Hans Remeeus PA0Q

ex PA1HR

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Acom 2000A EBS modification and installation of the optional fan

EBS (Electronic Bias System) MODIFICATION AND


In two parts:

A. Installation of the optional fan

B. The EBS modification


Do the two modifications on your own risk and be aware of the warranty of your linear amplifier!

If you have any doubts or questions about this, please ask your dealer or Acom in Bulgarye.


The fan can be mounted by the dealer or manufacturer on request, or you can deliver and install it by yourself.

The fan must be sized 119x119mm and can be powered from 12VDC, 24VDC, 100VAC, or 120VAC by the amplifier.

A 24VDC brushless type is normally supplied and/or installed by the manufacturer as an option.

The set is completed with 8 screws M4x16mm and appropriate nuts, and washers.


Safety First!

The amplifier works with high voltages up to 3000V which is LETHAL!

For your safety pull the amplifier power plug out of the mains wall outlet and

WAIT AT LEAST 30 minutes EACH TIME BEFORE you remove the cover of the amplifier.

Disconnect all cables from the amplifier (grounding last).

Do not touch any part inside while the amplifier is open because some residual voltages may still be present!


Pull the mains cord out of the wall outlet and wait about 30 minutes

to let the residual charges accumulated in the high voltage circuits to discharge.



In order to mount a fan, the amplifier must be opened.

Remove the top cover by removing all screws except the eight located in the tube exhaust area.

Check carefully whether the LID CROWBAR (mounted on the middle chassis wall) does reliably short-circuit the HIGH VOLTAGE when the top cover is removed.

Remove the HV power transformer. Observe the considerations from S.2-2 of the OPERATING MANUAL but in reverse order.

Then unscrew the four bolts and remove the air intake filter and protecting grid from the rear panel.



I bought the 12V Papst 4412 F/2GLLL IBL (IBL stands for IchBinLeise) with only 16 dB(A) sound.

This fan costs about 22 euro and is extremely silent!


Place the fan on the rear panel outlet. Direct its axis according to the air flow, so the fan would blow into the amplifier volume.

I also bought four Noiseblocker NB-SLICS vibration absorbers to prevent tremble from the fan onto the cabinet of the Acom 2000A, see the photo's below:









C A U T I O N!:  Pay attention to the air flow direction as indicated on the fan case.

The fan must suck air from the outside and force it into the amplifier.

Orient the leads in the shortest way to the inside.


Direct its axis with the dome facing inside the amplifier (in order not to touch the fan).

Align its mounting holes with the holes of the rear panel and the fan outside.




In case that the shortest way is inconvenient, use the for this purpose provided aperture close to the fan, piercing the blanking cable gromet.


Place the metal grid on the iternal wall of the rear panel, against the fan.



Mount the air filter on the fan rear, using the remaining four M4x16mm bolts.

Please note that it is very important to have the air filter installed since much more air will flow through the amplifier volume.


C A U T I O N!: Compare the type (DC or AC) and nominal voltage of the supplied fan with the indicated in Table 2-1,

ACOM2000A Individual Data in your Operating Manual.

If the fan is for 12V DC or 24V DC, connect it to the small PCB, located immediately below its aperture.

Fan's positive lead, which is usually red, connect to the board pad indicated "+", and the negative lead - to the other indicated "-".

If the nominal voltage of the supplied fan is different from that indicated in Table 2-1,

you must open the cover below TUBE DECK; see the photo's above.


(Photo by Acom)


(Photo by Acom)


And there, on the AIR SENSOR board, move the jumper for the appropriate voltage: 12V or 24V.

In my Acom 2000A there was no jumper as you can see on the photo above!


C A U T I O N!:

Where the yellow cross is signed on the photo above you have to scratch away the PCB-print with a little sharp knife.

Use a multimeter to convince yourself that the PCB-print is fully scratched away.

Then you can solder a bridge between the two round openings on the PCB, where you see the little yellow line on the photo above.

I did it differently! You can see it on this two photo's below:



I soldered a three-pins holder on the PCB and put a jumper for 12V operation,

in case another fan (24V) is installed later in time.

Again: do not forget to scratch away the PCB-print first!

N O T E   The external fan is running all the time while the amplifier is ON.

Using DC types is preferable since they are controlled at two speed levels, depending on the exhaust air temperature.

Running speed is accelerated above 90 degrees C (194F) and is automatically slowed down when the cooling air temperature drops below 80 degrees C (176F).

Full temperature scale on the RCU is up to 130 degrees C (266F), each bar weights 10 degrees C (18F).

The amplifier will work normally up to the scale end, and a ">" sign would appear if overflowed,

in order to warn you about a too high temperature.

A protection trips at 150 degrees C (302F) threshold.

The absolute maximum plate temperature at the ceramic-to-heatsink junction is 200 degrees C (392F) as stated by the tube's producer Svetlana.

If the exhaust air temperature is higher than 80 degrees C (176F) at the moment you switch the amplifier off,

the tubes and external fan are instantly powered off, but the internal blower would run long enough,

until the exhaust air temperature drops below 80 degrees C (176F).

A protection system monitors the air motion in the tube deck volume.

This would prevent the control system of being let down if the blower stops (otherwise it would measure a low-temperature but still-standing air above too hot tubes).

You would get a "LOW AIRFLOW" message in such conditions in order to prevent tubes overheating.

B. MODIFICATION OF THE EBS (Electronic Bias System) IN THE ACOM 2000A

I did the modification which fully eliminates the EBS circuit.

With the addition of a capacitor only the time is increased,

and with the addition of the jumper the action of the circuit for EBS is eliminated.

This will connect the emitter of the transistor Q4 in order to fully block the EBS operation,

i.e. the tubes will be operated with the high-level idling current always when the PTT is pressed,

whether the RF drive is present or not.

No additional capacitor will be needed in this case.


Put the Acom 2000A upside down and remove the four screws directly behind the front panel.

Put the Acom 2000A on its feet and remove the two screws on the upper side of the front panel.



(Photo by Acom)


(Photo by Acom)


This is the Interface board in my Acom 2000A (from August 2004)


Please look for Q4.


(Photo by Acom)

Add a wire jumper between the via holes pointed by two yellow arrows as shown with a blue line in the picture.


First solder one side of a wire.



Then the other side and keep the lead (bridge) as short as possible.

The EBS modification now is a fact.

ATTENTION: when you use an older type Acom 2000A (before 2002)

please follow the modification as on the photo below:


(Photo by Acom)


Reinstall the power transformer and the cover of the amplifier.

Observe the considerations from S.2-2 of the OPERATING MANUAL.

In my case the EBS modification was a significantly improvement of my transmitted audio!

I have recordings of before and after the modification.

So far I did not observe any increasement of the temperature.

Please enjoy your modificated Acom 2000A!

73, Hans PA1HR.