VHM-314 Bluetooth + Battery Upgrade on Old Guitar

Thread Starter

THRobinson

Joined Jun 7, 2018
13
Hey guys... I have some "basic" skill in electronics, but I do like to tinker.

I have a mid-'80s TERMINATOR by Synsonics guitar. For those who never heard of it, it was a 3/4 size guitar with a built in amp and 3.5" speaker. All-in-One guitar and amp for kids to learn on. I got one that needs some restoration and figured for fun I'd upgrade it as well. Most is straight forward, with some new hardware, new potentiometers, switches, etc... but the amp is where I may need me some guidance.

My workshop is still being re-built so haven't had a good chance to fully dismantle the guitar yet, but looks like it has 2x9v battery powering a 1.6w 8ohm speaker (which I plan to upgrade). I believe the batteries are running in parallel so I guess 9v with 800-900mAh. Small 2-way toggle turns the amp on/off. It also has a 1/4" headphone output jack.

Ideally, I'd like to replace the 2x9v batteries with something like a Li-Ion pack, maybe 7.4v. I see that some have 2 plugs [1 for power out (JST) and 1 for recharging (XH-3P)] and thought I could get a small board for USB charging and a board that forces the output to 9v. I could go 11.1v but not having luck finding a USB-C charging board for it.

Then I saw those small VHM-314 Bluetooth boards, about 3cmx3cm... Could play backing tracks off the phone while playing along through the built-in speaker. 3.7-5v DC.

So... the Bluetooth board, how do I go to mono? Left speaker only? Do I combine the two into 1? Power wise, can I use the same Li-Ion battery and split it's output into 2 and have a 9v and a 3.7v converter board? or go from the battery to the 9v board, then split the output from the 9v board and run to the 3.7v board and the amp? Should I keep looking for an 11.1v USB charger board?

It's a needless upgrade, but fun learning project is how I look at it. Having a recharge USB port, longer battery life, and ability to play with a backing track would be a bonus.

Rest of the upgrades need to be done because 30yrs old and falling apart.

Terminator-Cropped.jpg
 

KeithWalker

Joined Jul 10, 2017
2,354
Anyone? Any advice on batteries? or how to hook up bluetooth?
The simplest solution is to use a single 6080 Lithium Ion cell with a boost converter to step up the voltage to 9V.

https://www.aliexpress.com/item/324...a16526208563304946e0963!10000006944672899!sea

and a 1A single cell USB charger module.

https://www.aliexpress.com/item/400...d16526211327943596e2d3b!10000015268548939!sea

Adding Bluetooth audio to mix with the existing guitar input to the amplifier would take quite some experimenting to get it to work well. I think it's probably a waste of time and effort as the instrument can already be played along with audible music from any source. A separate Bluetooth speaker playing music from the phone would be much more practical and versatile.
 

Thread Starter

THRobinson

Joined Jun 7, 2018
13
boosting 3.6v to 9v, what's the downside? I assume there must be one... almost triple the output must mean it'll last 1/3 as long?

Though looking at the 6080... also about 3x the price, but says 6800mAh... so even at 1/3 it's still much more than 2x9v 400mAh combined.

Though also looking I don't see a 6080 with 2 leads... I assume I need 2, a 3-wire lead for the charger and a 2-wire going to the step-up which goes to the amp?

https://batteryclerk.ca/products/br...NlY7oPTOBt_3_wJOVq2DEfpWtdqg5mIYaApkTEALw_wcB
 

Thread Starter

THRobinson

Joined Jun 7, 2018
13
Or not... just found this on Google... does it look right? minus the switch... amp has a power switch already.

If so, back to the first question... how does a step-up from 3.7v to 9v, affect battery life? Ideally I'd like the guitar to last longer than it did on 2x9v alkaline batteries.

Option B is to buy 2x9v rechargable batteries with the built in USB, and if enough space use some sorta Y-adapter with a panel mount USB-C on one end.

c943a70545bb1ddc48c358ea228873c48026f433_2_500x500.jpeg
 

MrChips

Joined Oct 2, 2009
26,133
Battery capacity is measured in Amps x hours = Ah or milliamps x hours = mAh

A typical 9V PP3 battery has a capacity rating of about of about 550mAh.
Let's do some off-the-cuff calculations.
Suppose you want to deliver about 1W of power @ 9V. This would require about 110mA. Theoretically the 9V battery will last for about 5 hours. When you take efficiency losses and falling battery voltage, the useful life of the 9V battery would be about 3 hours.

