Channel output Issue Marantz 1150

[JamesWest...]

Hello dwellers of the Marantz subforum. I am seeking knowledge about a Marantz 1150 (not the D model). This amp is a friends, and they asked me to take a look at it. I have a rudimentary knowledge of electronics (mainly tinkering with them for fun) and can solder confidently.

A bit of Context:
Here's the story of what happened. I did some work on the amp for my friend because one channel was fuzzy and cut in and out. Cleaning the pots and switches with contact cleaner fixed it right up, but I also found the main power switch was toast (literally melted) so I swapped it and the snubber out for a new replacement kit I got off Ebay. After that I held onto it for about a week or two using it as my daily amp to make sure everything was Peachy-Keen, it worked completely fine with zero issues.

Then about 2 months later, I was contacted again by my friend saying that “One channel is cracking really loud”. I visited his house the next day, yep it was undeniably crackling from the left channel; I agreed to take it home to figure out what was wrong. Disassembled it figuring it would be the contacts of the output relay (which I had just cleaned instead of the suggested route of just replacing it, as I have seen mentioned on forums). After futzing with the relay, I turned it on and… I got audio out of both channels (Sparking a moment of jubilation. I had found the issue) ... then the cracking came back, and the relay immediately turned off. After leaving it off for a while to cool down, I turned it back on and the output relay would not click on; “Ok” I thought “I'll disconnect the left output board and see if the relay turns on.” It did.

So, that leads us to today at which I have been trying to figure out what is wrong for the past week and have not made much progress.

Anyway, the gist of it is,
There is something wrong with the left channel's DC offset causing the protection relay to not turn on. I know it is a problem with the left channel and not the relay circuit because the right channel and protection circuit function fine when the left channel is unplugged (inside the amp, not at the output to the speakers). I am trying to figure out what would cause this. I have read it could be a “leaky” transistor; I have also heard that there is no way to test for this with a multimeter (which is about the most sophisticated testing equipment I have).

Penny for your thoughts? I am keen to figure this out so any guidance would be most appreciated. If y’all need any further, or more detailed information I am more than happy to oblige.

Here is what I have tested so far,

• Diode mode multimeter test on output transistors (while removed from their sockets); both read within spec.

• Basic board mounted test of all of the component's resistances compared against the functioning output board for the right channel. (I know this is not super reliable, but I wanted to test as much as I could without aimlessly De-soldering components,) Nothing varied much between the two, and there were not any shorts that I found.

• I should also mention I have found evidence of transistor H713 having been replaced at some point, the traces were lifted and replaced with wire. (there is also a sticker on the back saying it had work done by someone back in 1986)

The files below are as follows:

-The whole amp schematic.

There are two different schematics in the manual they both say “Note: this schematic diagram applies to units manufactured for the European market” the only difference I noticed in my quick glance is that one is multi voltage (I chose the schematic that is not multi voltage because the unit I have is 120v only).

-The “p700” output circuit, board diagram and schematic.

-The Front and back pictures of the output board.

-And here's the HiFi engine link where you can find the full service manual (if you have an account). https://www.hifiengine.com/manual_library/marantz/1150.shtml

 

[AE Bob]

The absolute first thing I would suspect is intermittent / noisy differential amp transistors H701 and H702. Monitor the DC offset on that channel while you spray the transistor pair with freeze spray (duster with the can inverted), If the offset jumps wildly or comes back down, and possibly the protect relay closing, you have found the issue. I would then replace the differential amps in both channels, and set the offset and idle current.

 

[JamesWest...]

Thanks Bob,
Got back to futzing with it again this weekend, can confirm those transistors were the issue. I took them out (they both tested alright with the Diode Mode Test) swapped them into the known working channel and got the same protection relay not turning on issue. I have some KSC1845's leftover from a previous project I can swap in (I've read on the forum that these are acceptable replacements).

Quick question: Do you happen to know why they are thermally bound together? (Not that I won't do that with the replacements I was just curious.)

 

[AE Bob]

Those would likely work fine. If I find noisy / defective differential amp transistors in one channel, I always replace them in both.

 

[JamesWest...]

Ok… That didn't fix it.

It's still got a DC offset of 40V. In an attempt to diagnose it further I have been swapping components from the known good board to the dead one. Here is what I have swapped so far.
H702-H701 (As previously mentioned. Although I did swap them again for the ones from the other board just in case.)
H13
H17
H705
H708
H709
And Both output transistors.
I did notice previously that R740 (the twin 0.2+0.2 ohm “Noble” branded resistor next to the coil) reads 0 ohms. I didn't think much of it at the time as the other channel read the same, but now I'm running out of ideas.

I also noticed that R741 (the DC offset potentiometer) doesn't seem to change the dc offset at all. It measures fine with an ohmmeter though.

Could it be one of the old electrolytics? I have some in stock that are the right values, I'll go ahead and swap them out for new ones when I get a moment.

Thanks for all your help, Bob.
And I will certainly make sure both channels are matched with whatever I replace.

 

[AE Bob]

I thought you stated that moving the differential amp transistors to the other channel caused the problem to go to the other channel.???????????

 

[JamesWest...]

The issue did switch to the other channel when I swapped the differential amp transistors. Unfortunately, upon replacing them (in the dead channel) with the new ksc1845's the issue persisted, so that was not the entirety of the issue it would seem.

 

[amorr]

Did you gain match the 1845's?

 

[AE Bob]

I would look closely at the replacement transistors, and make sure they were correctly installed. If they are, I think I would swap the differential amp transistors to the other channel again, to see if I got the same result. If the channel with the original differentials in it always works, I would get some transistors with higher gain. The 2SC1327's specified in the schematic show T or U hfe range. This indicates a gain of 300 to 1040, which is reasonably high. Where did you get your KSC1845's? Their gain should be around 400, but if thy are fake, it could be anything.

 

[JamesWest...]

I got them From Mouser They are KSC1845-FTA (which I believe indicates gain?). I would agree with you and say "it was the new differential amp transistors" except that after it had the same dc offset issue with the new KSC1845s I installed the old 2SC1327's from the working channel installed and it still has the same issue.