Question about Amplifiers & Preamps
Ok fellow techs...I have an Marantz MA 700 U monoblock on the bench. Everything works great, but the customer complains about a clicking noise from inside the amp.
On the bench it will occasionally click...and then 30 seconds or so later, click again. I found that the front end section of the amp runs SUPER HOT, to the point where the board was baked. I watched the voltage points on the front end and they were pretty unstable, so I just shotgunned the whole front end ( it was baked that bad ) tested at 10 - 50 watts RMS continuous for around 2 hours and sent it out.
It came back a week and a half later, now it clicks occasionally. After much digging, I found that the clicking was actually.....hold on to your hats...when the output reaches 35 volts pk, a peak detector circuit clicks the relay over to the high voltage section of the power xformer primary slamming the rails up to + - 75 volts to achieve 200 watts RMS, then after 30 seconds or so....CLICK !... back down to 40 volts. for low power operation. Has anyone ever services one of these POS's before ?
I have eleven MA700's. Of the eleven, three produce random clicking and popping noises at the speaker output with no input. Clearly this is a very common failure mode for the MA700.
I have opened one of the "poppers" up on the bench, and with the buffer amplifier signal cable to the power amplifier disconnected, the popping noise remains, so it's definitely not in the audio path on the buffer amplifier board. The buffer amp is connected to the power amp via an electrolytic, so there's no problem unplugging the buffer from the power amp.
I do see somewhat random engagement of the high power rail mode for the amp, but this is *not* co-incident with the clicking coming from the speaker. Also, just as a side comment, if the amp is putting 23Vrms of audio on the speaker output as designed before that relay "clicks", you'd probably not hear it happen -- the speaker is already seeing several watts, and that is going to be louder than the relay. Also, when the relay switch back, it does so very quietly.
When the high/low power state switches randomly, the relay clicking is audible at very low volumes, but this is obviously not the design intent. Erratic high/low power switching problems are going to be nearby QU05, very likely the electrolytic CU08, the diodes DU01 and DU02, or QU05 itself. I'd suspect CU08 first.
The main amp PCB does show evidence of strong heating towards the front of the chassis, where the driver section is located. Unfortunately, it looks to me that the mechanical assembly is complex here; taking it apart looks to be quite a big deal and I don't actually need the amp, so I'm just going to close up. However, at least this exonerates the buffer amp and gives you a pointer as to how to troubleshoot high/low rail power issues.
If anyone locates a specific component that is responsible for the clicking and popping, I would very much appreciate hearing about it. Likewise, if circumstances lead me there, I'll post what I've learned.
fyngyrz comfyngyrz gmail com(insert @ and . as obviously needed)
Posted on Mar 14, 2013
I have a cople of MA-700 which randomly make a click sound; not the actual unit but on the speaker output.
Have found the service manual for the MA700's on the Internet and gladfuly will share it.
Wonder if you finally solve the issue for your client or if want to go deeper on the troubleshooting.
Posted on Jun 06, 2010
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