harman/kardon hk725 preamp rebuild

[z-adamson]

So i followed one of the solder pads to a wire....it's the fully jacketed wire which runs along side a bare, non jacketed wire. Typically when I see something like this, the non jacketed wire is negative and the fully jacketed wire is positive. Look at the pic next to the "E".....The fully jacketed white cable leads directly to one of the solder pads in question. I would guess that this is POSITIVE since it is the jacketed of the two wires which means that the corresponding solder pad is cap positive. Thoughts?

 

[z-adamson]

Looking at the schematic, C405 is on one channel and c406 is the same cap same function for the other channel. Since both of those lead to their corresponding 8.2k resistors at r401 and r402, use the leg of c406 going to r402 as your guide as to which leg of c405 to put towards its r401.

Looking at it that way, negative goes towards resistor, positive goes towards the jacketed wire. THANKS

 

[jheu02]

Sure it's not a non-polar or bipolar cap? That would explain no markings on the board, though most times the cap will have NP or BP on the label somewhere. If in doubt, you can put a NP /BP cap where a polarized one goes, just not polar for bipolar.

Looked at the scat and it is not marked for polarity at C405, 406, nor at the following:
C207, 208, 211, 212. There may be others too. Elswhere, the scat is marked for polarity. I'd almost bet they're NP caps as they appear to be used for coupling. Check those other areas and see.

 

[dlucy]

Sure it's not a non-polar or bipolar cap? That would explain no markings on the board, though most times the cap will have NP or BP on the label somewhere. If in doubt, you can put a NP /BP cap where a polarized one goes, just not polar for bipolar.

Looked at the scat and it is not marked for polarity at C405, 406, nor at the following:
C207, 208, 211, 212. There may be others too. Elswhere, the scat is marked for polarity. I'd almost bet they're NP caps as they appear to be used for coupling. Check those other areas and see.

The schematic seems to fail to note the polarity of the electrolytic caps, yes, but there is at least one electrolytic cap clearly label "NP" between the output transistors and the speaker protection circuit, so I'm guessing anything marked as electrolytic and not marked "NP" is actually a regular, polarized electrolytic. But I'm not there and I'm not an expert; just sharing some observations.

 

[dlucy]

Looking at it that way, negative goes towards resistor, positive goes towards the jacketed wire. THANKS

If you do discover this is right, you might consider noting the polarity (of all those you verify) on a copy of the schematic and posting it back to hifiengine or here to pay forward all that good work you're doing....

 

[jheu02]

it's interesting that you and I are looking at different scats for the H/K 725. I don't even see that circuit and the labeling system is different. The one I have is from hifiengine and is for a preamp only. Is yours an integrated as that circuit shows power amp as the board?

 

[RWood]

I recapped and relamped a 725 before discovering that it was the relay contacts that were bad. Mine probably didn't even need a recap. The relay subsequently - a few months later - got intermittent on one side again. If I still had the amp I would probably bypass the relay with jumpers. If the preamp is turned on before the power amp it really serves no purpose that I can see.

On those function controls - yeah, they're almost as bad as the little tone control presets. A great idea but poor execution. For mine, the key was to get access to the body of the switch so the cleaner could best make its way to the contacts. If you have to remove a board or some other hardware to get better access - do it. I also liked a non-residue cleaner better than DeOxit for this type of task. Then, frequent use (and lower humidity) seemed to make them happier.

 

[z-adamson]

it's interesting that you and I are looking at different scats for the H/K 725. I don't even see that circuit and the labeling system is different. The one I have is from hifiengine and is for a preamp only. Is yours an integrated as that circuit shows power amp as the board?

I dont think that schematic, the one that says power amp, is applicable to the 725 preamp.

Another question....does it matter which way resistors and transistors get installed? Do they an in and an out? A pos and neg?

 

[z-adamson]

I recapped and relamped a 725 before discovering that it was the relay contacts that were bad. Mine probably didn't even need a recap. The relay subsequently - a few months later - got intermittent on one side again. If I still had the amp I would probably bypass the relay with jumpers. If the preamp is turned on before the power amp it really serves no purpose that I can see.

I like this idea.

I will look into this.

Is the relay the component that is located near the on/off switch? The plastic box hanging off the side?

 

[dlucy]

I dont think that schematic, the one that says power amp, is applicable to the 725 preamp.

Another question....does it matter which way resistors and transistors get installed? Do they an in and an out? A pos and neg?

There is no polarity on resistors you install them any which way you like.

Transistors are quite a different story. Most (if not all) BJT transistors have three leads emitter, collector and base. You must put these three leads in the PCB correctly for it to function properly. Be aware that replacement transistors (e.g replacing a 2SC458 with a KSC1845) often have a different order on the leads than the original part. So, you have to be careful and be correct when replacing transistors.

 

[z-adamson]

Sure it's not a non-polar or bipolar cap? That would explain no markings on the board, though most times the cap will have NP or BP on the label somewhere. If in doubt, you can put a NP /BP cap where a polarized one goes, just not polar for bipolar.

Looked at the scat and it is not marked for polarity at C405, 406, nor at the following:
C207, 208, 211, 212. There may be others too. Elswhere, the scat is marked for polarity. I'd almost bet they're NP caps as they appear to be used for coupling. Check those other areas and see.

So if I use my polarized caps in place the original caps, and the original caps are non polarized that will not work?

But I can use non polarized and they will work well either way?

If I understood you right, that is what I will do considering that I am unsure and they are unmarked.

