Acoustic Model 116 Bass Combo DC on Output Help

[misterc57]

Hey Folks

Have a 116 that I need advise and help on. It had been thrown out by someone missing the 15" speaker. I suspect the speaker was good and removed and the owner had problems with the amp itself.

Before connecting a speaker I checked for DC on the output jack. At startup there is 22 VDC at the output which immediately begins to descend. After 30 seconds it fluctuates in the area of 10 to 35 mV DC.

I would expect to not see such high VDC at startup and closer to 0 mV DC at the output. Suggestions please!

I read 71.2 VDC at the large filter cap.

Also, I have yet to find a schematic for this model if anyone can help.

Looks easy to get to components. Here are some pics.

Thank you, MarkO

[g1]

It looks like an older unit so I would guess it has a cap to couple the output to the speaker. That would be why it shows some DC when turned on. If the cap is ok, it would not allow andy DC current flow through the speaker.

[g1]

Here is a 124/125 schematic that might help. There is a 115 power amp schematic on pg.4 that may be similar.

[Jazz P Bass]

You probably wouldn't see any Vdc with a load attached.

[misterc57]

Hooked up a dummy load and yes no move VDC. Reading under 1 mV DC offset. And yes the output to speaker jack is cap coupled (1000uF/50V).

I connected to my bench 12: speaker and all sounded good. The 115 schematic states "8 ohm min load" but the 116 back panel states 4 ohms at the output jack.

I think I have a 4 ohm 15" I can place in there.

Thank you all for the quick help!

MarkO

[misterc57]

I installed a Peavey Scorpion 15" 4 ohms (originally from a KB300) and notice rattling sounds on low notes.

I next tried a Eminence 15" 8 ohms 200W and have the same rattling sounds on low notes.

Rattling the entire first octave of a 4 string bass. The lower the note the worse the rattle.

Wondering if I have two bad speakers or if there could be an issue in the circuit. What circuit issues could cause this?

Thank you

[Delta362]

I'd suspect loose hardware as the cause of the rattle. I think it's a low resonance frequency of some sort causing that/those component(s) to rattle.

[TomCarlos]

Let's determine if this is speaker rattle vs something in the chassis. I would remove the chassis, put it on top of the cabinet, put a piece of foam or carpet under the chassis, and then retest. I would think the Peavey is the better speaker for testing as the KB300 was designed for low end. BUT... keep in mind, that speaker was designed more along the lines of full range as opposed to a bass speaker. I think the amp itself was rated for 75 watts. It should be able to handle a 4 ohm speaker.

[The Dude]

Do you have a known working speaker or cabinet you could try the amp with?

[misterc57]

The chassis is out of the cabinet, not touching the cabinet. The chassis has nothing loose in it.

The cabinet itself has no components inside it, aside from the speaker.

The speaker is attached by 4 clips. Only three of the clips are present. It is very secure but I wonder if this could cause a rattle?

I initially tested the chassis using my bench 12" speaker with a guitar, not a bass guitar and did not hear issues.

I have other bass combos but do not have any bass cabs to try out.

I do have one other old 15" speaker that may have come out of a Hammond Organ. I could try that one but would be concerned about power rating. Not sure of the speaker rating.

I believe this Acoustic Model 116 is rated at 75 watts into 4 ohms.

The KB300 was rated at 150 watts into 4 ohms and also had a tweeter.

I do not recall the origin of the Eminence 200 watt 8 ohm I have in there now.

I appreciate the continued comments.

MarkO

[g1]

The speaker is attached by 4 clips. Only three of the clips are present. It is very secure but I wonder if this could cause a rattle?

Yes it could be the reason. Especially as it is more pronounced with low notes.

[TomCarlos]

For the remaining four spaces on the speaker, use a wood screw (a good sized wood screw) to firmly attach the speaker to the cabinet. Try again!!

[misterc57]

I went back to the Scorpion. Total of 11 fasteners consisting of 8 self taping wood screws and 3 bolted brackets.

Issue is still there. I made a video at https://youtu.be/TbsbMPEuwkI

Hard to hear the issue on this video (more prevalent in person) but listen to the last note (a suspended low G) and there is "after" crackle while the note is suspended. Both speakers did this.

Thank you.

[Helmholtz]

Doesn't sound like a speaker problem.
Any noises from the amp when you give it a firm thump?
Do you have a scope?

[TomCarlos]

I believe the Acoustic 116 has a Preamp Output jack on the back. If you have another amp that has a Line In, see if you can send the signal to that amp and see if that buzz is present.

Update- The schematic shown is from the 114/115 amp. Focus n the Power Amp Out and Speaker.