc

Q6 B 1-3vdc  C 0vdc   E 0vdc

Q7 B 0.6vdc  C 3-11vdc   E 0vdc


Q3, Q4 and Q5 are TIP41

Q3 B 8.3vdc  C 27.8vdc   E 7.8vdc

Q4 B 14.8vdc  C 29vdc   E 14.4vdc

Q5 B 0vdc  C 14.3vdc   E 0vdc

Anything else to read?

[shooter]

might take a second, ohm the PI transformer, primary and both secondary's
If you have a scope, or access to a scope might be a good time to bring it into play

[uki]

PI transformer ohms

primary  31.7 ohms

secondary 5.7 ohms both sides

about same reading connected and disconnected.

no scope

[PRR]

Q5 base should not be ZERO. It should get a small positive voltage (half Volt?) through the transformer from the resistors. If truly zero it will sound like crap. Find where that's gone wrong.

[uki]

__ It should get a small positive voltage (half Volt?) through the transformer from the resistors.

Ok got new reading and indeed it does have about half volt, 0.48 vdc at Q5 base

[shooter]

without a scope you're down to building a listening amp to trace the signal or shot-gun each stage with new parts, testing after each stage is "be-build"

[uki]

A little more info about the amp, besides what is in the 1st post, the 3 input wires were connected to one input jack(probably a bad idea), the other 2 jacks were missing, fuse was blow.

Besides using a scope and shotgun method, what else could be done, what could be the most suspicious components? coupling caps ?

[shooter]

a listening amp might help.
headphone amp, small tube amp, old boom-box with a headphone jack will all work.


the idea is to inject a clean known signal, looped song, etc. into your bloke amp, then probe the listening amp along the signal path til you get the uglies.  then it's basically re-build that section and re-test..


shot-gunning parts, I always start with caps, both Ecaps and couplers, then active components, tubes, transistors, IC's. Resistors get ohmed and replaced if they have drifted, look stressed, crusty.

[uki]

Connect the output signal from the stage to the listening amp yes? ground and signal path to the listening amp input ?

[shooter]

you need a "probe" for the listening amp.  shielded wire cable, center conductor is the "probe" (signal), shield goes to ground.
you need a coupling cap between the probe tip and the center conductor in case what you're probing has DCV.  Any cap from .02 - .1uF should be fine.
this "probe" goes into your listening amps input, the output can be a speaker if it's another amp, or headphone amp you just wear the headphones to listen.


on a transistor amp i typically start at the base of the 1st transistor in from the input jack, then move to the collector, find the next stage, repeat. til you hear the distortion.


it takes some practice so don't just jump down a rabbit hole because something just got scratchy, crackly.  try and discern if that sound was your setup, or real

[sluckey]

this "probe" goes into your listening amps input, the output can be a speaker if it's another amp, or headphone amp you just wear the headphones to listen.

I strongly advise not to use headphones, unless you're willing to blow a hole in your eardrums.   

[shooter]

 
most headphone amps have a volume pot, ANY other amp, ya, a speaker is best

[sluckey]

most headphone amps have a volume pot, ANY other amp, ya, a speaker is best

Of course they do. And you inject a suitable signal into the busted amp, connect the listening amp probe to the base of the first xsistor, and adjust the headset volume for a pleasing ear level. Then you move the probe to the collector and BOOM! My ears hurt. About the same thing as you laying on the sofa silently listening to Made in Japan and your dorky little brother walks in and cranks up the amp volume!

Now if you were doing this with a guitar amp as your listening amp, about all that would happen is all the cats would disappear! And that's a good thing. Right!   

[uki]

Cool thanks!!  I'll do the suggested listening method ill take some time on it.

Oh hey I manage to get a "scope", well sort of 

https://manpages.ubuntu.com/manpages/bionic/man1/xoscope.1.html
 

[sluckey]

Oh hey I manage to get a "scope", well sort of 

Hmm, I'd keep looking.

