Adret 740A signal generator – fixing the lack of output signal

Styczeń 15th, 2012

The problem with Adret 740A signal generator was the disappearance of the output signal at the N output in the upper power range. Sometimes the signal was present for several hours, sometimes it would disappear for a few minutes after switching on, sometimes there was no signal at all.

However, at the control output on the back of the instrument the signal was present all the time (0.5-1.2 mW depending on the frequency and attenuator setting).

While searching the web I found the complete instructions for use with all the schemes on page http://boulayp.free.fr/f6iib/ (Page requires log in) from where I took the schemes.

What I found on http://www.bobford.plus.com/finished.htm was a false trail for me – even though it contains a description of repairing a very similar defect of Adret 740A.

I suspected the problem originated in output attenuator (page 19).

It works in such a way that attenuators of a specific attenuation range are switched off  by internal relays from the approach board (17a). +12V voltage on the relay means that the attenuation is zero, no voltage means nominal voltage attenuation.

To check the voltage on the specific relays, one has to check the voltage on the ribbon cable that connects the VHF board (33b) and the doubler (if you’re in possession of Adret version 740A to 1200 MHz) or is attached to the casing of the attenuator (for 560 MHz version).

On the VHF B board, voltage goes through another ribbon cable to relays from the buffers (LM324) on the approach board.

Unfortunately, the numbering of the connector 26 pin ribbon cables does not correspond to the numbering on the scheme, but to its mirror image, so finding the voltage going to the specific relay attenuator requires some attention.

To simplify the search, we set the output level of +13 dBm (maximum signal) which means that all the relays in the attenuator are powered .

In my case, it appeared that there was no +12 V voltage to the relay one of the attenuator 30 dB. The voltage goes from the LM324 (described as SN10 on the scheme). More specifically, the voltage goes from the operational amplifier located between pins 5,6 and 7.

When active, the amplifier should be about 12 V on pin 7,  4.2V on pin 6 and 5.2V on pin 5, the off-state voltage at pin 5 and 7 is 0 volts.

In my case, the voltage at pin 5 fell from 1.8 to 2.4 V, which resulted in an uncontrolled switching on 30 dB attenuator. Cursory inspection of the elements showed that the pin 5 resistor is 100 kΩ is OK and  there is no short circuit on the 0.22 μF, which suggested a failure of LM324.

After careful desoldering  of  LM324  (important note – printed circuits are sensitive to longer heating), adding a 14-pin socket (for easier replacement) and replacement of LM324, I noticed that after 10 minutes of normal operation, the output signal disappeared again.

I managed to find next 5 pieces of LM324 manufactured by different companies that exhibited similar symptoms – output signal sometimes was present, sometimes it disappeared.

After ruling out the microdamages of print tracks, what remained was -the last item – 0.22 μF capacitor, which could be damaged.

After replacing it with a new one, my Adret 740A started working properly! As it turned out – after connecting the 12VDCvoltage to the desoldered capacitor, it showed unstable leakage current of 3-10 μA and the ohmmeter showed unstable internal resistance of 1-4 MΩ, and the capacitance meter pointed from 0.14 μF to 0.16 μF depending on the temperature.

There are  many blue capacitors on the PCB, but I decided not to change them.

ADRET 740A has a good signal stability, which I have used recently in experiments with TRV at 24 GHz, as a 105.473 MHz signal source (x228 = 24,048) to the oscillator circuit from the 23 GHz radioline from the DMC (white box), used by me as a bacon control, as well as the oscillator for the conversion of 24 GHz/432  and 24 GHz/1296 (after changing the frequency of the generator to 107.320 or 103.579 x212 x228).

12 komentarzy to “Adret 740A signal generator – fixing the lack of output signal”

  1. luison 02 Lut 2013 at 12:42 am

    Hello from Spain and many thanks for your information about Adret 740 and your fixed problem.
    I`ve got one from ebay,as faulty too.the problem is:power up but it doesnt react to any key stroke and shows all the time 888888888 8888 8888 on displays
    Of course is a CPU board problem ,so I’ll start with caps replacing.Any idea if it doesnt work?

    Thank you in advance !

  2. kevin smithon 01 Lis 2013 at 11:47 pm

    i have a 740a with a problem where the display from switch on reads ‚740-25.1’ this has had a new memory battery which hag leaked and distroyed tracks which i repaired and got it working again, but now has this fault on it. it did come alive a couple of times but noticed the display interacted with other displays when setting frequency. also there was no output in this condition.

  3. Staszek SP6GWBon 17 Lis 2013 at 2:51 am

    kevin, the problem you are describing is new to me and I do not know how to fix it. However, I can send you the manual and maintenance pdf with boards schemes – just let me know it you are interested.

  4. Gary VK8BNon 29 Gru 2013 at 5:01 am

    Staszek,

    Do you have a copy of the English Fault diagnosis flow chart for the Adret 740a.
    The unit I have outputs incorrect frequencies and will not output any low frequencies.

  5. j.christophe F4ENGon 08 Cze 2014 at 7:38 pm

    hi,
    i’ve a pdf copy of the English Fault diagnosis for Gary if still interested.
    i’ve just finish to repair one faulty 20000 steps module.
    73s F4ENG

  6. j.christophe F4ENGon 08 Cze 2014 at 7:39 pm

    Sorry french Fault diagnosis.

  7. kevin smithon 24 Sie 2014 at 3:18 pm

    j.christophe F4ENG
    is it possible to send me the english version of the manual please as i have a fault with my 740a.
    many thanks,
    kevin

  8. vinceon 28 Lip 2015 at 3:33 pm

    both links no longer work

    can you email me schemes please

    http://boulayp.free.fr/f6iib/ .

    What I found on http://www.bobford.plus.com/finished.htm was a false trail for me –

  9. kazikon 08 Sty 2016 at 12:32 pm

    I have problem with my Adret 740 now, foult is in modulation unit. can you help me in documentation?
    regards,
    kazik

  10. Clemgillon 01 Cze 2016 at 6:20 am

    Hi,
    Does anyone know how to adjust the frequency calibration of the 740A? I cannot see it in the manuals I found.
    Thanks much,
    Gilles.

  11. Clemgillon 01 Cze 2016 at 9:30 pm

    I found it. There is a srew on rhe rear called „phase locking” and it tunes the internal 80mhz pilot.
    Easy.

  12. saeed(EP2LSH"on 03 Lis 2016 at 8:24 am

    is it possible to send me the manual please as my 742b have a fault which i need to repair.
    Vy 73 de ep2lsh

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