Problems when testing my Digilite

[Codeine]

Hi Everyone

After several months I successfully completed my Digilte build for 70cm and
connected it to a power supply and it was drawing 150mA...great..!.
After reading page 20 of cq-tv235 Initial DC voltage settings and I quote

Sliders of I & Q Bias presets R29/R30 should be set to midway initially
(2.1v on sliders) Set I & Q Level presets R16/R23 to mid-way. ( 4v on there sliders)

Mine are exactly the opposite R29 and R30 I and Q Bias have 4 volts on
the wiper and R23 and R16 have 2.1 volts on there wipers. Know either
I have made an error during construction or this is just a typo...?

It also says in cq-tv page 20 that the output leg of my MIMIC should be 4 volts
but mine is only 1.75 volts..! The led flashes twice a second when no data and
not in TX mode. All the regulators are correct and working and the FT2232H
interface is installed and working and the LED flashes ounce every a second
when in TX mode. has anyone else had this problem or have a solution..!

73

M0PDA

[g8ajn]

The readings were taken from my build and confirmed by someone else, so should be correct. Certainly set the pots to midway anyway, if there is not a range of volts from the sliders when moved I would suspect the half-rail regulators,U3 and U4. You should not have more than half ht i.e. 2.5 volts at the bottom end of the presets R16 and R23 this is adjusted by R29, R30. Adjust the sliders and check the change of volts on the IQ input pins 1,2,15,16 of IC4.
The 8volt line feeds the MMIC through L5 and the ht resistors R26,R41. The MMIC output dc is set by internal bias so should always be right.Which device are you using I wonder?
Most of the (thankfully few) problems with construction have been inadequate soldering on the pins of the ICs, the legs sitting actually above the solder pads. Solder bridges. Cracked smd capacitors.
I was thrown on my early discrete board build when one of the new axial inductors L2 was o/c.
Obviously with tolerances where different suppliers are involved some dc readings can be different but should be somewhere near.
Which IC type of IC1 are you using? There is a tight voltage range from the 3.3v IC2 and if there is noise on the waveform going into IC1 it could prevent proper operation.
If you haven't already, try the help page on OTHER/CORRECTIONS on the DL website.
Let us know if there is still no improvement.
Dave G8AJN

[Codeine]

Hi Dave

Thank you for your reply. It turns out that the reason for the wrong voltage on the output of the MMIC was that the dot on the PCB was in the wrong place. After consulting with a local amateur who has built a working Digilite we discovered that I had
soldered the MMIC in the wrong way round and so I removed and fitted a new one with the dot next to C30.The dot on
the PCB Ver 5.7 was next to JP3 and not next to C30.The MMIC voltage is know correct but the output of pin 11 on IC4 is a little low at 2.85 volts and should be 3.5 volts.

I also got him to check the volts on his working Digilite on the wiper of R29 R30 and his was also 4 volts and R23 and R16 was also 2.1 volts..! So apart from a minor voltage difference on ouput pin 11 of IC4 of 0.65 volts I think everything is ok and IC1 is 74ACT574. Finaly when I set the Digilite config to test mode "No Mod" will this allow me to test for signal strength and or quality
just using the output of the Digilite or will I need to fit my 70cm Ultram VCO in an airtight box of course on the input as well...?

Paul M0PDA

[g8ajn]

OK Paul, glad to hear you are making progress. The white dot was left over from a previous version(not released) that used a different device entirely, but the dot was not deleted in my CAD files by mistake. There is a post about this elsewhere on this site.
If you have a Spectrum Analyser available then the options in the Transmit program are useful for setting up and measuring but if not you are best to use the on-screen quality reading on your receiver instead. See Brians comments at http://www.g8ajn.tv/dlsoftware6.html.
As to whether you need the amplifier stages or not for setting up, I would suggest that an ac coupling into the receiver direct will work, but if it overloads the input you will need to loose couple it. The problem with putting a 5" wire on the output as an aerial is that it tends to spray signals in all directions and get reflections in all phases so can be tricky to keep steady enough to setup.
The same can apply to the oscillator board, it seems to be different for everyone, some can leave it lying on the bench without problems, others seem to need to place it in a lead-lined box three feet down into the garden!
Some voltage readings may be different because of the 70cm aspect, but I will check again and if the readings are incorrect on the pots here also I will alter the website accordingly.
Thanks for the feedback. Let us know what quality reading you get from the receiver when you get it all running as I don't have a lot of info on the 70cm version performance.
Dave G8AJN