I Need Help With A Wurlitzer Juke Box Tube Amp

[Grainger49]

My brother in law has a refurbished Wurlitzer model 1015.  Cool looking but takes odd 78 one sided records.

The amp was kind of refurbished and just crapped out the other day.  There is a wedding coming up and he wants it to work for afterward. 

Here is the set up.  It came stock with an octal full wave tube rectifier.  I am guessing that was expensive so it was replaced with a 5Z4 full wave, four pin.  I get the incoming B+ on the AC side, the plates but downstream I get about 1/2 a volt from the cathodes to ground.  There is a 20uF/450V cap between the 5Z3 cathode and ground.

I tried a newly bought tube, still the same voltage.

I tried to snug up the socket pins, still the same voltage. 

There is 377V correction 480V AC coming into the rectifier, between the plates, and next to nothing coming out.

Any suggestions would be happily accepted. 

 

[earwaxxer]

Hey Granger - Jay from Winsome Labs may be able to help you. From what I remember he got started by refurbishing such old tube radios, etc.

http://www.winsome-labs.com/index.htm

 

[Grainger49]

Eric,

I will contact them tonight and see if he can help.

I'm just so befuddled that the rectifiers don't seem to work.  They both light up, have incoming voltage and nothing on the output/cathode. 

 

[galyons]

Grainger, the original rectifier was a 5U4G.  The 5Z4 should work, although 5U4G's are vreadily available and cheap.  The B+ after the rectified goes to a cap.  Is the cap good?  If it is bad, it probably arc'd the original 5Z4 and your replacement. Do you have a tester?  If not, check continuity with a VM. Check the downstream components before you power up again.  You don't want to fry the power tranny.  Once you know you don't have any shorted caps, etc.  I would put in a 5U4G.

I attached a schematic that should help.

Let us know how you are getting on!

Cheers,
Geary

 

[earwaxxer]

Granger - Jay is a really cool guy. I bought 2 of his Mouse (Meuse?) He sent me another SMPS when I misplaced one of them.

 

[Grainger49]

Geary,

When this started I found the same schematic on the internet, couldn't find it today.  But I now have a great reprint of the manual.  I'm impressed with the engineering.  There is even a resistance measurement for the quick start relay coil.  That is good detail!

I was getting 480V between the plates.  So the high voltage winding is good.  I tried to test the 20uF cap but my meter only goes to 7uF on the capacitance measurement.  It went to overload, if it were shorted I don't think it would be overload.  I have tried on a bad cap. 

Tomorrow morning I will try the cap and see if the resistance climbs.  I don't know why I didn't do that this evening.  If it is the cap, I have some to use.  If it is the rectifier, I won't make it by Saturday.

Thanks!

Eric,

I'll be in touch tomorrow.

Thanks!

 

[Paul Joppa]

I think I was getting 377V between the plates.  So the high voltage winding is good....

I dunno - that seems pretty low for end-to-end. Pull the rectifier and measure ground to "plate" for each plate - I wonder if the power transformer is actually delivering on both halves.

Oh yes - forgot to say "re-flow the solder joints around the rectifier cathode and first cap" - always have to say that!  :^)

 

[Grainger49]

Paul,

Thanks, I will check the voltages and get back. 

This "kit" has been in service a while.  The old solder joints heat slowly, I have had to go over a couple and a wire broke loose as soon as I started checking it.  The old joints also don't seem to want to wick into the solder wick.  My metal solder sucker works the best here. 

No wonder that the sound was low and distorted, there seems to be no B+ anywhere.  According to the official schematic, posted by Geary and in the manual, there should be 320V DC right off the rectifier - going to the field coil of the speaker.

Geary,

I had the amp on its side when I plugged it in, there is no power switch, so I couldn't see the rectifier when I started it.  My eyes were on the meter to see what was happening there.

Edit, update:  I checked all the caps.  None are shorted.  I had a problem with the first cap after the rectifier.  I short it to ground, no spark, and when checking resistance to ground it immediately jumps to 4.07M ohms; no ramping up and no change after jumping there.  All others can be made to charge up to maximum readings.  This one doesn't charge it just jumps!

This is a 50uF at 400+V.  I might try cobbling in a motor start cap of 45uF@400V AC.  Anyone want to speculate on this substitution? 

PJ, You are right, the output of the two high voltage windings, measured plate to plate is 480V AC.  I had mis-remembered the voltage.

 

[Grainger49]

Ok, no one has suggested anything for a day.  I'll put in the oiler for the first stage and see what happens.

Edit:  No smoke, no explosion but the voltage is still off.

 

[galyons]

Most old caps are +/- 20%.  Should be fine! 
Cheers,
Geary

 

[Grainger49]

Geary,

I soldered in the oiler last night.  This morning I tried it with the rectifier.  Still about 5V DC coming out.

I think PJ nailed it.  I reread his post last night and saw that he wanted the rectifier out and readings to ground.  I got 5V AC and 475V AC to ground.  I think the transformer winding is shot.

Thanks guys!

 

[2wo]

If you have 475 AC, why don't you remove the rectifier and sub in a pair of diodes leaving the 5V disconnected...John   

 

[Grainger49]

I have been thinking of that, but the windings put out 274V and 5V.  I'm thinking that the 5V to ground is a bad winding.

Oh, thanks for the reminder of removing the 5V winding.  I would have forgotten it.

 

[2wo]

Yea, I'm thinking a fault in the 5V winding...John

 

[Paul Joppa]

A failed connection at the transformer centertap could do that, I think. Check winding resistance end to end and each end to CT.

 

[Grainger49]

Sorry, it is slow going.  A rehearsal and rehearsal dinner last night.  The Wedding is at 4:00 this afternoon.  I have told my brother in law that the transformer can't be replaced quickly.

I measured across the two high voltage windings, 89.1 ohms.  From one leg to CT of the HV winding is 46.8 ohms and on the other it is 42.9 ohms.  I'm surprised at that with the 5V to ground reading from one of them.

Just FIY, from either lead to ground is zero, nope they are open to ground.

 

[Paul Joppa]

OK, there is something wrong with the measurements (or else I have not understood the description). I believe you just said there is 89.1 ohms end to end, and both ends are grounded. In which case there would be zero ohms end to end, right?

The two halves appear to be fine - that amount of difference is not uncommon.

 

[Grainger49]

Sorry, both resistances were end to center tap.  From end to end it is 89.6 ohms. 

Either winding end to ground is open.

The center tap is not grounded, or I haven't seen where it goes to ground since it goes to the speaker plug.  It could be used for a through connection to ground.  Sounds stupid if ground is needed you can never unplug the speaker when it is on. 

Updated above.

 

[Grainger49]

Paul,

Using the old Bottlehead email I just sent a copy of the schematic with more detail than the one on the first page.

I also tried a PM with the image embedded.  I have no idea if either will work.

Let me know if it got through.

Thanks!

 

[Paul Joppa]

Sure enough, that's it - there is no B+ unless the speaker plug is inserted. I notice that the speaker field coil seems to be the only power supply bleeder; perhaps it drains the caps fast enough to prevent lingering noises when the jukebox switches off? I notice also the fast-heat filaments; the heater voltage is 9.5 volts until the output tubes are actually drawing current, then drops down to 6.3 volts via the relay that serves as the cathode bias resistor. Hard on tubes, but it probably does start playing much faster. At 50uF and given the low transformer resistance, the 5U4 is pretty stressed as well - designed to have the tubes replaced frequently perhaps?