Dragster DB 505 5 channel amp

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mekaiver

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Joined 2008
#1
Got this amp, and it was in protection mode. So i had a quick check and one of the output transistors was bad. Changed it for the same thing and powerd the amp again. Still red led on. So i checked if the SG3525 was working. It wasnt. I traced the PCB and found that a transistor (Q45) was bad. Changed that and also the SG3525. Now if powered up, the SG has VCC voltages and also 5V on legs 11 and 14. Checked with a scope, its a squarewave. After that measured the power FETs, pin 1 has that 5VDC square, pin 2 has 12VDC and pin3 is 0VDC. None of them seems to be shorted and all four have the same parameters. So something is happening, but still the red led is on, and no sound comes out from the amp. The output transistors have +/- 22VDC . A bit small or is it normal? Nothing heats up or anything. Any ideas? Maybe a schematic? Mr. Perry for shure has some ideas i hope.
 
#5
Both positive and negative voltages are ok. It uses ST 074C chips in preamp. Without the schematic, its pretty impossible to trace the signal on a 2 layer PCB. All the 4 chip outputs(074C pin 1,7,8,14) have the signal that i feed from the signal generator. Then i can trace some resistors and capacitors and then who knows where it goes, but near the output i picked the signal up again and it goes to a chip "15532" where the signal looks a bit like a squarewave instead of the sine i use.

A quick question from a friend - sinuslive SLA-1500. Says that one of the capacitors in the main power supply (C63) has a voltage of about 60VDC on it, but when he connects the subwoofer, the voltage will be about 100VDC, if speaker disconnected, voltage again normal, 60VDC. The other capacitor (C67) has always about 60VDC, no matter if the speaker gets connected or disconnected.
 
#9
Measured from GND Q42 and the results where :
Pin1: 13,0 VDC
Pin2: 12,3VDC
Pin3: 12,3VDC

Also cheked the nearby Q41 and:
Pin1: -13,0 VDC
Pin2: -12,3VDC
Pin3: -12,3VDC

Also that 15532 thing:
All pins show 0VDC if measured from GND.
 
#10
The amp may have an isolated secondary ground. If that's true, you have to use a secondary ground point (non-bridging speaker terminals or the secondary center-tap) as a reference (black probe).

You stated that you didn't have negative supply voltage on pin 4 of the 5532s. Do you have positive supply voltage on pin 8?
 
#12
Measured from SEC GND Q42 and the results where :
Pin1: 1,0 VDC
Pin2: 0,33VDC
Pin3: 0,33VDC

Basicly the same as before just subtracted the 12V i power it from. Measured the 15532 as following - one probe on pin4 the other on pin8, still 0VDC.

I also mention, that the transistor that was burned/dead , i replaced with a TO-92 case. I think its right because after changing that i had the SG working, but i might be wrong and something maybe tied to that.
 
#14
I could read from the old transistor 2G (Q45), so my database said its a BC850C, replaced that with BC550C TO-92. The pinout should have been on that SOT one from the left : BCE. According to that i tweaked the TO-92 legs. LED red on, but the SG 3525 started to give me that nessesary 5VDC squarewave, and i did get the voltage of 22/-22 to the output fets. Also tried some other NPN transistors there, nothing better. I also tried a C3198 and what i noticed with that, is when C and E are shorted, the green LED lights up, BUT at the same time the red is also on - they both are lit at the same time. Its a 3 leg LED.

The legs are not touching.
Yes, the Q41 did the same thing as Q42: same voltages as before - 12V.
The only thing that says to me, that the power unit is on, is the output voltage and that the SG generates squarewave.
 
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#15
With no remote, do you have ~12v on the emitter of the transistor?

Is the collector connected to pin 15 of the 3525?

Do you read anything near 30v with the black probe on the negative supply pin and the red probe on the positive power supply pin of any of the op-amps?

Confirm that you have rail voltage when you measure the voltage.
 
#16
Perry Babin said:
With no remote, do you have ~12v on the emitter of the transistor?

Is the collector connected to pin 15 of the 3525?

Do you read anything near 30v with the black probe on the negative supply pin and the red probe on the positive power supply pin of any of the op-amps?

Confirm that you have rail voltage when you measure the voltage.
Click to expand...

Removed the transistor. Without remote i have on C 12V.

No, collector is not connected to pin 15. At least i cant measure it from the collector pin. Might be going pass some components. It seems that the pin 15 is connected to GND via a small SMD capacitor (C52).

Yes, read 29VDC from the opamps. With rail voltage.
 
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