Marantz 4240 with defective channel (first post)

[Jackmot]

Hi! I hope I am posting this in the right section.
I acquired a 4240 when a family member died and I've been battling with the Front Left channel for a while now.
The problem is that out of nowhere the channel just starts to make a VERY loud crackling noise, this normally happens after 5-10 minutes after turning the 4240 on, but sometimes it happens instantaneously.
The noise doesn't vary with volume or any other setting and usually the noise blocks any other sound from that channel.
I've already cleaned all the knobs, checked the obvious components and tried swapping a few with no luck. I also went to a local repair shop that specializes in this types of receivers but they also didn't figured it out.

Any ideas will be appreciated

PS: I tried to attached a video of the 4240 connected to an oscilloscope, hopefully i attached it correctly haha.

 

[par4ken]

I've heard that sound often. Usually it's a dirty contact or switch. You would have to get inside and apply Deoxit to the switch contacts (try to get it to run inside the switches). Use it on the potentiometers as well. You should be able to do that without any technical knowledge, sounds like you might have done that already?

Obviously It could be some failed or failing component as well. The electrolytic capacitors can start to fail after so many years.

I suspect that your repair guy just doesn't want to bother to take the time try to figure it out (time is money). Being one channel only it should not be that hard to trace down. First you would need to determine if it's the amplifier itself or if it's occurring before that stage. I would monitor the input to the noisy amplifier with the scope, it the noise is there when you hear it on the speaker it's before the amplifier. If the signal is clean on that input then it's a fault in the amplifier.

manual here Marantz 4240 Stereo/Quadradial 4 AM/FM Receiver Manual | HiFi Engine

 

[Jackmot]

Yes, I've already cleaned all contacts and potentiometers and had no different result.
This might be a very newbie question but are the headphones jacks passed through the Amps? because the noise is also heard there

Thanks for the help btw

 

[par4ken]

Yes, I've already cleaned all contacts and potentiometers and had no different result.
This might be a very newbie question but are the headphones jacks passed through the Amps? because the noise is also heard there

Thanks for the help btw

The headphones are connected to the output of the amp. Checking the schematic they are connected all the time, not switched.

 

[Jackmot]

You're right, I just had another look to the schematic and found the Jack's. Then I think im out of ideas haha, there is no built in pre amp out on the 4240 and I'm not experienced enough to fully understand that schematic tbh.
Another thing I forgot to say is that a while ago I managed to find a line in the circuit board (not sure if that's how you say that in english, sorry) that grounds itself exactly when the crackling begins, I checked the components I thought could have something to do with but nothing stood out.
With this in mind, am I correct to asume that it is the amp what's failing?

 

[par4ken]

You're right, I just had another look to the schematic and found the Jack's. Then I think im out of ideas haha, there is no built in pre amp out on the 4240 and I'm not experienced enough to fully understand that schematic tbh.
Another thing I forgot to say is that a while ago I managed to find a line in the circuit board (not sure if that's how you say that in english, sorry) that grounds itself exactly when the crackling begins, I checked the components I thought could have something to do with but nothing stood out.
With this in mind, am I correct to asume that it is the amp what's failing?

That sounds suspicious. I would try to figure out where that line goes to. Checking the component drawings and referencing the schematic, might lead you to the faulty component/area. Look out for signs like puffy/swollen or discoloured capacitors.

Is that line you're checking on the amplifier circuit board?

 

[Jackmot]

That sounds suspicious. I would try to figure out where that line goes to. Checking the component drawings and referencing the schematic, might lead you to the faulty component/area. Look out for signs like puffy/swollen or discoloured capacitors.

Is that line you're checking on the amplifier circuit board?

I did tried to look for that kind of stuff but found nothing, I'll probably re-open it next week to look again.
Yes, the line is on the left amp circuit board

 

[jupp369]

As par4ken said: Elyt capacitors! The amp's are 50 years old. in my JVC's I had to change all elyt's, all over 50mu were leak except the big from main power. Change them all, maybe with a higher voltage, and don't touch poti's or transistors. Think you are experienced enough if you already know to use an oscilloscope. Shure you would not pay the price from a repair shop! Good luck!

 

[bill114]

I've experienced a similar issue with a Lafayette SQ-W decoder that had been re-capped. Turns out it was a failed transistor.

