Section 4
Troubleshooting

TROUBLESHOOTING DS-400 'INSTALLATIONS
Two basic troubleshooting methods are discussed in this section. The first method is a hit-or-miss type of system using a graph. This method is simple to use but it will not reveal all possible problems. The second system uses a flow chart and will take the problem to one particular circuit. Before using this system, it is highly recommended that you read and understand the section of this manual on "theory of operation". If attempting to locate a problem using these procedures is unsucessful, contact Digalog for any new information that may be available.

DEFINITION OF SYMPTOMS
OUT OF LOCK: In an "out of lock" condition, varying the DS-400 frequency setting will not change the radio frequency. A voltage measurement on the TP-2 will quickly determine a locked or unlocked condition; 5v for locked, Ov for unlocked. The first digit on the DS-400, will display a 2 for a locked condition and an L for an out-of-lock condition.

LIMITED RANGE: An "out-of-lock" condition at the top or bottom end of the band.

WARBLE: A warble is detected on SSB. This can cause SSB communications to be distorted or difficult to clarify.

RADIO OFF FREQUENCY BY A MULTIPLE OF 5KHz: Radio is in lock, but the operating frequency is constantly off by some multiple of 5KHz.

RECEIVES 2 OR MORE CHANNELS AT ONCE: When receiving, the same incoming signal can be heard on several frequencies.

WILL NOT LOCK WHEN POWER IS RESET: A radio may function normally after realignment, but when the power is turned off and thenAurned on again, it may not relock. The reason this condition can exist is as follows: (1) Power is first applied, and the 400 circuitry has no input on Coax #1 because the VCO has not begun to oscillate; (2) The 400 senses the lack of input and puts out a high (5v) state on Coax #2; (3) The VCO then oscillates at its maximum frequency, and because of inherent design its amplitude decreases at higher frequencies; (4) Therefore, the down mixer output is low and the 400 cannot get enough drive from the radio on Coax #1. The PLL chip does not get enough drive and the loop is unable to recover. The problem is solved by increasing the gain of the down mixer in the radio or input amplifier in the 400 or readjusting the VCO coil to a point where the VCO will not free run achigh in frequency.

LOOP FILTERING:
Symptoms of the loop filtering being incorrect may be: distorted SSB, warble on SSB, difficult to clarify SSB, or, in extreme cases, squeal on AM and bleedover may be present. In most cases the problems described above are cured by the adjustment of the variable loop filter This loop filter is between the center of Coax #2 and ground.
The loop filter is aligned for best SSB clarity. It the resistance of the trimpot is too low. the radio will warble on higher frequencies: and it the resistance is too high. bleedover may be experienced on lower frequencies.

GROUND CONNECTIONS:
For proper operation the 400 must have a good ground loop to the radio. Best results are achieved by connecting the coax shields near the VCO and connecting the black wire to a ground near the voltage regulator.

BROKEN PC PADS:
The pads under the PLL chip are sometimes damaged during chip removal. The 8v source is connected through one of the pads and the circuit must be complete through the pad to attain a locked condition on 8719 installations.

VCO OUTPUT TRANSFORMER ALIGNMENT:
On some chassis. mostly Uniden 858 and Palomar 500. misadlustment of the VCO output transrormer can cause warble on SSB. Slightly detuning the transformer slug will generally stop the warble on these chassis.

DOWN MIXER OSCILLATION:
Uniden 8719 & 8734 chassis requires removal of C70 or C89 to prevent the down mixer from oscillating.

ALTERNATOR WHINE
In mobil applications, 8719 and 8734 chassis have had an occasional problem with "alternator whine". This problem is cured by reconnecting the red wire (presently connected to the power switch) to the radio side of T1, T1 will then filter the alternator noise from the 12V power for the 400.