Fender M80 help needed

[Mooman]

Hi everybody - I need some help getting my old Fender M80 combo back in shape.

The main problem is this: I get intermittent output from the amp when powering up, but I'm not sure where the problem lies.

For example, I can power up the amp and everything is on and the channel select light works, but I don't hear that little 'thump' from the speaker, telling me it's receiving power. There is just no sound coming from the speaker.

Then if I plug into either of the input jacks and toggle the power switch on and off for a while, I may get lucky and hear the little 'thump' and all works fine. The speaker powers and I can use the amp as intended.

That little 'thump' of the speaker getting power I'm talking about should happen regardless of whether I'm plugged in to the inputs or not, but I can only seem to resolve it by using the toggle method above.

The odd thing is, when it's working the amp is great and sounds brand new. If I don't use if for a few days then try again, chances are it won't work and I have to use the power toggle / input toggle trick to get it to fire up again.

I enclose some pics of the circuit board for reference. I note that the diodes marked CR25 and CR26 have some scorching round them and the wire-wound resistors at positions R88 and R94 seem more chalky coloured than the others - so maybe a problem lies there? I don't know in which part of the amp these feature (preamp, power amp etc) so can't tell if there's an issue there.

Thanks to Enzo for sending me a schematic, but unfortunately I'm not that good at reading them so was hoping some kind soul out there might be able to help me out?

PS this is the export model as I'm in the UK and it's the stock M80 - no chorus. It's the grey face, red knob version in grey tolex not the one with the silver carpet covering!)

Cheers,
Jeremy

[Mark Black]

I'm going to bet on cracked solder connections for a couple reasons:

1. The toggling of the power switch is probably generating just enough of a surge to arc across some microscopic void in a solder connection.

2. I know from experience that all those old solid-state Fenders need a comprehensive solder retouch as a matter of course.

If it were on my bench I would remove the PCB (leaving as many of the wires connected as possible & marking those I had to disconnect). Then I would retouch (with added solder - not just a re-melt) every solder point at every PCB-mounted jack, switch, pot, LED, large and/or heat-producing components (including those resistors & diodes you mentioned as well as all heatsink-mounted parts), and any other point that looked suspicious or solder-starved. Be sure to hit the inrush limiter too if it has one. I also usually do all the QD terminals where the wiring attaches to the board.

This can only benefit the amp and will very likely cure your problem. Even if it doesn't you will at least know what it's NOT.

It is also possible the normalling contacts in an effects return jack might be dirty...

While the PCB is out is a good time to treat the pots with Caig MCL and switches & jacks with Caig DeOxit. Good time to clean the front panel too.

Best of luck!

[Mooman]

Thanks Mark, I'll give that a go and if that fails I may come back to the forum for more help! Cheers!!

[russix]

Mooman,

I'm having similar issues with a Fender amp(Power Chorus) somewhat related to the M80. It has intermmitent power/sound issues. It has similar scorch marked diodes as your pictures. Did resoldering contacts on the board help correct this problem?
If not, what helped you resolve this problem?

Thank you

[Enzo]

Those are five watt zeners and they naturally run hot. It is normal for the board to brown around them some. Doesn't mean you should not check the solder though.

I don't know what I sent you . DId you get the right drawing? The M80 and the M80 CHorus are not the same amps.

Before I go crazy soldering stuff, I like to isolate the problem.

First, do the headphones work? If so the problem is likely the phones jack itself.

Next connect a spare cord from the PREAMP OUT jack to the POWER AMP IN jack. If this restores the sound, then the power amp jack needs service or replacement.

In fact, when the amp cuts out, just plugging something in and out of the POWER AMP IN jack a couple times might wake it up. If so, that jack is suspect.

When it happens, connect the PREAMP OUT jack to another amp - is the preamp out signal still there? That tells us if the preamp is OK and if it is involved.

Likewise, plug a guitar or other signal source into the POWER AMP IN jack. That tests the power amp.

There is a large 10 watt cement resistor, 0.22 ohms, R88. Check the solder to it, and make sure its wires have not broken off at the board. Towards the center, it is the only 10 watter.

[badabing]

My son just gave me his amp to look at. Its a Fender M80. It used to work perfectly then one day he turned it on, the light comes on but there is very little sound coming out of the speaker. It doesn't matter which channel...its the same volume all the way up. Any suggestions out there? I checked all the simple stuff. IE: fuse, power cord, wires to speaker. I did take the unit apart and noticed the burnt area around the diodes as stated in a previous post. I also need an On/Off switch. One of the terminals came out when I removed one of the transformer wires. I need to know where to purchase one of those as well. If not, I'll just sell the amp for parts. Thanks for any help you guys can give!