trouble with Micromitter

[axeman2ooo]

hi
built the circuit for the Micromitter, but I think something is not right...
The instructions say:

"
(1). Set the transmission frequency using the DIP switches, as shown in Table 1. Note that you need to select a frequency that is not used as a commercial station in your area, otherwise interference will be a problem.

This is done, no problems

(2). Connect your multimeter's common lead to TP GND and its positive lead of to pin 8 of IC1. Select a DC volts range on the meter, apply power to the Micromitter and check that you get a reading that's close to 5V if you're using a DC plugpack.

This is done, no problems. Using a plugpack selected to 5V

Alternatively, the meter should show the battery voltage if you're using AAA cells.

(3). Move the positive multimeter lead to TP1 and adjust the slug in L1 for a reading of about 2V.
"

This is the problem area. reading is 0.6VDC and adjusting the slug makes no change. I have no signal to the FM, but everything seems fine, checked the tracks with the diagram perfectly, making sure with a blade that nothing is touching. measured the diodes, i think they are ok...

any ideas? driving me insane...

 

[Russlk]

Can you post the schematic, or a link?

 

[axeman2ooo]

sure
sorry about the mess, it wouldnt PDF for me, so attached the html files etc in a zip for you

 

[audioguru]

Where did you get the pcb and L1? Did you make the kit?
Maybe the polarity of the varactor diode VC1 is backwards.

 

[axeman2ooo]

bought it from dicksmith electronics (they get them from silicon chip)
will look into swapping the VC1 around, can barely see it, and no stripe on it! :S

 

[audioguru]

VC1 is a diode. Measure it!

Are you Down Under? Are you getting springtime now? Here we got just a little snow today for the first time this winter.
G'day mate.

 

[axeman2ooo]

tried it around both ways, same reading
measured the diode (VC1) when it was off the board (damn hard to see!!!) and it measures OL on one way, and 0.6VDC on the other, so seems to be functioning (no idea where it got 0.6VDC from!)
could it be one of the other 2 diodes?
they seem to be in correctly... any ideas anyone?

 

[axeman2ooo]

oh, and does it really matter if the crystal is laying down on the resistors? mine is standing, and dont have the extra legroom to bend across now...

 

[axeman2ooo]

all diodes are checked, and all are now in the correct way, but the voltage at the L1 is still 0.5VDC
voltage on pin * is at 5V which is where it should be, so power is getting to the IC1, but after that, its all weird... checked points, doesnt seem to be wrong anywhere...
once it works, i am going to mod it to boradcast to 150M, but still cant get it going to 5 metres!

please help, anyone?

 

[axeman2ooo]

also, with this:

Crystal X1 can now be installed. This is mounted by first bending its leads by 90 degrees, so that it sits horizontally across the two adjacent 10kΩ resistors (see photo). The board assembly can now be completed by installing the DIP switch, transistor Q1, regulator (REG1) and the antenna lead.
is it important that it is on a 90* angle?

 

[axeman2ooo]

VC1 is a diode. Measure it!

Are you Down Under? Are you getting springtime now? Here we got just a little snow today for the first time this winter.
G'day mate.

i believe it is measured good
never measured diodes, so not too sure, but seems to be in the correct way
like I said, got a current on the IC1. just not correct on the coil...

 

[axeman2ooo]

just found this at "silicon chip" website:

I purchased and built the Micro-mitter kit from the December 2002 issue and I am having a couple of problems. First, in the test and adjustment section, it mentions to set the voltage at TP1 to around 2V by adjusting L1. All I seem to get is 0.61V, no matter what adjustment I make.

Second, even though I only get 0.61V at TP1, I still manage to get an output when I connect a stereo signal to the RCA input but the output is mono not stereo. Can you please help?
(W. P., via email).

The problem most people are having with this project is with the adjustment of the PLL voltage for the RF oscillator. The cause is usually that coil L1 is not wound in the correct direction. You must wind it in the direction shown in the photo, with the coil close to the PC board and the turns touching each other. In addition, the bottom of the former should be flush with the bottom of the PC board. If this is not done, the coil’s inductance will be too high.

will try this in a moment, and let you know...

EDIT: still no go, the coil matches the photo exactly (had it going in the wrong direction) and voltage is still the same (0.6VDC)

 

[audioguru]

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.
1) Maybe the frequency-selector switch is bad. Try another switch setting.
2) Maybe the slug in the coil isn't in far enough.
3) Maybe the three capacitors near the coil and varactor diode got mixed up.
4) Measure the voltage at pin 7 of the IC.
5) Measure the voltage at the voltage at the collector of Q1.

 

[axeman2ooo]

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.
1) Maybe the frequency-selector switch is bad. Try another switch setting.

Tried several different frequencies, all same result...

2) Maybe the slug in the coil isn't in far enough.

screwed as far as i can (about 7 turns past top) and out just as far, no change...

3) Maybe the three capacitors near the coil and varactor diode got mixed up.

all capacitors are checked, all are in the right places...

4) Measure the voltage at pin 7 of the IC.

reads at 4.6VDC

5) Measure the voltage at the voltage at the collector of Q1.

reads at 0.6VDC

 

[audioguru]

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.

 

[axeman2ooo]

ummm.... okay
except the instructions insist on it being 2.5 turns only...
you think that an extra turn will make it all work, or work better?
will it bring it to the required 2V?
remember the L1 is still not able to be adjusted...
i am rather concerned that it isnt adjustable, how would I be able to adjust the oscillation without it?
thinking of buying a premade L1 tomorrow, perhaps that will fix it

i cant tell from the schematic, is the 0.6VDC on the Collector of Q1 the expected voltage?
perhaps Q1 is faulty?

 

[audioguru]

except the instructions insist on it being 2.5 turns only...
you think that an extra turn will make it all work, or work better?
will it bring it to the required 2V?

It is easy to try it. Then maybe you will be able to adjust the core to get 2V.

thinking of buying a premade L1 tomorrow, perhaps that will fix it

L1 needs to be adjustable.

i cant tell from the schematic, is the 0.6VDC on the Collector of Q1 the expected voltage?
perhaps Q1 is faulty?

I think Q1 is working normally since pin 7 is high which turns on the darlington and it saturates with a voltage of 0.6V.

 

[axeman2ooo]

cant i get an adjustable coil from a store? seen them on older FM transmitter bugs etc (i think)
i found pin11 was not connected properly, so that is done now, but still same reading on the L1
silicon chip is replying to emails now, but so far nothing new

 

[audioguru]

cant i get an adjustable coil from a store?

You are going to ask at the store, "One coil for the Silicon Chip Micromitter of 2002 please". The store guy will say, "Huh? What value?" You will say, "I dunno". Store guy, "Goodbye."

Did you shorten the former on the coil?

 

[axeman2ooo]

former currently stands at 12mm above the board surface, without trimming it
thought that was a bizzare instruction...