 

[par4ken]

Also look for bad solder joints, cracks around component leads, might just need a touch up with a sorting iron. I would try to find the problem first before replacing all the capacitors, but definitely replace all the suspicious looking ones first. Once it's up and running you could continue on and replace the rest of them.

The amplifier boards contain two channels each, so you can compare them side to side, good to bad checking voltages etc.

 

[Jackmot]

Yeah, I think that my soldering skills are good enough so that won't be a problem but this is my first time working in a medium-high end receiver so I'm a little lost there haha.
I already re-solded all of the left amp so it's probably not a bad solder joint. I also tried to cross check some components as par4ken kindly suggested but fond nothing out of the ordinary, I'll probably re-check everything on tuesday.

I found a youtube video of a guy that has a 4270 with the exact problem I do and apparently his problem was that there is a pair of "BIAS" transistors (?) that should be of identical value, it certainly seems reasonable to check those specific transistors. Here's the video.
Am I correct to assume that the 2 pairs of transistors for my 4240 that the video is referring are H701-H703 and H704-H702?

 

[par4ken]

That would be a differential pair, yes they are H701-H703 and H704-H702. If you have the means to check them you could pull them out. Today many multimeters have a socket for measuring hFE of a transistor. Likely one has started to go "flakey", the two simply being a bit mismatched, I don't believe would cause that problem. By all means replace them with a new closely matched pair!

If you search Aliexpress you can find 20 piece lots of those transistors cheap(from China). Pick through them to find the best matches. If you are uncomfortable with purchasing from China (they have been known for selling counterfeit components) you can find sellers on eBay. One US seller has a substitute "KSA992FTA 10 New OnSemi transistors -- sub 2SA992 2SA493 2SA640 2SA722 2SA725"
https://www.ebay.ca/itm/124170723279?hash=item1ce926dfcf:g:2VgAAOSwgaZelzMQ
Just be careful setting the bias. Recheck it after the unit has be run a bit. As the transistors heat up the bias current will increase. If you set it too high the amp can go into thermal runaway and fry the output transistors!

 

[Jackmot]

It seems like I have a lot of research to do haha. Honestly, I have never setted up a BIAS before so I'll do extensive research before attempting anything. Any advice on this?
I think I'll buy the components in a physical store because I'm gonna go there in Monday anyways but THANKS for the link!!
PS: any brand I should avoid for the transistors/capacitors?

 

[par4ken]

The procedure is in the manual and mirrors that of the video that you posted. Only the component numbers are different.

If you're purchasing locally you will have to use whatever is available. It's OK to up the voltage rating of capacitors a bit. New capacitors are likely to be smaller than the old ones.
As shown in the video you should check the bias first before changing any components. Check the good channel as well for reference. You should just have to tweak the pot a bit to get the required values. Recheck after the amplifier has been on/operating for awhile and readjust if necessary (it will likely be higher). Instead of a VTVM it's best to use a Digital Multimeter.

 

[Jackmot]

So, I spent several hours between yesterday and today adjusting all 4 BIAS and DC-offsets. everything was out of specs but I think that they are all ok now, all my channels oscillate 1-2mV (both in BIAS and DC-offset), is that inside the accepted parameters?

The problems seems to be slowly getting better. The left amp tended to heat a lot and now it's way better! but I still have that crackling noise now and then.
I also noticed that the power supply tends to overheat a lot, I adjusted it to 35v as the manual says but there's no change in it's temperature, maybe that's part of the problem?

I also discovered that the guy of the shop changed one of my output transistors (an original 2SD315) to a new version (MJE15030). I did a little research on the internet and it seems like that it is an acceptable substitute (still, a little bit mad that he didn't told me he did that when I specifically asked him to tell me what stuff he changed on the receiver).
I haven't changed and component yet cause the shop i went to is supposed to have a warranty and since he charged for his services I intend to stop by the store on monday.

Hope everyone is having a nice holiday

 

[par4ken]

I'm guessing that the noise is from the repaired amplifier? I would've replaced both output transistors, not just the bad one. Now you are running with two different output transistors in that one side. Being a quasi-complimentary output stage, it's not critical to match them but it would still be preferable for them to be the same part number. Often the driver transistor(s) will blow as well and require replacement.

I wouldn't worry about the varying 1-2mV, that sounds acceptable. The fact that one channel was running hot illustrates what I was saying about re checking the bias. If left alone it might have resulted in another blown output transistor.

