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 Post subject: Wurlitzer Model 851 amp
PostPosted: Jan Tue 30, 2018 9:05 pm 
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Joined: Jan Tue 19, 2010 7:07 pm
Posts: 723
Location: VC Highlands, NV
A friend asked me to have a look at a Wurlitzer Model 42 jukebox. It has the model 851 amp which looks quite straightforward, but I could use some expert advice before getting too deeply into it.

1. The power cord is melted so that's the first clue.
2. The fuse is okay, but it's a 3A fuse and the schematic from Radiomuseum.org doesn't specify the correct size.
3. The 5Z3 heats up on my Hickok 605A, so I know the heater is okay, but both diodes test dead with no response from the meter.
4. Someone opened the amp at some point because all of the screws are missing. What I found was a lot of wax that appeared to be melted out of the power transformer, so I'm sure it overheated. The wax was on the bottom cover and on the wires coming out of the transformer.

So, can anyone advise what I should test? I'd like to verify that the power transformer is good before spending too much time and money on this thing.

Thanks,
Brian

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Playing with old radios in the mountains 2000 feet above Reno


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 Post subject: Re: Wurlitzer Model 851 amp
PostPosted: Jan Wed 31, 2018 3:49 pm 
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Joined: Jun Fri 15, 2012 2:22 pm
Posts: 101
Location: Wausau, WI
I rebuild a lot of these amps. First replace the power cord. Fuse should be a 2 amp fast blow. Remove all tubes from amp. Plug it in, if fuse does not blow then check the secondary voltages. Be very careful when checking voltages. There should be about 5 volts AC on pins 1&4 on the 5Z3 and about 1100 volts AC on pins 2&3 on the 5Z3. If you have those voltages you can then check the filament voltages for the for the other tubes. There is a quick warm up relay that sends about 10 volts on warm up and then goes down to 6 volts for normal operation. Most likely the reason for all the wax coming out of the power transformer was caused by shorted filter caps.


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 Post subject: Re: Wurlitzer Model 851 amp
PostPosted: Jan Wed 31, 2018 4:50 pm 
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Joined: Jan Tue 19, 2010 7:07 pm
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Location: VC Highlands, NV
Thank you! That's a lot of very useful info. I will proceed along those lines and see what I can find.

Brian

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 Post subject: Re: Wurlitzer Model 851 amp
PostPosted: Feb Tue 06, 2018 10:26 pm 
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Joined: Jan Tue 19, 2010 7:07 pm
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Location: VC Highlands, NV
Thanks to the excellent advice from rro61889, the amp powered up and responded properly after I finished putting it back together.

Thanks again!

Brian


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 Post subject: Re: Wurlitzer Model 851 amp
PostPosted: Feb Wed 07, 2018 1:35 pm 
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Joined: Jun Fri 15, 2012 2:22 pm
Posts: 101
Location: Wausau, WI
Great to hear that you got it playing again! BTW the 76 did not come with a shield on it from the factory.


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 Post subject: Re: Wurlitzer Model 851 amp
PostPosted: Feb Wed 07, 2018 1:45 pm 
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Joined: Jan Tue 19, 2010 7:07 pm
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Location: VC Highlands, NV
Yes, that extra shield did seem out of place without the base mounting. I'll remove it before returning it to the owner.

Brian

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