EMTRON DX1b - linear power amplifier


Arrival
Dx1b arrived well packed with a transformer installed on its position, protected with foam GU74b, and with two missing screws in the tank compartment. Missing screw in the same place is visible also on the Dx1b photo placed on the official Emtron webpage - so maybe it should be like that- hi, hi. How it was handled during its journey you can see below.... Bent base and bent thin wall of the band switch compartment are clearly visible.



While opening a cover I removed, by chance, a small EBS jumper which is placed on the Main Board on soldering side of it (black plastic strip). Surprisingly I found it on my table and finally I discovered the place where it should be placed. - so, pay attention.


Timer malfunction
No matter how long you were waiting "Ready" LED was not activated - so the amplifier was blocked. Emtron sent me a schematic and photographs of the main board for easy location of the C7 - electrolytic capacitor (preferably tantalum kind) responsible for delay time along with the instruction on how to repair it. I have replaced C7 and the problem is solved. The attached photo shows the location of C7 - maybe one day somebody else has the same problem so do not panic - you already know the solution...

Dan from Emtron sent me clear, detailed instructions how to manage this issue:

- with all precautions due to a high voltage presence , partly open cover so that there is access to the top front corner of the control board.
- locate two pins (T110) with yellow wire, the top ones in the corner next to the analog meter.
-power up PA and after 3 minutes check the voltage at pins against the ground (chassis): it should be less than 1V - if 12 Volts remain indefinitely it means that timer creates a problem.
-identify C7 (yellow cap) and remove it. Without C7 "Ready" LED might come immediately and this shows that C7 must be replaced. At this stage, you may operate Dx1b but wait a minimum of 3 minutes to warm it up
- if the fault persist without C7 it is likely U5 (LM555) has to be replaced.



BEFORE REMOVING PA's COVER
MAKE SURE THE PLATE VOLTAGE HAS REACHED ZERO!!


Arcs on the band switch
While on air on 24 MHz I have heard a spark sound - during close examination presence of the black carbon on the band switch was discovered. Emtron sent the new set of the band switch and one nice sunny Sunday I replaced it.


To replace the band switch the front cover with switches and tuning reduction mechanism must be removed...... "Piece of cake" if you have proper tools, unfortunately Dx1b is not sold with its...



Here we are - the cause of this malfunction is not known and it never happened again (knock, knock).




One mystery thing - what is the function of this fourth slice of the switch band. This nice wire around a half of this slice switch (160,80,40,30,20m pin) is connected to nowhere... think DX1b was a prototype for experiments...




Illumination of the Ip meter

Noticed that bulb inside Ip meter is not operational I opened the amplifier and removed meter from its position and after opening a transparent cover I found broken bulb connection wire. Soldering solved the problem.



Soft start
I liked the moment when you switch this PA "ON". No "bang", lights were coming softly - sign that this process is under control. Think it is the "state of the art" on the market.
Unfortunately, after over one year break in operations due to permanent problems with the power supply in "Salone" (Sierra Leone), soft start control is out of order.
Call to Emtron - they promised to send a new soft-start board. In the meantime Andy DK6AN sent me spare parts - they reached Freetown after two months.... Since Vcc on Pin 9 was well below 12 and other components were OK I exchanged, following Andy's advice, the IC TDA 1085C. Vcc on PIN 9 is 15.6 V now as in the book, I mounted soft start into DX-1b switched on - lights and fan went softly but: meters' lamps are far away from white, the fan is working as quiet as ACOM does, and plate voltage only 1800V. Seems to me the triac is not fully closed - my knowledge is not beyond that. So, took it out and I am in the same position as before.

Phone to Rudi (Feb 2009) who promised the new one - "sorry I have forgotten about you" - at least he did not claim that package is on the way..... waiting just for 6 months - package missing. Exchange phone conversation and e-mails including Dan - next date Easter 2010 "package on the way" this time to my Polish address - bad luck, August 2010 soft start did not reach Poland as well. Following Rudi's advice I connected Amps trafo direct and I am temporary using amplifier without soft start- works well.

NEW August 2011!!
After so many months I decided to do something with this soft start issue. Peter VK2, responding to my post on Emtron group advised me to check these small capacitors as well... . So, on 14 August 2011 on a very rainy Sunday, I started checking capacitors - unsoldered one "leg" from the ground and use an ohmmeter. I started with C10 since it is connected to Pin 5 where the switch "ON"- "OFF" is connected to. The first "shot" and BINGO - C10 - 100nF is faulty. The new one was placed and we are done! Triac is fully closed - only 2 Volts exist on MT1-MT2 pins. Finally !! Hope it is the end of this soft start story.
Think I could resolve this problem much earlier if only the proper documents on this subject exist. Having access to the proper scheme, not this one with errors which you may find on webpages, scheme with voltages on important pins should be a standard.

