Wurlitzer 2000 - do I have a blown output transformer ?

(Disclaimer: I don't have extensive theoretical radio knowledge!).

I rebuilt the amp 10 years ago and it has been fine. However, I accidentally turned the volume up too high and it now has an extremely loud hum (unaffected by the volume control) with no trace of the pickup signal. I have a valve tester and a cap tester so checked the valves and rectifier - all were good. I then checked all the electrolytics and these too were good as were the speakers. Could this be a blown output transformer? Are there any checks I can do on the output transformer - likely resistance readings etc ? If the output trans is blown, could I substitute a spare version from a W1100 ? Thank you for any responses.

On 02/23/2014 8:03 AM, pinbore wrote:
(Disclaimer: I don't have extensive theoretical radio knowledge!).

I rebuilt the amp 10 years ago and it has been fine. However, I accidentally turned the volume up too high and it now has an extremely loud hum (unaffected by the volume control) with no trace of the pickup signal. I have a valve tester and a cap tester so checked the valves and rectifier - all were good. I then checked all the electrolytics and these too were good as were the speakers. Could this be a blown output transformer? Are there any checks I can do on the output transformer - likely resistance readings etc ? If the output trans is blown, could I substitute a spare version from a W1100 ? Thank you for any responses.


Unplug all the tubes except for the 5U4 and the 6L6s. Do you still get
hum? OK, then unplug the 6L6s. If it still gives a loud hum then you may
indeed have a failed output tranformer - winding shorted to ground. You
should be able to detect that with an ohm-meter. the plate leads of the
6L6s connect directly to the output transformer - that should have
infinite resistance to the chassis with the Yellow wire from the output
transformer unsoldered. See schematics for the 530 Amp. Of course if
C-41 or C-42 are shorted then you will get either R-77 or R-78's
resistance showing to ground.

Report back with your findings.

I doubt that a 1100 transformer will work well in your machine, ten year
earlier desing - won't be quite as good at best.

If you can't find the schematic for the 530 amp I can email you one...

John :-#)#

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Many thanks for your interest and thanks for the offer - I do have a 530 schematic.
I still get the same hum even when just the 5U4 is left in. There are no shorts to ground across R77 & R78.
I then suddenly realised I have a (working) W1800 (and schematic) which uses a similar output to speakers arrangement so I was going to take like-for-like resistance readings between the two amps.

On 02/24/2014 5:00 AM, pinbore wrote:
Many thanks for your interest and thanks for the offer - I do have a 530 schematic.
I still get the same hum even when just the 5U4 is left in. There are no shorts to ground across R77 & R78.
I then suddenly realised I have a (working) W1800 (and schematic) which uses a similar output to speakers arrangement so I was going to take like-for-like resistance readings between the two amps.


Now try disconnecting the B+ connection to the output transformer...

John :-#)#

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(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

On 02/24/2014 5:00 AM, pinbore wrote:
Many thanks for your interest and thanks for the offer - I do have a 530 schematic.
I still get the same hum even when just the 5U4 is left in. There are no shorts to ground across R77 & R78.
I then suddenly realised I have a (working) W1800 (and schematic) which uses a similar output to speakers arrangement so I was going to take like-for-like resistance readings between the two amps.


Oh, and another trick is to connect the 2000 audio output transformers
B+ lead (disconnected from the 2000 amp) to the 5U4 in the 1800 amp and
listen for hum on the 2000 output - the 2000 amp is NOT powered up for
this test, just eh 1800. Don't forget to connect the two chassis
together unless you enjoy HV shocks!

John :-#)#

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(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

Thanks for the tips John. I noticed that the output transformers and fader switches for the 1800 and 2000 are identical (same part numbers) so I might just try a complete substitution.

On 02/24/2014 3:56 PM, pinbore wrote:
Thanks for the tips John. I noticed that the output transformers and fader switches for the 1800 and 2000 are identical (same part numbers) so I might just try a complete substitution.


I figure better to do a simple test than to rip out a transformer and
install in the other amp...and find the problem is a shorted bleeder
resistor or some such other random cause.

John :-#)#

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(Please post followups or tech inquiries to the newsgroup)
John's Jukes Ltd. 2343 Main St., Vancouver, BC, Canada V5T 3C9
(604)872-5757 or Fax 872-2010 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

I think you might be crediting me with more electronics knowledge / ability than I have. Probably better that I take it to 'the menders'. Again, thanks for your help.