Fender M-80

[pchilson]

Hi,

I made a bonehead mistake last evening and now need some help.
I was going in to replace the power led and accidently shorted across one leg of the led and a leg of the power switch. I don't know whether I can post the schematic here or not. The amp is the Fender M-80 1x12 combo non Chorus with the black felt carpet covering.

Anyways, a little diagnosing to maybe give a clue where to look.
This beast has an effects loop. Plugging guitar into the Power Amp In gives no sound. Plugging into the PreAmp Out into another amp gives no sound on the Normal channel but does produce sound on the Drive channel. Plugging into the Headphone jack produces no sound. All the time when on the amp produces a steady loud hum that is not affected by anything.
I will post some pics this evening of the inside showing the two points that shorted. If I can make a simple part replacement on this its worth it.

Any help appreciated.

Thanks

[52 Bill]

I made a bonehead mistake last evening and now need some help.
I was going in to replace the power led and accidently shorted across one leg of the led and a leg of the power switch.

I hate when that happens!

Start by reading the voltages of the power supply.

There will basically be two power supply sections. One will be the high voltage supply for the power amp, and the second will be the low voltage supply for the op-amps. Check your schematic and compare readings to what is listed.

As for simple part replacement, I'm gonna guess multiple active devices will need to be replaced. At least it still powers up without blowing the fuse.

[Enzo]

I know it is after the fact, but that is why we unplug stuff before we work in it.

You caused 120VAC to run into the low voltage circuits. Who knows what was damaged. COUld be a couple things, or could be a ton of things.

Bill is right. Make no assumptions, approach it as if you didn't know how it was damaged. It doesnpt work, and apparently more than one part has issues. SO start with power supply - there are +/- high volts and +/- low volts. Without them, nothing works, so get that right first.

I am betting you lost a low voltage rail... at least.

Inspect the underside of the board closely. Look for places where a copper trace might have burnt open like a fuse.

[pchilson]

Thanks guys. I will get a chance sometime today to get back in and see if I can find anything.

Funny part of this is, the power led has never come on since I have had the amp but after this incident I pulled it out and powered it with a 9v battery and it works just fine. This is the sole reason for me being inside the amp to begin with. Why would the led be operable but not operate as it should in an operable amp?

[Gtr_tech]

cracked solder.

[pchilson]

Here are some shots of the PCB.
There are no burnt traces on the underside of the PCB, all looks normal.
Again, when I power it on I get this semi loud steady hum.

This shows where I shorted from LD4 to leg 3 of the power switch.


This show a larger context, leg 3 of the power switch follows the white wire to TS1.


A closer shot of TS1. I don't know what TS1 is, I only see it on the layout and not on the schematic...

[Enzo]

TS-1 - Thermal Switch 1 or Thermal Sensor 1. It is a NC, open when hot, thermostat. Look at the wiring, it is in series with the AC mains lead. It is on the heat sink. If the heat sink gets hot enough, the TS opens and shuts off power to the amp.

You have touched together the AC mains and essentially Test POint 1 - TP1 - on the schematic. You sent 120VAC into CR22,24, and for that matter back through the transformer to the other two diodes. At that moment that AC got rectified and onto the filter caps C48, 49. PLus that higher voltage spike also would have gone on through R94,95 into zeners CR25,26 and their caps C50,51.

This voltage is a lot more than any of those parts want to see, so any or all of those caps could have been damaged. either or both zeners could have collapsed, and the places those power supply rails go - the power amp and the preamp circuits - could also have been damaged. Only way to know is to look.

SO once again, we suggest start at the start. it doesn;t blow fuses, that;s good. What voltage is on the high voltage rails? That is at TP3,4. WHat voltages are present at the low voltage rails? TP5,6.

[pchilson]

First column is the test point from the schematic
Second column is the value at the TP from the schematic
Third column in red is my reading at the test point

