Marantz 140 Power Amp – Intermittent Fluctuating Right Channel

[Flaxmill19]

Hi All,

I have an interesting and probably very difficult problem to overcome, this has a very long history, so here goes.

I am the original owner of this 140 amp and a 3200 pre amp, not long after I purchased the pair in the late 70’s I somehow (don’t ask, I cringe every time I think about this) connected the output of the 140 (via the 3200) to the output of another amp (lower power output, probably about 25 watts). And I assume that an output voltage from the other amp found its way into the output of the 140 amp. Because from this day on, normally a short time after turning the amp on, the right channel voltage would fluctuate about 5 to 10dB, this can be heard through the speaker as a loud pulsing sound. If you switch the amp off and on the fluctuating will go away, normally until the amp is turned off and left (say overnight) and the turned on again. The volume and amp gain controls have no effect on the fluctuating. Here is the curve ball, this does not happen all the time, say once every 5 to 6 times you turn it on.

I then gave the amp to my sister who’s used it for a few years and then stored it under the house for about the last 15 years. I recently (eight months ago) reclaimed the amp & pre amp. I cleaned them up and re-capped (except the 13000 x 2 uf main power filter cap) them both and up until last weekend all has been fine, in fact excellent, I love the sound of these amps. BUT out of the blue the fluctuating has started again, this is most frustrating as I thought replacing the caps had cured the problem, not to be. I suspect the main output transistors, but have no evidence only a hunch as these would have seen the incoming voltage (from the output) first.

I am about replace the filter cap in the 140 and some other cap changes on the 3200. So prior to the commencement of this work I would like to ask our experienced Audiokarma tech’s (or would be tech’s) if they have any ideas on what this could possibly be? I suspect because of it being so intermittent it will be very difficult to diagnose and I don’t want to get into replacing components just for the sake of it.

So any suggestions and/or ideas would be greatly appreciated.

Tony

 

[Lavane]

I hope you figure it out , I really like my 3200 and 140's. The big dual cap in the 140 isn't available anymore. You will have to use two separate caps.

 

[Robisme]

Does the 140 do this by itself? Without the preamp connected?

I repaired a 140 that had an intermittent problem. Turned out to be a diode.

It's most likely NOT the output transistors.

Rob

 

[Lavane]

My 140 problem was a failing capacitor. It would run fine for about 20 minutes then the volume would drop and be distorted, sometimes it would run fine for hours before the problem would return. Robisme may be right about the outputs being OK.

 

[Flaxmill19]

Hi All,

Thanks for your comments Lavane & Rob.

Well, wouldn’t you know it, the amp did it tonight, so I decided to have a play and see what happens, this was my observation and what the amp did.

1) I turned the amp on via the 3200 and almost immediately the fluctuating started in the right channel.

2) I turned the right amp gain up and down and noticed that the fluctuating increased as the volume decreases, only slightly (maybe 1 to 2 dB), but never the less a decreased.

3) I then removed the right interconnect cable from the 3200 and the fluctuating increased in volume, it was fluctuating from -5dB to almost 0dB on the amps right VU meter.

4) I then adjusted the right gain and the fluctuating almost disappears with the gain at maximum and increases as you reduce the gain.

5) I then continued to adjust the gain and after about 2 minutes, there was a slight pop sound from the right speaker and the fluctuating stopped.

6) I then reconnected the right interconnect cable and played some music at a low volume, then after about 1 min the fluctuating started again, I turned the volume up and the fluctuating disappeared and I did not hear the fluctuating again.

The only comments I can make is that the fluctuating seems to be on the increase, as I said earlier it been running fine for eight months and in the last five days it has done the fluctuating four times (I run this amp virtually every day, at an average of 3 to 4 hours per day).

The fluctuating almost seems to be associated with heat as the amp warms up the fluctuating seems to disappear.

My concern is trying to locate the problem, because of its intermittency, testing may prove fruitless, so I am looking for ideas, or other experiences that can allow me to make a decison to test and/or change out components.

Tony

 

[ChrisMarantz]

The fluctuating almost seems to be associated with heat as the amp warms up the fluctuating seems to disappear.

