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trouble with Micromitter

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axeman2ooo said:
former currently stands at 12mm above the board surface, without trimming it
thought that was a bizzare instruction...
They said they had two coil formers and you must have gotten the short one.
 
okay this seems strange to me, dont know if this will help identify in any way
almost all pins have an out of 2.5VDC, except 2 or 3 of them which are closer to 5VDC
some pins read 0VDC, these are pins 6 (Ground) 10 (RF Ground) 18 (D3) and 14 (X'tal Oscillator)
checked the components around Q1 again, but nothing seems to be strange...

the VC1 has strange readings, with 2.5VDC on one side, and 0.6VDC on the other checking with the multimeter on current flow

what would happen if I pull it and just bridge it open with a jumper? i know, the answer is prob a bad one, but if one side has required volts, then i cant help thinking it is faulty...

would you know the voltages that should be at the Q1 and VC1 pins? if you do, could you tell me? i am thinking something is busted in this area...
 
axeman2ooo said:
the VC1 has strange readings, with 2.5VDC on one side, and 0.6VDC on the other checking with the multimeter on current flow

what would happen if I pull it and just bridge it open with a jumper? i know, the answer is prob a bad one, but if one side has required volts, then i cant help thinking it is faulty...

Assuming VC1 is a varicap diode?, then it's supposed to be reverse biased - and why would you be trying to read voltages with the meter set to a current range?.
 
okay not sure what you mean by reverse biased means
and i meant with the diode setting, not a voltage reading (DC or AC) it is the -->|-- setting
 
axeman2ooo said:
okay not sure what you mean by reverse biased means
and i meant with the diode setting, not a voltage reading (DC or AC) it is the -->|-- setting

Oh, you mean the diode test range! - that probably sounds correct then.
 
yeah, i never knew what it was called, just that it uses the diode symbol, so i guessed it was for that...
still no closer to finding out why this sucker isnt working yet
needing it for a house party in 1 week
 
Was your meter set to measure DC volts or "diode test" when you were measuring 0.6V at the test point that is supposed to be 2.0V?
It must be set to measure DC volts.
 
okay this seems strange to me, dont know if this will help identify in any way
almost all pins have an out of 2.5VDC, except 2 or 3 of them which are closer to 5VDC
some pins read 0VDC, these are pins 6 (Ground) 10 (RF Ground) 18 (D3) and 14 (X'tal Oscillator)
checked the components around Q1 again, but nothing seems to be strange...

the VC1 has strange readings, with 2.5VDC on one side, and 0.6VDC on the other checking with the multimeter on current flow

would you know the voltages that should be at the Q1 and VC1 pins? if you do, could you tell me? i am thinking something is busted in this area...
 
You shouldn't measure points on a powered electronic circuit with a meter set to measure current. Because then the meter is a dead short and will damage some parts. A current meter is supposed to be connected in series with something to measure its current.

VC1 has its anode connected to ground (0V) so one of its pins should measure 0V.
 
IC was measured with a pin on earth, checking its DC volts reading on the track next to pins
VC1 has one pin reading high, and the other reading very low, but not 0
have removed, cleaned under it and reconnected, but still reading 0.6VDC
this is driving me crazy, i just want to get it off my tabe now...
 
audioguru said:
If the IC is doing anything, it is trying to reduce the frequency of the PLL oscillator by reducing the voltage to the varactor diode which increases its capacitance.

Try making a coil with one additional turn to increase its inductance to reduce the frequency of the PLL. Then the voltage at the test point should rise when the IC tries to increase the frequency.



Hi,
I have the identical problem, did you ever resolve it?
Alan
 
Silicon chip micromitter

I've just built one of these kits from Dick Smith electronics too and have the same problem. However, I have got it transmitting OK. I wonder if the varactor diode is the right type. It may be in incorrect substitute. I have the data sheets for the supplied one and the one on the Rohm app note.

The thing is, for the loop to be in balance the value of the bias voltage to the varactor at about 0.6 volt may be about the right value. Increasing the L1 inductance may make sense as someone else suggested. I am running at 87.7 MHz but I dont know if the loop is balanced or right on the edge of control. That would be my question at the moment.

Maybe at the 107 MHz end it might run with a higher DC bias since capacitance REDUCES with volts, and this would happen at a higher freq. I can get about 5 metres with the attenuator and at least 25 metres without, using an old Sansui receiver in the garage as a test machine. I have access to high bandwidth oscilloscopes and a spectrum analyser too.
 
Problem of less than 1 volt at TP1-solved

If anyone still interested in this fault, reply here as I understand the issues from first hand experience.
 
Hi Geoff,
Please tell us your first hand experience with fixing the problem with The Micromitter.
 
Micromitter- Low TP! symptom

The low value of volts at TP1 indicates that the closed loop frequency control is not operating. I'll bet that most builders having this trouble are trying to use the 88MHz end, and that makes sense because most would think that is a quiet part of the band. (I have a friend who bulids FM transmitters in the 10kW range for community radio, and he suggest that in Australia, this part of the band is less suitable because it is used for low power intentional transmitters)

I had 0.6 volts at TP1, so I tried running at 107.9 MHz and the voltage at TP1 suddenly jumped to about 4 volts. I could then easily trim to the 2 volt spec. I chose to run at 106.9 MHz which is silent in Melbourne Australia. It looks to me like the range of adjustment of L1 is not enough to cover both ends of the band, and if you need to use the low end of the band, L1 may need another turn. I can't say whether that won't then have a too high value though, you will need to experiment. The 22 pf cap could be increased a little for the 88MHz end, but this will have the effect of reducing the range over which the varactor diode operates, and reduce its freq range. I am using a 6v battery too, so I set the volts at TP1 at 2.5 volts using the slug adjust.

In short, if you have 0.6 volts at TP1, it does not mean there is a problem, try the 107MHz end.
 
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