Greetings! Need some help with a resurrected Rock-Ola 440

[jukenorman]

Hi Tony,
I was going to suggest something similar to Ron but without any unsoldering. Unplug the magnetic pre-amp, hum still there? Short out your auxilliary lead (with no pre-amp), hum still there? I think the bottom part of your cut-out had the the upper handle on that side - it looks like it may have been extended for some reason.
Norman.

[gic440]

I'd tried running it without the pre-amp and yes, hum still there. Same if I used the other pre-amp. And I finally did remove (unsolder) my aux leads and yes, hum is still there.

Unplug all lights and animation motor, lift stylus from record during playback - hum still there. A steady low hum that doesn't increase with main volume (or maybe it does but only slightly from min to max, whereas music volume would be painful halfway through... so I consider that no change where the hum is concerned).

I really think it's coming from the amp transformer, somehow. If I plug the amp into a convenience outlet (instead of the relay-controlled outlet), I can hear a much fainter hum of the same frequency - granted, I'm focused on picking it up at that point, otherwise I'm sure it would be indiscernible from the cabinet's own standby hum. It sounds like this is being amplified once the mute relay is turned off for source playback.

I also just confirmed that I have good solid component / chassis ground continuity all the way to the mains plug.

[jukenorman]

Hi Tony,
I wonder if you (we?) are barking up the wrong tree here and what you are experiencing is not electrical hum but mechanical hum? Give that suspect transformer a solid whack on the steel core with a small hammer with the transformer powered up. Does that make any difference?
Norman.

[Ron Rich]

--and / or, or un-plug the cart, and see if it persists ?? Ron Rich

[gic440]

Norm: Ha, at least we're on the same page: while I had the amp out to remove the aux cable I checked the transformer mounts to be nice and tight. Never thought of smacking one before... alas, it made no difference.

Ron: Forgot to mention that specifically yesterday but yes, I've unplugged the cart. I've isolated every other source of noise/feedback I can think of and the steady hum remains.



So, having removed the typical/easy, I guess percolates two things to maybe consider if this is indeed internal to the amp: 1) The replacement values of power caps I got from Bill are 2200uF/63V. The originals on schematic are 1500uF. I understand power caps are one of the few times where increasing the uF can be OK but... 1500 to 2200 is a 50% increase, well beyond design/component tolerance. Is that too much, maybe? 2) At some point before my ownership, the input wires to the AVC board were connected wrong: the ground and R channel wires were swapped! Sadly, I stupidly did not realize this until after I'd recapped the amp and powered it on (and only when it still sounded like the same garbage, did I finally realize the wiring I'd assumed was left OEM-correct was, in fact, not! Sigh)... could those input wires being swapped, have damaged the caps (or something else)? For what it's worth, both channels sound identically clear so if something was damaged, it's not obvious.

[jukenorman]

Hi Tony,
The electrolytics being a bit higher in value wouldn't be an issue, tolerance on these would have been high in any case. An issue at the AVC can probably be ruled out as well, not least because if there was a problem there, you would expect it to vary with volume.

I was reading back through the thread and one thing struck an alarm bell. You said that you replaced the selenium bridge rectifier in the power supply, presumably that was with a silicon one? I think that in the amplifier it should be a silicon one anyway? But have you checked the DC voltages, are they OK?

Norman.

[gic440]

Hi Norm,

Hah, we're still on the same wavelength. When I read about the power rectifier issue that indeed made me check the amp's rectifier as well. I was pleased to discover it's putting out a steady +/- 53 volts, just slightly over the schematic's 52. I've adjusted the bias pots per the manual to read 26.4 volts, although I can't seem to get the mA dialed into the recommended range per room temperature - instead it's just slightly high.  

I did pull the amp and double-checked all this among other things again before posting. One thing I noticed that seems weird to me, is the input pins read at 3V when the amp is plugged in at the bench (and thus no input signals connected). Where would that voltage be coming from?


