Owner`s manual

Remove the input connections from the power amplifier and insert the RCA shorting

plugs. If the hum remains, the hum is obviously being created internally. Defective

component possibilities in increasing order of probability include the power transformer

(really unlikely), rectifiers (vacuum tube or diode), a tube with heater to cathode internal

shorts (in other words, a defective tube), or open electrolytic capacitors (most likely).

Here’s how it works: The purpose of the transformer is to take that 120 VAC from the

line mains (via the line cord, power switch, and fuse) and convert it to both higher AC

voltages (for the tube plates) and lower AC voltages (for the tube filaments and bias). The

rectifier (weather ss or tube) converts that AC voltage from the transformer to DC

voltages necessary to power the tube amplifier circuits and provide the necessary

negative DC bias to the output tubes (the filaments are usually fine with AC). The

rectifiers convert the AC to DC by inverting only the negative half of the AC waveform

(converting it positive) – what’s left is a huge 120 cycle (notice that it is 2 X 60 Hz)

single polarity ½ sine waves. Those electrolytic caps, choke, and power resistors

following the rectifier are there to smooth out that ripple. The filter choke almost never

fails oh but that Quad electrolytic capacitor – they don’t age well and are the most

unreliable component in your Stereo 70. That is why the most common cause of HUM is

an open capacitor.

1. Hum in One Channel Only

a. Reverse the 7199 driver tubes (small tubes on the driver board)

• If the hum moves to the other channel, replace the 7199’s

• If there is no change go to the next step

b. Reverse the two output tubes from one channel to the other

• If the hum moves to the other channel, replace the output tubes

• If the hum remains, the driver board is probably defective and should be

replaced. As I noted earlier, the entire 7199 driver board assembly is failure

prone and therefore we recommend replacing it with either our upgrade module

(CAE P/N PC-3U) or for a dramatic improvement in the performance of your

Stereo 70 our modification module (CAE P/N PWB-3A).

2. Hum in Both Channels

a. Remove the 7199’s (small tubes in the driver stage)

•

••

• If the hum disappears, replace the 7199’s

•

••

• If the hum remains

replace the Quad Electrolytic Capacitor. Unfortunately

replacements are very unreliable. However CAE now offers a small Quad

Capacitor Replacement module (P/N PC-S7U) specifically designed for the

Stereo 70 that contains a composite of modern reliable capacitors. Both the

voltage and capacitance ratings are raised giving higher reliability and

performance.

D. HUM (with a preamp connected)