Electronics > Repair
Understanding and Repairing Clock Generator on Adret 742A UHF Generator
<< < (3/10) > >>
PerArdua:
Interesting posts, thanks both.

A clear indication that the filter is supposed to be quite selective of the 4MHz component, thanks for that DLJ. Interestingly I spotted your board's Q3 is the opposite way to mine, using another MPS3640 according to the schematics from t&f's website here https://archives.doctsf.com/documents/feuilleter_document.php?num_doc=52087&ref=39759.

Not to put you on the spot DLJ, but would you feel up to measuring the inductances of T3? As this would involve removing it from the board I'd totally understand if you'd prefer not - no pressure meant at all. I'll be trying the separate 4 MHz crystal next week in any circumstance, though I am suspecting the route of my results is an incorrectly rewound T3. A slight niggle in my mind is that the schematics show an offset centre tap on T3, but the parts list suggests it is even (15+2x7 turns). Maybe artistic licence, but maybe not.

Flashing red = bad, flashing orange = good? That's a nice choice of colour haha. ;D

Timeandfrequency, a fascinating video - I think that is our CPU board going through the wave soldering process as 0:23!
timeandfrequency:
Hi PerArdua,


--- Quote from: PerArdua on January 12, 2023, 10:18:59 pm ---I think that is our CPU board going through the wave soldering process as 0:23!
--- End quote ---
Wow ! You are probably right.

The SM tells that the yellow LED 'occupé' (no mention of orange colour, but for sure it's the right LED) flashes when the µP is busy. So DLJs CPU board is probably in perfect working condition. The 'busy' LED is driven by the 'BUS AVAILABLE (BA)' signal which is at pin 7 of the MC6802.

It might also be interesting to mention that the MPS3640 (Q3) has a much lower current gain than the BC560C (Min value is 30 @ Ic =10mA  vs 380 @ Ic = 2mA). fT is twice as high for the MPS3640. Those parts cannot be considered as equivalent but, nevertheless, the B560C seems to be a better choice.

Indeed, having a deeper look into that section of the schematics shows that Q3 is actually set as an emitter-follower (also named 'common-collector') : this stage provides no voltage gain but only current gain and therefore its function is to decouple T3 from the logic gate input. The less current is drawn from T3, the better it is possible to keep its Q value at a high level, that speaks for better selectivity. So even if T3 is in good health, drawing too much current (and that's also true for the primary windings) creates a burden that hampers its ability to extract the 4 MHz frequency from the broad spectrum signal that comes out of the LS196.

To summmize : T3 might be faulty, but if it's not the case, that means that the surrounduing parts are involved in creating the mess.


And if the failure persists after changing those surrounding parts, the mess might be located below the parts, that is inside the PCB.

You explained that the Ni-Cd battery on your CPU board leaked and that the electrolyte spilled in the area where T3 & Q3 are located. Unfortunately, it might happen that the electrolyte also soaks into the PCB, which ruins its insulation resistance. The electrolyte is now trapped inside the lamination layers and it has to be removed from there.
Maybe other forum members have significant experience about PCB restoration after electrolyte spillage and could provide useful hints. If you want to head towards that direction, maybe that creating a dedicated thread on that topic would help to gather the largest possible audience.

If I had to perform such a restoration, I would proceed as follows. Soak for at least one hour the PCB into an ultrasonic cleaner filled with isopropyl alcohol. This has to be done outdoors  (caution : flammable vapor). Replace the alcohol at least twice and when done, bake the PCB in the kitchen oven for 4 to 6 hours @ 50°C, with (forced) convection turned on.

A bit of SPICE
Applying an FFT to the signal you recorded, which is coming from the LS196 output, confirms that T3 has to act as a rather selective bandpass filter to catch out the wanted 4 MHz carrier. The second strongest signal in the (actually infinite) spectrum is the 6 MHz peak @ -3 dBC and then the 2 MHz peak @ -6.5 dBC.
DLJ:
Hi PerArdua,

I have tried to measure the impedances of the transformer windings.

I am not super confident of these measurments, but I have repeated the measurments and observed the same results.
Measurments taken using a Marconi TF 2700 (out of calibration by several decades  :D).
One winding of 4.5uH
second winding of 7.5uH and 13uH, giving a total of 20uH

I wonder if the Adret description "2x7+15" does not refer to turns, but instead indicate uH, as in one winding of 2uh, and a second consisting of 7uH and 15uH.

Good luck
DLJ
George Edmonds:
Hi Per Ardua

I really did not want to be involved with this conversation, but as there appears to be a total lack of understanding as to the role of T3 I am reluctantly doing do.

T3 is a simple L/C tuned tank circuit with a resonant frequency of about 4mHz, it forms the band pass filter shown on the block diagram, now given that the parallel capacitance is 164pf the main tuned winding must be in the region of say 11uH, Spice not needed, just a bit of basic tuned circuit theory.

The use of a tapped primary and a small secondary is done to maintain as high a Q (narrow bandwith) of T3 as is possible.

I have tried to make some measurements of T3 from a 742A that I own, these are out of circuit measurements on T3 alone, although repeatable they do not make complete sense.  They were made at a frequency of 100kHz using a Peak LCR45, possibly at still too low a frequency to be accurate, measurements made using the 1khz of a Marconi 2700 were complete nonsense, especially for the secondary.

I found entire primary was 14.97uH
Primary tap to one end was 3.97uH
Primary tap to other end was 8.85uH
Secondary was 3.2uH

Yes I know that these do not add up correctly, but that is what they measure on a virtually new Peak LCR45 and on a number of other LCR meters that I have that operate at 10kHz and above.

If T3 is operating correctly you should be able to adjust it for maximum smoke on the secondary at 4mHz.

With Adret you need to decide if you want to use a signal generator or to spend your time repairing one.  I have repaired a number in the past, but now the two that I own sit in my store unloved and unused.  I use Marconi Instruments kit only for a signal generator as these just simply work.

The best thing that Marconi Instruments ever did was to put Adret of its misery.

G Edmonds G6HIG
George Edmonds:
Hi Per Ardua

Further to my last I have now had time to look at my printed Adret manuals.

T3 consists of an input winding of 7 Coils with an over wind of 15 Coils, the entire winding is in parallel with 164pf to form a resonant tank circuit at 4mHz.

The output winding is of 7 Coils.

Adret use the term “Spires”, which translates from technical French as Coils.

My measurements also confirm the above.

G Edmonds G6HIG
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod