Mystic 478 Right / Left Issue

[Keith Gorham]

Having  some audio problems on this unit that I am attempting to get working again.

Slight hum that is controlled by the volume control until record starts playing, then I do not hear the hum.

One side sounds fine, the other side is high pitched. crackling, static  / noise in the audio that comes and goes rapidly.  Never has any bass.

I have done / found the following:

Speakers are fine by switching wires on the back. (no change)

Input to amp is fine by switching right and left channels. (no change)

swapping the two cards inside the amp. (no change)

Any ideas or tips to try to narrow down the problem area?

Amp is the 52280-A

[Ron Rich]

Yep-- If you don't have the skills/tools, necessary to trouble shoot this amp, send it to someone who does--NOW--the symptoms you describe indicate something is failing, and if "left alone", could cause catastrophic failure ! Turn the power OFF, and leave it off till it's fixed ! Ron Rich

[jukenorman]

Hi Keith,
Ron is 100% correct! But one thing I've found with these Rock-Ola amplifiers (and I have previously mentioned it) is that they are very prone to poor solder joints - especially the sockets on the printed circuit boards and the plugs that they mate to. So get the soldering iron out if you have one!
Norman.

[Keith Gorham]

Thanks Norman, I will check the the solder joints out.

Ron, Norman, any recommendations for who to send the amp to for repair?

[Ron Rich]

No sir-- Might have, if you had filled out your "profile", as you were requested too---
Ron Rich

[Keith Gorham]

Found the problem. One of the ICs on the preamp board is bad. Victory Glass has the ICs. The only other issue is the slight hum until the music starts. Do you think replacing the caps would solve that problem? I have heard of two places in the USA that work on them. Jukebox Friday Night and Rockola. I may reach out to them Monday about working on the amp. Any other ideas?


PS I hope I have filled out the profile as requested.

[Ron Rich]

Hi,
Cool-- glad you foundit ! Replacing caps is never, a "bad" idea-- looking for poor connections (cold solders/ mating pin problems) is not either !
Sure- See our FAQ's section-- repair person's lists are there !
Almost-- you just need to "sign your name"--
Ron Rich

[Keith Gorham]

Update, I the part I found on Victory Glass was the socket, not the IC. They do list the preamp card for $79. The chip is marked as 48934(Rock-ola part number) and RC7940. I cannot find any reference to this chip anywhere. If I correct to assume an custom IC by Rockola?

[Ron Rich]

Hi Keith,
Yep-- anything's "possible"- Is the chip a LSI ? Try contacting RockOla--- Ron Rich

[Keith Gorham]

From the pin out and specs on the diagram it appears to be an NE5532

[Keith Gorham]

Picked up this chip from Micro Center and both channels are working.

NTE Electronics NTE928M LM358 Low Power Dual Operational Amplifier

Seems like one channel is louder than the other but sounds great. Next step balance the two channels.

Reached out to JukeBox Friday Night and was told he does not work on the 52280 and 54510 amps anymore due to parts issues.

[Keith Gorham]

Found the source of the hum. Found three bad capacitors on the power supply inside the amplifier.

Question, Two of the capacitors rated for 40 V I had 25 V caps on hand. Same rating otherwise. I have the correct ones on order but wondering since the circuit shows only 15 V I am OK short term with these two in place for a week or two.

[jukenorman]

Hi Keith,
Glad you found your problem! If the working voltage is 15V, the the 25V ones would be fine!
Norman.

[Ron Rich]

Keith,
Are you reading what's in there, or the schematic / parts list ? i would be leery of the lesser rated caps, if the book calls for 40 volt caps-- Ron Rich

[Keith Gorham]

Parts list says 40v. Schematic shows they are on an 15 V circuit.

They will replaced soon.