Trouble Oscillation qcx 40? #40m #alignment #problem #qcx40
开云体育Hi, Gianpi This is almost certainly an audio problem. Problem is, everything after the FST3253 is audio. So we have to somehow "divide and conquer", determine the point where the tone enters the audio chain. The first, and easiest to use, separation point is the volume control. Does the tone vary in volume with the volume control, or is it still loud with the control at minimum? 73 Jim W4JED On 6/11/22 09:56, info@... wrote:
Hello this IK1TTD Gianpi and I have build a qcsx 40.... But at the power on I got a loud tone (much higher then 700 hertz described in the trouble shooting. It is impossible to wear the earphone as the tone is incredible loud, and can be heard also without earphone... I try to start the alignement, and the tone is always present, when I exit from the Align process, and I plug the antenna, I cannot listen anything except the continuous loud tone. Can you help me please ? Kind 73.- |
开云体育Hi, Gianpi The next thing to check is whether or not the volume control
thinks it's a volume control. Measuring from the three lugs on the
control to ground, one should check zero ohms to ground, the
opposite lug should check roughly 5000 ohms to ground (it's a +-
20% part) and the middle connection should sweep smoothly from 0
to 5K as the shaft is rotated. Volume control connections and an
occasional broken control were somewhat frequent problems
initially. I eliminated the problem with mine by putting it into a
box with a separate front panel, with the connections made with
stranded wire. My radio box also has 2 6-volt NiMH battery packs
in the bottom for 12 volts, and a charging/charge control
connector on the back. 73 Jim W4JED On 6/11/22 18:50, info@... wrote:
Hello James and thanks for your reply. To reply to your question yes, also with the volume control at zero the tone is loud and clear.... Increasing it, along the rotation seems it's vary just a little bit but always loud. I'm finishing my trip in Florida in 2 days, and next Tuesday I will be at home, and if helps. I could make a short video to give you more info about. Let me know in the meanwhile if there is something else to test it. Talk you soon. Best 73 de IK1TTD Gianpi? |
开云体育Hi, Gianpi, Could you run the resistance checks again, only with no power applied? There was something strange happening there, but it could have been the way the meter reacts to mixed signals. I would expect the "near" connection to stay the same, regardless of where the shaft is rotated to; the middle connection to vary from almost ground to whatever the "near" connection reads, and the third connection to always read ground. This is one of those cases where an oscilloscope would help a
lot. If the volume control can be unplugged from the circuit, try that and see if it still "squeals". 73 Jim W4JED On 6/14/22 14:06, info@... wrote:
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开云体育Hi, Gianpi That sounds more reasonable. At this point we have 2 possible sources - Q7 or IC9A. I think the easiest way to eliminate one is to lift one end of C21. Carefully, these little ceramics are a bit fragile. If it still squeals, lift the drain connection of Q7. One of these should have stopped the squeal. 73 Jim W4JED On 6/14/22 18:18, info@... wrote:
With no power I have:? |
开云体育Gianpi, I think that the squeal starting when the frequency display starts means it needs the microprocessor to be in "operational mode". That means SI5351 started, and all the RX/TX voltages set to RX. Not sure if it will help with troubleshooting, but I will keep it in mind. For our next trick, put C21 back and remove R29. That's our next break point, dividing the radio into two pieces again. If the squeal comes back, we need to look in the 700 hz bandpass filter area. If it stays gone, we are looking at the mixer or the sideband reject circuit. 73 Jim W4JED On 6/15/22 19:20, info@... wrote:
Jim, I guess we have a small progress ! I have removed C21 and the loud tone is disappear...but nor completely... |
开云体育OK, Gianpi. It looks like you do pretty desoldering work. For our next plug in, pull out - put R29 back, and remove C20. We're sneaking up on that bear. Also, just for giggles, check the voltage on pins 1 and 7 of IC8
and IC9. 73 Jim W4JED On 6/16/22 12:54, info@... wrote:
Thank Jim.. see in this video what happens after having put C21 and removed R29 |
开云体育Hi, Gianpi The voltages are OK. There will be some variation because of resistor tolerance, etc. I was looking at the picture (thanks for the really sharp video) and realized that there was nothing in the holes for C14, C16, C18, C23, and C33. Is it possible that you missed page 28 in the assembly manual? These 5 capacitors are all installed on one page, and are all 1 nf. C23 would be the reason for all the hiss - if it was present it would roll off the frequency response of the audio driver stage, preventing hiss. 73 Jim W4JED P.S. Back when I was working up North (mid 1980's) I was pulling
my hair out over a VCR with no color on playback. Finally I asked
the other bench tech to look at it. He walked over, glanced at the
board, pointed, and said,"how about that broken corner on that
board?" The board had a piece broken off one corner, about a
centimeter on the top and left sides. There was exactly one foil
path interrupted by the break. It was the feed from the tape head
preamplifier to the color process circuit. One little piece of
fine wire later, and the unit had color.
On 6/16/22 19:08, info@... wrote:
Thanks Jim... here the result of desoldering C20 |
James Daldry W4JED