Owner`s manual

14
problem is further back (towards the source). Next, place the RCA shorting plugs into the
phono input and select that input. Once again, power up and observe. The presence of
hum now points to the phono stage inside of the preamp. Otherwise it is due to turntable,
arm, cartridge grounding and / or shielding.
If from the above series of tests you identify that the source of hum is internal to your
preamp, the most likely causes are the tubes or power supply. If the hum is only present
in one channel, switch all the tubes from channel to channel to confirm. If the hum moves
with the tubes, they should be replaced. If the hum remains with the same channel, or is
present in both channels, there are only a few components responsible. (I will assume you
have eliminated the tubes per our earlier discussion). In order of likelihood take the
following steps in sequence and check the results after each step:
• Replace the rectifier tube (in amplifiers so equipped).
• Replace the electrolytic capacitor. In Dynaco equipment and in many others, the
electrolytic capacitor is a 4” (approximately) tall 1.5” diameter metal “can”. It has four
terminals (usually) and is connected to ground via four “tabs” that feed thru a cutout in
the chassis. These capacitors are notorious for failing. When they fail open, hum results
and when they fail in the short mode, fuse tripping results. A telltale sign that they are
either defective or well on their way to becoming defective is a brown power that is
evident near one or more of the terminals. If you see this power at any time, order a
replacement.
Other sources of hum, especially with DIY constructed preamps are improper grounding
(internal ground loops) or shielding. The range of possibilities is simply too great to
address in this paper and therefore I recommend contacting us directly for personal
guidance.
If at this point you have not eliminated the hum, it is time to call in a skilled technician.
You may feel free to contact us here at CAE ([e-mail]
) or find a local
technician experienced in tube technology.
F. HISS, CRACKELS, POPS, AND OTHER DISHARMONIOUS NOISES
Hiss and other noise can come from any number of sources but mostly the signal tubes
(7199’s) themselves or their cathode or plate resistors. In feedback designs, it becomes
difficult to identify the effective stage because the loop obscures the test. The first step is
to insure that the hiss is originating internally. Once again insert the RCA shorting plugs.
If the hiss remains the most likely cause is either the 7199’s or other components on the
driver board. If the hiss is in one channel, try reversing the 7199’s. If the hiss follows the
tube, replace the 7199’s. If it does not, there is certainly a defective component on the
driver board. Isolating the exact component is somewhat difficult however given the
many reliability problems with the stock board, we would recommend that you replace
the driver board. CAE offers two replacements – an upgrade (CAE P/N PC-3U) and our
premium full modification board (CAE PWB-3A). Visit our web site for more
information and feel free to download the instructions.