Roland JC-120 HELP!

[zulu89]

Hello!

And first of all, thank you in advance for any guidance or help anyone can provide me with.

I recently picked up a dirt cheap 1984 Roland JC-120. Why was it dirt cheap? Because it didn't have any speakers!

I've installed some replacement speakers (Eminence Tonkers) as I couldn't find any direct replacements.

But I have a problem.

There's no indication of what the polarity of the wires is coming from the amp.

For the left speaker (perspective looking from the front) there's a red and black wire, and for the right speaker there's an orange and brown wire.

I've tried scouring the web for information but it's all a bit confusing, as it seems Roland changed the wire colours quite a bit.

My assumption is that the 'bright' colours (red and orange) are the positives and the 'dark' colours are negatives (black and brown).

I hooked it up like this. And on turning the amp on there was a rather loud popping noise. Which I assume is the current surge causing the speakers to move.

Just a little concerned that if I've wired one or both of the speakers in reverse polarity it'd do some damage, especially with the popping.

I thought I could possibly tell if the polarity was wrong simply by listening to the amp (it does sound sweet) but a friend rightly pointed out that if only one of the speakers was out of polarity it'd be hard to tell as the phase would cancel out due to one being right.

Anyways, if anyone could give me any guidance on what might be the right polarity of the wiring, or maybe a 100% test to work it out. I'd really appreciate it.

Can't wait to get gigging and recording with this amp.

[teemuk]

Should be as you wired it but there are many revisions of those amps with different wire colour schemes so I can't vouch on it.

A simple test would be to use a multimeter to measure DC resistance to ground from the wires. Negative wires should show zero (or close) ohms while the positive wires should show a far greater value.

[zulu89]

Should be as you wired it but there are many revisions of those amps with different wire colour schemes so I can't vouch on it.

A simple test would be to use a multimeter to measure DC resistance to ground from the wires. Negative wires should show zero (or close) ohms while the positive wires should show a far greater value.

Thanks for the quick reply, I'll give the multimeter test a go and see what I come up with.

Do you know if the popping when I turn on is normal? My other solid state amp does it, but nowhere near as loud. Bit concerned about blowing these brand new speakers.

[DrGonz78]

Well go one step further with that multimeter and clip the leads to the each end of the speaker terminals(speaker not connected). Check if there is an DC voltage present on the output stage of the amp and that is what you are most fearful of blowing speakers. That popping is typical in many if not all SS amps. However, sometimes it can be something else but very rare for it to be something other than power switch arching. Do a search on this site about that popping issue and it taught me a lot when I had this question.

[g1]

The only concern is whether the phasing (polarity) of the speakers is the same. If they are both reversed, that will be ok, as they are still in phase with each other. If they are out of phase, you will lose bass, and hear more mid-range.
Another way to check the phasing is to try reversing the leads to one of the speakers. If you lose bass response, then you know the speakers are now out of phase (incorrect). Make sure the chorus or vibrato are turned off when doing this test.

[zulu89]

The only concern is whether the phasing (polarity) of the speakers is the same. If they are both reversed, that will be ok, as they are still in phase with each other. If they are out of phase, you will lose bass, and hear more mid-range.
Another way to check the phasing is to try reversing the leads to one of the speakers. If you lose bass response, then you know the speakers are now out of phase (incorrect). Make sure the chorus or vibrato are turned off when doing this test.

I've just found a service manual that lists the polarity of the wires as:

Red - Left channel +
Black - Left channel -
Orange - Right channel +
Brown - Right Channel -

So I guess I assumed right and everything is ok

Just got to figure out a way to get rid of the 'turn on' pop.

I've heard of people installing little boxes with what I assume is a capacitor in, to delay the power going to the speakers, and therefore stopping the 'pop'. Is this something anyone is aware or or could provide me with a link to buy?

[Jazz P Bass]

As to the 'pop' at turn on, unplug the power cord from the wall outlet, turn the amp switch on & then plug the power cord in to the outlet.
If it then does not 'pop' it is a on/off switch issue.
Please advise & then we can take it from there.

[zulu89]

As to the 'pop' at turn on, unplug the power cord from the wall outlet, turn the amp switch on & then plug the power cord in to the outlet.
If it then does not 'pop' it is a on/off switch issue.
Please advise & then we can take it from there.

It still popped, but not as loud as it has done before.

[Jazz P Bass]

The 'pop' will not hurt anything.
If it is really annoying you could build a relay circuit to turn on the speaker after a specified delay.
Does the amp make any noises when it is powered down?

[imix500]

I've rebuilt a couple 60's and 120's and my experience has been replacing all electrolytics helps.

[zulu89]

The 'pop' will not hurt anything.
If it is really annoying you could build a relay circuit to turn on the speaker after a specified delay.
Does the amp make any noises when it is powered down?

There's no 'pop' when I power down, but you can hear the noise of the speakers relaxing, which I suppose is normal.

I would like to get rid of the pop ideally, does anyone know of somewhere else that sells these pop delay relays? David Navone doesn't ship to the UK it seems.

[teemuk]

The "pop" doesn't have to be a switch issue at all. There's always an AC component bled into the signal path from power supply rails when the amp gets powered on and filter caps charge from zero volts to the operating voltage. All this might be just a tiny "pop" output from preamp but its further upstream amplified by the power amp.

This can be counteracted with a muting or even some sort of speaker -disabling standby switch circuit but not many older amps bothered so the "pop" is kinda inherent to them. If it's extremely annoying turn down master volume when you power on and off and it might help a little bit. Most muting circuits often try to achieve a pretty much the same function anyway.

All in all, I wouldn't really worry too much of it. Some amps just do it.

[Jazz P Bass]

Here is a nifty little circuit compliments of Tomi Engdahl.

[zulu89]

Here is a nifty little circuit compliments of Tomi Engdahl.

Thank you very much for this!

And thank you for everyones help, it's greatly appreciated.