Rock-Ola 437 Ultra Amplifier issues

[ajmills522]

Hello,

A little background: I am somewhat new to working on amplifiers... I have worked on about 10-15 jukebox amps and have had pretty good success but I have a few that are really throwing me for a loop. I have a meter and have a pretty good understanding on how to use it but I do not have a scope or a signal generator.  I wouldn't know how to use them even if I did. 


Right now my biggest hurdle is the 43500-A amplifier in this Rock-Ola 437. The amp had new capacitors in it when I got it but everything else looked original. It had no bass what so ever and the right channel was there but very quiet.  Sounded like a ham radio through a tin can.  Terrible. 


What I have done so far:   

• Pulled the AVC board & Driver board so I could work them over.  I pulled the legs on all of the resistors and replaced about 1/2 of them on each board because they were 10% or more off.
  
• I pulled the transistors and statically (checked resistance and diode check with the meter) and they were all very similar. I didn't find any bad or shorted transistors. 
  
• Pulled the Power Output transistors out and they were all basically the exact same readings on the meter.  I put new heat transfer paste on and reinstalled them making sure the isolators where in place for the legs.
• One of the 4 Bias Pots were bad so I put a used 350 ohm pot in that worked
• Pulled the magnetic preamp boards out and checked all of the resistors. Everything checked out. Most with-in 1% of spec 
• The filter Caps are all 1500uf and new.  I removed the old canisters are completely from the circuit.  I used terminal lug strips vs how the pervious person did it by just using the old caps as the lugs. 
• Replaced the bad volume pot
• Reflowed ever single solder point in the entire amp including the tone board

Tried to set the bias and cant achieve the numbers that the sematic shows so I did the best I could.  I have each channel set at about 30 Miliamps and 23 volts.   The power at the bridge rectifier does go up or down depending on how the bias pots are adjusted but even before I set the bias I could not get up past 49 volts. Diagram shows I should be at 52v.

After all of that I put it back in the Juke and the sound improved drastically.  I have clean treble and solid low bass... 

HOWEVER... the right channel is supper quiet when the amp is set to Stereo.  Left channel is rocking.  If I switch it over to MONO... the left and right speakers sound even and the sound is good.  But the left channel sounds much better on stereo even with a basically dead right channel.

If I try to adjust the magnetic pots on the preamp the right channel is basically unchanged.  Left as you can guess you can hear it perk up quite about half a turn on the pot. 

I cleaned up the wiring on the back of the Needle Cartage... bent in the plug sockets on the amp side to make good contact with the tone arm plug.  If I rig up some wires and switch the left input to the right and vise versa while its playing a record... the sounds seems to be even and work on both sides. 

I went back in and replaced all of the transistors in the amp with the exception of the R&L Q3 on the AVC board (ran out), the power outputs, and the driver transistors.  All 4 in the Preamp boards where replaced.   NO change. 


Should I just bite the bullet and buy new outputs and driver transistors?  They all test the same from one side to the other (L&R). Resistance reading and diode test that is.  If so I have found a new old stock pair of drivers on Ebay for for about 15$.  I was told 2N3791 for the outputs on the heat sync and swap out R61 & R64 to 4.7 ohm resistors and re-set the Bias.  Thank you jukenorman ! 

I also cleaned the Mono and Stereo switch itself but no change.  Set to Mono sound comes out of both the R&L speakers. When switch it back to Stereo the left channel volume increases (without turning up the volume) and the right side is maybe 1/4 as bright or loud... easily noticeable. 

Sorry for the book... I just wanted to give as much info as I could.  THANK YOU in advance for any suggestions you may have.

[jukenorman]

If you have a good left channel, you should be able to cross the left channel to the right at the mono/stereo switch (or balance control) to check the driver and output section of the right channel. If this is comparable to the left channel, it would show that your problem is before the mono switch. You should be able to do a similar test at the output of the magnetic pre-amp. Basic, I know, but it's a start and it should help to narrow down the problem area at least.

Norman.

[ajmills522]

So r61 and r64 are 2.4 ohm 2W the ceramic resistors in the chassis of the 43500. I swapped out those 2.4 ohm resistors with 4.7 ohm resistors and I am not picking up a amperage reading at all now when I try to adjust the bias.

I'm considering just putting the old transistors back in it but I cut the ceramic resistors out and now cannot find 2.4 ceramic replacements. I can find 2.4 Metal Oxide Film Resistors or 2.2 ohm ceramic 5w but not 2.4 ohm ceramics. Not sure what to do?

From what I can tell it did not help my left channel so the issue must lie in the preamp area.

Man these things can be awfully frustrating for newbies.

[jukenorman]

Did you install the new transistors as detailed in the manual, ie did you fit insulation kits? Any of the resistors you mention should be fine - although I would tend to use a 2.7 if I didn't have a 2.4 but I doubt that would make much difference.

Norman.

[ajmills522]

Hello,


Quick update. I figured it out. I reverted back to the original set up with 2.7 ohm 5w resistors. I was able to adjust the bias again so I decided to call it good. Went over the whole amp again with my meter and I just cant find anything wrong with it. So I hooked it all back up and still had a dead right channel.

I checked the tone arm again, replaced the cartage with a back up that I had, and did the old wire pull and giggle and found that the Volume control cable would make the sound do wild things if I did it just right so I pulled all of the wires from the socket and resoldered them back in. I had already done this once but didn't fully remove the wires the first time. I reinstalled it and it didn't help the right channel but it did correct the issue I found by wiggling the cable.

Just out of curiosity I flipped the "Low Bass" switch on and off down on the volume control box down inside of the cabinet where the volume pot. Depending on what position I had it in... the right channel would come back or go away! I have done this a million times but I am guessing that cold solder in side the volume plug was keeping me from hearing any difference.

Not sure if its an issue with the switch or the circuit up in the amp but I like how it sounds with it set how it is. I am going to call it good at this point. I am curious as to what the issue is but I've spent so much time and effort getting it working and sounding good... I am ready to be done. It sounds great and everything works. Thanks for all of your help jukenorman