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February 24, 2024, 04:35:33 PM

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Need schematic for Dwarf Guitar amp by Twenty First Century

Started by guy with a soldering iron, February 16, 2023, 09:02:30 PM

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guy with a soldering iron

Working on repairing an old 1974 amp that "used to work." I've been noting and putting together a parts list. I've identified most of the components I think, but then I started reading the different ohm ratings of the resistors and got spooked I could choose wrong... The board has hand soldered traces, looks cool and I wanted to clone it.

I like that it holds and can run off 24-C batteries.


guy with a soldering iron

Should I choose .25, 2, or 3 watt ratings on the resistors when the color banding is correct? Also what is this part I don't know what to search for? it has 3 legs and labeled 2N3819 and I found a much different looking part on google.

Tassieviking

The 2N3819 is a JFET transistor, you are looking at an old style TO-9 one in your amp, the new ones look different in the TO-92 casing.
You can still buy the old style one if you shop around, but I would not.
New ones are more likely better quality.

https://electronic-studio.com/2N3819-JFET-N-Ch-25V-005A-035W_1

https://www.digikey.com/en/products/detail/nte-electronics-inc/2N3819/11647880
There are no stupid questions.
There are only stupid mistakes.

g1

Are you making a parts list for the clone, or for the repair?
The fault should be located and replaced before any general part replacement.

dmeek

I can't see all of it very well but here's a rough schematic

You cannot view this attachment.

Tassieviking

I think I spy a shorted track ?You cannot view this attachment.
There are no stupid questions.
There are only stupid mistakes.

guy with a soldering iron

Quote from: g1 on February 17, 2023, 02:51:21 PMAre you making a parts list for the clone, or for the repair?
The fault should be located and replaced before any general part replacement.

Agreed, It was in a non working state, I believe due to a broken connection to the  pot center of first picture. Also I shorted a diode while probing and plugged in <arg!> 1N4003 is the diode, but again not sure about the power ratings I need for this little amp?

guy with a soldering iron

BTW, the schematic you made is really helpful "dmeek!" It helps me get some of what is going on. I've never done a schematic before and I only am this far... Still wondering about watt and volt values for the disk caps labeled 104Z, with a single same size disk that is labeled .05uF at the input jack.

Loudthud

104Z is likely a 0.1uF cap. That's 10 followed by four zeros pico-Farads.

guy with a soldering iron

Thanks Loudthud, would I be correct in assuming those are for cleaning the signal? They all seem to near the inputs and outputs. And one is connected directly to the center lug of the pot.

Loudthud

"Cleaning the signal" is not something you're going to find in a textbook on electronic technology. They are likely used as general circuit components throughout the design.

Ceramic capacitors are low cost capacitors. When compared to the ideal capacitor, they are inferior at audio frequencies because they change capacitance with Voltage and temperature. Film type capacitors perform better in this area. Ceramic's good points are low cost, small size and very low impedance at high frequencies.

All these characteristics are taken into account by the Electronic Engineer who is designing the product and must weigh cost against performance of the product. Often, it is the Bean Counters in the back room that exert an overriding influence on design decisions.