Rock-ola 440: bass suddenly "dropped out" and now missing

[gic440]

Hi all,

Been enjoying the 440 quite a bit and trouble-free for the past many months, but had a weird problem appear.  In the midst of playback (from my hard-wired aux input) the bass / low frequencies suddenly dropped out.   I rebuilt/recapped the amp in 2020 and it's been fine up until Sunday.... here is the status since then:

- Volume works, smooth static-free change across full range, with nice clarity and no distortion. 
- All four speakers are working fine.   There is just no more "oomph" from them, especially the 12's.
- Does not make a difference if I use the aux-in I added, or original phono inputs.  
- The bass level control makes little-to-no discernible difference in tone.  Treble works as expected... if anything treble seems exaggerated now that bass is gone. 
- If I turn the volume to max you can finally feel a little oomph in the speakers... but let's be real, it should be (and was) almost painfully loud long before you'd ever get to "max".   Now it's more like... a loud radio. 

I do have a another volume control from a parts amp, and swapped it for testing... it's hard to be certain but it seems like it has slightly more "oomph" than the other, though nowhere near the painful wall-rattling output as before.  But it does seem slightly different, and again came from a parts amp so now my question is: could the PEC (capacitor bundle) in this volume control affect bass output that much *if* in fact those caps have finally popped out of spec after all this time?   I didn't replace them when I rebuilt the amp because that sealed PEC bundle was a mystery.   But studying the schematic, and looking at pics of remote volume controls online (because for some reason THOSE used the discreet components), I think I understand what to replace IF in fact that's the problem.  But, I don't want to waste the effort on that, if something else makes better sense.

As it stands, everything sounds clean and crisp and there is some bass there, it just doesn't "thump", so I think the amp is still mostly good overall, I just don't know what the bass-specific parts would be.    As always, any guidance would be appreciated!

-Tony

[jukenorman]

Hi Tony,
All the lower bass comes from the 12" speakers - what about the plug/socket going to the door speakers, solidly connected?

Norman.

[gic440]

Hi Norman,

Yep, the lower speaker plug is nice and tight. The lower speakers work fine; if I unplug the upper speakers they sounding just about identical in tone and volume, just no bass.

-Tony

[jukenorman]

I don't know, seems strange that the problem affects both channels? You could try wiring directly from the amp to door speakers (even just to one?) to prove whether the fault is in the amplifier or crossover etc.?

Norman.

[gic440]

Yeah, that's a key clue: both channels are affected. So it's something common to both.



Which gets to one of the reasons I was suspecting the volume control... I'd googled and there was a thread somewhere else about a Seeburg or AMI with the same issue and it was a volume control.   So looking at the schematic for the R-O 440, I see this:




Notice how it says "Low Volume Bass".   I'm not sure what that means ("bass at low audible volumes" or "bass in low quantity", but why would bass of any sort exclusively be filtered through the volume control?  (Note: the switch shown in the circuit just above the label "low vol bass", is actually the cancel switch.   Which... I am not even sure how it works because there doesn't appear to be any wires connected to that switch, but it DOES cancel playback when pressed!)

You'll note all the resistors and capacitors shown in that circuit.  Turns out, on the control built into the amp, they are sealed in that module apparently now-forgotten-once-known-as a "PEC".   If you look at a remote volume module, they are all discreet. 




So anyway, today I did an experiment: I took my spare/parts volume control, and disconnected the PEC to see what if any change there would be:



When I use this as shown, there is very little change... actually, if anything it seems a little louder maybe.  But it definitely did not affect the bass any worse!  At any rate it seems the presence / absence of this PEC module isn't affecting anything much, suggesting they may in fact now be defective.   But are they meant to impact the bass in the first place?  Hmmm....


But to your suggestion Norman, how would you suggest I connect the speakers to bypass the crossovers, tone board, etc...?

[jukeboxmarty]

This may seem too simple but....

Are you sure everything is in phase?

[gic440]

I'm pretty sure they're in phase. Put another way: if they're OUT, they've been that way so long I never noticed, ha!

But in all seriousness: after playing for an hour or more, the bass literally evaporated halfway through a song. I was in another room while it was playing and then it seemed as if it got quiet, but before I could investigate my family started making noise and then distracted me for the next two hours. By time I got back to the juke it was still playing but I could tell it didn't sound quite right.

So, nothing changed as far as speaker / output connections are concerned. I reseated them and such of course but that hasn't helped. If they're presently out of phase then they always were even when they sounded great.

[jukenorman]

Hi Tony,
I don't have much documentation on your amplifier, just a poor incomplete pdf so I'm afraid I cannot be specific as to how you would wire a speaker direct. But the speaker distribution unit and the crossovers are external to the amp. so isn't there a plug/socket on the amp. that carries the outputs? Run a couple of wires from there to one of 12" speakers. You would only need to check one channel.

Norman.

[gic440]

Thanks, Norman. That does make sense... I'll give it a shot (since I don't think I have the right caps on hand for the volume PEC anyway) but it might be some time before I suss out the connections and make jumper wires, etc.

[gic440]

I think I fixed - or at least, identified the problem!

The schematic snippet I posted above, indicates the Tone Control Board could affect things too, right?  I mean part of its job literally concerns bass! Now I was pretty sure I rebuilt everything, but I went back to my parts amp and took another closer look.  Oops, Oh wait, that's right: the TCB is somewhat tricky to remove since there isn't much slack in the lead wires, which have these weird snap-in-and-solder lugs that are kind of a PITA to deal with... and since none of its caps were electrolytic, appearing instead to be tantalums which conventional wisdom says do not fail... Long story short: because of the above factors, when I rebuilt the amp a couple years ago, I left the Tone Control Board alone.

But now that I have a problem pointing in this direction, it's finally time to deal with removing it, because at least I have an extra on hand in that parts amp.




I removed the TCB from the parts amp, and swapped it into the good amp for a test.  As soon as the stylus touched the record, I *immediately* knew the BASS BOOM was back! Yay!

So it seems I should finally rebuild the TCB (and probably the Volume control PEC for good measure) as well. While the caps on these parts may not be electrolytic, I guess they don't last forever either.    But I've got to round up the parts list and order them first of course.  I'll follow up once that's done.

For what it's worth - which is more and more, apparently - seems like for a newbie, having a parts amp is invaluable.   I only bought it because the 440's (unique?) photocell AVC module was missing in my original amp and unobtanium everywhere...  so when a "spare untested amp" was listed on ebay that still had the AVC photocell module, I had to buy it, even though I wasn't keen on the cost of buying (and shipping) an entire amp for one stupid tiny piece.  But ever since, as a test/ reference/ practice/ parts source, it has paid for itself over and over, so that might have been a forced move of the universe imparting wisdom