Rock-ola 480 amplifier has no audio after recap and service

[jollyjon]

I have 2 Rockola  480 amps that had a terribly loud machine gun buzz. First I started testing every electrolytic cap and replace any with bad capacitance or ESR. Never repaired an amp before and thought that was all it was. After checking voltages I discovered one amp had odd voltages of + and the other with -. I found one had faulty mosfet transistor for driver board and the other amp just needed a resoldering of the driver board. I used both amps to try and help narrow down where the problem was located but it still took me an unbelievable 75 hours during 2 months. In hindsight I felt if I was a 10 year old kid delivering newspapers I could have made enough money to pay a technician and still had money left over. 
My voltage levels were all okay across the amp except for the mute board that had voltage levels of mute mode. I thought that may be the reason why I heard no music. My vinyl mech was removed and replaced with CD player when I bought it 20 years ago as it is. I found that when I disconnected  one of the wires to the mute board, the voltage returned to normal play levels. It but have been disconnected when I bought it and I must have reconnected it during the 75 hours I spent on it going over every little detail.
My last problem remaining (hopefully) is there is still no sound. Not sure what voltages levels I should be getting where signal goes in driver board (maybe 0.5V?) and not sure what point(s) to test at. What ever one amp has the other likely be the same. I think I may have did something to them because I did hear music thru one channel when I started to find what was causing the loud buzz in the beginning. Each jukebox had a bad channel. I doubt I damaged all 4 driver boards and likely did something that I shouldn't have - like when I reconnected the wire to the mute board, it would have been right if it still had the original vinyl mech and player.
Any ideas or tips on how to narrow down what the problem could be?  The CD inputs are wired into the volume control and bypass the pre-amp as there is no use for it like a record player needs it.

[Ron Rich]

Sir,
I commend you--- But, I think you are chasing rainbows-- Ainna no way I can even start to guess , what was done by someone else-- let alone by you in 75 hours of "working on it" for two months ! Ron Rich

[jukenorman]

One thing that you haven't mentioned above is the power transistors, maybe a good place to start?

Norman.

[jollyjon]

Hey Norman, I thought the MOSFET's were the power transistors and they were all tested. I removed every transistors on the driver boards and the heat sink section. Only one was faulty and that was replaced. I didn't do any modifications to it at all. I spent all the time going thru every part to ensure it was original like. When putting parts and cables back in after testing, I think I attached the mute wire to original position for a record player so I detached it to leave mute off for the CD player. I took heaps off photos before starting any work to ensure everything was put back like it was (but obviously I missed the mute wire). I'm just an amateur but I don't think it's 4 faulty drivers. I tested every transistor, capacitor (except 4 tiny ceramic ones for each driver), diode, resister and checked every trace and solder joint a driver board except the PEC-8. It may be a common part that both channels use - like a faulty volume control pot but I checked the resistance on mine in both amps and both are good.

I wish I knew what voltage levels for a CD signal I should be getting and exactly where to check so I could narrow down the location by following the signal path (a path I need insight to head down).

[Ron Rich]

How about reading the output of your CD player for voltages ? Most CD player DACs produce a "line level" voltage-- Ron Rich

[jollyjon]

mine has about 0.180VAC when music playing + or - 0.030V and about 15mVDC. Is that about normal? My levels aren't anywhere near those of a record player when I test a couple of spots from schematic. I'll try and go from the volume control where the cd line signal starts from and work my way thru til the driver output line that should be 12VAC and note them all done for reference. I hope it will be something simple before the driver board, maybe the tone control board in which both channels are effected by a common part or line.

Here's a link of the schematic for any that interested in taking a look.
drive.google.com/file/d/15ItcnK_Zkwvt2Lt0O377MOeIgRN2jAcW/view?usp=sharing

[jukenorman]

The original power transistors were 2N3055's and not FET's!

Norman.

[jollyjon]

Okay Norman, you made it clear to me the first time. I'm an amateur and saw a photo of a FET that matched my 2N3055 and wrongly made an assumption. It's not how it looks, it's what actually is inside that counts. I'm learning things everyday - just not fast enough.