More info about Emtron Soft Start is here.






160 meters band
Having installed and tuned Spiro D-162 inverted V for 80/160 meters I connected Dx1b and there was no way to tune PA on 160 meters in the usual way. Plate voltage dropped to 1600-1400V low power out 200-250 strange reaction during tuning. Mail to Emtron and advises - "connect another antenna for 160m" (it is normal that every ham has two antennas particularly for 160m), "maybe PA is too close to the antenna", "install the ferrite ring on power cord", "connect dummy and see how it works". Installed chock on power cord changes nothing. Fortunately, 9L5A team arrived for WW CQ CW 2009 Contest so we connected PA to their dipole, and the results were the same. A few months later I bought MFJ dummy and the dummy shows output power 650W - adequate to the power IN; but Dx1's meters were still showing strange figures. I came to the conclusion that metering circuit is affected by 1.8Mhz RF. No diagram or sketch of the Dx1b metering system exists in Emtron files, I checked suspected places (blocking capacitors) for "cold soldering" no success. I have nine bands PA with only eight bands workable.... it is not too bad, so if only one would work?.


Playing around I opened SWR meter box, just to have any idea how it is made.


Update: as a part of the preparation for CQWWCW 2010 I decided to go once more through possible points where RF could exist and influence the metering system. Once more, all blocking capacitors and wires were re-soldered. Apparently, it helps since I can use Dx1b on 160 meters now, but as a matter of fact, I have no idea what the problem was... Contact with DL during Contest 2010 proved it.



Dx1b - is it possible?
This photo IS NOT a result of the "paint shop" activity. It is a real image of my Dx1b ....... note some value of the Ip and Vp when an amplifier is unplugged. "Cold soldering" of blocking capacitors on meters pins caused this malfunction.



Loss of BIAS
After a normal warm-up with READY led activated when switched from STBY to OPR two GREEN LEDs were activated and making things worse Ip jumped up to 1A (full scale of Ip meter). Switching back to STBY changed nothing the only way was to switch PA off from the mains.

Dan from Emtron advice that only lack of BIAS may cause so big Ip and he sent me following instructions :

Method 1.
- power up PA and check voltage at BIAS pin (white wire) - normal value about - 120V.
- warm-up observing BIAS
- if BIAS voltage is dropping to lower value eg -50V it means that grid is shorting to cathode once getting hot -TUBE is faulty

Method 2:
- disconnect BIAS (white wire) from the board
- disconnect HV from supply module
- check resistance between white wire going to grid and ground - power on and monitor resistance - if drops below 2.2kOhm - TUBE is faulty.

Having reviewed these suggestions I started checking BIAS issue step by step with operation switch in STBY position.

1- voltage on BIAS pin with white wire connected to the board- ops.. value -1 Volt should be -120V
2- white wire disconnected from the board, BIAS value on the BIAS pin -120V - bingo it should be like that. It means we have to look for troubles at the other end of the white wire..
3- HV disconnected from the supply module
4- resistance between white wire going to gird and ground - app 75 Ohm (NOT CORRECT -too low)- no changes during warm up
5- I have removed the tube and checked resistance between tubes pins – no shorts
6- resistance between white wire going to gird and ground - app 75 Ohm - no changes despite the absence of tube.
7- it is obvious that components in the tube compartment, not the tube itself creates this problem
8- opened small cover at the bottom side of the cabinet and checked the resistance direct on tube's pin 1 (grid ) (without tube ) - 70 Ohm - on white BIAS wire 75 Ohm
9- analyzing scheme two suspect components were identified - a blocking capacitor on BIAS white wire and two parallel capacitors connected to RF IN COAX.
10- since it was impossible to determine with Ohmmeter which component is faulty once everything is connected I decided to make the end of 50 Ohm resistor floating so that there is no doubt what is measured at the time.
11- to make 50 Ohm resistor floating - not connected to the ground, I unscrewed both from the chassis, unscrewed soldering point, of course, white BIAS wire must be disconnected from the board, the same brown wire RF in, unplug nine pins connector lying on the fan and the last disconnect the shield of the RF IN coax from the ground - easy access on TX/RX relay - see photos for reference.