Instead of doing any voltage conversion you want to run the amplifier at the battery voltage.

A typical 18650 Lithium ion battery is rated at 2200mAh @3.7V. Then you have to find a way of safely charging it.
A better solution would be to use a compact USB power bank. These are available at 10,000mAh and up. They have the advantage of having the charging circuit already built it.

For optimum power efficiency in an audio amplifier you want to be looking at a Class D amplifier.
Since this is for a guitar you don't need two channels. A mono amplifier will be a better choice.

Just to give you an idea of what is available in a Class D mono amplifier, have a look at the PAM8302 IC.
It will run off 2.0-5.5VDC or straight off a 3V battery.

1652643687794.png

https://www.adafruit.com/product/2130
 

KeithWalker

Joined Jul 10, 2017
2,354
Was trying to find a 9v step-up, not a variable output one but rather one set to 9v and came across this... looks like it has a USB charger, battery holder and 9v output all in one? Probably too big though... when at home I need to get some better measurements... but yeah, 1 or 2 battery holding options.

https://www.ebay.ca/itm/383776099170?hash=item595ad68b62:g:NLAAAOSwt5hfkQOm&amdata=enc:AQAHAAAA4EPkAMhQBcZVegxzE9KJEf3fsBzYW5tVdt8v+tk6gW9yCwehBRsyxo8sCZhYDbVbjxh0W64rCvFpYlMfGPybphegyaLe32X4utalRHEs627ej3SICXAtXvHPgsUdypQnKE4+iNeJiYIRDQjifzadgpWr2qgRvZvcWnbscVwFTGPJm1khrqPfVu680SsNdqYx/Gmj2U6Xhv70z+omga9fFWg19Q0MJBsZNE6lw+pujEi7vpQbEMvQuj/7SCtebWAQnsWCbrQq07AjHGZQRVbl3GSPRvLr1ZSdae6LlB07KKrU|tkp:BFBM7MbCj5lg
That will do the job. If it will fit it will make the upgrade very simple.
MrChips solution may work but there is probably not enough signal output from the pick-up to drive a replacement amplifier directly without any additional matching and amplifying circuitry. I noticed that there are two controls and a pre-set potentiometer that work with the existing amplifier too. They would need to be connected somehow if the amplifier is replaced.
 

Thread Starter

THRobinson

Joined Jun 7, 2018
13
Oh this guitar has an amp built in already, with a 3" 1.6w 8ohm speaker, and from what I can tell 2x9v batteries in parallel.

It's from the mid-80s, and I plan to do an overhaul of it. It's in good shape but as with many cheap 30+yr old guitars some of the hardware needs replaced. I plan to upgrade this with good parts (not the best obviously) like tuners, tremolo, bridge and a speaker with more clarity/range (though shallow because only 35mm thick). Also twin rail humbuckers with splitting that are single coil sized.

Power wise, I'm looking at a limited amount of space. That's the issue.

I think... the simplest idea would probably be the best.

Buy a pair of 9v batteries with built in USB ports on the side/edge. Gotta find ones where the side of the USB is the side of the "-". Cut a slit in the existing battery cover big enough that the recharge cables fit through. Then when need to charge, use the "Y" splitter that comes with the batteries.

Sadly, not as cool as having a USB port on the front, but, I know the batteries will fit and designed to be charged with USB and 9v output.

Dumb question... on a 9v battery, which side is "-" supposed to be? Battery in front of me says "-" is the big side and "+" the small... but looking for rechargable batteries, I'm seeing both...?

...also seeing some 9v batteries saying 6000mAh... that seems a stretch.

51jl8qwvbGL._AC_SX425_.jpg51DD3xHOLLL._AC_SX425_.jpg
 

Thread Starter

THRobinson

Joined Jun 7, 2018
13
@KeithWalker I plan to keep the amp as-is... designed to fit the guitar and the vol/tone pots attach to it (though need replaced).

I do have a small foot pedal kit for distortion, that would probably fit, but no room for the extra knobs and not sure how to split the 9v power between it and the amp. Well... maybe... it's a 1 knob "FUZZ" with the on/off switch. Fairly small... wonder if instead of a pot, a 3-way switch would work. off-half-full....?

https://www.taydakits.com/instructions/one-knob-fuzz
 
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