[jollyjon]

I wonder if the 2 trimpots on each driver could have something to do with having no audio. According to the service manual, the center voltage for output line must be set to 0V (which it is spot on 0mV), and the other is for idle current and must be set to 3.3mV. Problem is my trimpot reading measures in the negative voltage, -1.5mV to -8mV. Not being able to increase it to the positive range, I just set it to -3.3mV.
What's the likely hood it could be a cause?

[jollyjon]

I don't think I did anything, but now I have music thru one channel. The other channel signal works all the way when I switch the left and right input cable to the driver boards. Could be a problem in the heat sink section of power transistors or the driver board or the cables. I will swap them around to narrow it down further tomorrow and post the out come.

[jollyjon]

I now have sound but the quality isn't as good as I was expecting. Now sure what a music is supposed to sound like thru a 40 odd year stereo system as opposed to something from this century. It seems my mini HiFi is almost as loud and it has 6inch speakers. The jukebox has 6 inches but is also has 12in so I was expecting it would pump out a lot more volume. It had been sitting in garage unused for 17 years and can't remember how it sounded back then.

[Ron Rich]

Are the speakers connected-- properly ? Are there any "hanging wires" from the speaker terminals ? Is the needle OK ?
Should have much more volume then you describe, or, my understanding, of what your saying !
Ron Rich

[jollyjon]

I had removed the speaker wires many times switching between speaker of jukebox and speakers from other jukebox and it makes no difference. There is no corrosion on any of the speaker terminal nor is there any hanging wires. Not sure what needle you are referring to. If it's what's used for playing a record, it's NA as I'm using CD input via RCA plugs wired to the volume pot (and old signal wire removed). It sounded really loud back in 2005 when I last heard it (it's not a fact - it's only what I recall. All my DC voltage are as they should be (around 32VDC + and -). With the output line I measure about 1VAC (+ or - .5V playing along with music) at max volume. Yeah, something's not right but what? All my speakers measure almost 8ohms from both jukeboxes and both are connected to the default terminal 5 for phono speakers only.

[Ron Rich]

Sorry- You have now gone "above my pay grade"--I have never worked on that model amp-- I did not realize you were feeding a CD in--
My GUESS, would be, since it raised in volume as the power was cut--AVC problems ?? Ron Rich

[jollyjon]

UPDATE: I thought the speakers in jukebox were all 8ohm. I measure left 6in 7.3 ohm, right 6in 7.8ohm, left 12in 5.9ohm and right 12in 5.7ohm. When measuring at speaker terminal on the audio distribution assembly, I get 0.7 ohms for each channel and when I disconnect there wires from the terminal I get 6.0 ohms for left and 5.9ohms for right. Is that normal? I gave all the speaker terminals and the terminals on the crossover board a good sanding with fine grit paper and retouch every solder joint on the crossover board. I measured the left 8uF 35V cap as 0.40 ESR and 7.9uF and the right with 0.46 ESR and 8.9uF. For a standard E cap I was expecting about 3ohm ESR for an 8uF cap and I thought an ESR of 0.4 was much better than expected (maybe too good), and unaware if non polar caps should have a different ESR than bipolar ones.

The audio signal from CD player measures about 150mVAC (+ or - 50mVAC while playing with music). The schematic shows a signal voltage of 0.17V when all controls at max level and the test record playing but mine has no record to compare with but I'm just assuming that since the CD signal I do have is about 150mV that it may be fine with signal strength.

I wonder if the idle current control has any effect to increase the volume greatly. The trimpot adjustment is supposed to be set at 3.3mV but mine only has a range of about -0.2mV to -4mV. I do not have a positive range so I set it at -3.3mV instead and wonder that since I can not get it in the positive range that maybe it's the causes of low volume. I turn it down to 0.2mV to get it as close to positive as I can while playing but it appears to not increase the volume at all. As little flaws can sometimes appear in manuals, I'm not certain if it's even suppose to be a positive reading.

When I switch of the jukebox while it's playing, the volume will rapidly increase to triple volume for about a tiny 1.5 seconds. I've only tried that at a low volume though so I don't know if it increases by a set amount or if it's a percentage increase. I don't know if (or how much) sanding the speaker terminals will make a difference with volume and sound quality but I'll have to wait until tomorrow to check as my housemates are now asleep.