Hartke A70 problem

[rextor]

Hi guys, i'm just a newbie in repairing amplifiers so please help me with this problem.
I have 2 units of Hartke a70 but i will focusing first on the one that produces a loud sound upon powering the amplifier on.
I have attached the measurements i made with respect to chasis. I tried these measurement same as Mr. Lee_ranaldo's thread.

[J M Fahey]

Hi guys, i'm just a newbie in repairing amplifiers so please help me with this problem.
I have 2 units of Hartke a70 but i will focusing first on the one that produces a loud sound upon powering the amplifier on.
I have attached the measurements i made with respect to chasis. I tried these measurement same as Mr. Lee_ranaldo's thread.

Open/unsoldered/cracked_track_or_pad C35 .

Resolder it with a good hot iron and add a little extra solder, try to get shiny bright looking soldering.
If not enough, replace it.

Just for kicks, measure and post the DC voltage at the speaker "hot" out (junctions of R5/56); you supplied all *around* the one we really need

[rextor]

Open/unsoldered/cracked_track_or_pad C35 .

Resolder it with a good hot iron and add a little extra solder, try to get shiny bright looking soldering.
If not enough, replace it.

Just for kicks, measure and post the DC voltage at the speaker "hot" out (junctions of R5/56); you supplied all *around* the one we really need

Hi J M,

C35 looks open, i've changed the two capacitors. measured junctions R5=0.02vdc, R56=-0.3vdc. i tested and it sounds like still humming and a loud sound went out upon turning the power.

[J M Fahey]

Sorry but your answer isn't clear:

Hi J M,

C35 looks open,

Ok, what did you do about that?

i've changed the two capacitors.

Which ones?

measured junctions R5=0.02vdc, R56=-0.3vdc.

The speaker "hot" out (junction of R54/R56) is a single point, so the answer is a single voltage, referred to ground, you post 2 different values ¿¿¿???
Maybe you are posting voltage *across* each of them?

i tested and it sounds like still humming and a loud sound went out upon turning the power.

Do you still have a +30V DC rail or you recovered proper -42V?

What is "a loud sound"?
Hum/buzz/hiss/feedback/whistle?

Thanks.

[rextor]

Hi J M,

Sorry for this reply, I replaced both the capacitors C35 and C34.

The speaker "hot" out (junction of R54/R56) is a single point, so the answer is a single voltage, referred to ground, you post 2 different values ¿¿¿???
Maybe you are posting voltage *across* each of them?

-You mean R64/R56, I've got -0.2vdc


Do you still have a +30V DC rail or you recovered proper -42V?

-I recovered -42V.

What is "a loud sound"?
Hum/buzz/hiss/feedback/whistle?

-I'm hearing a "bog" sound upon power up then a continues buzzing sound and a "bog" sound again after i turn it off.

Thanks.

[J M Fahey]

If the "bog" sound you mention is what we call a "thump", probably involving the speaker cones visibly moving forward or backward for a second, that's normal.
The amp is "stupid" for a couple seconds after turn on, some same at turn off, because inside voltages have not yet found the proper value, or have just lost it.
Nothing dangerous at all.

Now the buzz: is it unbearably loud, as loud as music or worse, or it's just annoying, lost in the background when you play?

Does it respond to any control?

If you turn everything to 0, what happens?

If your multimeter has a low AC voltage scale, (2V or 200mV), measure how much AC voltage (noise) you have at the output.

Cheap multimeters with only, say, 200 and 750VAC aren't sensitive enough.

[rextor]

If the "bog" sound you mention is what we call a "thump", probably involving the speaker cones visibly moving forward or backward for a second, that's normal.
The amp is "stupid" for a couple seconds after turn on, some same at turn off, because inside voltages have not yet found the proper value, or have just lost it.
Nothing dangerous at all.

Now the buzz: is it unbearably loud, as loud as music or worse, or it's just annoying, lost in the background when you play?

Does it respond to any control?

If you turn everything to 0, what happens?

If your multimeter has a low AC voltage scale, (2V or 200mV), measure how much AC voltage (noise) you have at the output.

Cheap multimeters with only, say, 200 and 750VAC aren't sensitive enough.

Now it's worst..when i tried to measure the terminal T1 and T2, the buzz was lessen so i thought it was only a cold solder or loose connection but when i resoldered it and tested it again..the buzz sound becomes loud and becomes unbearable. Everything is on 0 level.
I measured around 7vac on my output going to speaker.

[J M Fahey]

Ouch!!
7V noise is unbearable.
Short across R41 *or* unsolder and lift from the PCB 1 leg from R39 or C50 .

Any change on the noise?

[rextor]

Ouch!!
7V noise is unbearable.
Short across R41 *or* unsolder and lift from the PCB 1 leg from R39 or C50 .

Any change on the noise?

Now the buzz sound is completely out.. 1.2 mvac is the output on speaker terminal.
I unsoldered 1 leg of R39.

[J M Fahey]

Good!!

So we must now discard thepower amp as healthy, the buzz is getting injected into it.

You must trace it backwards, towards the input jack.

¿Does any control affect it?

What's connected to the pin labelled "Audio_out"?

You *might* have something as simple as a broken ground connection to the preamp.

Post the rest of the circuit.

If possible, at higher resolution, or if it's a PDF, post it here in full; as is is almost unreadable.

[rextor]

Good!!

So we must now discard thepower amp as healthy, the buzz is getting injected into it.

You must trace it backwards, towards the input jack.

¿Does any control affect it?

What's connected to the pin labelled "Audio_out"?

You *might* have something as simple as a broken ground connection to the preamp.

Post the rest of the circuit.

If possible, at higher resolution, or if it's a PDF, post it here in full; as is is almost unreadable.

Hi JM,
this is the link from Lee_ranaldo's thread for the schematic diagram;
http://music-electronics-forum.com/t15848/

the controls doesn't affect it.
pin3 is connected to the Audio out.

[rextor]

FIXED...

Hi JM,

thank you for your support..it is now fixed.

resoldered and traced back from R39, resoldered also the two capacitors C35 and C36. Also, found broken potentiometer for the Limiter.
No more buzzing sound. Everythings back to normal.
thank you again JM..

[J M Fahey]

Good