Nagging intermittent hum on 2285B - want to remedy

[jasong]

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.

Would faulty caps explain the intermittent cutting in and out, and also the apparent chassis ground issues? If I do recap, I'd like to recap the whole power board and order all the parts so a local tech can do it. I'm not confident in my cap-swapping abilities.

At the very least, I'd love to solve the intermittent ground hum problem.

 

[Gigapod]

Would faulty caps explain the intermittent cutting in and out, and also the apparent chassis ground issues?

There are three possibilities here: bad caps, bad solder joint and/or bad contacts.

If I do recap, I'd like to recap the whole power board and order all the parts so a local tech can do it. I'm not confident in my cap-swapping abilities.

At the very least, I'd love to solve the intermittent ground hum problem.

If you do recap, I suggest recapping the entire receiver, not just the power board. But a minimal job would be the entire power board, I agree.

I think you have made the right decision in getting all the parts and having a local tech do the unsoldering/desoldering work.
- If you get the parts yourself you skip the inevitable delay in ordering the right capacitors, which would mean the local tech would just keep your receiver for a week or two for nothing.
- If you bring the local tech the receiver, the schematics and all the replacement capacitors, that provides an extra motivation for him to do the job well and quickly.

 

[EchoWars]

I'd say the group mind has it pretty much covered. If it is due to an aged cap in the regulated supply, replacement should do it. A bad solder joint or a broken ground wire is also a possibility, but the betting money is on old caps.

 

[jasong]

If only I could figure out the source of the intermittent hum - perhaps a bad solder or contact - I'd be set for the short term. I'd be very happy if that hum was completely gone. I wish there were a way to pin down the fault more accurately.

I can take some time to reorder all the capacitors and get that all set up, that will make it more simple for a local tech to do the swaps. It seems like the unit saw some time in storage and is otherwise in good shape. I think all the tech did when I got it serviced was just solder a little on the main board or the power board anyway. Recapping the entire unit would be a huge job, I don't know if I want to take preventative maintenance that far unless I have to.

In the meantime, I think it'll be good to order replacements for all the caps on the power board and try that for starters.

Thanks for all your comments guys, I really appreciate your interest. I want to preserve this special amp, but I've never been proficient enough at electronics soldering to hope to confidently replace all the caps on the board or accurately diagnose faults.

 

[jasong]

More symptoms:

Actually the hum does come through on the FM, but it is not affected by the volume knob. When I change back to phono, the volume knob makes the hum louder.

So: the hum is present on the radio modes and in the AUX mode, although not controlled by the volume knob. In phono mode, the volume knob increases the strength of the hum.

I wonder if it's worth hassling the electronics shop again...work that 90 day warranty.

 

[Gigapod]

Hi Jason,
These symptoms are consistant with excessive power supply ripple on one of the regulated lines, which is usually caused by a bad electrolytic (one or more of the caps on the PS board).
It would be easy to track down to a specific capacitor using an oscilloscope, a tool that any electronics repair shop has, but most individuals don't.

 

[jasong]

Ah OK...supposing I had access to a scope, how would I go about testing the caps?

 

[Gigapod]

The scope allows you to "see" (and measure) the ripple on a power supply line. So from the schematic diagram, one would just test the various regulators on the PS board and check which ones have ripple at their output. Then of course replace the capacitors around it/them.
But then again, if you recap, you do the entire board.

 

[jasong]

Yeah a power board recap is definitely in the near future for me.

Edit:

But why is the hum intermittent if it is a failing capacitor? Does the intermittent hum match a typical failure mode for the caps?

 

[Gigapod]

Yeah a power board recap is definitely in the near future for me.

Edit:

But why is the hum intermittent if it is a failing capacitor? Does the intermittent hum match a typical failure mode for the caps?

Yes, when the electrolyte dries out the capacitance will drop to very low levels, and the value will be quite unstable but always well below spec.

Actually I would make that near future very near, because if any cap shorts it will take out a few more components with it, and that becomes quite an expen$ive repair.

 

[jasong]

Yes, when the electrolyte dries out the capacitance will drop to very low levels, and the value will be quite unstable but always well below spec.

Actually I would make that near future very near, because if any cap shorts it will take out a few more components with it, and that becomes quite an expen$ive repair.

Yeah I should look at ordering replacement caps pronto. Don't make me paranoid!

Edit: do caps C801 and C802 (see schematic) act as filter caps? I'm trying to read up on the EE of the electronics, Oh if only I'd paid more attention in my EE classes.

I'm just trying to wrap my head around why the hum only affects certain settings of the amp if it is caused by the caps on the power board.