Having a can of freeze spray ready to go, leave the cover off the amp and get ready for the problem to show up again.

 

[Flaxmill19]

Having a can of freeze spray ready to go, leave the cover off the amp and get ready for the problem to show up again.

Hi All,

Thanks Chris, although I have heard of this method, I have personally never used it. Are you able to offer an explanation as to what I should be looking for? I assume if the fluctuating starts, I spray the right channel components and watch for any rapid defrosting component?

Although my comments about heat stemmed from what I thought was the fluctuating occurred when the amp was cold, or the fluctuating went away as the amp warmed up. Also the fluctuating never seems to occur if you are running the amp hard. But if you have the amp on just play quite background music, sometimes the fluctuating will start. So maybe the amp is not generating enough heat to kill the fluctuating. Make any sense, or am I just confusing the hell out of people:scratch2:

Tony

 

[WhammaJamma]

Tony,
It sounds like a low frequency oscillation. I have been holding off recapping my Pioneer SX-1010 until I get more information on the effects of modern components on the physical layout of the circuits. I am afraid that some of the specs on the new recommended components will introduce self oscillations but oscillations way above the audio range of frequencies. I have seen this problem in other receivers I have modified and it was not easy locating the oscillating circuits. Getting to your low frequency oscillation you say it did this before your recap job.

As a first step I would confirm all ground points are tight on the amp channel in question. Check all screws and intereconnect connectors. If there is an HF oscillation causing a motor boating oscillation tightening up the grounds may reduce, change, or stop the oscillations. Be sure the power connections to the amp assembly are sound. I am not familiar with your amp and I am assuming it is modular in design here. Post some photos if you can.

BTW, how is the Rotel working? I hope all your hard work paid off on it!
-D

 

[Flaxmill19]

Hi Duane,

Thanks for your comments, I have attached a couple of photos, I have the 140 amp ready to freeze spay it (as Chris suggested) when the fluctuating occurs, guess what? It hasn’t fluctuated since I set it up like this. I will be patient though.

Yes, the fluctuating was there prior to the re-cap.

Regarding the Rotel, it’s going well, the transistor pairing still needs to be reviewed and fine tuned. Thanks to your info I believe I can pair these better and I now understand why the newly fitted transistors blew. The Marantz is my primary amp and the Rotel is my backup for when the Marantz is on the work bench, can’t live without my sounds.

Tony

 

[rcs16]

Nice clean amp.
it could be anything from a cold solder joint,component(s),connections.pot(s). It could be oscillating as well, won't know until you hook up a scope and look.
I will see if I can find the SM and better advise.
Rick

 

[WhammaJamma]

Rick has some good info there Tony. The oscilloscope is a great tool for visualizing the signal path when it oscillates and can help locate the problem. But yes, that is a beautiful amp and it is modularized. Those white molex plugs that interconnect the modules via the wiring harnesses ARE to be suspect. The pins are likely tin plated steel. All you need to do is pull them from the board and inspect the board pins and the wires where where they go into the pin inside the white shell (sometimes lots of green oxidation present here). If all looks good deox the pins and pop the connector on and off a few times so self burnish the contact points. Inspect and clean ALL of those plugs! Ideally if you want service free connectors they should be gold flashd/plated pins.
I worked for a company years ago that built computer controlled manufacturing machines that were installed in industrial environments. The computers would fail causing production line shut downs and expensive service calls by our techs. I replaced all high power molex plugs and pins with gold flashed stuff and the problems went away. I mandated kits be assembled and sent to all customer locations with installation instructions. This saved our customers thousands of dollars and us embarrassment on reliability issues. Since then I always suspect those tin plated connectors.

-D

PS: I have a Heathkit AR-1515 that I am using at the moment here in the RV. When I got the receiver one channel would get really really hot but played fine. No sign of any problems except the heat sink burning a fever. The problem was poor connectivity of the tin plated molex connector supplying power and ground to the channel. The channel was oscillating around 150 kilohertz or so. Two squirts of deox and it has been playing fine since.

