Marantz 300DC Peak Light Issue

[lovablevietb]

My Marantz 300DC's right side peak light keeps flashing only at 1/3 turn of the knob. Its only the right channel that keeps flashing. Does anyone know the problem and how to fix this? Thank you

 

[chazix]

I should probably keep quiet, because I don't know the answer. With luck, perhaps someone will come along who does. Meanwhile, though, I'm too intrigued to shut up. Lord help me, how I love a diagnostic challenge!

I took a look at the 300DC schematic. At my level of comprehension (always iffy), it's not obvious exactly how the peak lights are supposed to function. But, I think they might trigger in response to the level of current in the output transistors, and that could reflect a serious problem.

One thing I can see from the schemo is that the peak light circuit is not adjustable. So, either there's really something amiss elsewhere, and the peak circuit is reacting to it, or the peak circuit itself is faulty.

I think it would be a good idea to see whether the symptom occurs when the speakers are disconnected. If it does not, then I'd suggest checking the speaker wiring very carefully. If you don't spot anything wrong with the wiring, you might consider re-connecting the speakers with the left and right connections (at the amp) swapped. and checking whether the same channel's light flashes.

Do you hear anything amiss when the light flashes?

Cheers,

chazix

 

[petehall347]

are the meters acting normally ?
if so look at qn06 qn04 qn26 cn02 .and all resistors in that part of the led drive circuit ..
might just be a cold solder ..
might be best to check dc offset and bias first ...

 

[jblmar]

I too have the 300DC. There are a series of transistors that make up the Peak LED circuit. Can't help you via email or on the forum, but if decide to send it out for repair, I can repair it for you,

 

[lovablevietb]

are the meters acting normally ?
if so look at qn06 qn04 qn26 cn02 .and all resistors in that part of the led drive circuit ..
might just be a cold solder ..
might be best to check dc offset and bias first ...

Yes, the meters are acting normal. Its the peak light on the right channel is blinking like crazy when there's stronger bass notes. I will check on the DC offset this weekend. I'm not really good with electronics, that is why I asked. Thank you

 

[lovablevietb]

I should probably keep quiet, because I don't know the answer. With luck, perhaps someone will come along who does. Meanwhile, though, I'm too intrigued to shut up. Lord help me, how I love a diagnostic challenge!

I took a look at the 300DC schematic. At my level of comprehension (always iffy), it's not obvious exactly how the peak lights are supposed to function. But, I think they might trigger in response to the level of current in the output transistors, and that could reflect a serious problem.

One thing I can see from the schemo is that the peak light circuit is not adjustable. So, either there's really something amiss elsewhere, and the peak circuit is reacting to it, or the peak circuit itself is faulty.

I think it would be a good idea to see whether the symptom occurs when the speakers are disconnected. If it does not, then I'd suggest checking the speaker wiring very carefully. If you don't spot anything wrong with the wiring, you might consider re-connecting the speakers with the left and right connections (at the amp) swapped. and checking whether the same channel's light flashes.

Do you hear anything amiss when the light flashes?

Cheers,

chazix

Thank you for your response. I really dread that it isn't anything major, I'd be real disappointed if it had to do with the output transistors

 

[petehall347]

do test it with no speakers connected .if it seems ok then swap your speakers and see if problem moves to other side . if so its some problem with the speakers..

 

[chazix]

After doing the no-speaker-connected test, and if the light still flashes during that test, there is another diagnostic step that might be DIY-able. The step I have in mind can't rule out very many possibilities, but it could pretty definitively determine whether the light driver circuit itself is faulty and giving a "false positive".

It looks like it might be practical to swap the connections for the two channels of the peak/meter board. I haven't been inside a 300DC, so I don't know how hard it is to access the appropriate circuit board (labeled PN00), but if it's accessible, the test would go something like this:

• Unplug the amp from AC. Probably best to leave the speakers disconnected, too.
• If the amp has been running recently, give it at least 5 minutes rest for the main filter caps to discharge.
• Open the amp and locate PN00, the circuit board that drives the peak lights, meters, and what I assume is the protection relay.
• On PN00, locate the two twin 9-pin connectors, JN01 and JN02.
• Carefully remove the plugs from the JN01 and JN02, noting which plug came from which connector.
• Re-connect the plugs, but swapped (i.e., the one that came out of JN01 goes into JN02).
• Power up the amp and see whether the same, or the opposite light flashes.

If the same light flashes, it means there is at least a minor fault in one of the power amplifiers, and the amp should really go for repairs before it's used any further. But if the opposite light flashes, it means that one of the light driver circuits is faulty, and very unlikely to do any harm if it's ignored. (Well, it might be hazardous to mental health...)

Cheers,

chazix