Having problems with NEScaf kit
October 20, 2011 - 21:31 — smviking
Hi!
I built a filter a few weeks ago and it works great. I told a friend of mine and he ordered and built a kit too.
He was not as fortunate as I was and his filter is dead! We've have checked the pcb for problems like soldering, wiring, etc and tested the regulator, the clock and the audio amp and they are all working fine but no signal comes through the filter, i.e. the SCAF chip is not working.
The only thing we can find that's out of spec is the Vreg/2 voltage. Vreg is 9 V but Vreg/2 is only 3.7 V. We have checked R17 and R18 and both are 10k. This is a mystery as a voltage divider with 10k + 10k should give 4.5 V.
However, according to the spec sheet the LTC1060 should work with a Vcc as low as 2.5 V. There must be another problem??
The following measurements were made with the LTC1060 removed. Coming back to the voltage divider R17/R18. The voltage over R17 is 5.3 V and over R18 it's 3.7 V?? Disconnecting R18 from ground Vreg/2 should be basically the same as Vreg but not so - it's only 7 V!!??
IOW, with the LTC1060 removed Vreg/2 is still drawing about 2 mA? We have disconnected C1 and C5 with no change.
Pleas HELP!
73 Bert, VE3NR
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Comments
Problems with NEScaf kit
Hello, Bert.Sorry to hear your friend is having trouble with his NEScaf kit.Thank you for the detailed description of your troubleshooting thus far. With IC1, C4 and C5 removed, there should be nothing connected to the Vreg/2 line (except of course, R17 and R18). The fact that when you disconnect R18 you do not see 9v on that line tells me that there MUST be another path from Vreg/2 to ground via some resistor. I suspect that there may be a short between adjacent pins of the IC1 socket. This is assuming you have used a socket. Normally, with the IC removed, pins 5, 12, 15 and 16 should be shorted together and NO connection to any other pins. Please check carefully around the IC socket. There should be no path from those pins to any other point on the board. In particular, you should not see any resistive path to ground from those pins with the IC removed. Good luck, Bruce, N1RX
NEScaf problem resolved
Working together with Bert, we have been able to bring the recalcitrant filter to book. It was ulimately traced to some questionable solder connections and a mis-wired switch. All is well now, and Bert reports the filter is performing nicely. -Bruce, N1RX
Can't get NEScaf filter to work
Hi,
I built my NEScaf filter in February of last year and was never able to get it to work. Due to a new job and a move, it got set aside until now.
I am using my K2 for testing it. The symptoms are that in Bypass, I can hear the sidetone from the K2 coming through the headphones fine. When I switch the filter power on, I can "barely" hear the tone, but only when the bandwidth pot it set to the 2 o'clock position. Adjusting the K2's audio gain has absolutely no effect on the signal coming through the scaf- it is the same volume whether the K2 volume is turned up or down.
I removed the scaf chip and measured the output of the clock (measured on pin 10 of the empty scaf IC socket. I confirmed 8 v p-p square wave with my scope and the frequency is adjustable from approximately 41-92 kHz as measured with a counter.
I have inspected all solder joints with under my microscope and all look acceptable. Also I checked SW1 and the dual gang pot wiring and to the best of my efforts it looks like what is called for in the manual.
Here are voltage measurements with all chips in:
LM386 pin 6= 12.23VDC
LM555 pin 8=8.78 VDC
SCAF chip pins 6,7,8=8.78 VDC
SCAF chip pins 5,12,15,16=4.38V.
I have spent several days on trying to troubleshoot last February and several hours tonight and am at a loss. Any advice would be appreciated.
Thank you.
Chris
Hi, Chris.Sorry to hear about
Hi, Chris.Sorry to hear about your trouble getting the filter working. I am sure we will be able to set it right. First off, your DC volatges look fine. The 9v regulator is working, and you have the proper Vr/2 values on the SCAF chip. Also, you have confirmed the 555 clock generator is working properly.
Moving on. For some reason, you seem to have a very low level of audio getting through the system. First, remove the SCAF chip from it's socket. You can easily check the LM386 amp by touching the input pin (3) with a metallic srewdriver or other probe. This should induce some AC hum which you should be able to hear on the output. If you do hear the hum, do the same thing by touching the probe to the other side of coupling cap C9. You should be able to hear the hum when you touch there also. If not, then I suspect C9 has failed open. If you -do- hear it there, then try touching the probe to pin 13 of the SCAF socket. This is the output pin. Again, you should hear the hum, but somewhat lower. You may need to turn up the on-board volume trimmer, R14 to it's maximum to hear it.
Using your oscilloscope, you can check the input capacitor (C17) by using your K2 as a signal source. Connect the filter to the K2 While receiving a signal, or use the sidetone, and check with your scope on the ouput of C17. This is the junction of C17/R2. You should see the signal on the scope there. If all of the above tests show the components tested are working, then the problem must lie in either the wiring of the switch or dual gang pot, or the SCAF chip itself. After satisfying your self that the controls are wired properly, you could contact Dave, NT1U using NT1U at hotmail dot com for a replacement chip. Good luck! Bruce, N1RX
NEScaf problem resolved.
Chris, WD4PBJ was able to get his filter working. After we checked several voltages and confirmed the audio and clock generator were working, he gave it another good "looking over". Here is a message he sent me last night:
Hi Bruce!!
I just believe I got it working and it's the last thing I would think of. As I was pulling the PCB out of the enclosure, I noticed at least one solder pad was close to a mounting hole. I had used thin hex standoffs, but thought why not put small nylon washers in between the standoffs and underside of the PCB.
Now I have normal audio levels through it and can tune the filter. I guess that's what it was. Thanks again for all your help!!!
Chris
KD4PBJ