Nagging intermittent hum on 2285B - want to remedy

[jasong]

Hi all, I have had a nagging problem with my 2285B since I bought it. Every once in a while, it decides it wants to hum, and a 60 Hz hum starts. Sometimes turning the unit off and on remedies the hum, sometimes leaving it off for a while does the trick. It was humming so much that I sidelined it for a while.

Last month I took it to the shop to get repaired...when I brought it home, the humming was still there. So I took it back. Brought it home a second time, and it worked...but it still hums occasionally! The first time, the repair note said "soldering on circuit board" "function test runs ok" and the second time, the note just said they ran the unit.

So. Seeing as it's a PITA to take it down to this electronics shop for whatever they do to it, I'd like to ID the problem precisely. This seems like a good place to ask some knowledgeable folks for ideas how I could permanently fix the humming problem. I am relieved for every time it goes away on its own, but I fear that the problem will never go away completely unless I get to the root of it and fix whatever is causing the hum.

Help, please? Anyone? EchoWars?:scratch2:

 

[Gigapod]

Hi jasong,
Did you ever get it recapped? On 30-year old vintage gear, there are two recurring problems: bad solder joints that have finally oxidized to the point that they really show up, and bad caps.
Testing for bad caps is pretty simple really: just recap the whole thing.
Similarly for bad joints, you just redo all the solder joints, one by one, until the problem goes away.
You are lucky in the sense that you can very easily isolate the problem to one of the boards:
1) Do you get the hum only when playing FM? Then it's an FM problem.
2) If you separate the amp from the preamp, and use an external amp, do you still have the hum? Then it's a preamp or power supply problem.
3) Similarly if you use the amp section of the 2285 separately and still have hum, then it's something to do with the amp board or the power supply.
4) Testing the big power supply caps is easy with a scope and a dummy load...
Recapping is becoming more or less compulsory for classic Marantz receivers, there is no way around the fact that they are 30 years old or more. They are quality units and it shows when you open them up, but you can also see the capacitors bulging...

 

[jasong]

Hi Gigapod, thanks for the reply.

It hasn't been recapped that I know of. I am aware that that will likely be a necessary thing to do. I got the unit in 2001 and I am unaware if any prior cap servicing has been done on it; I'd guess not. I'm not confident enough to do them myself; I would prefer to research it, order new caps, and take it to someone more skilled to replace them.

1. The hum seems actually to attenuate or disappear when using the FM, but I'm not certain of this.
2. I haven't tried an external amp. How would I wire it up to test this? I only have one other receiver/amp (a Hitachi receiver).
3. Again, not sure how to go about this. Can I do this with the Hitachi?
4. I have neither a scope nor a dummy load...

I haven't cracked it open to actually look at the caps. I have the service manual, so I can correlate caps to where they are on the schematic drawings and board layouts, but I'm not sure what does what and which is which (I wish I'd paid more attention to the electrical engineering classes .

My unit is a little beat down, but it still runs well, and I really like it. I'm willing to sink a few bucks into recapping it if it will prolong its lifespan significantly. But in the short term, I just want to get rid of the intermittent hum. I wish there were ways to physically test the caps without a scope and dummy load. *shakes tiny fist at multimeter*

 

[Gigapod]

Hi jasong,
Well, crack'er open and take a look inside, it won't hurt. It will also give you an idea of the amount of work required to recap the whole unit (a _lot_). Quite honestly I wouldn't attempt a 2285B recap as my first one... not even as my second one... actually I wouldn't try it at all...
Sometimes it's obvious to see a bad cap, sometimes not obvious at all.
Here's my webapge on recapping: http://gigapod.free.fr/re-cap.html
Yes, you can test the preamp and amp separately with your Hitachi... if your Hitachi also has separate preamp and amp (usually found in MOTL and TOTL receivers). All you need is a couple of spare RCA cables.

While you have her open, try an'take a few nude pics and post them here! :thmbsp:
:worthless:

 

[jasong]

Nooooo I am not going to attempt a full recap myself! That would be insane. I would only recap failed or near-fail caps as necessary.

Ah OK! I read your page earlier this morning and enjoyed it. It was encouraging to see that replacement is possible, yet troubling to consider extracting and reinserting capacitors by myself at home.

Haha you want nudie pics? I'll try and hook some up in a bit here.

Next time it hums, I'll do the tests you describe with the other amp. Thanks for the ideas!

