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Marantz 2230 Repair - H757 and H755

M

MarcD

AK Subscriber
Subscriber
Hello everyone. Long time lurker on this site but I think first time posting! So first of all, I want to express my gratitude to many of you as I have learned so so much just by reading on this site. And reading and reading some more.

So, about 10 years ago or so I was just starting to look into vintage receivers/integrated amps. I had just purchased a Kenwood KA-6000 for $150, which I still have and love. Then I was walking down my street and a nearby neighbor had put his 2230 on the curb! I was familiar with the model by this point, so I stopped in my tracks and ran home with it. I didn't even know enough back then to start it up with a DBT, but guess I got lucky. I replaced the dial lamps and deoxit on the pots and put it in service, mostly as a turntable amp, and it sounded pretty darn good. It then went to my basement room with a tape deck and mostly sat while I put the Kenny up in the living room. Fast forward to maybe a year ago and fired it up and there's considerable low end distortion in both channels. I isolated it to the P750 boards by using the pre-amp out another amp and it sounded great (and I haven't recapped anything yet on the preamp side). So, I've cobbled together my parts list of replacement caps and such, and read and read some more about how to rebuild this wonderful machine, mostly here on AK. So again, thank you.

So, now my very specific questions...(you can skip the intro above if you want)...

On the P750 board there was as service bulletin 2230-2 mentioned in the forum here that replaces H757 with a 18ohm resistor and 150 ohm 25 degC thermistor that ties into the heat sink of H755. The resistor lead broken off when I was fiddling with it, so I need to replace it.

Q1, Do I need to replace it as such, or can a more modern diode do the job now? (i.e. get rid of the thermistor).

Q2, I don't know if I'll ever get the thermistor out as it is glued into the H755 heat sink. Can anyone recommend a replacement (if needed, depending on the answer to Q1). I'm having trouble understanding all the thermistor parameters available. It's supposed to be 150 ohms at 25 degC. I assume PTC, but can someone please confirm this?

Q3, As luck would have it, I think my H755 (2SC959) is bad. I get OL between B-E. (so far I've only tested in circuit though, I will take out and try again). But this leads me to my question - any ideas how to remove that heat sink that seems to be glued on? I don't want to break it.

Q4, With a replacement 2N3440, do I need the heat sink? I've read that maybe I don't. But maybe I need it depending on the answer to Q1, for the thermistor.

Q5, There's a whitish pad underneath H755 (and 752, 756). Can I reuse this? Any recommended replacements? I don't know what to call it, is it just a thermal stand-off? I can't find anything that matches.

Q6, do I need thermal grease when I reassemble H755 in its heat sink?

That's it for now. Thanks very much if anyone has any insight they can share.

Marc
 

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I've read H757 being replaced with a single 1N4448 diode. The H755 heatsink should allow for some prying and dislodging from the transistor (you should see an opening in it). I always reuse the heatsink and a dab of compound always helps. You can also reuse the pad/stand-off I believe there are pads for other packagings that may work for a TO39 such as the TO-5 which seems more readily available.
 
Thanks so much for the reply. Very helpful. I read that too about the 1N4448 but wasn't sure if it was applicable with the thermistor thing going on. I'll give it a try. Thanks again.
 
Check your serial number. If it is above 37801, you don't need to do the H757 upgrade. Definitely use the heat sinks. That transistor produces a lot of heat.
 
Thanks Steven. I saw that, but I read it a little differently. Mine is a much higher serial number (170xxx), and it came with the thermistor contraption already. So I think the lower numbers were the only ones in need of that repair, but the factory installed it after 37801. That said, I looked up the 1N4448 diode and it looks like the current changes with temperature (there's a chart on the last page of the spec sheet), so I'm speculating that's how it does the trick. Makes for a much cleaner and easier installation anyway.

Any further insight is welcome. Thanks again.
 
MarcD said:
Thanks Steven. I saw that, but I read it a little differently. Mine is a much higher serial number (170xxx), and it came with the thermistor37801 contraption already. So I think the lower numbers were the only ones in need of that repair, but the factory installed it after 37801. That said, I looked up the 1N4448 diode and it looks like the current changes with temperature (there's a chart on the last page of the spec sheet), so I'm speculating that's how it does the trick. Makes for a much cleaner and easier installation anyway.

Any further insight is welcome. Thanks again.
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I think we are saying the same thing. Units after serial number 38701 should not need an upgrade there. The factory built them with the upgraded design.
 
Got it, thanks, I see we're saying the same thing. Perhaps what I wasn't clear on is that my resistor broke. And my H755 was bad so I've already dismantled it from the heat sink and hence took out the thermistor being held in the heat sink by the near 50 year old glue. So I'm wondering if I can just replace H757 with the 1N4448 diode instead of trying to put it all back like it was (anything to avoid glue).

And not to complicate, but upon further reading here on the AK there seems to be a disagreement in another thread about whether it should be one or two 1N4448's replacing the original S3016R Varistor. If you think that matters much please let me know, I'd appreciate it very much. As it is, thanks for your replies.

Marc
 
Ahhh. Got it. The original is destroyed. I knew I had been down this road before on AK, and couldn't remember how it was settled. I looked back at my threads, and, sure enough, Tom Williams (Catrafter) suggested that the thermistor be replaced with a 1N4448 diode. If he thinks that is a good sub, that is good enough for me. In my case, the problem ended up being a bad transistor, and I put the old thermistor back in. So, being in the spot you are in, I would go with the 1N4448 and see if the bias looks stable after it warms up. :thumbsup:
 
Awesome tip, thanks. I just searched for Catrafter and 1N4448 and got loads of further reading (and can see you have indeed gone down this road before). I'm going with it. Thanks again for the help.
 
Did you put the diode back in the bracket on the heatsink or leave it? I'm dealing with the same issue
 
It's been a while but if I remember correctly I replaced H757 (18ohm resistor and 25deg thermistor contraption) with a new 1N448 and just soldered it back into the board, and is not inserted into the heatsink. The combination 18ohm resistor and 25deg thermistor contraption goes away.
 
MarcD said:
...I replaced H757 (18ohm resistor and 25deg thermistor contraption) with a new 1N448 and just soldered it back into the board, and is not inserted into the heatsink. The combination 18ohm resistor and 25deg thermistor contraption goes away.
Click to expand...
Can confirm that's the way I've done it in multiple units, and have seen no issues.
 
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