TP1	29.9VAC	      [COLOR="Red"]0[/COLOR]
TP2	29.9VAC	      [COLOR="red"]29.9VAC[/COLOR]
TP3	+40VDC	      [COLOR="red"]+42.6VDC[/COLOR]
TP4	-40VDC	      [COLOR="red"]-30.9VDC[/COLOR]
TP5	+16VDC	      [COLOR="red"]+16.91VDC[/COLOR]
TP6	-16VDC	      [COLOR="red"]-16.72VDC[/COLOR]
TP7	5mVAC	       [COLOR="red"]0[/COLOR]
TP8	19.8mVAC       [COLOR="red"]0[/COLOR]
TP9	110mVAC	       [COLOR="red"]0[/COLOR]
TP10	610mVAC	       [COLOR="red"]0[/COLOR]
TP11	4.40mVAC       [COLOR="red"]0[/COLOR]
TP12	106mVAC	        [COLOR="red"]0[/COLOR]
TP13	1.55mVAC       [COLOR="red"]0[/COLOR]
TP14	MIN 10mVAC    [COLOR="red"]0[/COLOR]
TP15	225mVAC	        [COLOR="red"]0[/COLOR]
TP16	+8VDC	       [COLOR="red"]+8.3VDC[/COLOR]
TP17	+3.4VDC	       [COLOR="red"]+3.51VDC[/COLOR]
TP18	-8VDC	       [COLOR="red"]-8.44VDC[/COLOR]
TP19	+25VDC         [COLOR="red"]+26.7VDC[/COLOR]
             2.45VAC        [COLOR="red"]57.8VAC[/COLOR]
TP20	-25VDC         [COLOR="red"]-28.6VDC[/COLOR]
             2.45VAC	       [COLOR="red"]0[/COLOR]
TP21	5.12VAC	       [COLOR="red"]0[/COLOR]
TP22	4.90VAC	       [COLOR="red"]0[/COLOR]

I should note there is a semi loud hum at the speaker that does quiet down after a time while I'm taking these readings but the speaker magnet gets quite hot. I hear no cracking or popping while probing the test points.

[pchilson]

At Test Point 1 which is CR24 I get a zero reading. Would that indicate that the power supply rail CP5 took the hit? I get non zero readings at the others CR21, CR22 and CR23.

[Enzo]

Disconnect the speaker. Measure the speaker wires for DC. Is there DC voltage there? Look at your speaker cone, when you turn the amp on, does the cone move one direction and stay there? DOn't leave it on more than a second. If there is DC there, work without a speaker until you get it back to normal.

That zero on TP1 is suspect. Either half the PT winding is open, or there is a lack of continuity to it, or the reading is faulty. And that lack would add hum, but not really that loud and wouldn;t heat the speaker.

If there is no DC on the speaker then the circuit might be oscillating above freqs you can hear causing the speaker to overheat.

[pchilson]

Disconnect the speaker. Measure the speaker wires for DC. Is there DC voltage there? Look at your speaker cone, when you turn the amp on, does the cone move one direction and stay there? DOn't leave it on more than a second. If there is DC there, work without a speaker until you get it back to normal.

That zero on TP1 is suspect. Either half the PT winding is open, or there is a lack of continuity to it, or the reading is faulty. And that lack would add hum, but not really that loud and wouldn;t heat the speaker.

If there is no DC on the speaker then the circuit might be oscillating above freqs you can hear causing the speaker to overheat.

Speaker moves out and stays there.
DC at the speaker wires measures 42.1VDC

TP1, I measured at the diode as the schematic shows and also at the spade connector from the power transformer for CP5. Always zero reading while the other diodes are non zero.

[pchilson]

If I need to replace the power transformer the original Fender part number is 027668.
Where would I go about trying to find one of these?
Are there other aftermarket power transformers that I could use?
I don't know if thats what I need but would like to get some sources if I do.

Thanks

[Enzo]

Power off, note where the wires go and pull the power tranny secondary wires off. Now measure continuity through the windings - open means bad.

0027668000 is still in my parts listings, listing for $35-40 (price is a couple years old at $35) Any Fender dealer or parts house could order it for you.

DC on the outputs needs to go. Check the power transistors on the heatsink. ANy shorted? I might worry about the IC driving them more, frankly.

[pchilson]

Power off, note where the wires go and pull the power tranny secondary wires off. Now measure continuity through the windings - open means bad.

0027668000 is still in my parts listings, listing for $35-40 (price is a couple years old at $35) Any Fender dealer or parts house could order it for you.

DC on the outputs needs to go. Check the power transistors on the heatsink. ANy shorted? I might worry about the IC driving them more, frankly.

Thanks for the help Enzo. I will do these things and report back when I can.
Would the "IC driving them" be the IC chip MC1436? How do I check the transistors for shorts?

thanks

[pchilson]

Power off, note where the wires go and pull the power tranny secondary wires off. Now measure continuity through the windings - open means bad.

I measure no continuity. I guess I have blown the power transformer?

DC on the outputs needs to go. Check the power transistors on the heatsink. ANy shorted? I might worry about the IC driving them more, frankly.

I don't know the procedure to check these.