 

[jasong]

OK here are the nudies

Alright:

Plain nudie inside

Big caps and left half of P700 board

P700 schematic drawing

P700 general arrangement drawing

P800 board photo

P800 board again

P800 schematic and GA drawings (sorry, didn't rotate before uploading)

 

[Gigapod]

Alright:
...
(explicit pics included)

Hey, nice pics, jasong. :thmbsp:

Are the big capacitors marked 6800uF x 2? If they are, these are pretty much unobtanium nowadays, but easily substituted by a pair of much smaller (physically) 6800uF/63V capacitors.
I didn't see any obviously bad capacitors in the pictures posted, although a number of the smaller electrolytics seems to have suffered from slight sleeve shrinking; these are inexpensive and not-too-difficult to unsolder/resolder.
Visually, it seems it was not recapped before.
The transformer is H-U-G-E, in typical (good) Marantz fashion. A great receiver design, no doubt.
I can only say that such a beauty is worthy of the best recapping and general check-up that you can afford (FM section tuning, general cleaning and bias and offset adjustments), the sooner the better.

 

[kalpol]

source

Where are you folks finding good quality capacitors etc.? I've heard of lots of cheap stuff flooding in from overseas and I've had so much bad luck with Radio Shack that I'm sure not getting any from there.

 

[Gigapod]

Where are you folks finding good quality capacitors etc.? I've heard of lots of cheap stuff flooding in from overseas and I've had so much bad luck with Radio Shack that I'm sure not getting any from there.

Basically *all* small passive electronics components are made outside the US nowadays, with a good percentage made in China (yes, cheap labor...). Since I live in France I don't think my local suppliers would suit you, but I ordered stuff from Digikey (in the US) in the past and didn't have any major problems.
Also, first and foremost you could check some of the Audiokarma sponsors...
:ntwrthy: :ntwrthy: :ntwrthy:

 

[jasong]

Hey, nice pics, jasong. :thmbsp:

Are the big capacitors marked 6800uF x 2? If they are, these are pretty much unobtanium nowadays, but easily substituted by a pair of much smaller (physically) 6800uF/63V capacitors.
I didn't see any obviously bad capacitors in the pictures posted, although a number of the smaller electrolytics seems to have suffered from slight sleeve shrinking; these are inexpensive and not-too-difficult to unsolder/resolder.
Visually, it seems it was not recapped before.
The transformer is H-U-G-E, in typical (good) Marantz fashion. A great receiver design, no doubt.
I can only say that such a beauty is worthy of the best recapping and general check-up that you can afford (FM section tuning, general cleaning and bias and offset adjustments), the sooner the better.

Yes, the big caps are marked 6800 μF x 2. I anticipated they would require the double recap as with other Marantz units discussed on this board. :\

Hopefully idle current and offset have been done already during servicing; as soon as I get my hands on a multimeter I am doing both for sure (hopefully a multimeter is as good as the VTVM the manual refers to).

Recapping as you say is worthwhile given the overall properties of the amplifier. I know I'm going to have to bite the bullet someday :\ If I had better gear or access to gear, I would test the performance of the capacitors regularly.

 

[namahealani]

I had a hum on my Marantz 2270 that was hard to figure out. I have my TV/DVD/VHS sound hooked into AUX on the 2270.
Whenever I had the TV on and the amp in the PHONO position I got the hum.
So if the hum is there when the TV is on turn the TV off. That might be it.

PS- This solved another AKers hum problem in another post. Not sure what causes it though. Maybe the DVDs DAC?

 

[Gigapod]

I had a hum on my Marantz 2270 that was hard to figure out. I have my TV/DVD/VHS sound hooked into AUX on the 2270.
Whenever I had the TV on and the amp in the PHONO position I got the hum.
So if the hum is there when the TV is on turn the TV off. That might be it.

PS- This solved another AKers hum problem in another post. Not sure what causes it though. Maybe the DVDs DAC?

Or: ground loop? (sticky thread here)

 

[jasong]

OK, some testing

Or: ground loop? (sticky thread here)

OK it just started humming again, so I ran some tests.

Putting the PRE OUT from the 2285 over to the AUX IN on my other amp: result was audible hum through the audio output of the other receiver. The hum would actually start BEFORE the 2285 made its little "click" thing where the speakers come on. (When I switch th 2285 on, it takes a couple seconds before the sound clicks on).

