Recapping advise for an old amp

[Charles_b]

Thanks for you replys guys, definitely helpful.

 

[JeffS7444]

I recommend getting some el-cheapo caps off ebay.

Please no: For a hobby type project of this scale, the cost of genuine Elna Silmic, Nichicon Muse or other audiophile-annointed product purchased from the reputable sources like Digikey, Mouser or Newark/Farnell is minor compared to the labor needed to install them.

 

[restorer-john]

I know it's naughty but the guy is a newbie and needs to cut this short.

It's not going to be pretty anyway, I can tell. 4400s are tricky beasts.

Go have a look at the topology. It's not a beginners amp to work on...

 

[Charles_b]

I have similar amps but without the 'a' suffix, they do suffer from an occasional dropout and and some cracking/popping noises. I'm pretty sure it happens due to the oxidation of the speaker protection relay contacts as if I crank up the volume it goes away. If your issue is similar I'd visit the relays first.

If it is the speaker relay, would replacing with these solve the problem? https://www.ebay.de/itm/111870297679

 

[restorer-john]

If it is the speaker relay, would replacing with these solve the problem?

There's two speaker relays. Use the speakers B (the unused one) terminals. If the crackle doesn't return, it was the speakers A relay. If the crackle does return, it wasn't the relay.

Use processes of elimination strategies, not blanket shot-gun approach techniques.

 

[gvl]

There's two speaker relays. Use the speakers B (the unused one) terminals. If the crackle doesn't return, it was the speakers A relay. If the crackle does return, it wasn't the relay.

Use processes of elimination strategies, not blanket shot-gun approach techniques.

For me both A and B have dropouts, B is worse than A as it's not usually in use. Both relays can be shot.

 

[Doodski]

It's not going to be pretty anyway, I can tell. 4400s are tricky beasts.

Go have a look at the topology. It's not a beginners amp to work on...

I imagine it will be a typical Denon PCB, easily lifted traces if overheated and not treated delicately. It maybe /probably has service bulletins out on it too. But on the flipside it should be reasonably easy to disassemble and reassemble because Denon thinks this stuff through. You might be right on getting a experienced techy to do the proper repair.

@Charles_b What is your experience with troubleshooting and repairing electronics circuitry? Do you have the the proper rated resistor to discharge a power supply and do you know how to use a oscilloscope and sine wave generator if you need to go that route? I would contact Denon and request any service bulletins that may be out on this unit too. If you tell Denon you have 3 of them and one is needing service they will probably try and help by supplying literature if it is available. Strange noises like these can sometimes be caused by ICs or transistors. @restorer-john has a point that you may cause damage to the unit(s) and these are nice units and should be taken care of.

 

[Vini darko]

Please no: For a hobby type project of this scale, the cost of genuine Elna Silmic, Nichicon Muse or other audiophile-annointed product purchased from the reputable sources like Digikey, Mouser or Newark/Farnell is minor compared to the labor needed to install them.

I was suggesting cheapo caps for learning practice not in the denon. It's far too easy to damage traces when first learning to desolder.

 

[solderdude]

Very common issue. Crank up the volume when it happens, if it clears the congestion it's likely the relay. You can likely restore it if replacement isn't available, there are videos on YouTube.

tapping on the relay(s) also can help locating the error but sometimes tapping 'cures' something else on the PCB where you might think it is the part you tapped on it may well be another part.

You can check semiconductors with coldspray (or canned air held upside down)

 

[Charles_b]

There's two speaker relays. Use the speakers B (the unused one) terminals. If the crackle doesn't return, it was the speakers A relay. If the crackle does return, it wasn't the relay.

Use processes of elimination strategies, not blanket shot-gun approach techniques.

Unfortunately, crackles with both A and B terminals

 

[Charles_b]

I imagine it will be a typical Denon PCB, easily lifted traces if overheated and not treated delicately. It maybe /probably has service bulletins out on it too. But on the flipside it should be reasonably easy to disassemble and reassemble because Denon thinks this stuff through. You might be right on getting a experienced techy to do the proper repair.

@Charles_b What is your experience with troubleshooting and repairing electronics circuitry? Do you have the the proper rated resistor to discharge a power supply and do you know how to use a oscilloscope and sine wave generator if you need to go that route? I would contact Denon and request any service bulletins that may be out on this unit too. If you tell Denon you have 3 of them and one is needing service they will probably try and help by supplying literature if it is available. Strange noises like these can sometimes be caused by ICs or transistors. @restorer-john has a point that you may cause damage to the unit(s) and these are nice units and should be taken care of.

I have seen one cracked solder joint that I need to take care of. Apart from owning a capacitor ESR checker, I don't own nor have any experience with an oscilloscope or sine wave generator. I was hoping it was a cap as they are easy to diagnose/replace. If its not something blindingly obvious like a bad cap or bad solder joint, then it's probably beyond the scope of my competence and I would probably have to get it professionaly fixed. Money is a bit tight at the minute (isn't it always!?) and I didn't want someone else to have the same bad experience that I did by selling a malfunctioning amp on ebay (the fault only came to light 30 days after I received the amp so I was unable to return)
Many thanks

 

[Charles_b]

tapping on the relay(s) also can help locating the error but sometimes tapping 'cures' something else on the PCB where you might think it is the part you tapped on it may well be another part.

You can check semiconductors with coldspray (or canned air held upside down)

So semi conducter produce heat that evaporates quick then?
Cheers,
Charles

 

[Charles_b]

It's not going to be pretty anyway, I can tell. 4400s are tricky beasts.