12- use Ohmmeter to check suspected components - in this case, package of two capacitors showed 10.7 Ohms - separation of them and close inspection reviled that 750pF 500V is faulty, you can even see small burned whole on it.
13- replace the faulty capacitor with "any ceramic one with the value from 1.5-10nF preferably 1000V is OK"- as advised by Dan. Do not forget restore all connections!!
DONE DONE!
I know this one is overrated but this is what I could reach here in Salone

14-GU74b appeared be OK, amplifier is again fully operational.



The protection current adjustment.
In connection to the case described above, I raised the question about the current protection circuit which is in this amplifier.
Dan from Emtron have sent the following instruction/explanation:

"The protection current is adjusted by POT6, marked IPTRIP.
The best way to adjust it is to inject a current into pin TP2.
If you have an adjustable power supply that can give 1A, connect it to TP2, and with the amplifier running and READY, increase gradually current, starting with 0,5A, until it goes into FAULT.
The moment this happens tells you the limit.
You can use 0,8A, and it should not trigger the protection, then use 1A-1,5A, and it should trigger.
It is up to you how you adjust the upper limit a low value (1A or even 0,9A) protects better, but if it triggers, it resets the start-up timer, and you have to wait 2 minutes.
Instead of injecting a current, you could use a voltage.
There is a 1 Ohm resistor from TP2 to GND, which means 1V=1A.
If you unplug the white wire from the IPTRIP pin (top of the board, in the middle), then you can apply +1V to TP2 but, there is no current.
You adjust based on voltage, then put back the white wire."
During all these adjustments, the amplifier is powered up but leave it in STBY mode."


I see this instruction as very valuable, and it is why I placed it on the web page - Dan, thanks for your outstanding support!

Transformer AC INPUT CONNECTIONS
Extract from "Operating manual EMTRON DX-1b":
" Point 7.4. The Emtron DX-1b will normally arrive pre-set for the power source of thy country the amplifier has been sold to. However if operation to another mains voltage is required, the appropriate connection changes will be needed. Figure 2 shows the transformer connections for 120V, 200V, 220V, 230v, and 240V operation. If a change is required, this should be done only by a qualified technician, after taking all the necessary safety precautions..... NOTE: For 120V operation, change fuse to 20A rating."

After that you have Figure 2: "Transformer AC input connections" and the photo of transformer under the title: "The 240V connections is shown below"



I am the person who reads manual from the first to the last page - not only tuning or "knobbing".
What I noticed is:
- Figure 2 showing various connections for 120V, 200V, 220V, 230v, and 240V is the most confusing info in this manual. It is not what is installed into the real amplifier. The only proper info is on the photo with instructions showing where to "Move brown wire:" for 240V, 230V, and 220V.
- Fan connection according to figure 2 is going to Pin 120V on trafo - in my Dx1b fan is connected to the same wires as the transformer is - it means direct to mains through a soft start. The yellow wire which is connected to trafo's 120V Pin is connected to nowhere at the other end.....

Recently (May 2021) I encountered a DX1d's manual on the net - and in this manual the Figure 3: TRANSFORMER AC INPUT CONNECTIONS (page 13) there is a proper drawing of the trafo's pins - the same as my handwriting remarks on the left.
DX1b meters assembly diagram
The figure shows the wiring of analog indicators.
Both meters are protected against too high level of a signal by diodes. Note blocking capacitors 10nF/1kV attached to the each pin of the indicators.
Top meter marked Vp shows three values: Plate Voltage - Vp, Output Power - Pr and Refelcted Power - Pf, one at the time. Values are selected by switch "METER" connected to the indicator by Violet wire.
The voltage Vp is taken from the voltage divider in Power supply Module. Divider consist of six 330kOhm resistors + 10kOhm resistor. Lets make simple callculations: 6x330k +10k=1990kOhms. Plate Voltage is 2600V so 2600V/1990k=1.3V/kOhm, it means that on 10kOhms resistor exist 10 x 1.3=13V which indicator shows.

The second indicator - Plate Current Ip - it shows a voltage drop across the 1 Ohm resistor in Power Supply Module ( White wire)


While browsing the internet, I came across the "Emtron DX1d manual" . It is more "rich" than the Dx1b manual; it includes a detailed description, schemes, and adjusting procedures. It may be helpful in case of any malfunctions or tube replacement. The drawing with "transformer AC input connections" is corrected, but unfortunately, the scheme of the soft-start printed with the errors I described in the chapter: "Soft start". .

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