Original by:
Jörn Bartels 49088 Osnabrück dk7jb@yahoo.de
Repairing the OCXO MORION MV89A
Introduction to the repair report by Andreas
Andreas Zimmermann DH7AZ describes in this nice report the repair of several OCXOs of the brand MORION MV89A.
The OXCO MV89A has excellent phase noise in the low frequency range.
The short term stability and long term stability are also very good and can easily keep up with OCXOs that are 100 times more expensive.
Error Signs
We have observed the following typical error patterns with the OCXO MORION MV89A:
- Output level very low
- Heater does not work
- Heater does not being regulated
Open OCXO non-destructively
2 methods to get to the inside of the MV89.
a) You can heat the base with a heat gun or a small (!!) gas burner until the tin melts and you can remove the lid. The latter (gas burner) is easier for my taste.
b) You can also use a fine, sharp blade to scrape/peel away the solder seam - similar to linocut or copperplate engraving. The base can then be carefully levered out with a narrow screwdriver. This is not entirely successful without leaving marks on the housing, but these can be repaired later with flat-nose pliers.
There are apparently several variants of the MV89A, I have some here with a thin, sheet metal-like base, but also some where this is designed as a solid metal plate.
Further differences can also be found in the layout, Rudi, DC2YF has a newer MV89A, where the ominous decoupling capacitor is placed quite centrally, which is quite hostile to repair. The rest of the layout also seems to have undergone occasional changes.
Error pattern: Output level too low
Many MV89s "suffer" from an output level that is too low.
In this case, the coupling capacitor on the output pin is defective.
It is a chip capacitor rated for (presumably) 10nF. In 2 of my patients, one of the "solder caps" was brittle (so-called crack).
The repair consists of installing a new capacitor.
The decoupling capacitor is obviously placed in a mechanically very unfavorable position close to the output pin.
There are newer MV89s where this C is located more centrally on the circuit board, but failures have also been observed here.
Such cracks are caused by mechanical stress.
The output level specified in the data sheet is only achieved by one of my Morions.
Whether a deviation of several dB (>4dB) downwards already indicates an imminent total failure is currently still a matter of conjecture.
Position of the decoupling capacitor, seems to break from time to time.
Rudi (DC2YF) told me that there are newer MV89A where this C can be found in a different place.
Error image: MORION no longer heats
A Morion no longer wanted to heat. Here I found the cause to be 2 cold solder joints on the voltage divider resistors for the NTC/OPV control circuit.
After soldering, the MV89 reached its steady state after approx. 8 minutes.
I was able to measure 80.0 degrees at the NTC.
This corresponds quite well with the information that can be found on the reversal point of SC quartz crystals.
Note: It is advisable to also replace the heat-conducting paste that has (almost certainly) become dry. To do this, unscrew the retaining plate of the 4 heating transistors, carefully remove the old paste and apply new paste.
Main board: The power amplifier can be seen in the first quadrant in blue, next to it in red are the (here) incorrectly soldered voltage divider resistors of the heater control system
Another fault was cold solder joints on the
Voltage divider resistors for temperature control.
Really "dry joint" here!
When lightly tapped with a measuring tip to ring through, there was sometimes even contact.
The exact position is marked in red in the picture above. There are 2 resistors, a larger value and a smaller value connected in series for more precise narrowing. This is a very clever way of avoiding a potentiometer that is susceptible to interference and TC if the NTCs cannot be selected precisely enough.
Error pattern: Heater keeps heating
There was another error in the control system of an MV89. The OCXO did not stop heating, I was able to measure >90 degrees.
A cold solder joint had formed on one of the two solder joints of the NTC.
This fault could also be fixed by re-soldering.
Regarding the "heating fault": the 2 wire ends of the PTC can be seen on the right-hand morion.
Here, too, I had a cold solder joint on a patient.
You can also see here that there are apparently several Morion versions, one had a thin flanged sheet metal base, while the model on the right has a somewhat more solid base plate.
The inner oven
By the way: The voltage divider resistors for the temperature control are apparently "hand-picked", I have found different assemblies of a large and a smaller R in two MV89s.
Presumably the NTC parameters are quite scattered and a potentiometer was too susceptible to interference or had too large a TC for the people in St. Petersburg.
I then turned my attention to the inner oven.
Here the same types (BD677) are used as heating transistors as in the outer oven.
Only the current consumption is much lower. The final temperature is only reached in the removed state with very good thermal insulation, of course, the preheating by the outer oven is missing, so to speak.
I was able to measure a switch-off temperature of approx. 83 degrees, so 3 degrees above the temperature of the outer oven seems plausible to me.
I avoided dismantling the Morion for a long time, but I can see that the actual oscillating quartz is still in a hermetic round housing.
In my case it is silvery, but I know from another OM that it is brass-colored.
At some point I did open the inner oven - I was too curious! There are no big surprises, but I was a little surprised to see 2 toroidal cores.
One serves as a resonant circuit, the other is a decoupling transformer.
Above is the view of the inner oven: 2 circuit boards, in between a somewhat airy sheet metal housing with 2 heating transistors.
Ditto, different view
Inner oven with "Add-on parts".
Here a decoupling transformer.
The internal sheet metal parts once again enclose the quartz in the round housing.
In contrast to the outer oven, the inner one is not "airtight" soldered.
This is what the inner oven looks like when opened. The quartz is in the presumably/hopefully vacuumed round "box". In some Morions this can also be brass- colored, here it is silvery.
More pictures
Main board with the 4 heating transistors, low-noise voltage regulator (10V), etc.
On the left is the tin can of the outer oven - it sits between the 4 heating transistors.
Complementary push-pull output stage.
Another repair
When repairing an OCXO, Andreas was able to find and rectify another fault.
An OCXO did not stop heating.
The reason was a break in the conduction line on the wired NTC.
The picture shows this at the edge of the large soldering eye.
Pictures from Raimund DL3OP
I would like to thank Raimund DL3OP for these interesting pictures and the permission to include them in this document.
Circuit diagram of the Morion MV89A - reversed by Raimund DL3OP
Phase noise measurements
Here is an older measurement of two Morions against each other:
Two 10 MHz Morion OCXOs with downstream quartz filters were measured against each other.
Here only as a supplement (measurements at 100MHz):
Here my two best self-built 100 MHz oscillators were measured against each other.
The crystals used were cheap AT crystals from Reichelt, which have been extensively selected.
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