Hi, First see the “reference” url below for the previous (first?) announcement of this problem.
I seem to have the same problem with the K8031 (high level spikes on the trace, sometimes several vertical divisions), but in my case the problems only occur with the timebase on 4 microsecs/div or faster. Also there seem to be a lot more spikes if the triggering is “on” (or perhaps I do see them better). The spikes only seem to occur when the trace is vertically exact in the middle.
[b] * Was there eventually a solution to this problem?
Was the AD convertor indeed defective?[/b]
Thanks in advance.
Hi, I did replace now the TDA8703 (AD convertor) and the phenomenon is still there, exactly as it was before (not worse, not better).
So, can conclude that the AD convertor is not the problem?
I have seen though that the application of the TDA8703 in the K8031 kit is slightly different than what the datasheet suggests:
Vccd (digital supply voltage) and Vcco (output stages supply voltage) are not separated in the K8031 by an extra RC network (22E/22nF) as suggested in the datasheet.
The capacitor between Vrb (pin 4) and Vrt (pin9) is 150 pF in the K8031 in stead of 47pF in the datasheet.
In the K8031 there is an extra resistor between Vrb (pin 4) and Vrt (pin9) of 330K, which is not present in the datasheet.
The polarity of the capacity between Vrt (pin 9) and Dec (pin 5) is reversed in the K8031 with respect to the one in the datasheet.
I do not know if this slighly different application of the TDA8703 can cause such spike problems?
Can the OPA just before the AD convertor cause such problems?
Indeed, Philips has made some modifications to the suggested circuit in the datasheet (dated 1996 Aug 26). Maybe they have also noted the tendency of this circuit to generate some spikes.
In the original datasheet (dated March 1991) there was no 22R resistor and no 22nF capacitors from pins 18 and 19 to GND.
The capacitor between pins 4 and 9 is 150pF due to the test results. Beter performance than with the suggested 47 pF.
The polarity of the tantalum between pins 5 and 9 is reversed due to that pin 5 is more positive (about 0.6V) than pin 9.
In the application schematic is 100pF from /CLK pin 17 to GND but in the text is written:
“If one of the clock inputs is not driven, then it is recommended to decouple this input to DGND with a 100 nF capacitor.”
The extra resistor between Vrb (pin 4) and Vrt (pin9) of 330R is to increase the gain (reduces the internal reference voltage).
Maybe most important modification to reduce the spikes is to add the 22nF capacitors from pins 18 and 19 to GND. Also disconnecting pin 19 and adding the 22R resistor between pins 18 and 19 may be good idea.
I started with adding the extra decoupling of VccO: a resistor of 22E between VccD and VccO and an extra capacitor (I used 100nF) on VccO. To my happiness the problem seems to be solved. I could not reproduce it anymore.
I did not make any changes to the PCB itself, in stead the following was done:
removed the TDA8703
bended up pin 19 so that it will not make contact in the socket any more
soldered (on the IC pins near the IC body) a resistor of 22E between pins 18 and 19
soldered (on the IC pins near the IC body) a capacitor of 100 nF between pins 19 and 20 with as short as possible leads
placed back (the modified) TDA8703
saw that the problem was solved!
I would like to stop further modifications to the K8031 kit I have here, since the problem is solved with above modification, if that is ok with you VEL255.
I’m glad to see that you got the problem solved by adding those two components.
All this is very valuable information to us how to proceed if similar problems occur.
Yes, there is no need to do any other modifications because the problem seems to be now solved.
bended up pin 19 so that it will not make contact in the socket any more
soldered (on the IC pins near the IC body) a resistor of 22E between pins 18 and 19
soldered (on the IC pins near the IC body) a capacitor of 100 nF between pins 19 and 20 with as short as possible leads
placed back (the modified) TDA8703[/quote]
I have the same problem and tried this solution but it did not work. Also now when I use the 1V/Div or 3V/Div buttons I get no signal at all but all lower settings work fine. Is the TDA8703 damaged? What else can I check?