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Marantz Pm 493 amp issue - power cycles on/off - repeat

Homer4beer

Homer4beer

Well-Known Member
Hi guys, I picked up a Marantz PM493 system for free out of the classifieds today. The owner upgraded and was simply giving it away for free along with the matching CD, Tuner and double tape deck along with the original manuals and remote.

When I arrived (after about a 55min drive) he tells me it works fine however occasionally turns itself off then after about 10 secs seconds it turns itself back on again. Then so on and so on.

Sure enough he powers it up and it works fine for several seconds (I hear sound through both speakers) then does as he describes. So after driving all that way and finding it in excellent condition other than the fault I thought "for free I'll take it anyway and have a crack at fixing it."

I did enjoy fixing my 1060 amp so I gathered this can add to my learning experience :)

Any ideas??

I found the service manual on Hifiengine

https://www.hifiengine.com/manual_library/marantz/pm493.shtml

qAYENH.jpg


RX3A2e.jpg



Thanks guys! :beerchug:
Homer
 
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Is it actually the power cycling on/off or just the protection relay clicks on/off ?
From the schematic it looks like the power is supplied just through a normal mechanical switch, not operated with relays and such.
 
Power seems to be going on and off. The amp goes completely out as does the tape deck and tuner which is powered through ribbon cable on the back.

Then it comes back on.
 
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petehall347 said:
monitor dc offsets .see if one or both rise prior to it clicking off
Click to expand...

Checked.

Left 14mV, Right 17mV

Funny thing is since I've run the amp on it's own without the tape deck and tuner connected by the ribbon cable it has not turned itself off and on. Perhaps one of the other components is faulty which is why the amp cycles on and off?
 
Getting back to this unit now I have some more time I've found some other odd things. The amp does switch itself on and off by itself without any other components plugged into it by the supplied ribbon cables (the tuner and tape deck). I've tried the CD input and have the same issue although nowhere near as frequent. I also have found that both channel do not work all the time and I believe it may be a faulty relay. The two resistors R709 (Right channel) and R710 (Left Channel) have signal coming through them using an oscilloscope even when I do not have any sound coming out of the headphones. Sometimes the left channel is not heard but I hear the right channel, sometimes the opposite. There doesn't appear to be a pattern as to when I actually hear the signal or not, although once I do hear the signal on both channels it seems to stay.

Testing the signal with an oscilloscope at the J702 ribbon cable junction after the relay seems to support the suspicion the fault is in the relay. When I don't hear sound I have no signal, when I do hear sound there is signal. When the power cuts out there is still the correct voltage coming out of both bridge rectifiers so I do not believe it is a mains issue.

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Hi Homer4beer, long time.

The intermittent power off/on could be a bad solder or wire connection
probably in the power supply area. You've probably lost power to IC's
QU01,,,QG01 which provides power to the volume LEDs etc...
Next time it powers down, measure voltages at connector JS07, hopefully
there is a little tab on the connectors, check all eg, +/-6V, 12, 15, 33,,,
Might be a dying voltage regulator...

The intermittent dropped channel is almost certainly the relay. The
contacts become pitted. Using the cro, if you see audio into the relay
but no audio out of speakers then replace the relay. Some people try and
clean the contacts but it's a waste of time, end up doing more damage.

I picked up a PM493 also, was surprised at how good it sounded that I
tinkered a little, some audio caps, updated the preamp opamp. For what it
is it did sound better than most in same class. Worth fixing.

Volts.JPG
 
You've put a big smile on my face with your post MBZ :biggrin:

I really appreciate your input. I'll have a look tomorrow at what you suggest. Yea it's a really good little amp when it's working.

Cheers. :beerchug:
 
The checklist is,

+5.9V...JS07 pin 4
-6V.....JS07 pin 5
+12V....JS07 pin 11
+14.5V..JS07 pin 12
+32.8V..JS07 pin 10

-14.8V...D805 cathode

The regulators are chained together, so if you find one out of
spec it may be caused by a upstream component, so you should test
all voltages.
 
mbz said:
The checklist is,

+5.9V...JS07 pin 4
-6V.....JS07 pin 5
+12V....JS07 pin 11
+14.5V..JS07 pin 12
+32.8V..JS07 pin 10

-14.8V...D805 cathode

The regulators are chained together, so if you find one out of
spec it may be caused by a upstream component, so you should test
all voltages.
Click to expand...

+5.9V...JS07 pin 4 = +4.2V then -0.7V when it powers off
-6V.....JS07 pin 5 = -6V and still -6V when it powers off
+12V....JS07 pin 11 = +11V powered off +0.5V
+14.5V..JS07 pin 12 = +13V and into mV range when off
+32.8V..JS07 pin 10 = +32V then down to +18V when it powers off

-14.8V...D805 cathode = -14.9V regardless of off/on

D804 14V when on then nothing when off

D806 +30V constant power
R802 +40V constant power
R801 +4.2V then -0.7V when it powers off

Q804 pin 1 14V then drops to nothing when it shuts itself off
 
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First up, I'm not real confident that the schematic is OK, appears t be some errors.
Two things look odd,
Q801 or Q802 is suspect, their output feeds the +12V and +5.9V supply, both down.
Q809 looks suspect, input and reference are ok.

Suggest check for dry solder joints around Q801,2,9, Happy that you reflow.

Maybe someone can chime in with possible replacements,
Q801...NJM7815...MC7815C??
Q802...2SD1666...TIP31A??
Q809...NJM78L18A...NJM7818FA??

Check for dry solder joints first. Maybe a pic if any doubt.
 
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