Acom 1000 tube replacement
Please don’t attempt tube replacement if you are not experienced. The high voltages in this amplifier are deadly and if you are not careful it could prove fatal.
This step-by-step instruction is about the tube replacement and idling currents re-adjustment of
ACOM1000 power amplifier. Please follow all details closely since the work is difficult, with many
details to keep track about, and might be dangerous too. Do not hesitate to contact your local
dealer or nearest ACOM’s service for any advice.
W A R N I N G HIGH VOLTAGE!
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 before ensuring as described below that no residual voltages
- REMOVING THE TOP COVER
a) First pull the amplifier power plug out of the mains wall outlet. Wait at least 30 minutes (or
more if the amplifier has been just powered up).
b) Using a Philips-2 screwdriver, unscrew 9 pcs of “eco-fix” flange-button head screws to release
the top cover (2×3 pcs on each bottom-side edge, and three more on the rear-top edge of the
box). Don’t unscrew four screws that hold the grid above the tube. Keep all screws and washer for
the mounting procedure later.
Lift the rear edge of the cover slightly (to 2-3cm) and then pull the cover backwards, in order to
release it from front-panel’s chutes. Remove the top cover. Do not touch the part inside for now!
You will see inside:
- the MAINS PCB that is mounted on an aluminum sub-chassis, just above the big HV transformer;
- the HV rectifier PCB in front of the MAINS PCB (with four big diodes, one wire-wound resistor,
and one disc ceramic capacitor on it);
- the HV filter PCB, located just below the HV rectifier (with 8 electrolytic capacitors on it).
c) Before proceeding, please make sure that no residual DANGEROUS VOLTAGES are present in the
amplifier. For this purpose take approximately 1m of WELL-INSULATED lead (adequate for 3000V).
Bare it to about 1cm at both ends.
Tighten one end of the wire under the GROUND STUD (on the rear panel). Only AFTERWARDS,
holding the wire BY THE INSULATION, at a distance of minimum 10cm (4 Inch), touch with its
second end consecutively all parts on the HV rectifier PCB, especially:
- the HV output J3;
- the wire-wound resistor R12 on its both sides;
- the four HV diodes on their both sides.
With the same wire end, on the MAINS PCB touch the diode D19 on its both sides (it is located at
the PCB edge, near the powerful transistor that is installed on the chassis).
- REMOVING THE SCREEN ABOVE THE RF DECK
Using a Philips-1 screwdriver, unscrew 15 pcs of countersunk-head screws to release the
aluminum cover above the RF deck. Lift and pull the cover to remove it.
Repeat step 1(c) about residual voltages, this time touching with the grounded wire the copper
strip of the plate anti-parasitic suppressors and the tube anode.
Check whether the HV LID crowbar makes a reliable short-circuit across the HV wiring when the
cover is missing.
- OLD TUBE REMOVAL
W A R N I N G HIGH VOLTAGE!
Prior to continue you must be absolutely sure that there are no residual voltages in the amplifier –
see (1) and (2) above!
Be careful – the tube may be VERY HOT!
Prior to remove the tube, you have to uninstall the 16mm (2/3″) wide copper strip with UHF antiparasitic suppressor, that is installed between the tube plate and the big ceramic DC-blocking
capacitor (near the plate choke). Use an 8mm-opening wrench to unscrew a hexagon-head screw
(M5 brass) from one side of the capacitor (where the connection to the plate choke is made via a
soldering lug). Please hold the capacitor for its ceramic body while unscrewing it, in order not to
let the second screw to loosen.
Using a Philips-1 screwdriver, unscrew an M3-screw from the plate capacitive divider (this is
aluminum / Teflon cylinder installed near the tube, on the tube deck). This way, the entire copper
strip, along with the UHF anti-parasitic suppressor is released and can be easily removed from the
Unclip the anode spring clip from tube cap, while pressing the tube to the bottom with a finger.
Now unthread the temperature sensor (which seems like a small transistor), out of the rubber
chimney. Then unthread the chimney out of the anode heat sink, together with the anti-parasitic
suppressor and copper strip (please keep track not to bend the temperature sensor leads).
At last, pull the tube out of its socket, using force directed to TOP ONLY, since bending may
destroy tube socket.
- INSTALLING A NEW TUBE
C A U T I O N
Examine new tube pins before inserting it in the socket. They must be straight and not inclined in
respect to the ceramic base, otherwise any bent pin could damage the socket’s spring contacts,
and thus the socket and/or the tube may become unusable.
a) Put carefully the new tube ON the socket, without neither pressing nor inserting the pins. Keep
track about the pins orientation. There is a larger gap between two pins (a pin is vacant) and it
must coincide with the corresponding socket’s gap. Be extremely careful about the tube screengrid ring, not to bend any of the socket’s spring contacts. If needed, slide the tube carefully up and
down while you align these contacts. After that, count the ring contacts: there must be 16 of
them, closely fitting OUTSIDE the tube screen-grid ring. This ensures that no spring contact has
been damaged during installation.
b) Align tubes pins so that EACH one would lie precisely in the gap, and at the width center of the
socket’s spring contacts. Use very small rotation movements during this alignment. Do not lift
tubes during this operation in order not to disorder screen grid ring contacts (otherwise repeat the
previous step (a).
c) Using a force of NO MORE THAN 50…60N (about 5…6kg or 12…15Lbs.), in VERTICAL DIRECTION
ONLY, push carefully the tube (holding its plate heat sink) in order to insert the tube in the socket.