 

[pfontherun]

Mouser.com is still my preferred place (USA). Fistells is great too for Denver residents. Anyway, I too have a 2285B and you can find just about every cap in there I reckon on mouser with exception of the dual section 6800s. Find my post called, if I recall, "2285B recap." I had to replace both of mine with 4 singles. Original replacements are long gone I'm sad to say. Oh-- sometimes dimmer lights do really goofy things on stuff. Is your amp plugged into a circuit in your house with a dimmer light switch? Just a theory.

 

[jasong]

Mouser.com is still my preferred place (USA). Fistells is great too for Denver residents. Anyway, I too have a 2285B and you can find just about every cap in there I reckon on mouser with exception of the dual section 6800s. Find my post called, if I recall, "2285B recap." I had to replace both of mine with 4 singles. Original replacements are long gone I'm sad to say. Oh-- sometimes dimmer lights do really goofy things on stuff. Is your amp plugged into a circuit in your house with a dimmer light switch? Just a theory.

Hey, thanks for the post. I'll concentrate on mouser.com to start with, I had read about that in other threads.

I'd like to start by recapping the power board, starting with C801 and C802. The service manual lists them as 470 uF and 330 uF, 63 V. The part numbers listed are EA47706310 and EA33706310, which evidently specify the capacitance (47/33) and voltage (63).

Dumb question: Are capacitance and voltage the only two ratings that need to be matched on capacitors? Are there any other criteria that set caps apart? Tolerances? Do those part numbers specify anything more than that?

I read in a thread somewhere about the double caps and replacing them with two 6800 uF caps wired in parallel...which is what I'll likely do also.

And no, I don't think that circuit is on a dimmer, although there are dimmers nearby. I've had the humming problem since I bought the amp.

 

[restorer-john]

what you guys need is an ESR meter to stop all this guesswork. you can test any cap in circuit (with the power off) and locate the suspicious ones quickly.
get yourself one of these:
http://www.dse.com.au/cgi-bin/dse.storefront/45a03fd104e326c82740c0a87f9c071d/Product/View/K7214

The best esr meter I have ever used and a really easy kit to build.
john

 

[Gigapod]

...
do caps C801 and C802 (see schematic) act as filter caps?
...

They are the main smoothing (filter) capacitors for all the negative and positive regulated voltages on the 2285B. IOW they are very important.

BTW there is surely a mistake somewhere in the Marantz schematics, a resistor must be missing somewhere before zener diode Q809...
OK, I found it (R816) on the PCB layout picture but it's really missing on the schematics.

A little late to signal it to Marantz engineers, though!

 

[jasong]

They are the main smoothing (filter) capacitors for all the negative and positive regulated voltages on the 2285B. IOW they are very important.

BTW there is surely a mistake somewhere in the Marantz schematics, a resistor must be missing somewhere before zener diode Q809...
OK, I found it (R816) on the PCB layout picture but it's really missing on the schematics.

A little late to signal it to Marantz engineers, though!

Hah yeah I couldn't find C805 or C811 (IIRC) on the schematic and thought I was going insane until I looked at the PCB. It seems there are a few items on the PCBs that are not indicated in the schematics.

Hey now, what's this about an ESR meter? Maybe I should check one of those out.

 

[restorer-john]

ESR meter

I reckon everyone on AK should have one (maybe I should get commission on this from dick smith). All the fantastic well meaning people here spend too long guessing cap problems and then doing a wholesale replacement of all the caps only to not fix the real problem and waste a stack of money and time.
The kit is a Silicon Chip design (our only great electronics magazine in Australia) and Dick smith have produced it for kit sale. Dick smith's kit is pretty good- ironically, you may want to upgrade a few of the caps in the kit to nichicon FG's and possibly a few other components, but it is an awesome test device. I have not even flattened one set of batteries in two years so it is economical to run. I'll post some pics of the unit and the design later today.
john

 

[jasong]

Eee, I don't know if I'm into building a meter. *sigh* I should just practice up on soldering and get good enough to do these things myself.

The meter sounds like a fantastic tool anyway, but probably the best way to increase the lifespan of the unit is to recap. I don't doubt that these caps will fail sooner than later, even if the unit did spend time in seasonal cold storage or something.

The intermittent on/off nature of the hum on my amp gives me the nagging suspicion that there might be a bum solder somewhere, but...I don't know. A bum cap seems to explain it, but I'd just like to know for sure.

Like just this second, the hum started...shut the unit off, turned it on, and it was gone.

 

[Gigapod]

...
Hey now, what's this about an ESR meter?
...

Hi restorer-john,

You are quite right, using an ESR meter is a good way to find faulty capacitors, even though they may "look" good. A worthwhile investment for somebody doing a lot of recapping.

I found this page from the original designer of the ESR meter, it seems apart from the kit from Australia there is also a fully-built, pro version of the ESR-meter (available here in Europe):

http://members.ozemail.com.au/~bobpar/esrmeter.htm

 

[jasong]

Whoa...which capacitors to pick...there are lots of different kinds at mouser. Looks like I should read a bit more.