crunch pzi250.2

[Hexhclass]

This amp came in with bad outputs.I replace them and drivers.If i let this amp idle for about 40sec the outputs i replace will get hot.I think i have a bias problem in that channel..someone had already try to repair this amp and give up.

good side
TIP36c
1: 0.186
2: -30.94
3: 0.153

TIP35c
1: 0.524
2: 31.01
3: 0.153

Bias transistoC1815
1: -0.756
2: 1.097
3: 0.123

Bad Side

TIP36c
1: 0.526
2: -30.94
3: 0.015

TIP35c
1: 0.056
2: 31.01
3: 0.018

Bias transistoC1815
1: -1.105
2: 1.146
3: 0.397

 

[Perry Babin]

Are you sure that the bias transistor isn't defective? The Vbe of more than 1.5v is excessive.

 

[Hexhclass]

I don't think C1815 is the original bias transistors.I will change it.

 

[Hexhclass]

I drop a new C1815 in there no change.

 

[Perry Babin]

With 1.5vb-e, the transistor should be saturated and the collector and emitter should have essentially the same voltage.

Are you sure that the bias transistor is supposed to be a transistor with an ECB configuration?

Amps that use the TIP35/36 often use a transistor like the MPSA06 or 2N4401.

Confirm that the collector and emitter of the bias transistor connects to the bases of the driver transistors.

 

[Hexhclass]

The emitter is connected to the base of the (A940) driver transistor through a 47 ohm resistor.The collector is connected the the base of (C2073) through a47 ohm resistor.

 

[Perry Babin]

Short the bias transistor collector to emitter and confirm that the channel works normally except possibly a bit of crossover distortion.

Measure the voltage on the bias transistors with the black probe on the emitter and the red probe on the other two legs (no collector-emitter jumper).

 

[Hexhclass]

With the jumper in place the channel works ok.


With no jumper
Black probe on center pin of emitter resistor
emitter -1.144
collector 1.178

 

[Perry Babin]

Black probe on the emitter of the bias transistor, not the emitter resistor.

 

[Hexhclass]

This is before the transistors have time to heat up
emitter and collector 2.378
emitter and base 0.710

 

[Perry Babin]

If there is a resistor between the base and collector, decreasing its value should reduce the bias current.

 

[Hexhclass]

yes, 3.9k
I will try some lower value and see what happen

 

[Hexhclass]

I put a 2.2k in there and the channel seems to be working good

 

[Perry Babin]

Run it hard for long enough to get the amp to thermal and re-check the bias when hot. You should also check the idle current several times as it heats up to make sure that the bias hasn't drifted too far.