Hartke HA3500 Output Voltage

[jonbo335]

Hello, I have this amp for repair. There is 1,5 volt on output, I can't obtain zero volt, moving svr301 full left tension drop to 1,2 volt. All transistor and diodes seams to be ok. Power transistors are 2sc5200/2sa1943. Any suggestion ?
Thanks,
Emilio

[nickb]

Rather than go to and fro with many posts try this. Set SVR301 at the best position and leave it. Now measure the voltage of the C, B & E of every transistor in the power amp and annotate the schematic with that information. Also check the power supplies and both ends of SVR302.

[jonbo335]

Hi Nick, Thankyou for replay
here is my measure
N side:
BC 82 Volt
BE 0,5 volt
CE 82 volt
P side
BC -79,5 volt
BE -0.5 volt
CE 80 volt
not appreciable differences
SVR2 -1,7 volt both sides refered to ground
output -1,2 volt
at the PSU +80,4 -80,4
Chears,
Emilio

[Jazz P Bass]

'N' side?
'P' side?

Which transistors are you measuring?

Note: The BC/ CE voltage looks suspect.

[jonbo335]

N side for NPN 2SC5200
P side for PNP 2SA1943

[nickb]

Hi Nick, Thankyou for replay
here is my measure
N side:
BC 82 Volt
BE 0,5 volt
CE 82 volt
P side
BC -79,5 volt
BE -0.5 volt
CE 80 volt
not appreciable differences
SVR2 -1,7 volt both sides refered to ground
output -1,2 volt
at the PSU +80,4 -80,4
Chears,
Emilio

The voltages on SVR2 are wrong. They should be about +0.65V at one end and -0.65 at the other. Measure the volts on both sides of D301 and D302. They are supposed to be connected together on the 0V side so measure on the leads in case there is a bad joint.

[jonbo335]

The 2 Ge diodes are ok, connecttion to input ground and on sides +0.65 and -0.65, problem should be around SVR2 both side remain at -1.5 volt

[nickb]

The 2 Ge diodes are ok, connecttion to input ground and on sides +0.65 and -0.65, problem should be around SVR2 both side remain at -1.5 volt

Sorry jonbo - I've been saying SVR302 when I meant SVR301 - can you please check that instead?


Can you also please complete marking the schematic will the voltages of on the CBE of ALL the power amp transistors - not just the output ones. Thanks.

[jonbo335]

Ok, I 'll do tomorrow. On SVR1 i have about +300 and -300mV on sides and on SVR2 -1,5 Volts both sides

[Dave H]

Ok, I 'll do tomorrow. On SVR1 i have about +300 and -300mV on sides and on SVR2 -1,5 Volts both sides

+300 and -300mV is OK. There should be about 60mV across SVR2.

I see C304 is NP (Non Polarized). I think C302 and C306 should also be NP (or reversed). If the output is set to 0V then the transistor base current through R381 will cause a voltage drop putting the transistor base and C306 +ve terminal at a -ve voltage. The left hand base of the transistor pair will be at the same -ve voltage so C302 is also reverse biased. Could a leaky reverse biased capacitor be the cause of the output offset?