Fender PD-250 Passport has low audio output Right Side

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

What does the BIAS and OFFSET adjustments control? The reason I am asking is it looks like they could have been moved from their original factory set positions.

[budm]

Re: Fender PD-250 Passport has low audio output Right Side

The BIAS pot is for setting the current flowing through the output transistor (with out input signal) in idling condition, the set point is set for Class A/B mode for this amplifier.
The OFFSET adjustment pot is for setting the DCV at the out put to be at 0V so at no signal the speaker will be seeing 0VDC on the speaker coil.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

So assuming these are set right should I suspect a bad transistor or capictor. Need to test transistors without voltage applied?

[budm]

Re: Fender PD-250 Passport has low audio output Right Side

You do need to test the transistors and diodes (do the search on how to test transistors and Diodes), resistors (check the values and its condition) without power applied to the unit. Any suspect parts that tested bad should then be removed off the board and test again.
This is not really for beginner that do not have enough knowledge yet on how the devices function. Do you know how to read the schematic and have proper tools?

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

yes I can read the schematic and I have a solder iron and DMM. I will report back later on what I find. Thank you for the help if you can think of anything else please let me know.

Thanks
Tom S.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

As i increased the volume on the right right side output channel, I noticed that the sound became distorted and not clear. It sounded more like "thud" sounds then it did music. Which leads me to believe that when Q217 was replaced that the electrical characteristics of that one and original are not the same and therefore either the offset/bias or both needs to reset for this unit to function properly. Is that a good conclusion or is there more that I should be looking at.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

Looks like my problems went from bad to worse well checking things out today on the amp board. Looks like if the power amp to power supply is connected the protect light comes on. Otherwise if it is not plug in the unit is fine. Please help!!!!! Thank you

Tom S.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

After doing more component testing and checking, there seems to be extra components added to the board and some of the diodes are missing. I am playing with the idea of replacing the whole board or just all of the components which ever is cheaper. Once I get that done I am going to need help to set the OFFSET for the amp's left and right sides and also the BIAS adjustments without the help of an O'scope. Any help would be appreciated.

Thanks
Tom S.

p.s. I want to use this in parades this summer hooked up to my truck to play music and make announcements.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

I have two blown output transistors. One one the left channel and one on the right channel also had some thermal damage to the emitter resisters on both channels. I have also found that the caps c212 and 213 that were supposed to be 10uF 63v had been replaced with 4 1200uF 63v caps. So I was wondering should I keep pulling the transistors and caps off of the board to check or just order the new output transistors?

Thanks
Tom S.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

Need help to adjust BIAS and OFFSET on this amp now that I got it working.

Thanks
Tom S.

[tfs2015]

Re: Fender PD-250 Passport has low audio output Right Side

Everything is working good now, except a pop sound is heard when unit is shut off with speakers plugged in. Thanks everyone for the help.