Fender FM 65R slight noise at turn on

[DrGonz78]

So I am working on fully repairing this amp but ran into a bit of a snag. So the symptom is simply a bit of DC on the output in the first 2-3 seconds that it powers on. You can almost see it on the speaker as it pushing out just slightly. The amount of DC at power up is very small 800mvDC and I measure about 400mvAC where both die down to nearly 0mv after 2-3 seconds. After that the amp sits at 2mv DC and is passing clean signal. Also, to note this is not that pop noise that amps make and it really is a sizzle/hum that lasts for about 2-3 seconds as it first powers on and then the amp works perfectly.

It was basically repaired after I replaced Q11 and some open resistors. The noise is not coming from the pre amp and is definitely coming after preamp out jack. I have checked out the mute circuit but all of the components in that area are good. However, TP24 says it should be -15.5vDC but powering up the amp shows nearly 0vDC at TP24. Measuring on the other side of D38 show -16v. Not sure really if the mute circuit is to blame or what.

I have looked at AC ripple off filter caps but one the amp is up running it is 80-100mvAC ripple. All solder joints have been looked over and should not likely be the culprits.

Any help trouble shooting this would be greatly appreciated thanks.

http://music-electronics-forum.com/a..._schematic.pdf

[J M Fahey]

Not a problem so not repairable.

Very normal, dont know why it worries you, Iīm certain you must have repaired hundreds of SS amps by now.

It happens because to have 0V at the output, the input differential pair must detect the difference between input and output voltage and correct it.

Problem is the NFB circuit includes an electrolytic cap, which must be charged before it "puts itself out of the way" .

Also large filter caps must get charged from 0V , so until that they pull a large current, which creates large ripple.

All this is charged and working properly after a couple seconds, just what you see.

You *may* compensate for this, in fact modern audiophile amps are *fully* symmetrical to avoid that, just check DIY Audio projects or Bob Cordellīs book, but the Guitar World is far more simple or conventional.

And as Enzo often says, you are supposed to repair a non working amp, not to reengineer it.

[Jazz P Bass]

TP24 says it should be -15.5vDC but powering up the amp shows nearly 0vDC at TP24. Measuring on the other side of D38 show -16v. Not sure really if the mute circuit is to blame or what.
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TP 24 at zero volts indicates the 'Mute' function is On.

Insert a plug in either input jack or the power amp in jack & you should see the -15Vdc. (Un Mute)

[DrGonz78]

Well to me this turn on noise just does not sound healthy. Selling this amp off to a guitarist will be harder with this strange sound when you flip the on switch. Of course I monitored the AC ripple closely and yes there is large ripple when it firsts starts up. I even checked out the filter caps (ESR & Capacitance) and they seem to be just fine. So of course I do not wish to re-engineer the amp, but still this turn on noise does not sound normal at all. I will have to spend a bit more time on it just out of curiosity, but it might just be what it is.

Thanks Jazz I get it now in regard to the mute voltages.

[g1]

This may be completely unrelated, but several oddball problems with this series of amps (such as offset and hum issues) have been solved by replacing the differential pair.

[Enzo]

Over the years we had a few units come in for warranty repair, and we told the customer it had a defective serial number. We replaced the serial number and now it works.

[Jazz P Bass]

I have seen many bixarre turn on/ turn off symptoms caused by the main power rail capacitors being 'slightly' off.

If you have two new ones on hand, throw them in.

[DrGonz78]

Well I think Juan hit the head of the nail " Not a problem so not repairable." I changed out differential pair with a better matched set, no difference. Also, threw in some different filter caps, no difference. The noise seems a tad bit quieter though but it definitely still there when the amp wakes up stupidly. Oh well the thing works great after that 2 second sizzle/hum sound. Thanks for bearing with me on it and for the advice, much appreciated.

Edit: I am still looking to replace the serial number as to complete the job officially!!

[DrGonz78]

Well I replaced the serial number but it still bugged me that this thing made this start up noise. I know this sort of thing could be normal but I just had to crack it open and take one more swing. I elected to replace Q11 as my thought was that R83 had been stressed to the point that it went way up in resistance. While in there I got to thinking that most of the blown resistors were around Q9 (R76 = Burned up, R80 = open) and R81 was open with a shorted out Q10. So I buttoned it all back up and at turn on the DC offset rush only hit 200mv or so settling back to 2mv. Before the amp amp at turn on had about 1v dc offset in rush and it was causing the 2-3 second noise. Plugged in a speaker load and viola total silence at power up with a very tiny thump as the DC offset corrected towards zero. So it was either Q11 or Q9 that was culprit or both. I would probably lean towards Q9 as being the most likely to have been damaged in the amps original failure. Perhaps even Q9 had a hand in the amps original failure. Who knows?? Either way that horrible start up noise is gone!

[down1tone]

I’m having this same problem with a Fender Frontman 65r I picked up used for about 50 bucks. Loud hum for 2-3 seconds, can even see the speaker puff up, definitely doesn’t look/sound healthy. If I were gonna try to repair, where do you think I should start?

[DrGonz78]

Confirm the symptom is about 1vdc hitting the speaker for 2-3 seconds. Use voltmeter to confirm this with speaker removed. Open it up and check those fusible resistors R80-83.

[Mr.Migoo]

Hi had the same problem solved by changing 10V Zener D17. It was dead.

[Mars Amp Repair]

I have also had issues like this but with power down. Traced to the +15 and +15V supplies decaying at different rates. This was on a couple of Ampeg bass combos. One was a BA115A.
It has a mute ckt for power up, but it doesn't handle power down. On a few I added a timing network to slow down the action of the mute ckt so it mutes thru the decay of those supplies & another I added a FET Triggered off of the 2ndary AC to handle the power down.
Now I understand we don't typically 'reengineer these things, but in this case I was able to quickly fix the issue for anyone who was complaining about the issue...AND BTW, charged accordingly .
Haven't had the issue with this amp tho.

[Mr.Migoo]

to complete my upper post: To solve the problem I finally changed all fuse resistors (R80-81-82-83) around Q10 and Q11 and the 2 transistors. Only one of the resistor had a bad value when testing, and both of the transistors looked ok with a voltmetre diode test.