[R-390] Audio HUM WAS: B+ short in RF deck

[Dennis Wade]

Charles, I'll answer your questions inline.

On Sat, Dec 13, 2014 at 4:29 PM, Charles Steinmetz <csteinmetz at yandex.com>
wrote:
>
>
>  Can see with the scope very large 120 cycle ripple on RF/IF B+ line at the
>> B+ fuse and plate of  V201 as well as the plate of the 1st audio amp.
>>
>
> So, there's your problem.  (But, what is "very large"??  How many volts
> peak to peak?)


Measured 70 V p to p at the 1/8 A B+ fuse

>
>
>  V601 and 602 were both swapped out for new tubes with no change.
>>
>
> Do you really mean the audio tubes, V601 and 602, or the rectifiers, V801
> and 802??  (The accumulated observations do not implicate V601/602, so
> there would be no reason to pull them.)


No, I really meant V601 and 602.  I have a stock handy and just swapped
them out for giggles, and to conclusively eliminate any odd failures.

>
>
>  Checked L601, 602 and 603 for DC resistance with a VTVM, they are within
>> spec.  Can't check the inductance.
>>
>> Caps C603 and 606 were previously replaced by axial caps installed on
>> octal
>> headers and plugged into the original sockets.  33uf/350 and 45uf/350 caps
>> were used.  They were checked with the VTVM and do not appear shorted or
>> open. Can't test for leakage at rated voltage.
>>
>
> If there is large ripple at the B+ fuse, it means either (1) something is
> wrong with V801, V802, L601, L602, C606A, or C606B, OR (2) something is
> drawing massively too much RF/IF B+ current, OR (3) something is drawing
> massively too much AF B+ current.
>

When I started having the fuse blowing problem it was running the tube
rectifiers.  I kept the tubes in until I cleared the short, then installed
solid state replacements (the SigmaTech product).  No change.  Just for
giggles I put the tubes back...still no change.  It's not the rectifiers.

>
> Any RF/IF B+ current drain sufficiently large to cause excessive ripple
> should blow the 1/8A RF/IF B+ fuse, and would also drag the B+ voltage
> down.  So, (2) seems unlikely.
>

Plate voltages that I've checked in the RF deck and AF deck are normal.
I'm pretty certain there is no excessive current drain going on for that
reason.  And no fuses being blown in case you were wondering.

>
> Similarly, any sufficiently large AF B+ current drain would also drag the
> B+ voltage down.  Does your AF B+ still measure about right?  If excessive
> current were being drawn by V601A or B, it would fry R606, so V601 is not
> the problem.  Excessive current could conceivably be going through V603 or
> V604, or there could be leakage from the primary of T601 or T602 to
> ground.  So, if the AF B+ voltage is low (or just to be sure), pull the AF
> B+ feed from Terminal 2 of L603 and see where you stand.
>
> If L601 and 602 are within spec for resistance, they are almost certainly
> not the problem.  L603 is not implicated by the symptoms.
>
> SO:  By elimination, you most likely have a problem with excessive current
> drain on the AF B+ supply, OR a problem with V801, V802, C606A, or C606B.
>

So I'm back to C606 (or possibly 603).  Someone suggested that I bridge
(carefully of course) a 45 uf cap across the B+ line and see what the hum
does.  I remember now I used to do that as a kid, thanks for the reminder.
Now to look and see if I have a suitable cap to bridge....

Thanks for the analysis Charles.

          Dennis


>
>
>
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-- 
"If they trust you, it is an extraordinary privilege, and you simply can't
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      - A. Alan Post  1914-2011.  California Legislative Analyst 1949-1977.

-------------------------
Dennis L. Wade
KG6ZI
Carmichael, CA