[Milsurplus] AR-88LF Oscillation

[Al Klase]

Hi Joe,

I'm glad to hear you're making progress with that radio.  The circuits 
in the AR-88's were very carefully designed, and the audio amp is no 
exception.  This could be the cleanest audio in any boatanchor, so I'd 
be hesitant to mess with it.   The values for the plate and screen 
resistors on the 6SJ7 coincide closely withe the "textbook" values in 
the Radiotron Designer's Handbook, Third Edition, which calls for 250K 
and 1.5 meg.  I'd put the 1.5 meg back in.

I'm working from plain AR-88 documents, so be a little careful here.  It 
looks like the screen bypass, C-112, and the plate-supply decoupling 
cap, C-113, share the same bathtub.  If there's a bad ground on the tub 
or internal leakage between sections, you could have a embarrassing 
feedback path.  I guess I'd diconnect both caps and replacewith separate 
0.22 uF's.  Then try swapping out the tube if you still have a problem.

Good luck,
Al

.


On 4/25/2010 8:30 PM, Joe Connor wrote:
> I'm working on a nice AR-88LF with Royal Canadian Air Force markings. Any help would be greately appreciated.
>
> I ran into something strange in the first AF stage that I need help with. R-38 (1.5 MEG) had been removed from the circuit and replaced with a 4.7 Meg resistor. When I put a 1.5 Meg unit back in, I got a low frequency oscillation. With some trial and error, I eliminated it by changing C-112 from .25 mfd to .05 mfd. With the .05 mfd in the circuit, the voltages in the 1st AF seem fine and the set plays very nicely.
>
> Any idea what's going on? Should I leave the .05 in place?
>
>                                Joe Connor
>
>
>
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-- 
Al Klase - N3FRQ
Jersey City, NJ
http://www.skywaves.ar88.net/