U47 Clone Power Supply Dead
« on: February 11, 2019, 05:49:15 AM »
Hi guys,

hop you can help me!
I got a U47 clone with the 408 Tubes and Poctop PCBs.

This morning I tried to power it up but nothing happened.
LED and Mic are dead.
Since the LED and the PSU have a separate output from the transformer each
I suggest it is the input winding on the transformer that's fried?
My only concern is: why? Should I check the rest of the mic, the cable etc.  for shorts or inconvenient things?
Maybe some of you have had this in the past and could help troubleshooting things.

Thanks a lot and have a great day!


Re: U47 Clone Power Supply Dead
« Reply #1 on: February 11, 2019, 06:09:19 AM »
maybe the primary of your transformer is cut (if the fuse is ok)
try to measure between the two wires of the primary with an ohm-meter
if you have an infinite value then your transformer is broken

try also on the secondary (unsoldered from the transformer) to check if there's a short (you should read more than 5 kohms)
change it for a higher power value


Re: U47 Clone Power Supply Dead
« Reply #2 on: February 11, 2019, 03:19:45 PM »
Thanks for the quick reply!
Indeed I got a cut on the primary and shorts on the secondaries.

This cheap Chinese stuff makes me cry.
Does anyone have an idea why it failed? Is it just "worn out" because of cheap isolation or could there be a fault in the mic and PSU circuit itself?

Does any one of you have an idea where I can get a decent transformer that survives more then 2 years of daily work?
It's a 200V and 9,5V with 0,1A and 0,5A. Probably the 9,5 voltage could even be something else. It's just for the light.

Cheers and thank you very much!

Re: U47 Clone Power Supply Dead
« Reply #4 on: February 12, 2019, 04:41:56 AM »
sorry, let me explain more clearly
to check if there's a problem with the mic circuit, you have to measure  the total resistance before the first filtering stage of your HT rail, in this case before the first 1k resistance. (or before the inductor if you inserted one...)
put the black wire of your meter to the ground.
you should read around 5kohms
otherwise there's an issue somewhere...

if no short:  underpowered transformer , 100mA is not enough because each 408a takes 50mA plus the filtering stages and
 the dropping resistor for the heater...
choose at least 300mA/200V...
« Last Edit: February 12, 2019, 05:11:21 AM by granger.frederic »


Re: U47 Clone Power Supply Dead
« Reply #5 on: February 12, 2019, 07:25:38 AM »
While I haven't seen the particular circuit the OP is referring to, I think it's safe to assume that the two heaters are connected in series. Anything else would be nonsensical. Even without taking into account that it's probably underheated as well, 100mA will do fine.


Check the rectifiers/diodes to make sure they are ok.  If there is no secondary side fuse, add one. For some reason almost any mic supply I see doesn't have these.

Possible reason could be voltage peaks on the mains voltage that the wire or isolation couldn't withstand, maybe lightning, ...  I doubt that it's a secondary side fault, a short here doesn't kill a transformer in an instant, it's a cooking process that doesn't go unnoticed.

Re: U47 Clone Power Supply Dead
« Reply #6 on: February 12, 2019, 07:48:52 AM »
in this circuit the heater current/voltage is  provided by the HT through a dropping 5 or 10w  1,5k resistor...
300mA min for max reliability ... (i wouldn't go under 200mA)
 you'll probably have to readjust the filtering resistors in the psu...
« Last Edit: February 12, 2019, 07:58:16 AM by granger.frederic »


Re: U47 Clone Power Supply Dead
« Reply #7 on: February 13, 2019, 05:37:46 PM »

Thank you so much for all your help. That's DIY spirit! Really appreciate it.

I replaced the transformer and the PSU is up and running again.
I re-calibrated the unit. At first it was at 112V, now it's back to 105V.



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