Hi all, I'm new to the forum and this is my first post. I've got an sx-750 that I've had since 1976 which I bought when I was stationed over in Germany, it still plays nicely. And I've got a pair of cs-99 speakers to go with it. I also have a ct f7171 but it's not working anymore.
Now to my problem, was at a local flea market this weekend and came upon a Marantz 1120. I knew I'd be kicking myself for the next six months if I didn't take a chance on it so I bought it. Got it home, hooked it up and it plays, sounds nice, but when I turn up the volume past a certain point it shuts off, the relay kicks off. I does it sooner if I have the loudness switch engaged. So if it gets to loud or to much bass it shuts off until I turn the volume down then it comes back on.
Both channels play equally well on both system 1 and 2. No distortion that I can tell. If I keep the volume low it does not shut off, I had it playing for hours at a lower volume. I also tried it with one speaker at a time and it still did the same thing on either speaker, shuts off over a certain volume. I took the cover off and had a look inside, it looks like it has never been messed with and other than a bit of surface rust on the transformer where some of the paint has chipped away it looks very clean.
I've been searching around the forum a bit to try to find something similar to my problem without any luck but I did find something about the DC offset which I thought was pretty simple to check with the tools I have, a multimeter. The reading I got wasn't steady, don't know if that's normal or not, it would fluctuate between 4 and 6 mv on each of the four speaker hookups.
So I'm wondering what to do . I'm not afraid to poke around inside and I'll be very careful while doing it if anyone can give me a clue.

I suggest you find a local repair tech and let him fix it, unless you are qualified to repair it yourself. I've been told that it is pretty easy to cause additional damage if you're not sure of what you're doing. If you are comfortable working on it, there are plenty of folks here that can provide some assistance.
Good Luck!

Well, if you are willing to try to adjust the DC offset make sure you are in the dc mode plug the probes into the left speaker output and adjust R517 for 0 vdc, then switch to right channel and repeat process adjusting R518 for 0 vdc. The volume should be at min.

What DVM are you using?

This is just a suggestion and you should use proper electrical safety techniques for troubleshooting. There are potentially lethal voltages inside the chassis.

Well, if you are willing to try to adjust the DC offset make sure you are in the dc mode plug the probes into the left speaker output and adjust R517 for 0 vdc, then switch to right channel and repeat process adjusting R518 for 0 vdc. The volume should be at min.

What DVM are you using?

This is just a suggestion and you should use proper electrical safety techniques for troubleshooting. There are potentially lethal voltages inside the chassis.

+1 on what roggom said...In addition I would mark the position of the controls then I would "run" them end to end of rotation several times before trying to adjust...Just return them to the marked position then adjust.

Also it would be a good idea to check and adjust the "Bias" voltage...But you will probably need a service manual so you can find the correct test points.

IIRC and this is just from memory...Marantz suggests that you measure the bias voltage across two series connected resistors in the collector and emitter circuit for the output transistors...

Edit: Just looked in my Marantz folder and have a hand written note...So not sure if it is 100 percent correct...

But to adjust the bias voltage...Set volume control to minimum...Then measure voltage across R553 and R549 and adjust R 523 for a reading of between 11 and 18 mV DC.

Then measure the voltage across R550 and R554 and adjust R524 for the same range of 11 to 18 mV DC.

The best thing would be to clean them with some DeoxIT cleaner...As long as you have it apart it is a good idea to clean all switches and controls.

Cleaning the offset controls just might correct your problem...If not there is a lot of Marantz knowledge here at AK.

I am sure that it will not be long until you have that amplifier making some very nice :music::music::music::yes::yes:

Hi all, and thanks for the replies. Ok, first off a local tech, there doesn't seem to be any around here unfortunately. If I could drive it over and hand it in I would, it's a pretty heavy unit, damn near tore my arm off transporting it to my vehicle.
I've ordered some Dexoit D5 which hasn't arrived yet. I'm kinda leery about squirting this stuff around although from what I've read it's the best. None of the controls are scratchy and both channels seem to be the same volume with the balance at neutral, and there is no distortion that I can tell.
The DC offset... I've found the schematic and I have seen the DC offset and the Bias pots, the problem..... where are they on the actual unit? I can see four pots, I can tell that they are twins of each other. What I can't tell is which is which? The board is not marked in any way and I'm no expert at reading the schematic to be able to transfer what I'm looking at to inside the amp. I'm not even really sure I should adjust them at all because when I did the DC offset check it seemed to be pretty good to me, all channels between 4 to 6 Mv, it fluctuates a bit, volume at minimum and selector at an unused position.
I found out a few new things. First off it will shut off the relay even with no speakers connected. It just needs an input signal, without the signal it won't shut down.
Another thing i discovered is I can keep it from shutting down, I thought maybe I had figured out what I was screwing up on. My CDP has it's own volume control, I had set it at like one third and then used the volume from the amp from there. What I did was turn down the volume on the CDP to zero, turn up the volume on the amp all the way to max, then turn up the volume on the CDP until it shut off the amp then turned down the volume on the CDP a bit, all this with the speakers off of course. After this I can turn it up pretty loud without it shutting off, I didn't try it to max though, was kinda afraid to but I probably should have just to hear.
Found out that the CDP has two outputs, fixed and variable. I move it over to the fixed thinking that I had it hooked up wrong at the beginning but it didn't fix the problem, it made it worse. After that change I could only turn it up maybe a quarter way before it shut down. moved it back to variable. Does this indicate anything?
Man it does sound pretty nice, I want to get it working properly.
The DVM I own is a Sperry DM-4400A