And for anyone else reading or keeping score at home, the title holders from the 425 showed up and are gorgeous and pristine, but to my crushing shock and dismay will NOT fit because they are almost 2" longer...!? Now, both holders are for 10 records / strips, so how could there be this kind of discrepancy?! Well upon forced closer inspection it turns out the "louvered" appearance of the 440-style holders isn't just for looks (to angle the strips slightly for viewing) as I assumed... but because the angled over/under cuts in the molding actually allow the edges of the strips to overlap, and thus condense. Whereas it turns out the "flat" holders like the 425 not only don't have an overlap, but also about 1/8" additional between each strip.  Sigh... what a stupid oversight on my part.

It's such a pity the louvered holders weren't made of the older style's better plastic that didn't yellow, because the injection slide mold that made the louvered part with its void over/under cuts must have been rather complex - and those same over/under cuts are beyond even ambitious 3D printing - so there's no hope they'll ever be remade.  I guess I have to somehow fabricate my own.

[jukenorman]

Hi Tony,
I can see that these title strip holders really do badly discolour. Might it be possible to illuminate them from above using LED ribbons, maybe at both sides of the title board - is there enough space? You could then just spray paint the plastic title strip holders!

I think we have exhausted the simple remedies to your amp hum - an oscilloscope would probably be required to pinpoint the source.

Norman.

[gic440]

I'm afraid since the title holders are pressed tightly flush against the glass, that framed illumination wouldn't be effective because it would be obstructed. Not a bad thought though: if LED strips could be discreetly hidden and not blocked, front-lighting repainted holders would cure the unavoidable "yellow when backlit" issue. Maybe it's possible to drop the title holder frame to recess them a bit below the glass... room to do so isn't the issue, but the holder frame also doubles as the seal bracket for the glass so doing this properly would at least require fabricating a new frame, or a removable add-on shim for the existing one. Hmmmm. Stuff to think about!


I do have a couple o-scopes: An old Heathkit analog scope that IIRC has a few issues at certain ranges, but otherwise works. And a cheapy handheld digital project scope that does some basic stuff. I've used both for troubleshooting pinball circuit boards and understand the basic concepts, but am not well versed enough to exactly know how/where/what to look for on my own just yet. But if you can point me to a guide relevant to these amps, I'm happy to start that adventure.

[jukenorman]

Hi Tony,
On the amplifier schematic, there are circled values at various points - these are the AC readings you should be getting for a specific input (see amp notes). With zero input (grounded), you should be seeing no AC signal at these circled points. With your oscilloscope, you should now be able to see the hum and where it originates. Is it 60Hz or 120Hz for example? The inputs and outputs of the driver boards might be a good place to start.
Norman.

[waxcupbeerbox]

www.ebay.com/itm/133682293776?ul_noapp=true

Tony! Someone just posted some clean-looking title strip holders on e-bay- should fit your 440. They don't look very yellowed.

[gic440]

Norm,

That makes general sense. In the past few days I pulled out the Heathkit scope and finally found the OEM calibration manual... I repaired a broken trace and ran the calibration so it seems to be working much better now! But as I've got project collision going on, it might be a while before I get back to this. I'll update once I do.


Waxcupbeerbox, Unfortunately, that listing is mine - trying to recoup what I paid for those since discovering they won't fit the 440. An expensive mistake. See my previous explainer here (or there). Appreciate the lookout tho...

[gic440]

Hi all,



Been quite a while but I thought I'd follow up as I finally have some big/meangingful news. But first a note to say that since I last posted, the 440 has been a big hit amongst family and friends, especially on those rare days we were able to host parties and such during the Covid era. On each occasion I saw my daughters' friends utterly fascinated with the format and mech - one of them basically accosted the machine during a party and was trying to suss how it all worked, which made my day. That's what this is all about!



But anyway. The big news is by some kind of divine miracle, or perhaps mere intuition/intervention, I secured a full like-new set of title holders! They even have the stamped indexing for all unique selections! So I can finally banish the backlit nicotine poopstains and show off this box the way it was MEANT to be seen! Well, "upgrades" and musical taste notwithstadning of course.





Been pleased that all my repairs and fixes have held up pretty well over the last year.   I appreciate the help and knowledge from this forum.

-TL