dragon 64 problem

[centrespot]

Results with BASIC ROM's removed



_______________Reset switch _______Working Dragon ____________________Faulty Dragon

S0 (pin 25) _____not pressed ___________0.90v______________________________ 0.18v
S0 (pin 25) _____held in _______________4.62v ______________________________4.43v

S1 (pin 26) _____not ____________2.2 to 3.3v fluctuating ______________________2.13v
S1 (pin 26) _____held in _______________4.67v ______________________________4.47v

S2 (pin 27) _____not pressed _____2.2 to 3.3v fluctuating ______________________0.14v
S2 (pin 27) _____held in _______________4.67v ______________________________4.44v

This again tells me that IC40 , IC34, IC37 are all ok and I need to look at other areas supplying IC39

[centrespot]

The voltage tests on IC39 with it still fitted are :


___WORKING (FAULTY)___WORKING (FAULTY)

1_____4.56 (2.02)__________40___ 5.08 (4.87)
2_____1.59 (2.02)__________39___ 4.39 (1.01)
3_____4.56 (2.02)__________38___ 1.15 (1.34)
4_____4.56 (2.02)__________37___ 3.68 (1.03)
5_____1.56 (1.61)__________36___ 1.18 (1.04)
6_____1.80 (1.73)__________35___ 0.80 (3.41)
7_____1.74 (1.85)__________34___ 1.82 (0.66)
8_____1.98 (2.08)__________33___ 1.40 (1.84)
9_____3.45 (3.57)__________32___ 1.34 (0.96)
10____4.40 (4.62)__________31___ 1.34 (1.69)
11____1.77 (1.93)__________30___ 2.30 (1.24)
12____1.84 (1.63)__________29___ 1.34 (2.04)
13____2.43 (2.50)__________28___ 1.76 (1.61)
14____2.33 (2.50)__________27___ 1.27 (0.14)
15____4.06 (4.57)__________26___ 1.27 (1.16)
16____2.48 (2.64)__________25___ 0.18 (0.17)
17____3.09 (3.51)__________24___ 2.29 (3.21)
18____1.45 (1.59)__________23___ 2.27 (1.47)
19____1.93 (4.67)__________22___ 2.27 (1.92)
20____0.00 (0.00)__________21___ 2.22 (1.82)

The voltage tests on IC39 with it removed are similar on both WORKING & FAULTY except for :


___WORKING (FAULTY)

33____0.47 (0.00)
30____0.10 (0.00)


I have assumed that the voltage differences found with the chip in are either due to internal operations or pin 30 & 33 have an effect.
Pin 33 & 30 goes to IC33 and the other 7 chips (memory chips?).
I now find myself down a blind alley !!!

I checked around some other chips and found a difference in voltages on IC11 pin 20 the good Dragon is 3.3v the faulty Dragon is 1v

Can anyone make sense of this or am I just confusing & compounding the problem ?

[centrespot]

Hi All,

I have checked my results and noticed an error

6_____1.80 (1.73)__________35___ 1.11 (0.80)

This leads to another question pin 35 output of 0.8v goes to IC33 pin 8 via IC36 pins 2 & 3 and then R83 which drops the voltage to 0.72v.?

I have just checked the voltages on all the RAM chips and seen that IC22 differs to the rest.

Pin 2 is 1.01v instead of 1.6v and pin 14 is 0.47v instead of 1.6v. Would I be on the right track to suspect IC22 and try to replace it ?

Would the machine work if I just removed it ?

Cheers