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Marantz 18 Transistor

D

Douglie

Member
I have a shorted Q14 power transistor. Can someone tell me the best place to get a matched Q13 and 14 set? Is there any place that would have the original motorolas? I got these transistor part numbers off another post. All I’ve seen online so far are some Mexico Motorola’s and unbranded pieces and no reference to matching. Thanks.
MJ21194G
MJ21193G
 
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You can find matched pairs on eBay, I just looked!

I use eBay as my source because Hawaii doesn’t have Vintage Repair Stereo Stores or any outlet for things like this!

John
 
Thanks. I saw those but I wasn’t sure if they would be any good. Glad to hear you’ve had a good experience. Any vendor you would recommend? I’ve seen a couple posts on other non marantz forums that weren’t good.
 
Thanks, Tony, The link is loaded with good info. It will be very useful as I start to fix my problem as it looks like you had quite an adventure fixing yours. Mine started when I was working on the phono board and accidentally switched a 1khz signal in the phono input from 4mv to 400mv while the volume was way up. I didn’t realize It until the the R42 resistor at Q14 caught fire. I had the receiver outlet connected to heat sink resistors so all was quiet until poof. Anyway after replacing the resistor I started up on a variac and compared voltages on the other channel to the bad channel. On the Q14 side, I immediately get voltage from the -36v rail down to the emitter side of Q2. On the other channel and the Q13 side of the bad channel, I don’t get voltage until about 90v ac input which is enough to trigger Q2 and Q4 to activate the bias voltages. So I’m going to start by replacing Q14 and Q10 and go from there. I could easily have damaged some other things but it’s hard to tell at this point. I will buy all the transistors you suggest so I won’t have to stop and go. Do you have any other suggestions at this point? I really hope I haven’t screwed the whole thing up. It had a very nice sound and very low distortion across the band. I have a marantz 7 and 8 and it sounds comparable to them. Or as good as my tinnitus tainted ears can tell. Thanks, Doug
 
I replaced the two power transistors, adjusted the bias, etc, and she fired right up. On the bench oscilloscope both channel outputs were on top of each other and it sounds great. It does 40+watts at <0.05 THD. For what it’s worth, I bought two of each of the transistors and one PNP and one NPN were close in Hfe, around 100 on my Peak gizmo. The old NPN was shorted and the old PNP was 149. I didn’t replace the upstream transistors because the ones I got had gains that were all over the place and since the receiver worked fine I left them. (My dad once told me I was good at getting things to work but not necessarily actually fixing them for good. Oh well....) I did another thread on the transistor variability. I really appreciate everyone’s help from the forum.
 
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