Fender Sidekick 10 Humming

[patlaw]

I have a 1983 Fender Sidekick 10, which has to be the simplest amp in existence. It has a single 3300uF/25V snap-in filter cap. The amp is humming (120Hz) with the volume and master pots down. The ESR on the filter cap is .27 ohms, and the value is 3100uF. The amp works fine but for the hum. I don't have a schematic, but I'm not sure how much one would help. The only other time I've had a similar problem that wasn't the cap was a leaking diode in the full-wave bridge rectifier.

What else should I look for? Should I shotgun the 36 year old cap anyway?

[g1]

Is the solder good on that cap? Are there low voltage rails that might have bad caps?

[patlaw]

Is the solder good on that cap? Are there low voltage rails that might have bad caps?

It was soldered well. I took it out. It's hard to tell on the LV rails without a schematic. There aren't a lot of electrolytic caps in this thing.

[Enzo]

It is already out? Then sub some other cap into the amp and see if it makes the difference. I wouldn't waste time trying to make a head puzzle out of the cap specs. A 4700 would be fine if you lack 3300s.

[patlaw]

It is already out? Then sub some other cap into the amp and see if it makes the difference. I wouldn't waste time trying to make a head puzzle out of the cap specs. A 4700 would be fine if you lack 3300s.

Certainly an option. Have you experienced a cap that measures value and ESR correctly but doesn't work in the circuit? The cap is a very low profile snap in, so I'll probably just order one and troubleshoot from there.

There's something in a TO-220 package that's bolted to the chassis and the heatsink. What is it? Something to do with temperature monitoring? Darn I wish I had a schematic. This thing is electronics 101.

[52 Bill]

There's something in a TO-220 package that's bolted to the chassis and the heatsink. What is it? Something to do with temperature monitoring? Darn I wish I had a schematic. This thing is electronics 101.

That is probably the output chip, I don't think that there is anything else mounted to the heatsink.

[Enzo]

5-legs? That would be your power amp IC.


Have I seen a part that seems to measure OK on a meter but doesn't perform in a circuit? I am sure I have. My earlier point was I can take any cap remotely close from my drawer and tack it in there just to find out if I am on the right trail. If it seems to cure the hum, then I was right, and I can either solder it in permanently or order a more suitable part. If it does not make th difference, I can then move on to other ideas. If I just guess and order the part, it may be right, or I may just be delaying the troubleshooting process by a week or two.

I rarely worry about the capacitance value of a cap, that usually isn't helpful. ESR is more useful. But all those meter tests are done at very low voltages, not real world circuit voltages, so such tests are of limited value.

[Enzo]

And I just checked on the Fender web site, they do list the Sidekick 10 as in their archived schematic collection, and all you have to do is call them and they will send it to you.

[patlaw]

That is probably the output chip, I don't think that there is anything else mounted to the heatsink.

The output chip is certainly mounted to the heat sink, but so is this TO220 device. I'm not at the shop or I'd get the number off it it.

[J M Fahey]

Post a closeup picture, specially showing the TO220 thingie bolted to heatsink .
Old + cheap + single supply + 10W makes me suspect it´s a TDA2003/LM383 or similar.
But suspicion is not enough, so, ask for the schematic, post closeups, *read* what is written on chip.

[Enzo]

SIngle ended 10 watt, I might even suspect a TDA2030, but it will be a simple amp akin to the Squire strat pack type. one maybe two op amp ICs and a power amp.

[patlaw]

The TO220 device has D1190 printed on it. It could be a 2SD1190 or something else. It's mounted to a tiny PCB with wires coming from it. It is mounted to the screw that's holding the heatsink to the chassis. I have no idea what it is.

Here's the power module. It's an HA1388.

[Enzo]

Check the voltages around it. I might suspect it is used as a voltage regulator for the preamp.

[J M Fahey]

There's something in a TO-220 package that's bolted to the chassis and the heatsink.

which causes the logic answers:

That is probably the output chip, I don't think that there is anything else mounted to the heatsink.
.
5-legs? That would be your power amp IC.
.
The output chip is certainly mounted to the heat sink, but so is this TO220 device.
.
Post a closeup picture, specially showing the TO220 thingie bolted to heatsink .
.
I might even suspect a TDA2030

then a picture is posted with NO TO220 THINGIE ANYWHERE ON THE HEAT SINK.
And we have a **12** pin IC which was never ever mentioned before.

Proper troubleshooting data requires proper input.

...........

1) replace that huge missing cap which I *guess* is the main filter cap

2) post a somewhat further away picture showing the full PCB, component side.
I want to know whether we have an output capacitor connecting speaker to chipamp or it´s being used BTL (read the datasheet) with no caps involved.
They have different troubleshooting procedures.

3) to save some time, follow speaker wires and tracks to see how it connects to output IC.
Options are:
a) one leg to ground, the other to some IC pin (12 or 7) through a capacitor, I would expect 1000 to 2200uF
b) both speaker wires to IC , one to 7, other to 12, no big electrolytics involved.

NOTE: one of those connections will most probably go through a headphone jack, and plugging a headphone breaks the circuit.

4) again to save time,measure voltage to ground at IC pins 10, 12, and 7 .

FWIW datashet suggested layout is strikingly similar to what is shown on PCB, specially the large cap relative to IC body.



I would not be surprised at all that "Fender" (actually the OEM supplier) used basically datasheet suggestions, ... after all that´s exactly what Leo did in his time

[patlaw]

Juan, I am more than happy to provide pics when I get back to the shop. These are just what I had available. I don't have to right cap to install, so I'll get one, install it, put the amp back together, and take pictures. I may even have a schematic by then, if Fender provides one. Thanks!