Mark Bass Low Output

[drewl]

UGH!
Another one of these where the power amp seems like it's okay, but I'm only getting about 50 watts out of it clean.

This one is a CMD 151P, much different than the Little Bass, which is only schematic I've ever found.

I don't know if there's another driver stage after the preamp, which is putting out around 7 volts RMS, that sound about right?

The main board has a couple TL074's which I replaced and an NE556 timer, I'm not sure if these are for amplifying, or are part of the protection circuit.

[glebert]

There are multiple versions of CMD 151P, which one? Jeff Berlin, Mini, something else? The Mini alone seems like there have been four versions. Does it have an effects loop that you can drive with an external source and see if you can get more clean power?

[drewl]

YOU-REEK-A

So looking at the preamp, it looks pretty similar to the schematic available.

If you turn to page 10, I have a nice signal coming from the master volume pot through IC5 (4 on this unit).

So I should have 100 ohms between the output test point and pin 7 of the output connector, but I don't!



As you know these units aren't very family friendly to work on, hard to get at components etc. So I pulled the preamp out and checked for signal at pin 7, barely anything there!
I tried to follow the output signal trace to see where it goes, or if it passes to another stage, but that was proving to be difficult, so I said screw it, and jumpered a 100 ohm resistor from the test point to pin 7 of the output connector and ........

.................. VOILA'

massive amounts of signal screaming out of the power amp, as it should be.

[glebert]

I have a Markbass MoMark that I similarly had to do a jumper fix until I motivate myself to order some surface mount TL072 op amps. On mine it was the stage just before the master volume.

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[alexradium]

problem in these amps is those smd elco caps,the 1uF and 2.2uF get bad after some years,one of them pulls the limiter circuit down,hence the lack of signal in preamp output.