Marantz PM66SE internal fuse blows repeatedly

[dymaxion]

Hi there,

I've had my lovely PM66SE for many years and suddenly the internal fuse leading into the transformer blow. (20mm glass TD 1.6A 250V)

I replaced it and it woudl work for a few hours then blow again... then blew 4 more in space of a few minutes... etc I've now gone through around 15 fuses and am tempted to leave it on permanently (once it's running it's happy for quite a while)

Symptoms:
NORMALLY when starting up, the amp charges up, then flips a relay and sound starts up.
NOW: The relay flips on and off intermittently (sound on/off), then after a few tries the fuse will blow.

However after replacing a few fuses (maybe when it gets warmer?) it settles down and doesn't blow up and works for ages (until I turn it off)

Any ideas what could be wrong with it?
My initial thoughts would be capacitors died and shorting out?
or Transformer is dead and needs fixing? [note I'm stabbing in the dark here!]

I'm happy to take my multimeter + soldering iron to it, but am not sure where to start. Any help appreciated.

I haven't a clue whether the problem is related to the one this person had?
Marantz PM66SE


Regards,
Spencer

[poobah]

Spencer,

Find the thread named "Relays in amp". There is another guy with a similar problem... the thread is active right now.

[dymaxion]

Thanks poobah, just had a read of that thread.. not sure if it's related....

I get the feeling that my relay clicking is more symptomatic of the root problem... but I guess still could be that my relay is duffed up. However given that the fuse blowing seems to be not directly related to the relay switching it seems more consequential of the nornal power up routine..

Anyway eagerly awaiting a response :-)

[anatech]

Hi dymaxion,
Was the original relay a fast blow, or a time delay (slo-blo) type? Did the other fuses blow with a large silver or black area or a little "poof" on the glass? The speaker relay is not at fault, it is possible it can be damaged.

Try to disconnect the speakers and turn the unit on. If the fuse does not blow, measure the DC offset at the output terminals.

-Chris

Poobah, I just tried an experiment. My dog can count just fine! - darn.

[dymaxion]

They original fuse was a Time-delay (20mm glass TD 1.6A 250V)
and the next 16 fuses i've blown have been the same as replacements. When the fuses blow, then tend to glow nice and red then go off with a flash of bright light leaving behind a nice black charred smudge on the glass :-)

I've currently been running the system stable now for about 16 hours....enjoying some music...

I suspect as soon as I start turning off or moving it around then it will die on me and I've only got another 4 fuses to play with!!! .... Ok did a bit of moving around... the relay did click off a few times, but took some readings...and it's still playing away happily now.

Not sure what sorta voltage you were expecting, but I got nothing across any of the speaker terminals... tried my multimeter on a 1.5V AA battery seemed fine. Unfortunately the maximum sensitivity setting on my meter is 2.5V Full scale deflection in 0.05V intervals.

Anything else to try?

So far I've found the following:
1) Relay clicking off and on doesn't necessarily blow the fuse
2) When the system is warm it is more stable and FAR less likely to blow a fuse.
3) Moving the amp around physically can cause the relay to click off, and can also lead to fuse blowing.
4) On a cold start tend to blow fuse after fuse.

Some or the above could just be a red herring! HMmm...

[mastertech]

look for solder joint fault in the relay circuit

[anatech]

Hi dymaxion,
mastertech is on the right track, except I'd be looking in the output circuit of one channel.

Discharge the supplies and get to the bottom of the circuit board. Look carefully at the solder connectiond for the bias control transistors and the outputs. The crack can be so fin you can't see it, so if you don't see anything, resolder those areas. Use a little electronic solder flux and solder - sparingly. Too much solder is a bad thing.

-Chris

[dymaxion]

Thanks

I had been reading up on diyAudio and thinking that the problem could be with the softstart circuit.

http://www.parttimeprojects.com/audi...rtwebpage.html

So I'll follow your suggestion and checkout all the soldering.
Wonder why it would have been damaged with age.... i guess could have been some small torsional forces on the joints causing cracks when I moved the amp around a bit recently.

Will see what I can find...

[anatech]

Hi dymaxion,
There have been many brands where the soldering of the heavier components on the board is defective. This is very true of parts that mount to both the board and heatsink.

-Chris

[mgf_doughboy]

Originally posted by mastertech
look for solder joint fault in the relay circuit

I've had the exact symptoms of the original post.

Today I removed relay LN01 and reinstalled it. Q751 was also loose so I redid that too.

It's been working perfectly since.(fingers crossed)