[Amps] L4PS Xfmr buzz

[Michael Tope]

On 2/16/2010 6:23 AM, Jim Thomson wrote:
> Date: Mon, 15 Feb 2010 09:41:56 -0800
> From: Michael Tope<W4EF at dellroy.com>
> Subject: [Amps] L-4PS Transformer Buzz
>
> I just installed the Harbach PM-400 in the L-4PS power supply for one of
> my Drake L-7 amps after I popped a rectifier diode in the old bridge:
>
> http://www.harbachelectronics.com/main/page_products_drake_l4l4bl7.html
>
> Electrically, the amp and power supply appear to work perfectly with the
> Harbach board. Plate voltage and RF output are as expected. There is one
> problem, however. Last night about 20 minutes after I fired up the power
> supply I noticed a loud 60 Hz buzz coming from the supply.
>
> ## whoa, stop.   did it buzz  b4 u added the harbach mod ??
>    

&& Not that I recall.

>   Pressing on
> the sides of the cabinet or changing the physical orientation of the
> supply didn't seem to help. When I switch to the low-voltage setting on
> the L-7, the buzz stops. Also if I let the supply cool off the buzz
> stops for a while and then returns after the supply warms up. Looking at
> the supply in the dark, I saw no evidence of sparking. Additionally, the
> amplitude of the buzz seems to be totally unaffected when I go from
> standby to full RF output power.
>
> Any ideas on what might be causing this problem?
>
> ## try disconnecting the HV  AC   sec leads [ both of em]  from the
> FWD.. tape em.. then turn the supply back on, then toggle  between
> SSB / CW position.   I think Carl is right, the xfmr is trying to tell u
> something.
>    

&&   See below

> ##  If it still buzz's  with sec disconnected, then at least you have isolated
> the harbach mod / FWD  and  8 x caps, etc.  It may be mech vibration,
> how loud ..is 'loud'  ??   Are you sure it's not metal work, 2 x bleeders
> [ toss the bleeder's, then ur cab will run stone cold, and B+  will NOT
> rise at all].. or mounting posts for bleeders , or something below
> chassis, either breaker etc ?
>    

&&    You can't toss those two "bleeders", Jim. They are part of a 
voltage divider used to produce cutoff bias for the tube. In addition to 
supplying cutoff bias, they do serve as incidental bleeders, but they 
are redundant. The capacitor bank in both the stock supply and the 
Harbach board has a bleeder across each electrolytic, so the big 
wirewounds aren't needed for the bleeder function. Their primary purpose 
is for the cutoff bias.

&&    The buzz is loud enough that it overcomes the noise of the fan in 
the L-7 even when the supply is sitting on the floor behind my operating 
desk. Right now since I last opened up the supply and re-tightened 
everything, the supply stays quiet for about 15 minutes before the buzz 
starts to build up. The extra heat from the 50K resistors (about 30 
watts each) might be a contributing factor, so its possible that the 
problem won't occur if I disconnect the Harbach board (I would have to 
wire those two resistors directly to the transformer secondary to 
simulate the heat input from the high voltage DC across them when the 
Harbach board is in the circuit.

> ##  this buzzing is one problem I have never heard of in a L4-PS,
> and  I have  3 of em .
>    
&&  Someone sent me an email off list saying that 30 years ago they had 
similar problem with an new L-7 power supply.

> later.... Jim  VE7RF
>
>    

Thanks for the suggestion, Jim.

73, Mike W4EF........