Mosconi Gladen one 120.4

[Achim1409]

Got this amp for repair. Everything looks fine, but the amp goes after a few seconds in protect. Audio sounds good, during play, no issues.
After about three or for times of testing, it stucks in protect. Maybe a cap???

I have no diagrams for this amp.

 

[miniman82]

Sounds like an issue with the protection circuit, what does it have for a power supply chip TL494?

 

[Achim1409]

Here are some pictures

 

[Achim1409]

Its a SG3525A

 

[Achim1409]

On Pin 1 of SG3525 i have 1,05V
Pin2 4,19V
Pin 10 4,19V
It in Protect.


When not in Protect,
Pin1 1,05V
Pin2 4,19V
Pin 10 0V

 

[Perry Babin]

Is the circuit anything like the LRX series of audison?
AUDISON LRX 1.400 SM Service Manual download, schematics, eeprom, repair info for electronics experts

 

[Achim1409]

No, its different on the first sight. D13 and D15 are not in circuit. Pin 1 is connected to a smd resistor and cap.
Pin 2 are resistors only. I will try, to find where its connected.

 

[Achim1409]

I checked something and i expect the DC offset protection.

How the SG3525 works? Specially Pin 10, is this pin pulled high from external? If it is like this, i have the same voltage on the transistors in front of the Speaker terminal.

 

[Achim1409]

Hmm, i lifted Pin 10. Anyway stays in Protect, but PS is still on. Found the transistor 2GW,
where the emmiter is connected to that pin. This one is pulling high. Base has 5V in that state. Between emitter and base is a 10k resistor...

 

[Achim1409]

I lifted the transistor circled right below, the amp stays on. The Resistors left, get hot. The lower one has voltage across 8,3V, the upper 8,6V.
On my ampere meter, the amp draws 0,6amps, then goes up to 1Amp, after half a minute down to 0,6 again.


I expected 1 amp, because when the amp was working, it draws at starting 0,6amp, then when 5 seconds are gone, 1amp and start play music.

 

[Achim1409]

That means, current sensing, and i have different currents on positive and negative. Because the resitors are ok, both have 98.6 Ohms.
Checked all emitter resistors , 0V at all.. Could it be possible, that a rail cap is damaged?

 

[Achim1409]

According manual, the amp should have 1 amp idle current. If it hit the caps circled below, the current drops to 0.6amp, after a second or two, the current goes up to 1 Amp aggain.

 

[Achim1409]

These caps have 3,3uf 50V, can i use for testing 4,7uf?

 

[Achim1409]

ok, i think i found the problem. It was a cold solder joint on a resistor..now, the current stays at 1amp.

 

[Chucloo]

Hi, I have similar issue. May I know which had the cold solder and needs to reheating. Thanks in advance.