HELP a brother out! I need some advice on how to fix my Marantz Model 18 scope.
- Thread starter PooPoo
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- marantz model 18 scope transistors
I see you are using Hitachi transistors like I did. Just mount and connect them like the originals. After you mount them onto the heat sink measure with an ohm meter across the center lead (collector) of each transistor and the heat sink. If you have any reading other than an open circuit then recheck the insulating kit (mica washer and plastic screw sleeve) for that transistor. Proceed with soldering short lead wires from pcb to each transistor lead watching carefully for correct configuration (consult the schematic for that).
And make sure the exposed metal face/side of transistor is oriented face down towards the heat sink/mica insulator.
I wonder?
PooPoo
Active Member
Nope not yet... this ain’t working. On the board can someone tell me which hole is E, C, B
Is this the correct pinout on the board?
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PooPoo
Active Member
My bad Bk has to take a break on this fix before I start smashing this scope. Why won’t it work like it should urghhh need my cotton candy..
Yes, it is the correct pinout. I see what appears to be mica washers between the heatsink and transistor, did you use the plastic screw sleeves? Were the mica washers intact when you installed them?
Have you done what ausman1000 suggested above?
PooPoo
Active Member
Plastic screws? No I didn’t use plastic screws just these metal ones
Am I using the wrong type of screws?
Yes the mica seems to be intact and tranny not touching metal.
No, plastic screw sleeves. They are installed in the transistor screw holes before you install the screws. It, like the mica washer, is used to prevent the screw from grounding the transistor to the heatsink.
Steven Tate
CEO Flat Forehead Soc.
The important measurement that ausman1000 is talking about is with transistor mounted, measure with continuity tester between the heat sink and the center pin (collector) of the transistor. It should be an open circuit. If the case or attachment screw are touching the metal heat sink, you will have a short to ground. The plastic sleeve in the screw hole and the mica washer are to prevent this. Ask me how I know. 
Forget trying to mount them under the heat sink, get that thought out of your head. Also, you have to use flying wire leads to connect transistor leads to the pcb. You cannot just solder transistor directly to the pcb because the hole arrangement on the pcb is not ECB or BCE. Two of the leads are swapped and I can't remember which ones that is why you will have to consult the schematic.
It's been awhile since I looked at the schematic but I don't recall any of the flying wires crossing each other in the view shown. It kinda surprised me because flying wires wouldn't be necessary. I'll have to look again...
RV59
Active Member
In your picture, it's BCE from left to right, not ECB
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I will take a pic of the finished product and post it here when I get a moment this weekend. Since I used the same Hitachi transistors you just need to copy what is in the pic.
I have a late and early version of the 18 so I will look at both. There may be a difference in the transistors used and thus a difference in lead configuration.