Regulated power supplies often run hot, so that might be normal. Capacitor replacement wouldn't hurt. When things run hotter than I like I like to add better heatsinking, if possible.

 

[Jackmot]

Yes, the noise is form the repaired amplifier. It might not be a terrible idea to buy another MJE15030 output transistor, it's fairly easy to replace.

Thats great new about the 1-2mV variation,thanks! does that mean that we need to rule out that the noice is from the differential pair transistors?

The parts that are getting hot on the power supply seems to be the H801 capacitor (2SD315D) and specially the R809, that resistor is hot as hell! haha

 

[par4ken]

I wouldn't rule anything out. But yes if you want replace the other output with another MJE15030.

H801 is a series pass transistor, the full regulated current passes through it and R809. You could replace the resistor with one of a higher wattage. My thought is that the heat likely is normal. That regulates the power to the low level stages.

 

[tonyE]

Honestly, that's a pretty old unit. My recommendation is that you take it and have it REBUILT, not just "recapped". Your technician likely didn't want to touch it. Ideally, you'd have someone rebuild it for a set fee... not pay for the hour.

That's what I'm doing with my vintage receivers... I did the 2325, the Sansui G-7500 and the Akai AS980. Right now I have a Marantz 4215 in the shop as well.

The problem with these old units is not just that the capacitors are bad, but many transistors, diodes, relays, etc... have deteriorated. So they require a through rebuild... take out the boards, rebuild them one by one, clean out the chassis, replace the lights (no LEDs for me), relays, calibrate the tuner, the decoders, bias the power amplifier boards, etc...

The bill for the 4215 is coming around $800 and that it because it's in pretty good shape and I get a "returning customer" discount.

The bill for the 2325 was $1150.

Now, one thing, IMHO, the Marantz 2240 was not one of the good sounding receivers... the 2220, 2270/5, 2325 were good sounding, but the 2240 just didn't have it. I used to have one and flat out gave it away. I don't know how they did the 4240, the 4215 is really good sounding. In any event, those old Marantz's were conservatively rated.

Another thing... the PC boards on these things are old. Look for lifted traces.

Oh, another interesting tidbit. The Marantz quad keep the power on for all four channels, all the time... unlike the Akai AS980 that switches the power ON/OFF for the rear amplifiers and thus handily drives the front channels with more power. However, this makes the Akai more complicated (relays, logic, etc...), the Marantz is simpler.

Bottom line, fixing one thing at a time is just asking for non stop issues... is like replacing a single pair of power vacuum tubes... you usually replace all tubes! So unless you really know what you are doing, let someone else do the work.

I should note that these units are wildly overpriced in the used market. You might be able to sell it as parts and buy a more modern unit that sounds much better. Something like a Kenwood or Luxman integrated from '79 or '80.

Last... did your unit come with the SQA1/SQA2 logic module and the wired remote?

Have fun.

 

[par4ken]

I'm not sure that I would recommend a complete rebuild, in large part due to the cost involved. The capacitors are the least reliable component (after the lamps). The semiconductors are generally very reliable.

The capacitors of about 10µFD and less can easily be replaced with Wima MKS2 POLYESTER FILM CAPACITORS. The are available in 50V and 63V versions, either of which is suitable in most applications. They are about the same size as the electrolytics that they would be replacing. Replacing the electrolytic capacitors that are in the signal path will help to improve the sound and they should never require replacement again.

Marantz cheaped out, at least on their lower end models in that they used quasi complimentary output stages. In contrast complementary symmetry output stages are more linear but require matched NPN and PNP output transistors. The PNP devices were more expensive so manufactures opted for the quasi symmetry output stage instead, using only NPN transistors.

I have a 4230 kicking around, I was using it in the bedroom. I had to repair one of the amplifier channels as well. The 4230 has a quasi complimentary output stage and single ended power supply and so requires a large value capacitor in series with the output (3000 µFD). I also have a Sansui QR-3500 it sounds better than the Marantz. It has a complementary symmetry output stage. Both units still function with (mostly) original capacitors. As time permits I may recap both those units.

The 4240 has a bi-polar (+-) power supply and so it has no need of the output capacitor, so it's up a step in sound quality (as well as power) from the 4230. My 4230 also lacks the differential input stage of the 4240 and 4270.