Putting a signal from the other receiver to the MAIN IN of the 2285 results in plenty loud audio, which doesn't seem to be controlled by the 2285 volume control. At any rate, sound comes through the MAIN IN of the 2285 just fine. The hum disappeared as I turned on the signal coming from my computer through the other amp and to the 2285.

So does this sound like it is a power board issue?

 

[jasong]

Or: ground loop? (sticky thread here)

That would be a possibility...my turntable gets a hum in its signal when I attach it to the chassis ground post...so I leave it off! For a while it didn't introduce hum, then it did again...I wish I knew what was causing these electrical fluctuations.

 

[Gigapod]

OK it just started humming again, so I ran some tests.

Putting the PRE OUT from the 2285 over to the AUX IN on my other amp: result was audible hum through the audio output of the other receiver.
...
So does this sound like it is a power board issue?

Hi jasong,
The test you performed above shows that the hum is before the power amp (which is just as good, easier to fix in principle).

Also the hum is not in the FM circuit. Good.

Does the hum volume increase if you increase the volume on the Marantz or not? (without any signal on input)
If not -> possibly power supply hum. If it does -> hum somewhere in the tone controls.

Can you post a pic of the complete power supply schematics?

 

[jasong]

Hi jasong,
The test you performed above shows that the hum is before the power amp (which is just as good, easier to fix in principle).

Also the hum is not in the FM circuit. Good.

Does the hum volume increase if you increase the volume on the Marantz or not? (without any signal on input)
If not -> possibly power supply hum. If it does -> hum somewhere in the tone controls.

Can you post a pic of the complete power supply schematics?

Yes, the hum volume does increase with the volume knob and can become very pronounced the louder it gets. However, as soon as I plugged RCA cables into the MAIN IN ports on the back of the 2285, the hum disappeared. Then extremely loud audio came through from my computer patch through the other amp

I still think there may be grounding issues as well though, as grounding a turntable introduces hum into the phono circuit, whereas leaving it ungrounded is quiet.

Anyway, for pictures, I have already posted the power supply board P800 and the main amp circuit board P700. If you email me, I can send you a macro photo of the entire amp's circuit. The service manual has illustrations of the following:

FM Front End P100
AM Tuner, FM IF, & MPX decoder P200
Phono Amp P400
Main Amp P700
Power Supply P800
Pre and Tone Amp PE01
Monitor and Filter switches PS01
Dolby Socket
Function Lamp
Dial Lamp
Antenna Muting
Speaker Switches

The service manual also has a complete schematic for the whole unit.

Please advise what other board schematics I can provide, if the P800 and P700 schematics do not already cover it.

And thank you for your help!

 

[alwayslooking]

Probably is not causing it but that beautiful monster needs a good bath. And any problems will be easier to find.

 

[Gigapod]

Yes, the hum volume does increase with the volume knob and can become very pronounced the louder it gets. However, as soon as I plugged RCA cables into the MAIN IN ports on the back of the 2285, the hum disappeared. Then extremely loud audio came through from my computer patch through the other amp

That's OK, it's normal and it shows the power amp is Ok and has no hum. Good!

I still think there may be grounding issues as well though, as grounding a turntable introduces hum into the phono circuit, whereas leaving it ungrounded is quiet.

That's normal too, the turntable input is a high-impedance input with a lot of gain, so a ground loop will easily get amplified to audible levels.

I would need you to test the volume pot with the output of your Marantz preamp connected to the amplifier in the Hitachi (and the speakers off in the Marantz, of course).

Anyway, for pictures, I have already posted the power supply board P800 and the main amp circuit board P700. If you email me, I can send you a macro photo of the entire amp's circuit.
...

Sending PM now...

 

[jasong]

Probably is not causing it but that beautiful monster needs a good bath. And any problems will be easier to find.

Yeah I was wanting to dust it when I had it open, but didn't want to wipe it with a damp cloth

 

[Gigapod]

Hi Jason,
Judging from the schematics which you sent me, I would say you could try the following very simple and relatively safe test: solder a 470uF/63V capacitor in parallel with C801 (note the polarity!!!!!!!); If the hum goes away, replace C801 and you are done.
If the hum doesn't go away, repeat with C802.
Ditto for C810 and C809.
In each case, be very careful with the polarity, otherwise, poooof! :tears:
If you are not current with your soldering skills, practice on a few pieces of wire before you go for the receiver.
If and only if you feel very confident, replace C801, C802, C810, C815, C809, C803, C813, C820 and C806 (basically all the electrolytics on P800) with exact replacements. You'll need the tools listed on my recap HOWTO.