Need Help with this Circuit Please

[tfs2015]

Re: Need Help with this Circuit Please

looks like right now I have to replace all of the transistors as most of them test bad. But I will have to pull them out of the board to test better.

Thanks for the info. I will let you know what happens.

[rievax_60]

Re: Need Help with this Circuit Please

Keep in mind that you can initially do some voltage checks before fitting the output transistors, Q215 Q216 Q212 Q214.
Doing this will likely avoid a lot of damage if a problem is missed.

[keeney123]

Re: Need Help with this Circuit Please

Yes that is right make sure your voltages are there. You never said if the -46 volts is there or not.

[tfs2015]

Re: Need Help with this Circuit Please

ok just a quick update before I turn in replaced transistors for the differential amp on the right side of power amp board and also found a bad zener diode and replaced that. Tested to see that I had rail output voltage of +52vdc and -52vdc at the rails of the drivers and output transistor which I have. Set offset at the speaker output right side for 0vdc. Installed the driver transistors Q227 and Q229 and test the dc offset at the speaker output right side, still near 0vdc and nothing is hot so far, Tomorrow I am going to install the output transistors and test. All I will have to order is a couple of zener diodes for the left side and a couple of transistors as well. Hopefully that will fix it. I really need to replace the circuit board with a new one made with better material as the copper traces on this one are so small and break real easy. I ended up having to splice some of them back together with small pieces of wire. I will update my progress once I test everything.

[rievax_60]

Re: Need Help with this Circuit Please

Sounds good. The Collectors of the driver transistors, Q227, Q229 should be + and - 4.7V as set by the zenners, Z205, Z206. The idle bias current POT TP204 should adjust the voltage between the bases of the driver transistors over a range of about 1V to 2V.
It is important to not have any poor connections on Q224 and nearby components.

[momaka]

Re: Need Help with this Circuit Please

When you fit in the new output transistors and are ready to test, use an incandescent light bulb in series with the amplifier's power line to limit the current going to the amp. That should prevent you from blowing up the transistors again if something is still wrong. You can even test the amp with the series light bulb, as long as the audio is set to a low level.

How to wire it:
https://www.badcaps.net/forum/showpo...4&postcount=70
Except where it says "power supply" on the diagram, this is where you would connect your amplifier's power cord.

[keeney123]

Re: Need Help with this Circuit Please

ok just a quick update before I turn in replaced transistors for the differential amp on the right side of power amp board and also found a bad zener diode and replaced that. Tested to see that I had rail output voltage of +52vdc and -52vdc at the rails of the drivers and output transistor which I have. Set offset at the speaker output right side for 0vdc. Installed the driver transistors Q227 and Q229 and test the dc offset at the speaker output right side, still near 0vdc and nothing is hot so far, Tomorrow I am going to install the output transistors and test. All I will have to order is a couple of zener diodes for the left side and a couple of transistors as well. Hopefully that will fix it. I really need to replace the circuit board with a new one made with better material as the copper traces on this one are so small and break real easy. I ended up having to splice some of them back together with small pieces of wire. I will update my progress once I test everything.

Sounds like your on the right track with the right support. Wires are as good if not better than traces. Just try not to cross the output and input signal wires and try staying away from the DC voltages with the signal voltages.

[tfs2015]

Re: Need Help with this Circuit Please

ok I ran into a problem while testing. With Q228 and Q232 connected I get output. When I connect Q231 and or Q230 the amp goes into PROTECT. So what am I doing wrong or hat did I miss? Keep in mind Z202 is removed because it was bad.

[rievax_60]

Re: Need Help with this Circuit Please

Did you do the voltage checks that I mentioned earlier?
Amplifiers usually go into protect because of DC offset at an output. When checking transistors, C to B leakage can be easily missed.
Z202 belongs to the other channel. A DC offset can be caused by leaving it out depending on what other components are missing. It is best to leave it in or bridge it out.

[tfs2015]

Re: Need Help with this Circuit Please

How do I bridge it out (Z202)? You do have the schematic to look at correct? Like I stated it works good with Q228 and Q232 installed. it is when I install either Q230, or Q231 that is when I have the protection problem.

[rievax_60]

Re: Need Help with this Circuit Please

Im looking at the fender_passport_pd_250_sm PDF from earlier in the thread. Bridging is simply soldering a piece of wire across something. There is no need to do it if it's not causing a DC offset at that channel's output.
Im puzzled about the effect of fitting Q230 or Q231. Double check that the base pins are connecting to the correct places.
You have not been responding to any questions.

[tfs2015]

Re: Need Help with this Circuit Please

Im looking at the fender_passport_pd_250_sm PDF from earlier in the thread. Bridging is simply soldering a piece of wire across something. There is no need to do it if it's not causing a DC offset at that channel's output.
Im puzzled about the effect of fitting Q230 or Q231. Double check that the base pins are connecting to the correct places.
You have not been responding to any questions.

Voltage checks all seem OK with -52 and +52vdc at the output rail like it should be. Checking Q231 shows bad need to order some new output transistors. Sorry for not answering question right away. I will try to be better with that.

Thanks
Tom S.

[tfs2015]

Re: Need Help with this Circuit Please

I have found an error in the layout of the board page 29 of the service manual that I need someone to check and verify. Q229, Q227, Q213, and Q211 all look they are installed with the face of the transistor (printed side) facing the heat sink (edge of the board). Could someone please verify this and let me know. Seems like the back of the transistor should be mounted toward the heat sink for the heat from it to transfer across. Help on this would be greatly appreciated.

Thanks
Tom S.

Happy Holidays

[rievax_60]

Re: Need Help with this Circuit Please

Yes, It looks like a mistake. The Emitter of BD139/BD140 transistors is on the left when looking at the printed side and the Emitter pin should go into the left hole to connect to the Bases of the output transistors.
Are the driver transistors mean to be mounted to the heatsink? I don't expect so.
If they do, they should be insulated.

[keeney123]

Re: Need Help with this Circuit Please

Yes, It looks like a mistake. The Emitter of BD139/BD140 transistors is on the left when looking at the printed side and the Emitter pin should go into the left hole to connect to the Bases of the output transistors.
Are the driver transistors mean to be mounted to the heatsink? I don't expect so.
If they do, they should be insulated.

Depends on who designed the board.

[tfs2015]

Re: Need Help with this Circuit Please

Good news Right Channel had a bad output transistor. I replaced it and now that channel is fine. Waiting on parts for the Left Channel. Once I get them in I will report back.

Thanks everyone for the help.

Happy Holidays,
Tom S.

[keeney123]

Re: Need Help with this Circuit Please

Good news Right Channel had a bad output transistor. I replaced it and now that channel is fine. Waiting on parts for the Left Channel. Once I get them in I will report back.

Thanks everyone for the help.

Happy Holidays,
Tom S.

That is good news.I will be waiting to hear if you connected them to the heat sink and how. Most output transistors are heat sunk, but will wait to hear from you.

[tfs2015]

Re: Need Help with this Circuit Please

Just a quick update. The Fender PD250 is almost back together and it is working perfectly. The output and driver transistors are all mounted to the heat sink with insulator material between them.

Thank you to everyone that helped with this project with suggestions and technical help.

Thank you and Happy New Year 2016

Respectfully

Tom S.
Wells, MN

[rievax_60]

Re: Need Help with this Circuit Please

Good to hear. You should carefully check the idle current.

[tfs2015]

Re: Need Help with this Circuit Please

How do I do that? I read somewhere that you can measure the voltage drop across the emitter resistor which should be around 2mV DC when the idle current is properly set am I correct or did I miss something?

Thanks
Tom