Schematic for Fender Sidekick Keyboard 60?

[Prairie Dawg]

This one's got a couple of burned resistors. Does anyone have a schematic handy?

Thanks all.

[Enzo]

Here

[Prairie Dawg]

Thank you. Much appreciated and very useful.

[mitch66]

My friend has the same 2 burned 100 ohm resistors...
Did you simply replace them and all was well? or was there some other fault which would cause replaced resistors to also burn ??

[Enzo]

I don't know that we determined which resistors he had burnt, did we?

Resistors are passive, they never burn up on their own. SOmethng else has to draw too much current through them.

[mitch66]

I don't know that we determined which resistors he had burnt, did we?

Resistors are passive, they never burn up on their own. Something else has to draw too much current through them.

Yes... I was thinking exactly the same thing... something else caused them to burn.
When the OP mentioned 2 burnt resistors... I was thinking it might be a known defect
with a known solution. In my case it was R85, R86. they are fried Black.

Has anybody else seen this before with R85 R86 ?

[J M Fahey]

R85/86 are not 100 ohms.

[Jazz P Bass]

Defect, schmeefect.

R85 & R86 are the source resistors (750 ohm, 1/2 watt) for the +15 & -15 Vdc rails.
Something downstream of both resistors wanted more than 1/2 watt of power.
So the first item is to isolate the rails from the rest of the circuit.
Are the zener diodes o/k? (D21 & 22)
Are the capacitors o/k? (C51 & 52)
The resistor/ zener combination is a 'poor man's' voltage regulator.
It is kind of strange that both the + & - went at the same time.
Again, it appears that something downstream (a failed opamp?) took out your + - 15 volt supply.

[mitch66]

Defect, schmeefect.

R85 & R86 are the source resistors (750 ohm, 1/2 watt) for the +15 & -15 Vdc rails.
Something downstream of both resistors wanted more than 1/2 watt of power.
So the first item is to isolate the rails from the rest of the circuit.
Are the zener diodes o/k? (D21 & 22)
Are the capacitors o/k? (C51 & 52)
The resistor/ zener combination is a 'poor man's' voltage regulator.
It is kind of strange that both the + & - went at the same time.
Again, it appears that something downstream (a failed opamp?) took out your + - 15 volt supply.

BTW the amp is still working even with blackened resistors but just a little farty/noisy.

Yes... I see that now on the schematic... somebody has been in there before and swapped in Brown-black-brown {100 Ohms} in place of R85 & R86 which then, over time, Blackened up. Now I'm wondering about OP-AMP abuse & How much this 100ohms value would have boosted up the rails supply ? hopefully not beyond +-18Volts ... {Max V for the M5218 }.

[J M Fahey]

750 ohm resistors there dissipate 500mW; so they are rated *just* what's needed, no margin for extra dissipation ... but they would probably have worked a *lot* od time.
BUT 100 ohm resistors will dissipate 3.24W and toast in no time.
It's a miracle they are still alive.

[Artly There]

Is this schematic still available? I try to download the one posted by Enzo, I only get a 'clear.gif' and no download. I had one of these amps donated to me, and it's fried in several places.

1) The bridge rectifier is melted to the point I can't make out the writing that used to be there.
2) The 2 resistors mentioned above R85, R86, are semi-scorched.
3) The two 0.5ohm 5 watt resistors have heat markings at the solder points, one is cracked. The lead of one of those 5 watt ceramics goes to one of the power transistors...is it likely that is what caused the power drain, a shorted transistor?

Those are the only points I see obvious heat damage.

Not sure what rectifier to replace with...or how to test the other components down-river. OK to swap out the Op amps with 1458's or some similar variety? The original Motorola 5218's seems to be kind of hard to get (if I need those too) Thanks for any help (Fender Sidekick 60 Keyboard Amp, SKK60)

[Jazz P Bass]

Fender Sidekick Keyboard 60 Schematic

Whew.
All of that damage, maybe you should 'donate it' back.
Anyway, here is the reposted schematic.

[Artly There]

Whew.
All of that damage, maybe you should 'donate it' back.
Anyway, here is the reposted schematic.

Thanks so much for that schematic, big help. As to re-gifting it, I've thought as much. Might end up that way still, but it's in nice shape except for the electrics so maybe worth a few bucks to try.

Not sure what the specs for the Rectifier are still...40v? DB-40C it says, outputting 33v looks like, but I google on that number DB-40C, don't get any hits. Must be ancient.

Thanks again for the schematic repost.

Robert

[J M Fahey]

it's in nice shape except for the electrics

He's so healthy, pity he's dead

Not sure what the specs for the Rectifier are still...40v? DB-40C it says,

I bet any 4A 200V bridge rectifier will fit.

The amplifier had a meltdown beyond normal because *clearly* somebody "repaired" it with an unburnable fuse, such as a turd of cigarette paper, a 20A one pulled from a car or a nail.
The point here is: how's the PCB?
If it's a carbonized mess, lifted tracks, a burnt hole in the middle, forget it (or turn it into an extension cabinet or mionitor).

If it's fine, all other parts can be replaced.

Build a lamp bulb limiter so you don't re-blow it many times and at least replace power and driver transistors and the emitter ballast resistors.

Forget the preamp ICs unless having a *good* reason to suspect them (so far you don't).

Test without speakers or any load until you start measuring reasonable rail voltages and 0V at the output.

Good luck.

[Artly There]

He's so healthy, pity he's dead


I bet any 4A 200V bridge rectifier will fit.

The point here is: how's the PCB?

If it's fine, all other parts can be replaced.

Build a lamp bulb limiter so you don't re-blow it many times and at least replace power and driver transistors and the emitter ballast resistors.

Test without speakers or any load until you start measuring reasonable rail voltages and 0V at the output.

Good luck.

Thanks greatly for the tips.

Here's a pic of the PCB...not too bad, nothing lifted. The resistors aren't all that charred, just their mounting points for the 0.5ohm one in particular.

The rectifier is melted though.