Wurlitzer 532 AMP hum

Hello all, I have a 1957 Wurlitzer 2104 that has the 532 AMP. The amp has been rebuilt and does work. The built in power supply was rebuilt also. 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. I finally figured out how to test it on the bench with only a single speaker hooked to the onboard speaker tabs. Still has the same bad hum controlled by the volume pot. So it has to be in the amp. Any suggestions of where I should look now. What could cause this. I will post a few pics of bottom of amp. If you need anymore information please let me know. Thanks

Hello oldguy3,

hard to tell from far away, you need to make some tests and measurements to isolate the problem.

If the hum strongly varies with the volume, the problem is located in the circuit before the volume control. That was the easy part.

If the hum is 100Hz, bad filtering of the anode voltages is likely the issue. Check the power supply components, even small ripple in the preamp or oscillator/demodulator circuit hurts.

If the hum is 50Hz, check for bad GND connections in the amp, shieldings, heating voltage circuitry or bad tubes. Hum induced by the filament cannot be easily detected with tube testers.

Looks like many parts have been replaced, how about the caps on the PCB? Was the amp ever working properly after it was reworked? If not, you should verify all component values, solder joints and cap polarities. Wrong polarity of foil caps can also cause hum. The outer foil acts as shield and should be connected to the node with the lower impedance.

If the amp was modified from cobra to ceramic cartridge, wrong component values or poor wiring might be a source of hum.

Rgds Jürgen

Ich kann das auf den Bildern nicht wirklich gut sehen aber es sieht so aus als ob man nur die sechs Kondensatoren der Siebung ersetzt hat .

Thanks for the replies. I will check that stuff out as soon as I can and report back. Dang bad cold has me down now. Another question on these Wurlitzer Jukeboxs. The connectors in this Jukebox have seen better days. A lot of them are suppose to have the guide in the middle of the connector to make sure they are put in correctly. All but maybe 2 of the connectors in this jukebox that piece of plastic that is suppose to be in the middle of the connector to guide it in correctly is broken off. Has anyone ever came up with a way to replace these guides with something so the connectors cannot be put in wrong. Thanks

These connectors can be bought new , google , 8pin round plug , they are in 5,6,8,9,11,12 pin available.

I got it tested. The hum is 52Hz. With Jürgen02 saying the following is there anyway short of replacing every tube to check to see if any of them are the source of the hum. Since he said Hum induced by the filament cannot be easily detected with tube tester?

"If the hum is 50Hz, check for bad GND connections in the amp, shieldings, heating voltage circuitry or bad tubes. Hum induced by the filament cannot be easily detected with tube testers."

I was told the amp was working good after the rebuild with no hum. I can not verify this because that is what he told me when I got it from him. I never seen it working. When I got the machine he had sent the amp off about a year before that to have it rebuilt. He had it where the Jukebox was in play mode and he hooked his phone up to the amp to play music. When I got it nothing on the jukebox mechanism would work. I got all of that pretty well going now and moved on to the amp where I am now. The unit does still have the cobra setup. I will have to check the caps on the PCB board. Thanks

Hello,
try to remove the first tube (12AU7) which is the oscillator/demodulator circuit for the Cobra system.
Any change to the hum? On which tube did you notice a change after replacing it? I am not sure what this amp is doing with open input, maybe some hum is normal when the oscillator is not running? Any comments from the experts here? You can try to connect the Cobra system and see if this affects the hum or experiment with the circuit I posted here a while ago. [forum.jukebox-world.de]
I saw in an earlier post that you are based in the US? Then I would expect to see 60Hz/120Hz hum, are you sure about the 52 Hz reading?
Rgds
Jürgen

I will try to remove the (12AU7) tube tomorrow and report back. The tube I have circled in red on the pic is the tube I changed. It might have made a little difference in the hum but very little. The hum is the same if there is something hooked to the input or nothing hooked to the input. I am pretty sure about the 52Hz reading. That is what the scope says anyway. Thanks

Maybe one of my earlier comments is misleading:

"If the hum is 100Hz, bad filtering of the anode voltages is likely the issue. Check the power supply components, even small ripple in the preamp or oscillator/demodulator circuit hurts."