[PRR]

That uses your sound card. A sick amp and and a sound-card can make disaster. There ARE $30 mini-'scopes that do not risk your PC.

I did manage to install it in a new MXlinux. Doesn't dance but sound under Linux or Oracle VM is goofy at best. Gad, this linux won't even use the same screen resolution twice.

[uki]

Gad, this linux won't even use the same screen resolution twice.

It does need the guest addition thing to be able to run on full screen or any resolution or window size you like.

Found another scope software. Pulseview looks more advanced than the xoscope.

[uki]

Got the Xoscope working with a function generator app on cellphone!

What if the signal is pulled from the headphones output from the listening amp to the computer? would that work w/o risking the computer?

[shooter]

it should
the problem is signal amplitude that's being monitored,(your broke amp), will/could be greater than the sound cards max input allows.
Even if it doesn't smoke the card because of internal protection, you won't have usable results/signal, since it's "clipped" to be safe.

[uki]

Ok now this looks very interesting, Buffer Hardware for xoscope.

I did the listening amp method, the first stage before Q1 and its base and collector sounds fairly clear.
Gets messy on the tone control stage, very noise and distorted as it gets close to Q2, base and collector are very noisy, I did check Q2 its working, it looks like something between volume pot and Q2 gone wrong, if I'm getting this right.


 

[uki]

Just realized the amp is about 40 years old, the cab was totally roten, no recovery.

I've replaced some of the electrolytic caps, those were leaking:

C5, C9, C10, C13, C16, C22, C24

although the amp is sounding better, there is still a strong hum. what to do?

[shooter]

hum is the same whether sand or glass.


what happens when you put a metal "box" over the board?
what happens when you kinda sorta "wave you hand" over the circuit board, or touch one of the pots?


the circuit is either picking up "Noise", creating it then amplifying, the PS isn't filtered/grounded very well


look at the jacks, are they grounding type?  do they indeed "close"?
 

[PRR]

Just realized the amp is about 40 years old...still a strong hum. what to do?

The schematic is practically 1960, though the parts are a bit later. But could be 50 years old. And not the best-built product of 1971; indeed it looks cheap.

Hum: what affects it? Physical location? Metal box/not? Change with knobs is a BIG clue where it is getting in.

[uki]

The schematic is practically 1960, though the parts are a bit later. But could be 50 years old. And not the best-built product of 1971; indeed it looks cheap.

You are probably right! This amp is been reissued with different names, but same circuit, musicians around here by the time it was popular called it "baguinho". Indeed it is a cheap amp !!

Hum: what affects it? Physical location? Metal box/not? Change with knobs is a BIG clue where it is getting in.

what happens when you put a metal "box" over the board?
what happens when you kinda sorta "wave you hand" over the circuit board, or touch one of the pots?
the circuit is either picking up "Noise", creating it then amplifying, the PS isn't filtered/grounded very well
look at the jacks, are they grounding type?  do they indeed "close"?

Last time i got the change to work on the amp it was on top of the bench, w/o the chassis, that is probably one factor;
Touching the treble and bass pots does alter the noise(no knobs in place), noise increase as volume goes up;
The board was doing some noise while moving the hand above it in the tone controls area, like from volume to tremolo;
Jacks are for sure another factor, one is missing didn't find one yet to replace, the other two only one have switch and the one that doesn't have switch still need ground. jack with switch on all inputs?
Had no idea about to put a metal box on top if it, with the amp in the chassis, noise was just a lil better.
The most of the noise is something like no ground noise or like when it comes from the electricity installation.
I did capture the frequency from the speaker with a tuner, it was giving 600Hz frequency.

[sluckey]

jack with switch on all inputs?

No. Use only one switched jack. Notice the different values for the mixing resistors? Those resistors and the one switched jack work together to kinda equalize the different input levels for guitar, mic, and organ.

[uki]

I did assembly the amp in the chassis to see if grounding it all would help, it works alright, no noise!

Thank you very much guys!!

[uki]

All done now !!