 

[z-adamson]

Sure it's not a non-polar or bipolar cap? That would explain no markings on the board, though most times the cap will have NP or BP on the label somewhere. If in doubt, you can put a NP /BP cap where a polarized one goes, just not polar for bipolar.

Looked at the scat and it is not marked for polarity at C405, 406, nor at the following:
C207, 208, 211, 212. There may be others too. Elswhere, the scat is marked for polarity. I'd almost bet they're NP caps as they appear to be used for coupling. Check those other areas and see.

Another thought....up to this point, the board has been marked with a "D" where the cap goes and the neg lead has been to the flat of the "D" and the pos line has been to the rounded side of the "D". So I took that to mean that the flat side equals NEGATIVE in every instance wether the cap was marked as well or not. Some of the original caps had the stripe down the side which means negative and every time that was towards the flat of the "D". Other caps did not have the stripe. Just and orange body with the specs written on it. I removed those and installed the Nichicon caps, which have the stripe to indicate negative and I would put the neg lead to the flat of the "D" and I assumed this would be correct. This last cap was a "O" instead of a "D" and no markings to indicate polarity on the cap and that threw me for a loop.

Have I been going about this the right way?

 

[z-adamson]

Double post

 

[RWood]

Is the relay the component that is located near the on/off switch? The plastic box hanging off the side?

It is towards the back, just left of center if I recall right. Yes, its in a plastic box with a clear top that comes off (and subsequently gets lost). To clean the contacts take a strip of porous card stock - like a non-glossy business card, soak it in cleaner and very gently put it between the contacts, moving it up and down. There are two sets of contacts, one for each channel. Resist the temptation to use something more abrasive as it could make them even more prone to corrosion in the future. Take care not to bend the arms that support the contacts.

Or avoid all this, follow the schematic and install two jumpers. You might even be able to follow the traces on the relay board - some are for the voltage to operate the relay and others are for the signal itself.

 

[jheu02]

So if I use my polarized caps in place the original caps, and the original caps are non polarized that will not work?

But I can use non polarized and they will work well either way?

If I understood you right, that is what I will do considering that I am unsure and they are unmarked.

The easy answer is:
You can replace polarized with non-polar caps, but in general, you cannot put a polarized cap into a position requiring non-polar.

The not so easy answer is "it depends." Installing a polarized cap "backwards" won't always lead to disaster because it really will depend on the amount of over voltage is getting reversed. If the caps you pulled had no markings as to polarity (some detailed photos of the actual caps might help here), and the board had no markings, and the scat has no markings, the SAFEST way to address the issue is to use non-polar (also called bipolar) caps to replace them with.

 

[jheu02]

Another thought....up to this point, the board has been marked with a "D" where the cap goes and the neg lead has been to the flat of the "D" and the pos line has been to the rounded side of the "D". So I took that to mean that the flat side equals NEGATIVE in every instance wether the cap was marked as well or not. Some of the original caps had the stripe down the side which means negative and every time that was towards the flat of the "D". Other caps did not have the stripe. Just and orange body with the specs written on it. I removed those and installed the Nichicon caps, which have the stripe to indicate negative and I would put the neg lead to the flat of the "D" and I assumed this would be correct. This last cap was a "O" instead of a "D" and no markings to indicate polarity on the cap and that threw me for a loop.

Have I been going about this the right way?

Boards have been screened incorrectly, schematics have been printed incorrectly. Running changes have been made during production and never get into the documentation or annotated on the schematic/SM. That's why it's important to compare what you actually see installed to what is called for in the service manual or on the scat. Many here take detailed before/after photos and or only replace one component at a time for that very reason. It's also good practice, when able, to only work on one section at a time and test afterward. That way if something goes wrong the troubleshooting process is much easier since you have limited what has changed between each testing.

I can tell what you're describing with the D and O, but can't say for sure if your deductions regarding polarity are correct. If anyone has one of these preamps in working condition who could take and post some detailed photos, that would help greatly.

 

[z-adamson]

Pics of cap. Says "CE-BP" in it....I take it that means that it is bipolar.

 

[z-adamson]

Here is a pic of a cap that is marked as "CE-BP" in a location on the board that is marked for positive and negative. Up to this point, I have removed 5 caps like this (marked BP, but board marked for + and -) and I have replaced with polarized caps following the boards markings thinking that I was doing the right thing.

Is it likely that bp caps were used where polarized caps could have been used, hence the inconsistencies?

 

[z-adamson]

As frustrating as it is that I don't have the right caps, I will take a step back and start over. I haven't soldered anything yet, but it will be a bit of a pain to figure out what caps I need as I don't recall not did I note down which were bp and which were not. It looks like the diagram I have spells this out though. Live and learn.

 

[jheu02]

CE is from the French....Capacitor Electrolytique....and yes, BP is Bipolar. It's a shame that the SM didn't list these (most do) and yes, it could have been a factory enhancement, which is why it wasn't marked. Many feel that using a bipolar cap in coupling situations is better for audio purposes. I don't know if HK had a lower model number in the series which may have used polarized caps and a common board. There's a chance that nothing wrong will happen since you followed the rest of the board layout, but then again... As you said, live and learn.

These would be good caps for C405, 406, 211, 212

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtZ1n0r9vR22e0BBN1kF1oiaRKDk3aXVQU=

As to "reverse engineering" to figure out which went where, you should be able to take the cap value and look at the schematic to give you likely places they were pulled from. Don't worry, we'll get it all sorted out.