 

[Flaxmill19]

Hi All,

Some success (I think), I switched the 140 on today and straight away I could hear a low frequency hum coming from the right channel, so I thought this is my opportunity, so I waited about 5 minutes and the hum increased in volume until the fluctuating started. I then freeze sprayed the area where the two driver transistor are, no change, I then sprayed the transistors below the drivers (H706, H707, H705 & H708) and the fluctuating stopped about 30 seconds after the freeze spray had been applied. Then after about 2 minutes the fluctuating started again, so I freeze sprayed H707 & H706, no change. I then freeze sprayed H708 & H705 and the fluctuating stopped as above. These transistors are very close to each other (in fact they may be touching), so I separated the two as much as possible and freeze sprayed H705, slight reduction, and then I freeze sprayed H708 and the fluctuating stopped again. I then touched and slightly moved H708 and the fluctuation varied.

I then turned the 140 off and on and no hum or fluctuating could be heard.
Of note, is that will all the checks the low frequency hum remained only the fluctuating came and went. So there may be a ground problem on the right amp board. Although it does disappear when you switch the 140 on and off!

So, my thoughts are now to replace H708 (2SA733 P.Q), these are hard to come by, does anyone have a recommended replacement?
If I did use a replacement would it be worthwhile to replace H705, H707 & H706?

Any assistance in the transistor replacement would be greatly appreciated.

I have attached the 140 schematic for interest.

Tony

 

[Robisme]

KSA733. These are EBC, so if you need ECB get the KSA733C

Replace all 4. I don't remember if they are all 2SA733's.

Rob

 

[mech986]

what part of the circuit are these transistors part of?

 

[Flaxmill19]

Hi All,

Rob I am grateful for that information.

KSA733. These are EBC, so if you need ECB get the KSA733C

Replace all 4. I don't remember if they are all 2SA733's.

Rob

Two of the other transistor are 2SC945, so I assume that a suitable Fairchild replacement would be KSC945?

Tony

 

[Robisme]

Hi All,

Rob I am grateful for that information.



Two of the other transistor are 2SC945, so I assume that a suitable Fairchild replacement would be KSC945?

Tony

Correct. KSC945c for ECB.

Rob

 

[WhammaJamma]

I wonder if you have an oscillation going on there Tony. Those transistors look to be in the feedback circuitry. You have components there to ward of HF oscillations. Maybe you should check the + and - 15V power at the zener diodes then the voltage at the two test points. There is a pot there also for setting I expect the zero signal output balance voltage. Do you have instructions on how to set the pot? In general you want 0VDC at the speaker terminals usually without speakers connected and no audio to the amp. You may have to Gogle the service manual unless someone here has the info. If properly adjusted your problem persists then think about swapping out some parts.

 

[Flaxmill19]

I wonder if you have an oscillation going on there Tony. Those transistors look to be in the feedback circuitry. You have components there to ward of HF oscillations. Maybe you should check the + and - 15V power at the zener diodes then the voltage at the two test points. There is a pot there also for setting I expect the zero signal output balance voltage. Do you have instructions on how to set the pot? In general you want 0VDC at the speaker terminals usually without speakers connected and no audio to the amp.

Hi Duane,
I checked and adjusted the DC balance & the Bias as per the service manual following the replacement of the Elect. Caps. I have purchase the Fairchild transistors (as recommended by Rob) to replace the four, so once they arrive and installed I will let you know how I went.

Tony

 

[WhammaJamma]

Sounds like you are on top of it then Tony. I get nervous when I see those temperature sensors around the transistors. I would want to preserve the original parts just in case!

-Duane

 

[Flaxmill19]

Hi Duane,

I very much appreciate all the information and help you have provided on my Rotel amp and now on the Marantz 140, regarding your comments on you last post about being nervous when seeing the thermistors around the transistors. Well I completely missed these when reviewing the schematic and now notice that a thermistor is mounted on the PCB between the two (what I thought) troublesome transistors.

So, I am now thinking that maybe the Freeze Spray was changing the characteristics of the thermistor and that the fault is further back in the audio signal path, what do you think?

Although, as mentioned previously the offending transistor, when fluctuating, I moved the transistor with my finger and this changed the frequency of the fluctuating, so this gave me great hope that this was the problem.
As you can see, I am not so sure, but maybe I just change the transistors out and see what happens!

Once again I would value your thoughts on this.

Tony