Wurlitzer 2104 AMP 532 hum

[youngjuke]

Hello all, I have a 1957 Wurlitzer 2104 that has the 532 AMP. The amp has been rebuilt and does work. The problem is there is a pretty bad hum. If the volume is all the way down there is no hum. If you start turning the volume up the hum starts and gets louder as the volume goes up. There is nothing hooked to the input of the amp. The florescent lights are not plugged in. I have tested all of the tubes and one of the small tubes did test bad. I replaced that tube and it is a little better but still a bad hum. Any suggestions of where I should look now. What could cause this. If you need anymore information please let me know. Thanks

[Ron Rich]

What does "rebuilt" mean to you? When was this done ? Who done it (pro, or someone with a "re-cap kit") ? Normally if it was something that "just happened" I would suspect a tube (BTW--a tube tester will not, in most cases say a tube that causes hum is bad !)--Next a bad cap-- I fi just worked on, suspect a bad connection--especially one to ground ! Ron Rich

[youngjuke]

The AMP was sent off to a professional about a year ago and all parts where replaced but not all tubes. The guy I got it from had the record changer mechanism in play mode. So he could turn the Jukebox on and hook his phone to the input Jack on the amp and play music through the Jukebox. He said there was no hum or very little at that time. I started working on the unit to try to get the complete Jukebox working. After getting the record changer working I hooked the amp and everything back up and the hum was there. Now I never heard the amp in operation before this point. So I can not say with all certainty that the hum was not there before. So I am in the situation where I just have to try to figure it out. So I was hoping to get some help from the experts to try to help trouble shoot it. I guess there's no way to do anything with the tubes besides buy an extra set to be able to trouble shoot by swapping out one at a time. For the hum loudness to be controlled by the volume pot is there more suspect tubes than others? Also the hum is bad enough it is not something anyone would want to just live with. It is not just like a little faint hum. Thanks Ken

[Ron Rich]

Hi Ken,
If you un-plug the cartridge from the AMP and un-plug all of the plugs going to the amp, do you still have the hum? Does this amp use an onboard, or off board, power supply ? If "off", did he also re-cap that power supply ? Ron Rich

[youngjuke]

Hi Ron, The results I have talked about so far is with nothing hooked to the input of the amp. I have the input wire unplugged. This amp has an onboard power supply so it has been re-capped. So are you talking about unplugging everything from the amp except the speaker plug and then see if there is a hum? Thanks Ken

[Ron Rich]

Hi Ken,
Sure--try that !

[youngjuke]

I tried doing that test but I really cant. I have the speaker plug in. I have the muting plug and the input plug unplugged. I cant unplug the chassis plug that goes to the record changer because if I do there is no power to the amp. Do you know is there a way to jumper the amp on the bench so it would have power? From the test above With only the speakers and the chassis plug plugged in it still has the same hum. I need a way to test the amp without the chassis hooked to it I guess. I am thinking the hum could possibly come from the amp or possibly the chassis. Could the hum be coming from the chassis? Thanks

[Ron Rich]

Hi Ken,
Gotz the schematic ?? Jumper connections are shown on all of them---
Ron Rich

[youngjuke]

Yeah I have the complete manual for the Jukebox this amp came out of. I did not realize they had jumper connections listed. I am not where the schematics are right now. Are they on the actual amp schematics? Thanks

[Ron Rich]

Hi Ken,
Yep----usually-- RTFM (LOL) !
Ron Rich

[youngjuke]

Hi Ron, My first true jukebox repair. I have worked on pinball machines, arcade, slot machines from EM to the latest models. Yeah I have read the manual from front to back. A lot of it 2 or 3 times. I have fixed a lot of stuff on this jukebox already to get to this point. Nothing worked on this unit except the turntable constantly turned when I got it and supposedly the amp worked good. Both lift arms where broke loose. I had to take the whole record changer unit apart grind down where the arms where pressed to the brackets. Then I had to line them up correctly and weld them. Then I had to do all kind of testing to realize the motor on/off switch and the record load switch where both bad. I got them replaced and then the motor got power. The thing is the motor did not work right so I had to rebuild the motor basically clean it out new bearings ect. Brushes where good. Now it works good. Bunch of other stuff I done to get to the amp. So I guess out of the 86 pages in the manual I missed the part of the jumper connections being on the schematics. Plus all of the stuff I have read online. Now if this was a 69 Camaro SS I had up on my rack I would not need no stinking manual. (LOL)! I will check the schematics out again and see if it shows them for sure. Also if I understand the jumper setup. Thanks

[e093116]

I had the same problem, it turned out to be the jumper plug for the remote volume control.

The plug would measure fine across the pins, but for whatever reason, when it was installed, it caused a hum.  I ended up removing the plug and soldering jumpers on the back of the socket inside the amp.

You might try touching/cleaning/removing/installing/rocking the plug and see if the hum changes.  If I just touched or tapped the plug the hum would change, which told me this is where the problem was.  

[Ron Rich]

Hi Guyz,
Yep-- a common problem--however I am not a fan of soldering any jack-- I would first treat it with Deoxit-5-- Same problem can occur with the plug for the speaker,especially in tube type amps where the "B+" is jumped by the plug. Replace the socket if this happens there--do not solder the socket here !

[youngjuke]

I found a post on the internet that showed what the jumpers are for this amp. Even after having that information I had to study the schematics to finally figure out how the schematics show the jumpers. I was looking for it to be in text form that would tell you. I read back through the manual and did not find anything that told how the jumper connections are shown. Unless I just missed something. I was looking over the schematics again and seen that there is a line drawn between the pins on the junction box connector for the jumpers. I still would not know that those lines where the jumpers if I had not found the post online telling the jumpers for this amp. Ok I got the amp fired up on the work bench with a single speaker hooked to the GND and 8 OHM speaker connection. The hum is exactly the same as is was in the cabinet. Any suggestions of what I should check first on the amp to try to figure this out? I did check the remote volume plug as Bill suggested.
There is actually not a plug. There is just two pieces of bare wire installed. I moved them and hit on the socket with no change at all in the hum. Thanks

[Ron Rich]

Hi Ken,
Gottem a "cap checker"--O'scope ? I would first suspect power supply caps-- how "old" are they ?
Ron Rich