acom1000 4m

Description: The reliability of the tuning aid on 70 MHz is somewhat complex. However, it appears to provide an adequate loading factor when the attenuator is activated. This attenuator is a frequency-dependent network consisting of a coil and two capacitors (L7, C17, C19) rather than a resistive attenuator. This design likely aims to balance the attenuation against the frequency-dependent impedance of the grid. The input Standing Wave Ratio (SWR) is acceptable during operation at 70 MHz, but it measures around 1:3 when the attenuator is engaged. If this SWR value causes a decrease in output from the transceiver, tuning with the attenuator may be challenging. The tuning aid functions by measuring the peak RF voltage on the plate (D1A, D1B), and there is no reason to doubt the validity of this measurement at 70 MHz. The tuning aid readout is triggered by detecting input power greater than 5 W, which introduces complications since the frequency-dependent compensations in the input circuit attenuate the 70 MHz signal reaching the input detector (D2A, D2B). Interestingly, the poor SWR caused by the attenuator benefits the input detector. The inability to detect input power could also impact the electronic bias switching (EBS) of the amplifier, as a 4m drive signal is more likely to fall within the EBS transition zone. It is advisable to disable EBS by short-circuiting R11 on the MAINS PCB and to readjust the bias setting (RP2) to approximately 120 - 150 mA. This adjustment will slightly reduce amplifier gain and increase the required drive power. The relationship between the plate RF peak voltage and the tuning aid readout may depend on whether the attenuator is engaged, potentially affecting the loading value if the attenuation level differs from that of other bands. This relationship may also rely on outputs from the input signal detector and the frequency detection circuit (INPUT-A), adding to the complexity. It is recommended to tune and monitor the plate peak RF voltage from the display at full carrier output. This value should be around 10%, but not significantly lower than the loaded DC plate voltage, which can also be observed on the display. To increase the plate peak RF voltage, loosen the load (turn right), or tighten it (turn left) to decrease the voltage, ensuring to re-peak the tune knob for maximum power after each adjustment. The ACOM 1000 features extensive protection circuits that will interrupt amplifier operation if a fault is detected. In the event of such interruptions, the following advice may help identify the cause: 1. The ACOM protection circuits react faster than those in other amplifiers. An intermittent connection in the RF feeder line or antenna may continuously trigger the protection, often reported as an ARC FAULT, even if the error is not measurable. 2. Automatic Level Control (ALC) power spikes can occur, particularly in modern transceivers with considerable attack time. If power is regulated, this can create a leading edge spike sufficient to trigger the ACOM's protection circuits. Disabling or modifying the ALC circuit may be necessary. One method to disable ALC is to apply a constant negative DC voltage to the external ALC input of the transceiver. Alternatively, operating the set at full output through an attenuator can mitigate this issue. 3. If tripping results from flash-overs in the tank circuit (ARC FAULT), increasing the load slightly (turning left on the LOAD knob) may help. Tuning is critical, as even minor adjustments can significantly alter loading, necessitating re-peaking of the tune knob for maximum power after each load setting change. 4. The ACOM is susceptible to Radio Frequency Interference (RFI).

The circuit design for the tuning aid incorporates a frequency-dependent network that optimally matches the impedance at 70 MHz. The use of L7, C17, and C19 allows for tuning adjustments that accommodate varying frequency conditions while maintaining signal integrity. The design's reliance on peak RF voltage measurements ensures that the tuning aid provides accurate feedback for optimal performance.

The input SWR monitoring is critical for maintaining amplifier efficiency, and the introduction of an attenuator modifies the loading conditions. The SWR value of 1:3 indicates a significant impedance mismatch, which can affect the transceiver's output and necessitate careful tuning adjustments. The relationship between the input power detection and the electronic bias switching further complicates the tuning process, highlighting the importance of careful calibration and adjustments to the bias settings.

The protection mechanisms built into the ACOM 1000 are designed to respond rapidly to fault conditions, ensuring the safety and reliability of the amplifier. Understanding the causes of potential faults, such as intermittent connections or ALC spikes, is essential for effective troubleshooting. The recommended adjustments and modifications, including disabling EBS and fine-tuning the load settings, are critical for achieving optimal amplifier performance and minimizing the risk of tripping due to undesired conditions. Overall, the tuning aid's functionality is crucial for efficient operation within the specified frequency range, and careful adherence to the outlined procedures will enhance the overall performance of the system.Whether the tuning aid can be trusted on 70 MHz is a bit of a complex matter, but wtih me it seems to provide a suitable loading factor, when used for tuning when the attenuator is activated. This attenuator is actually a frequency dependent network of a coil and two capacitors (L7, C17, C19), and not a resistive attenuator.

No doubt this has been done to balance the attenuation against the frequency dependent impedance of the grid. The input SWR is fine during operation on 70 MHz, but around 1:3 when the "attenuator" is inserted. If this SWR value causes your transceiver to decrease the output, tuning with the attenuator might prove difficult. The tuning aid works by measuring (D1A, D1B) the peak RF voltage on the plate, There is no reason to assume, that this measurement should not be valid on 70 MHz.

The readout of the tuning aid is triggered by the detection of input power (>5 W), and this is where the problems start because the frequency dependent compensations built into the input circuit attenuates the amount of 70 MHz signal reaching the input detector (D2A, D2B). Actually the bad SWR introduced by the attenuator aids the input detector. The lack of input power detection could also affect the electronic bias switching (EBS) of the amplifier, as a 4m drive signal is more likely to be within the EBS transition zone.

You are therefore encouraged to disable EBS by short-circuiting R11 on the MAINS PCB, and to re-adjust the bias setting (RP2) to around 120 - 150 mA. This modification will decrease the amplifier gain a bit, . and increase the required drive power. I assume that the computational link between the plate RF peak voltage and the tuning aid readout is dependent on whether the attenuator has been switched in or not.

The resulting loading value might therefore not be good if the attenuation level is different from that of the other bands. The link might further be dependent on the outputs from the input signal detector and the frequency detection circuit (INPUT-A) making the whole thing a bit speculative and complex.

I guess the thing to do is to tune and readout the plate peak RF voltage from the display at full carrier output. This value should be about 10%, but not much more below the loaded DC plate voltage, which can also be read from the display.

Load looser (turn right) to increase the plate peak RF voltage, or tighter (turn left) to decrease it (always re-peak the tune knob for max. power. after each change of the load setting). The ACOM 1000 has extensive protection circuits that will interrupt the amplifier operation, if a fault is detected.

Should you experience this, here is some advice to find the cause: 1. The ACOM protection circuits are faster than protection circuits in other amplifiers. If there is a bad (=intermittant) connection somewhere in your RF feeder line or the antenna it will continously trip (usually ARC FAULT), although it might not be possible to measure the error. These kinds of trips have often been reported on the YAHOO reflector, and always resulted from a fault in the feed line.

2. ALC power spikes. Many modern transceivers have ALC with a considerable attack time. If the power is regulated this could result in a leading edge power spike enough to trigger the normal protection circuits in the ACOM. You will have to either disable or modify the ALC circuit to counter this. One way of disabling the ALC is to provide a constant negative DC voltage to the external ALC input of your transceiver.

Another way is to run the set at full O/P trough an attenuator. 3. If the tripping is due to flash-overs in the tank circuit (ARC FAULT), then a trick is to load a little bit harder (turn left on LOAD knob). The tuning is quite critical. It only takes a few mm ’s of movement for the loading to change significantly (always re-peak the tune knob for max.

power. after each change of the load setting). 4. The ACOM is prone to RFI via