If you meet a stronger resistance, do not force anymore and go back to recheck (a) and (b).
The tube must fully seat in the socket.
C A U T I O N
After inserting the tube, please recheck all 16 contacts of the tube screen ring.
d) Orient the silicon rubber chimney (together with the anti-parasitic suppressor inside) so that:
- the small hole could be threaded over the temperature sensor later;
- the connections to the capacitive divider and to the ceramic capacitor could be installed to their
Now thread, by small steps in parallel, the chimney over the tube plate heat sink. Keep track not to
bend the temperature sensor during this operation. Clip the anti-parasitic-suppressor spring
contact to the tube plate and precisely orient the connection lugs to the capacitive divider nut and
to the big ceramic capacitor. Align carefully the rubber chimney, so that it firmly contact the tube
deck at the base (otherwise a part of airflow would be exhausted uselessly and the tube might
At last, install and tighten the screws to the capacitive divider and the ceramic capacitor, using
their original screws and lock washers (should have been saved at previous steps).
e) Insure that after the tube replacement all the wiring is restored to its former condition.
- IDLING CURRENT ADJUSTMENT
After each tube replacement, the BIAS voltage needs to be re-adjusted in order to align the new
tube regime. There is an information screen “Plate Current” on the LCD, foreseen to monitor the
plate current during this adjustment. The adjustment trimmer-potentiometers is located on the
a) Install the cover above the RF deck (15 countersunk screws). In two of the nuts on the middle
chassis panel, near the HV LID crowbar and the cover-presence micro-switch sensor, tighten two
small strip pieces in order to imitate top cover. Check that they are pressing the crowbar reliably in
order not to let it escape during next operations.
W A R N I N G HIGH VOLTAGE!
Be careful – HIGH VOLTAGES up to 3000V will exist permanently on the accessible PCBs which is
LETHAL! Keep away of the parts on the PCBs! Keep track that nobody could access the dangerous
parts and do not leave the amplifier without supervision during next adjustment period with the
top cover removed!
Connect a dummy load (or an antenna) to the output. Install all cables to the exciter as usually.
Now, feed mains voltage, turn on the amplifier and let the tube at least 15 minutes in STANDBY
mode. Please guard the accessible HV parts during this period!
Select same bands (preferably lower) on both the amplifier and the exciter.
Use a small-size, flat-tip, insulated screwdriver for next adjustments. On the MAINS PCB, find the
two trimmer potentiometers RP1 and RP2 (marked “BIAS1” and “BIAS2”). They control the idling
(zero-signal) plate current of the tubes for low- and high- RF-drive levels respectively.
Please note that RP2 will affect both idling levels, while RP1 – only the low level. That is why, the
high level is adjusted the first, and only then is set the low level (according to the procedure
b) Select the “Plate Current” information screen on the LCD. Go to the OPER mode. Apply very
small drive power in CW mode (1…5W). Following the plate current, adjust R2 (BIAS2) to reach
220mA (200…240 is OK); CCW rotation will increase the current. Don’t exceed 15 seconds with the
PTT on. If you need more, repeat the adjustment 1-2 minutes later.
c) Do not apply any drive during the adjustment of R1 (BIAS1). Select SSB mode, with the MIC and
OUTPUT regulators on the exciter set to minimum.
While monitoring the plate current, adjust R1 (BIAS1) to set the plate current to 70mA (60…80 is
OK); CW rotation will increase the current. This adjustment does not affect the previous step.
- REASSEMBLING THE AMPLIFIER
After the adjustment procedure is completed, switch off the amplifier and pull the amplifier power
plug out of the mains wall outlet. After five minutes or later, remove both “cover-presence”
imitators. Do not hurry to remove them since the residual voltages will be still high! Do not touch
any part inside during reassembling the amplifier, since residual voltages may exist and the tube is
At last, install the top cover again as described below. Be careful to align screws thread to the nuts
and do not force them since any captive nut on the chassis might be damaged.
a) Put the cover above the chassis, while holding its rear edge slightly lifted (to 2-3cm). Align its
horizontal and two vertical front edges to the respective front-panel chutes. Then push gently the
cover forwards, in order to insert its front edges into the front-panel’s chutes. Take care especially
to the bottom corners of the cover.
b) Screw in loosely all 9 pcs of “eco-fix” flange-button head screws. Use a Philips-2 screwdriver.
c) Pressing continuously the cover forwards and to the bottom, tighten all 9 screws properly.
Note: This article was taken from the GMDX Associations website in Scotland and full credit goes to them for this article.