A Few Questions Please

[Richs_Trains]

Hey All,

The last unit I worked on taught me a lot as it was the first one that came DOA and I was able to bring back.
I have a few questions to better my understanding:

People refer to the 'main rails' quite often, what and where are they usually located.

This unit (Marantz 4230) lit the DBT when I got it and I found the short by process of elimination. I removed power to different portions of the unit until it passed the DBT and then re-powered them one at a time to check each one. The main problem turned out to be a bad filter cap (first time for me) and at this point I had the power removed from the filter cap and the power from the transformer to the power supply board, it passed the DBT, but I also had dial lights, where was the power coming from?

Adjusting the amp without a scope requires a measurement at the junction of 2 resistors, but they are always sheathed in rubber to stand them away from the board, it there a test point, or a way to remove some rubber to get a good connection?

After learning what the rectifier is and does, is the series of diodes on the power supply boards another rectifier set up?

Can you troubleshoot and repair a power amp with the actual unit (I have two power amps that are not working
that I replaced in units, having them working would offer some spares).

Thanks in advance,

Rich

 

[bryans12v]

The dial lights usually have their own AC winding which is usually about ~7v I believe.

If I'm not mistaken, most marantz amp boards have posts that are connected to the emitters by trace. By looking at the component location diagrams that show the traces it's easy to see which ones to clip on to, sometimes you can just see also.

 

[runningdog]

Can you troubleshoot and repair a power amp with the actual unit (I have to power amps that are not working
that I replaced in units, having them working would offer some spares).

Did you mean repair an amp board without the actual unit? The answer is a qualified yes. It takes a power supply that puts out enough to power the amp and in many cases needs to provide 2 voltages to the amp board. Then a signal generator and an oscilloscope to test it with.

You can use a receiver carcass as a power supply. You just need to be able to get the same voltage(s) your amp board requires. Of course don't troubleshoot an amp from a 2325 using a 2230 as the power supply. I have a 2270 that is largely stripped of parts and there are wires with clips for all the connections to the amp board.

 

[runningdog]

People refer to the 'main rails' quite often, what and where are they usually located.

"Main rails" refers to the points where power is distributed to the rest of the unit. There are two or more of these points in a vintage Marantz. The filter caps are one. There's the power from the transformer going through a rectifier and to the filter capacitors. From the filter caps the power is distributed to the main amps. That point on the filter caps is a main rail.

The other "mail rails" are the output(s) of the power supply board. One or more wire pairs from the transformer go to the power supply board which rectifies and regulates the power for the other circuits. At the outputs the regulated power is distributed to the other circuits, including sometimes the main amps too when they use multiple voltages.

i think the term might be used more often for the first example? Because improving the conditioning of the power, maybe with bypass caps or a Pi filter, is a popular subject of discussion and those are applied at the filter caps.

This was just explained to me and corrections or clarifications are welcomed. Any parts that I got right are thanks to ak'r TimCartier.

 

[Steven Tate]

When you asked the question about main rail, I did a Google search and found this little educational discussion. It gives a little basic theory that goes right along with what Runningdog said.
http://www.audioholics.com/audio-am...or-switch-1/amplifier-and-power-supply-basics