This refers to the caps and resistors used for filtering the voltage.
A bad rectifier tube or transformer may also cause 50/60Hz hum.

Just one more option...

Rgds
Jürgen

Ok. I screwed up. Had the scope on the wrong setting. The hum is 120Hz. I did remove the 12AU7 tube and there was no change in hum. To answer an earlier question the PCB board has been recapped. I will put a pic of it up. So now with me finally getting you the correct information for the Hz of the hum. Where do you think I should start. So basically just go with what Jurgen02 said earlier about if it is 100Hz as a starting point? Thanks

Start to check if all anode voltages are correct and verify the components and the wiring of the rebuilt power supply. Note that the cobra system must be connected to get the 12AU7 oscillating.
What is the voltage of the hum signal at max. volume ?
Rgds
Jürgen

Jurgen, how would I go about getting this voltage of the hum signal at max volume? Would this be with nothing hooked to the input? What would be the best tool to use getting this voltage DMM or scope? I am new to Jukeboxes but really want to learn. More new to tube amps than anything. I guess I can take this reading at the speaker terminals on the amp just like I did with the Hz reading of the hum. Should there be a speaker hooked up when taking this voltage reading? Also you said the Cobra system has to be hooked up to get the 12AU7 oscillating. So does that mean the 12AU7 could not cause a hum unless the Cobra system was hooked up? Is my best bet to get the hum figured out on the bench with no input into the amp and then go from there? I really appreciate the help so far. Thanks

You can measure at the speaker with a scope (peak to peak) or DMM (AC/RMS voltage). It’s just to get a rough idea how much hum we are talking here to make sure we are not hunting a ghost. The speaker must be connected all the time, otherwise the amp. can be destroyed. I am not sure if the there is any influence to the hum by connecting the cobra, however with the cobra connected, you will get a more realistic situation at the bench test. To measure the anode voltages, the cobra must be connected as stated in the manual. When the cobra is not connected, the 2.5Mhz oscillator is not running so the anode voltage of the 12AU7 will be different from the value in the manual.
Since you are not experienced with tube amps, I would suggest you make the hum measurement first and discuss the result with the guy who made the modifications to the power supply. I am not confident that we can solve your problem here in the forum.
Best regards
Jürgen

Well I finally decided to send the amp off to get the hum repaired. The guy I sent it to said they had stuff all messed up. That it was not setup correct for Cobra or a newer cartridge. He said there were capacitors were they should not be. The wrong size caps and resistors. He said there was to much voltage getting into the amp and if used for a period of time would have burnt it up. Got it back and no hum at all on the bench. Now my issue is the Tonearm. I want to keep if as much original as I can. The wire is all messed up. I tried using some wire but when the tonearm is put into the system in the cabinet I just get popping and static. I get no sound if I touch the needle at all. When I had the Amp on the bench with the tonearm hooked to it there was no popping or static. I could also here when I touched the needle like I should. The only thing then was I would get a squel at almost the highest volume level. Would the difference be the amp and tonearm are not grounded when just setting on the work bench. My question is does what i described sound right with the wrong wire. Also does anyone make and sell the original type wire for these Cobra setups. I would like to get both wires. The one for the tonearm and the one to go from arch down to the amp. I don't even know if the tonearm wire was hooked up correctly when I got it. It looked like it had been messed with before. I will have to do some searching for that info. I have not been able to find anyone in the US that has that wire. Should I start a new post with this issue? Thanks

You need thin coax cables for the cobra system. [www.jukebox-world.de]
Regards Jürgen

Hello oldguy3

You can get the cobra cable here in the online shop.
The thin one from the pickup to the socket [www.jukebox-world.de]
The thick one from the socket to the amplifier [www.jukebox-world.de]

The shop will be active again from Monday.

Best regards - charly49

Thanks all, I will get that wire and cinch plugs ordered from here.