Hi ;
I haven't got this fixed yet but I have done a bit of research in the three months since i last posted. first off, i got some deoxit but I don't want to use it yet, I don't think it will solve the problem. I ordered a manual. I have gone over it. I know where the DC offset adjustments are on the amps and I know where the bias adjustments are on the amps.
I adjusted both of them and the problem still remains. I ordered some caps from digikey but I havent installed them. I dont think it will solve the problem either. I tend to think the problem is in the power amp sections, both of them. reason being, I narrowed it down like this, I pulled the preamp jumpers and connected directly to the amp with a source unit and the problem is still there, as I turn up the volume at a certain point the relay shuts off.
I have taken voltage measurements according to the service manual and all the measurements look pretty good, less than a volt off and consistent if taken from the power amps. The only voltage I found that was way off was the one that goes to the power on light, it should be 28 volts but shows as 44+ volts. it only powers the lamp though so I don't think it is the problem.
The amp shuts down and I take a measurement on the amp input leads to the protection circuit. My multi meter shows the DC offset going over into the volt section from the millivolt section so I think the protection circuit is doing its job, its shutting down the amp because of excessive DC volt. As long as the volume is low the DC stays nice and low, although it fluctuates a bit, but as I turn up the volume the DC increases until it goes high enough to kick off the relay. What should I be checking in the power amp sections to find the problem. From reading posts Im thinking the transistors, maybe the differential pairs? What would could cause the DC offset to increase beyond acceptable and kick in the relay protect as the volume is increased? Should I get some freeze spray and hit the transistors?

I don't have the service manual on this one so I can't speak of specifics, but this can be caused by bad electrolytic capacitors in the protection circuit.
Without the filtering action of the capacitors, the protection circuit senses the audio as offset and trips out.

Tom

Those are the caps I got from Digikey that I haven't installed yet. I was thinking to replace the caps in the protection circuit, just guessing of course, and so I ordered them. I'll give it a try and swap them in, maybe I'll be lucky.

Thanks Catrafter for helping me out. I recapped the protection circuit. It helped, I can turn the volume up much higher now without going into protect. I'm not sure it is totally fixed but it's much, much better. I can still get the relay to click off but now I would have to turn up the volume on the CD player to max and then on the Marantz to over three quarter volume before it shuts off.
It could be operating normal because of my setup here. I'm using a car CD player as the source. It's a high end unit, ampless "deadhead" type. I've got that hooked into the AUX and if I turn that up to max and then the Marantz to almost max the relay clicks off.
with the meter hooked to the speaker outs starting at low volume the reading is low in the millivolts. As I turn up the volume the DC increases and fluctuates wildly, going from the millivolts to the volts and back again until finally when I turn the volume high enough it jumps into the 2 volt area or above and clicks off.
Just would like some opinions on that, could it be just clipping? I don't recall reading that clipping would cause an amp to go into protect.
I'm happy though, I listened to it today and it sounds pretty damn good to me, definitely plenty loud and nowheres near turned up to where it would shut off, plenty of headroom.

one more question someone can maybe answer for me. I,m pretty ignorant with this stuff but I really want to learn, great reading here on this forum. I mentioned that one of the voltage measurements is out of wack. On the power supply there is a pin #3, it's supposed to read +44 volts, and it does, really close. It goes to some kind of diode relay, four of them hooked together in the protection circuit. It also goes to the power on light. Before it goes to the light it passes through a 390 ohm resister. After the resister it is supposed to read +28 volts but it doesn't, it reads the same as on the other side of the resister, +44 volts or so. The light doesn't work, the bulb is burnt out. Isn't the resister supposed to lower the voltage? Like I said I'm ignorant here, could someone tell me what I'm missing?

sorry for another post but I think I found the answer to one of my questions. It's the first sticky in the Vintage Solid State forum "Amplifier Sensitivity, Decibels, and You!"
I won't be trying that experiment again.

Re: your +44 Volts. I'm kind of guessing here, just based on your description of the circuit since I don't have the manual... the lamp is burned out, there is no current flow through the 390 Ohm resistor, so no voltage drop across it. You always measure the applied voltage across an open circuit.

Tom

Thanks Tom, I think you're right. When I first saw it I said "aha, that's my problem" , I got another resister of the same value and switched them. I got the same readings with the meter afterwards which surprised me really, I was so sure I had found source of the trouble. That's when I looked in the service manual and noticed it only powered the bulb. Now i have to find a 28 volt bulb.

soldered in a new bulb, now reading 27.3 volts. learned a bit more.

Now for the praise thread... very nice ... doing duty as a headphone amp at the moment and all I can say is very nice.