Go have a look at the topology. It's not a beginners amp to work on...

Exuse my ignorance @restorer-john, but what is a topology?

Thanks,
Charles

 

[solderdude]

So semi conducter produce heat that evaporates quick then?
Cheers,
Charles

All components not mounted on a heatsink (the vast majority) will become very cold for a minute or so when you spray them for just a few seconds.
Sometimes heating components can also give away improper operation.
Suspect mechanical components first (switches, connectors, relays) then look at cold solderjoints and solderrot (tiny circles in the solder) usually around components that become hot or can move or vibrate. Also look at bulging caps or leaking caps.
Some tantalum won't show physical damage but can leak (noise, DC offset drifts).

 

[tomtoo]

Exuse my ignorance @restorer-john, but what is a topology?

Thanks,
Charles

Its the way the amp is build in the elektronic way. It could be easy one tube in class a, but it could be much more complicated.
To be honest i dont think your idear is a good idear. If its realy a cap and you know the cap than get some experience in soldering and desoldering before. But if you dont know if its the cap without experience its not a good task. You can easaly brake more than its already broken. And that would be uncool with this amps.

Edit says: I have a 3 1/2 year electronic education. Yes germany is different. You know Vocational training. And i would not do this without some concerns.

 

[tomtoo]

Exuse my ignorance @restorer-john, but what is a topology?

Thanks,
Charles

I mean ask @restorer-john imo he has some experience in repairing amps. Even i repaired some, its realy good to have some experience.

 

[AnalogSteph]

Random crackling is usually caused by:
* bad solder joints
* failing semiconductors (failed bonding, no less maintaining full C-E voltage, leaky FET)

Excessive leakage in electrolytic capacitors shows up as popcorn (1/f) noise when subjected to voltage. A relatively modern amplifier like this should have little in terms of DC going on in the actual signal path, so caps wouldn't be my first suspect.

Really, nothing beats having someone with the requisite knowledge and experience working on one of these. Troubleshooting complex DC-coupled amplifiers is not entirely trivial.

 

[restorer-john]

Exuse my ignorance @restorer-john, but what is a topology?

It is the design of the circuit. In this case, Denon were obsessed with speed and low distortion. They used various innovative (some say foolish) designs in the early- mid 1980s. When they work, they are pretty good, but they are also fragile. I have a few POA-xxxx series amplifiers here and a few parts donors if you know what I mean.

@Charles_b Nobody here is trying to put you off repairing your amplifier, but you need to consider that the entire circuit is a high speed feedback system (well in this case a rather complicated system of feedback and distortion cancellation). By that I mean something that occurs at practically any point ends up affecting stages both before and after the "problem". That can be extremely confronting and lead to much head scratching.

The secondary rails are 142V from + to- (+/-71V) and dangerous. Any slip of a probe tip will make a big mess and likely write off the amplifier. Right now, it is working and original but has an intermittent issue. Intermittent issues are the worst problem as technically, when the issue is not present, there is no fault to be found. In other words, the fault has to manifest, be characterised and then be localised to a small part of the circuit and eventually pin-pointed.

I cannot stress enough that changing out components at this point in an effort to fix the problem is pointless.

I would be:

a) running the amplifier on a DBT (dim bulb tester) to limit current and use a sacrificial speaker for listening. Face the speaker away from you.
b) test the various rails (+/-15V and the decoupled +/-68-71V) for stability, then test the voltage stage (it's a totally separate circuit) separate from the power stage. I'd be feeding in a sine and probing with a 'scope the output of the voltage stage. If there's no intermittent issue there, you've cut your work in half- it's in the power stage. If the problem is in the voltage stage you can disconnect the 8A rail fuses, jumper the front end to the power supply and work on it without the risk of blowing up anything down the line.

Without a scope and familiarity with using one, you are flying blind. Without one, may get lucky and find an obvious problem, but you're also highly likely to kill the entire amplifier.

I assume you have the service manual?

 

[gene_stl]

One of the signatures here is "addicted to fun and learning". We all would love for you to learn about amps and fix yours. But you are probably trying to bite off more than you may be able to chew. Do you have a load resistor for the amp to drive? A signal generator. (those are available now as cell phone apps I suppose but I wouldn't use one) and oscilloscope? A trustworthy multi-meter? And know how to use all of them. Do you have test probes and interconnects for the above equipment? All this sort of thing takes a long time to accumulate. And a fair amount of money too.

I would take restorer-johns word for it that it is an easy to break, sophisticated design. And even though I got interested in electronics in 1959, I greatly prefer having the service manual in front of me when I work on stuff whether its a lawn mower , a piece of stereo equipment or an "optikeen scalometer" at work. (especially at work). Fortunately the innerscreen makes such manuals available for download more easily than used to be the case. Service manuals tell you what voltage should be at various test point, the procedure for setting the bias currents and other things you need to know or things that will reveal where your noises are coming from.

Sometimes you can , by searching the make and model number, run across people that have documented stuff they did which may show you the way forward. But I think you should find a competent repair shop that does audio and pay them what they require to repair this unit and start building amplifier kits. Walk and then run. jmo I would hate to see you blow this thing up and quadruple the cost of the repair or worse yet blow up some unobtainium semiconductors. Honestly if you need to ask "what's a topology" you are not quite ready to tackle a thoroughbred amp. The inclination is praiseworthy.

 

[Charles_b]

Perhaps you're both right and I need a rethink. It just sucks when someone sells you something that doesn't function the way it should do. Too many dodgy people selling broken things making them other peoples problems. Thanks for the advice