Marantz 2235b jumper question

[Nat]

A fellow teacher picked up a Marantz 2235b in good cosmetic condition at a tag sale. One channel seems to work but the other is utterly silent. What's odd is that the preamp out main amp in jumpers are not there, and the Marantzes I've had needed the jumpers to work if memory serves. Is there an internal switch or some other system at work here?

[boyon00]

My 2235B doesn't have jumpers and it works perfectly...

[Oerets]

Some marantz units have Main / Pre-amp RCA's that will do the connection without jumpers. When a male plug is inserted it will open up the connection being made inside the jack. They do go bad and a patch cord or jumpers then will be needed to make the connection.
Try hookong up a patch cord and see if this helps, if not then swap the RT/LF and see if the problem changes too.




Barney

[Jailtime]

My 2235B is fine without the jumpers. I suspect something else, like dirty pots/switches, or maybe worse, bad driver transistors or the like. I know that dirty pots and switches kept a channel on my 2235B dead. Deoxit and Faderlube in the Tape monitor switch fixed her right up.

[Nat]

I now have the reciever to fiddle with it, and used some patch cords to connect pre out and main in, and it works. But someone seems to have futzed around inside -- the DC offset pots were turned to their stops, as were the bias controls. FM is terrible also, so maybe they played with that also. So lots to learn. Fortunately there is a clear set of directions for the amp part at Irebuiltmymarantz (?). And the service manual is on HIFI Engine.

[SoCal Sam]

Yeah, those interconnects can be a problem. I spray contact cleaner directly into the RCA input and use a patch cord to work the interconnect.

[Nat]

With the jumper problem solved, and the DC offset at zero, I am attempting to set the bias on the 2235b, but I can't get a reading above .1 mV on either channel -- no reading at all except with an HP 3468A, which reads down at .08 mV, so low I thought maybe the meter had a problem, but two other meters confirm that the level is below their scale.
It seems to play reasonably well despite this...
Any ideas?

[catrafter]

Nat,
Is the meter set to read DC millivolts, volume fully ccw?
There is a wire that feeds +35V to the power amp (J729-J730) from the P800 power supply board, the expected voltages are on the schematic.

Tom

[Nat]

Now I'm thinking that the sound isn't ok -- was listening to AM and sounded fine, but listening to CD sounds buzzy and flabby. So I think there is something actually wrong, rather than my technique.

[catrafter]

The bias is to reduce crossover distortion, which is what I suspect you are hearing.
Is there +35 volts on J729 and J730?

Tom

[Nat]

Hmmm... Looking for the expected 35V from jumper 804 to 805, and only get 7.4V, even with the pot all the way up.

[EchoWars]

The bias is to reduce crossover distortion, which is what I suspect you are hearing.Doubtful. Feedback pretty much eliminates gross crossover distortion even with zero bias (though lots of higher-order stuff is generated in the process). Try it sometime with a scope and an amp with a differential input pair. You'll have a very hard time seeing any crossover anomalies.

Anyway, for sure all the supply voltages need to be verified, and the output scoped while the input is fed with a sig generator.

[Nat]

Well, looking at it in comparison with the illustrations on Irepairmarantz, its apparent that someone replaced the rectifier, and so perhaps it, or the power caps, or even more components, may have failed again. I've never had a piece of equipment with so much information easily available for it, so I'm going in deeper than I have before. I'm intensely nervous about power supplies but there is a knowledgeable ex engineer who is able to offer advice and suggestions.

[catrafter]

Doubtful. Feedback pretty much eliminates gross crossover distortion even with zero bias (though lots of higher-order stuff is generated in the process). Try it sometime with a scope and an amp with a differential input pair. You'll have a very hard time seeing any crossover anomalies.

Anyway, for sure all the supply voltages need to be verified, and the output scoped while the input is fed with a sig generator.

Ahh, I see what you mean. I hadn't considered the feedback path, I was just going by what I read in the National Audio handbook, section 4, which shows the effect of bias on the output stage.
Thanks Glenn. I learned something!

Tom

[EchoWars]

Bench-build a simple amp with no biasing and no feedback, and you'll see exactly what the handbook shows (I have the same book). But it simply doesn't apply to a consumer piece of gear with a closed feedback loop.

But that doesn't mean no bias is OK. Far from it. The purpose of feedback is more to ensure flat frequency response and lower output impedence rather than correct for gross errors in design or adjustment. The less work the diff input pairs have to do the better.