Fender M-80

[Enzo]

Each transistor has three legs. set your meter to ohms - resistance - and measuer resistance between the legs on each part. Are any shorted together? That means is there a low resistance between any two legs?

[pchilson]

There are 4 transistors on the board shown on schematic as Q3, Q4, Q5 and Q6

Q3 outer lug to outer lug = 95
Q4 outer lug to outer lug = 95

Q3 and Q4 no reading from either outer lug to center lug

Q5 outer lug to outer lug = .5
Q6 outer lug to outer lug = .5

Q5 and Q6 either outer lug to center lug = .5

Q3 and Q4 are TIP142
Q5 and Q6 are TIP147

Is this showing Q5 and Q6 as shorted?

[Enzo]

Yes. .5 ohm between all the legs won't be a good transistor. Note which type goes where and pull them all off the board and measure them out of circuit to be sure. 95 ohms between base and emitter is suspicious too. Since the parts are more or less in parallel, it is possible one is shorted and the other OK in the pair. And it is also possible the 95 ohms is not in the transistors on the other pair, but instead somewhere in the circuit.

[pchilson]

Does this suggest which components are failed?

"Plugging into the PreAmp Out into another amp gives no sound on the Normal channel but does produce sound on the Drive channel."

Can this be true with a dead rail on the power transformer?

I'm not smart enough to trace this out but it seems like it should suggest something to experienced eyes.

Thanks

[Enzo]

That indicates there is enough power supply for the preamp to function. Doesn;t tell us that power is OK though.

That will be a separate issue from the power amp.

[pchilson]

I am making an attempt to get the replacement power transformer, P/N 027668, ordered.

If there are other "blown" parts in the circuit, will putting in a new power transformer jepordize the transformer?

[Enzo]

This is where a variac shines. And lacking that, the traditional current limiting bulb trick we have discussed many times here.

As I recall, only one side of the transformer was open. That leaves the other half, so all the voltages should remain present, just the ripple will be more.

As long as the main rectifiers CR21-24 are not shorted, things should go fine. You can continue the troubleshooting with the half-working transformer. The +/-40 and +/-15 should all be there even with half the transformer out.

[pchilson]

Thanks for your help with this Enzo. I should know sometime tomorrow if the transformer is in fact still available from Fender. If so I will place the order.

[Enzo]

If it is not, there are similar parts from later models of similar power that should work. Any SS amp of a given output power will tend to have the same power transformer requirements.

[52 Bill]

If it is not, there are similar parts from later models of similar power that should work. Any SS amp of a given output power will tend to have the same power transformer requirements.

This is one of the models that has the transformer mounted through the back panel of the chassis. This will limit your replacement selection.

[pchilson]

I've been informed that the transformer is available for ordering. I will place the order tomorrow. I'll pickup on this again when I get the new transformer.

Thanks again guys.

[pchilson]

The transformer is now ordered. I hope to see it by end of next week.

[pchilson]

Today I received a Texas Instruments OPA445AP that is supposed to be a suitable replacement for U6 MC1436P1 in case that is non working condition as a result of the voltage going where it shouldn't...

Is the OPA445AP a suitable replacement for the MC1436P1?

I'm hoping the tranny shows tomorrow.

[Enzo]

COmpare the two. Looks reasonable.


OPA445 data sheet

http://focus.ti.com/lit/ds/symlink/opa445.pdf


MC1436 data sheet

[pchilson]

Received a package from Mouser today. I have changed out all the parts indicated below and then some. About $13 worth. I also put in a socket for the MC1436P1/OPA445 op amp so its simple to trade them out. Would there be any reason to think swapping these would change the character of the amp?

Still waiting on the power transformer to arrive. Hope to have this up and running again in the coming week.

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.