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Fender 112 Deluxe Plus

Started by FenderDeluxe112Plus, April 12, 2022, 04:39:01 PM

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FenderDeluxe112Plus

Hello, this is my first time posting on the forum, I have been using related posts on here to help me so far.  Apologies if I have posted this in the wrong place.

I have a Fender 112 deluxe plus that was blowing the internal 2a fuse on start up. The amp was also making a very loud buzz with all volumes at 0.
I disconnected the speaker and checked the output transistors. I found a shorted pair of tip142/tip147 so I replaced all four transistors using new micas and heat compound.  I pulled and checked the heatsink diodes which all tested ok with multimeter. I also replaced the .47 emitter resistors for each output transistor.  I checked the diodes in the power supply and Qs 3-9.

The channel switching was also playing up - turning on the clean and overdrive channels at the same time. I replaced the u4 ic after finding it shorted with a new 4560D and powered the amp up with a light bulb limiter. This seemed to have solved that problem and the channel switching works again. The bulb was glowing very dimly with no sign of shorts  and fuse not blowing, so I powered off and reconnected the reverb pan - still all good. Then I powered off and reconnected the speaker - the amp is still making the same very loud buzz. I have gone through the entire board with a multimeter checking for continuity between each solder joint and all seems well. I reflowed the solder on a number of joints including the filter caps just to be sure. The 4 ohm speaker measures as 3.97ohms, so I assume ok. Could this be filter cap/power supply cap related or an issue with the transformer? Or am I missing something else e.g. switching issues in the jacks? I know these amps are not worth much but I would like to get it running again.
Any help would be greatly appreciated!

Thanks,

Andy


https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Fender-Deluxe-112-Plus-Schematic.pdf

Enzo

SO the amp works with no speaker?  At least not blowing fuses?  Measure for DC voltage on the output.  Or look at the speaker cone when it makes the loud hum.  Does the cone move one direction and stay there?  That is DC on a speaker.  It will damage a speaker so no more than a couple seconds please.

DC on the output with no load will not draw large excess current.  Put a speaker or load on it and it will.

FenderDeluxe112Plus

Thanks Enzo,
Yes, the amp powered on ok without the speaker (using the lightbulb limiter) and did not blow the fuse.

When I connected the speaker the noise was apparent but did not blow the fuse before I turned it off.  The lightbulb remained at a dim glow throughout.

I will check for dc on the output and let you know.

Enzo

OK, if there is no DC, then my next suspects are the main filter caps.  The two standing side by side near the heat sink.  I tend to think they are OK, but may have cracked their solder.

The main rails are 40v each,  are both up to that?  Or is one substantially lower, like 25 or something?  That would indicate large ripple and lack of filtration.  Or certainly you can meter or scope for ripple.

FenderDeluxe112Plus

Hi Enzo,
Here is the reading across the speaker leads.

Thanks,

AndyYou cannot view this attachment.

FenderDeluxe112Plus

Test point readings

Tp3    39.10vdc
Tp4   -39.10vdc

Tp5 16.35
Tp6 -16.42

FenderDeluxe112Plus


Enzo

If TP 3,4,5,6 are right - and they are - then TP1,2 pretty much had to be.

OK, so no DC on output and rails are right.   You might look for ripple on the supplies, but looks like that isn't the deal.

Plug the guitar into the POWER AMP IN jack and dial it to zero.  Hum still?

FenderDeluxe112Plus

The buzz is still present with all volumes on 0 and guitar plugged into the power amp in.

Enzo

I meant the volume on the guitar itself, we were bypassing all the preamp.  I will assume you did that.

That puts the hum in the power amp.  CAn you scope or otherwise measure the ripple on the 40v rails? 

Q7 is the mute JFET at the input of the power amp.  SHort across it, does that kill the hum?  Or remove Q7.  Same question.

FenderDeluxe112Plus

Hello, yes that was with all volumes at   zero - amp and guitar.

FenderDeluxe112Plus

Unfortunately I don't have a scope.

FenderDeluxe112Plus

Tp7You cannot view this attachment.

FenderDeluxe112Plus

Tp8You cannot view this attachment.

FenderDeluxe112Plus

When shorting q7, should I wire across joining the three legs of the fet?