Modifying Fender Frontman Amp to be Battery Powered

[fenderman28]

I love the sound of this little amp. But I need to modify it to make it battery powered and don't want to achieve this with a 12V to 120 Volt inverter because of the efficiency loss with that method.
I have enclosed the schematic below. Can anyone tell me what type of modification to which points in this circuit would I need to do to achieve this modification. I don't have any specific battery voltage in mind but a 12 volt source would be perfect due to the variety of sources available.
Thank you in advance for anyone assisting me in tackling this problem.

Elizabeth

Schematic: http://electronicstudio.net/schemati...ematic_653.pdf

(the schematic has both the 220V Transformer (europe) and the "Domestic" 110 volt Transformer (USA) I have the 110 volt transformer)

[vintagekiki]

Only 12v to 110v (220v) sine wave inverter of appropriate power, factory or diy.

https://makingcircuits.com/blog/simple-sinewave-inverter-circuit/
1)
https://www.ebay.ca/sch/i.html?_nkw=pure+sine+wave+inverter+12v+220v
https://www.banggood.com/buy/pure-sine-wave-inverter-100-watt.html
2)
Or 2 pcs accumulator 24V because the Frontman is powered by ⁺/_- 27VDC (red color on the attachment)
When working with the accumulator, do not connect the Frontman to the mains voltage.

[dmeek]

Or 4 12V batteries, before the diodes for reverse-polarity protection.

[g1]

There are lots of different Frontman amps, which one do you mean?
How many batteries (and how big) do you want to use? How long do you need the charge to last?

[vintagekiki]

Schematic (FM25R) in post #1

[vintagekiki]

Or 4 12V batteries, before the diodes

If 24V batteries is before the diodes, the voltage drop across the diodes is 0.7V, so instead of 24V it will be 23V at best.
1)
When the batteries are before the diodes, secondary windings PT is a short circuit for batteries, so there is a probability that comes to damage.

[Dave H]

Or 4 12V batteries, before the diodes for reverse-polarity protection.

Will it work without the VAC voltage?
I was thinking it could be permanently muted.

[vintagekiki]

Will it work without the VAC voltage?
I was thinking it could be permanently muted.

This means that the mother board is disconnected from the PT (point P8-10).
And in that case, voltage drop across the rectifier diodes will be 2 x 0.7 VDC, so instead of ⁺_/- 24V it will be ⁺_/- 23V. (FM25R works at ⁺_/- 27 VDC).
Only the batteries will be protected from short-circuit, and the PT protect from damage.

[Dave H]

This means that the mother board is disconnected from the PT (point P8-10).
And in that case, voltage drop across the rectifier diodes will be 2 x 0.7 VDC, so instead of ⁺_/- 24V it will be ⁺_/- 23V. (FM25R works at ⁺_/- 27 VDC).
Only the batteries will be protected from short-circuit, and the PT protect from damage.

Sorry, I didn't explain it very well. I meant the VAC voltage connected to D20 over on the right of the schematic. Without VAC on D20 I don't think the circuit associated with it will work. VAC also goes to R46 and the footswitch circuit.

[J M Fahey]

Yes, 20VAC from the PT is needed both to unmute Q4 and make footswitch work.
Mute can be removed and Fsw modified, but both require "surgery".

Really shortest path is getting a sinewave inverter.

BEST efficiency would require a custom DC/DC converter, +12V to +/-24 (or 27)V to straight power amp rails, Fender Passport does something similar (I guess they create +/-35V or so) but you would still need to solve the Fsw and Mute problems.

[vintagekiki]

Without VAC on D20 I don't think the circuit associated with it will work. VAC also goes to R46 and the footswitch circuit.

You are right.
VAC on D20 serves to avoid transients during on / off FM25R. If transients do not interfere, remove Q4 and the problem is resolved.
https://www.manualslib.com/manual/1188574/Fender-Frontman-25r-Series-Ii.html
Footswitch will require minor cosmetic modifications.
When choosing a battery keep in mind that the FM25R power requirements is of the order of 75W