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 Post subject: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 5:21 am 
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Location: Garland, Texas
Hello,
I am helping a friend with a Wurlitzer Model 42 jukebox and I am having an issue with the model 851 amp. Was told that up until recently it had been working and the amp had been professionally rebuilt. In looking at it, it does appear that most resistors and all caps have been neatly replaced. The big issue is no DC. I am only seeing 5-6 volts AC for the filaments. I am suspect of the 5Z3 rectifier even though it tested well on my TV-11. The reason I suspect it is a] no DC voltage to the entire chassis and b) the voltages on the 5Z3 are: p2 560 volts AC, p1 416 volts AC, Filament 1 4.6 volts AC and 0 volts on the other filament.
I wish I had another 5Z3 or an 83 to try. Am I missing anything else it could be?
Thanks,
Rudy


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 5:50 am 
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are you bench testing the amp?---most jukeboxes have a relay that kills the high voltage until the record starts---NOT SURE ABOUT THAT MODEL
https://www.tubesandmore.com/sites/defa ... pg13-1.png

also are you testing with the speaker connected --the field coil is part of the hv circuit

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Gary "JUKEBOXMAN"


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 6:28 am 
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Hi Gary,
I am bench testing. I don't have the entire jukebox with me so no speaker. I just pulled the amp. I jumpered pin 1 and 7 on the speaker interface with a 3k ohm 10 watt resistor to replace the field coil and then hooked a PM speaker on pins 4 and 5. That should work for bench testing, right?
Rudy


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 6:47 am 
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yes i do think if i am looking at the schem right that should work-

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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 7:39 am 
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After finding a needle in the hay stack of tubes, I found another 5Z3, put it in and now have good DC! However audio playing thru RCA input from my mp3 is very very faint. Do you think it is because it needs more input than what my mp3 can provide? I would just hate to drive all the way across town where I don't have my test equipment and it do the same thing in juke. What's your guess? Keep troubleshooting it on the bench or should it work when in the juke?


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 1:26 pm 
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Joined: Jun Fri 15, 2012 2:22 pm
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Location: Wausau, WI
If the volume control on the MP3 player is turned up all the way you should have a reasonable volume with the 851. Another thing to try is to remove the grid cap connector on the top of the 6C6 tube, then touch the cap with your finger. You should get a very loud buzz out of the speaker, if you don't you need to do more checking.

One more thing. Are you using the Wurlitzer volume control box while you are testing? If you are you may not have very much volume. It is best to test the amp without the volume control connected.


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 4:28 pm 
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Joined: Jun Sun 15, 2014 11:04 pm
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Location: Coos Bay, Oregon
The quick test on the grid of the 6C6 should tell you a lot. If you get the loud hum, then likely the amp is okay. You can inject the signal directly into that spot for an additional test. If you do that, be sure the cap lead is connected as it completes the bias. In addition, be sure to short pins 2 and 3 on the input connector, if you apply the input signal at the input connector. Cheers, Russie


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 7:05 pm 
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I did the quick on the 6C6 and I could hear the hum, but still very faint. What would be the next logical thing to test?
Thanks,
Rudy


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Mon 08, 2018 7:29 pm 
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Joined: Jun Fri 15, 2012 2:22 pm
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Location: Wausau, WI
On schematic that is in the link above has an omission on it. On the speaker socket pins 1 and 2 should be tied together, if you check your amp I am sure this is already the case. To complete the B- circuit pin 3 of the speaker socket must be go to chassis ground. If you were using the original speaker you will notice that the original speaker plug has a jumper between pins 2 and 3.

Here is the corrected schematic

Image


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Tue 09, 2018 12:41 am 
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Joined: Jun Sun 15, 2014 11:04 pm
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Location: Coos Bay, Oregon
If you have an O-Scope, check for signal gain between the grid and the plate of each of the stages, except the 76, check its cathode for the signal.

Also, be sure the relay triggers to the lower filament voltage. If it is not triggering, check the bias on the 6L6G's and also check C-12. If C-12 is bad, don't replace it with a larger value. Replace it with a 15 or 16 uf electrolytic (whichever you can get).

Be sure all voltages are present and correct on each stage. If you have good plate voltages (as shown on the schematic), your connections of the 3K ohm resistor across pins 7 and 1 are probably correct.

Cheers, Russie


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Tue 09, 2018 1:21 am 
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Joined: Jun Fri 15, 2012 2:22 pm
Posts: 141
Location: Wausau, WI
Pins 1 and 7 for the 3K resistor is correct, but you still have to make sure that pin 3 of the speaker socket is jumped to pin 1 or 2 for testing.


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Tue 09, 2018 7:13 am 
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Location: Garland, Texas
Ok folks, not getting any closer.
The relay is triggering so I think the bias is OK. With a 3k 10 watt resistor on pins 1 and 7 and the speaker wires attached to pins 4 and 5 in the picture below, the relay works and I do get minimal sound from mp3. Each of you have suggested attaching pin 3 to chassis ground or to pin 1 or 2...I can't because pin 3 has 389 VDC to it from the thin red wire in the pict coming from the ceramic resistor (that looks stock). Am I missing something?
Like I said this was supposed to have been working until just recently and it had been professionally rebuilt. I have to take them for the face value that it was working, just can't figure out why now it is is not. Completely open to ideas as I am running out of them.

One odd note, when I was testing voltage values on the 76 tube and I touched the 2 pin with my VOM lead, the sound does get a bit higher and clearer but still minimal volume.
Rudy


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Tue 09, 2018 12:27 pm 
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Joined: Jun Fri 15, 2012 2:22 pm
Posts: 141
Location: Wausau, WI
On your photo the numbers on the pins are going the wrong way. Looking from the bottom of the socket the pins are numbered going clockwise. If you count the pins clockwise, pin 3 should be the center tap on the power transformer. Note that the schematic diagram shows the speaker socket from the top.


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 Post subject: Re: Need help with Wurlitzer 851 amp
PostPosted: Oct Tue 09, 2018 3:39 pm 
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Joined: Jun Sun 15, 2014 11:04 pm
Posts: 1305
Location: Coos Bay, Oregon
That change in the tone and volume is a good clue. Check ALL of the resistors and capacitors associated with the 6C6 and the 76 (especially R-14 and C-14).

Cheers, Russie


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