Not being one to read the manual (RTFM) I bought a used IP-18 and tried to use it to charge a gel cel - don't do this! Q4 will catch on fire! Q5 will be crispy too! Flames and smoke come out of the case! Q4/Q5 are 2N3393 which don't seem to be available any longer. I substituted them with some MPSA06RLRAG somewhat generic NPN amplifier transistors. The pin-out is different so observe any change.
As I have two of these units I compared the working and non working voltages and resistances until they were about the same. Here is the final chart of the 'stock' and the repaired units with Q4/Q5 replaced.
They both seem to be working at this point.
Heath HP-18
Some Voltage and Resistance Readings
Stock Repaired unit
Q3
E .7-16.1 .6-18.1
B 1.1-16.5 1.0-18.6
C 29.2 29.2
D5 + .6 .6
D5 - .7 .6
#3(open) 20.1 21.1 Terminal Strip shorting bar removed
Q4
E O 0
c 18 1.5-18
b .6 .6
Q5
E .7-16 .6-18.1
C .6-17.1 1.5-19
B .7-16 16.6-16 .6-18 (4.1-18 CURRENT POT)
Q2
E 1.2-16.5 1.0-18.6
C 29.2 26.6
B 1.7-17 1.4-19.4
D3
- 23.9 22.7 OK
+ 19.6 18.0
D4
- .8 3.4
+ 19.3 17.6
ACCROSS 20.1 21.1 OK
Q1
G 1.7-17 1.5-19.2
D 28.6 26.2
S 1.7-17 3.5-21
AC REF
GR-GR 34AC 32AC OK
RESISTANCE
METER ON 200K
Q3
RED TO +
BLK TO
E 0 0
B 4 (VARIES) 4.2 (Varies)
C 22.4 23.4
RED TO -
BLK TO
E 3.5 2.2
B (VARIES) (VARIES)
C 23.4 23K (VAR)
Q4
RED TO +
E 3.4 3.4
B 7.4 5.7-11
C 23.4 VARIES UP
