harman/kardon hk725 preamp rebuild

[z-adamson]

Next on my agenda is a rebuild of my hk725 preamp. As It is now, it will sound good out of one channel and sound not so good out of the other unless it is warm. So the guy that sold it to me left the preamp on 24/7 and it performed well all the time. I am not sure as to what would cause that, but I hope to fix that problem.

I plan to replace all e-caps, rebuild the power supply and do a full deoxit and faderlube of all switches and pots.

What should I add to my to-do list considering the problem of not sounding right until warm?

Lastly, what is involved in a power supply rebuild? This was recommended in another thread, and honestly I am not sure how to go about that.

 

[dlucy]

Rebuilding the power supply is merely replacing all or nearly all the components. High wattage resistors may have lost value or been over-stressed for too long, voltage regulating transistors or Zener diodes may have been too hot for too long, electrolytic capacitors may have been run too long at too high a voltage or lost their electrolyte fluid....

You also might have ceramic or film caps on the power supply board. I'd say those are the only components I might _not_ replace during a rebuild. But I'm crazy and those parts are dirt cheap these days, so I'd replace them, too, if I were doing the rebuild.

I've got to do the same to the hk825 I bought last year but haven't found time for yet.

 

[z-adamson]

Rebuilding the power supply is merely replacing all or nearly all the components. High wattage resistors may have lost value or been over-stressed for too long, voltage regulating transistors or Zener diodes may have been too hot for too long, electrolytic capacitors may have been run too long at too high a voltage or lost their electrolyte fluid....

You also might have ceramic or film caps on the power supply board. I'd say those are the only components I might _not_ replace during a rebuild. But I'm crazy and those parts are dirt cheap these days, so I'd replace them, too, if I were doing the rebuild.

I've got to do the same to the hk825 I bought last year but haven't found time for yet.

Hmmm, the power supply is a board? I was under the impression that the power supply was a component especially considering that the eq that I just built and the preamp that I am going to rebuild don't have any board entitled "power supply".

That said, what is the part in the following pic? I thought this was the power supply.

 

[jheu02]

That's just the transformer. The power supply will have the rectifiers, caps, etc. whether the board is labeled as such or not, there will definitely be more components than just the transformer. The service manual is available for download at hifiengine.com. Free to register. I show a different power supply for the 725 preamp than that posted above.

 

[z-adamson]

Is the transformer servicable? Does not seem to be. How would you bench test one?

So it would be logical to replace everything in the power supply it would seem.

 

[dlucy]

Most people won't be able to service the power transformer. It is one or more tightly wound coils of specially-coated wire with a precise number of wraps of the wire.

You'd have to be very careful and practice good electrical safety, but to bench test it you would disconnect the secondaries (the wires on the side leading to the PCB) and measure the voltages they produce when the primary (wires on the side leading to the wall plug) is plugged in. The schematic will tell you what voltages you should see on the secondary side.

The voltages you measure when the secondaries are disconnected from the PCB should be a bit higher than those listed on the schematic since the secondaries are not loaded down with the resistance of all the components on the PCB. If the schematic says "red and red wires are 24 V", then you would expect to see at least 24 V made between the two red wires and maybe as much as 30-40 volts.

Don't go messing with the transformer when it is plugged in if you don't know basic electrical safety, though.

If the transformer is blown, you'll have to replace it rather than repair it.

 

[jheu02]

You don't need to do anything with the transformer. The issue with working better once warmed up is probably poor solder joints somewhere in the bad channel. Part of a restoration of older gear will include a careful look at the solder joints, especially around the power supply or any transistors that typically get warm...ones with heat sinks. Best to touch up all of them, but even check the joints at the RCA out jacks. Use a good magnifying glass and bright lighting. Again, and I can't stress this enough with this vintage of HK gear, replace all lavender or dark blue sleeved Matsushita caps you find. The green TK ones typically checked fine for the limited testing I could do, but I replaced them anyway.

 

[z-adamson]

You don't need to do anything with the transformer. The issue with working better once warmed up is probably poor solder joints somewhere in the bad channel. Part of a restoration of older gear will include a careful look at the solder joints, especially around the power supply or any transistors that typically get warm...ones with heat sinks. Best to touch up all of them, but even check the joints at the RCA out jacks. Use a good magnifying glass and bright lighting. Again, and I can't stress this enough with this vintage of HK gear, replace all lavender or dark blue sleeved Matsushita caps you find. The green TK ones typically checked fine for the limited testing I could do, but I replaced them anyway.

I plan to replace ALL aluminum e-caps which as I understand it should cover the lavender problematic caps you speak of.

When it comes to repairing compromised solder joints, what do you do? When you say "best to touch up all of them" are you saying solder over the preexisting solder or remove and resolder?

 

[z-adamson]

Most people won't be able to service the power transformer. It is one or more tightly wound coils of specially-coated wire with a precise number of wraps of the wire.

You'd have to be very careful and practice good electrical safety, but to bench test it you would disconnect the secondaries (the wires on the side leading to the PCB) and measure the voltages they produce when the primary (wires on the side leading to the wall plug) is plugged in. The schematic will tell you what voltages you should see on the secondary side.

The voltages you measure when the secondaries are disconnected from the PCB should be a bit higher than those listed on the schematic since the secondaries are not loaded down with the resistance of all the components on the PCB. If the schematic says "red and red wires are 24 V", then you would expect to see at least 24 V made between the two red wires and maybe as much as 30-40 volts.

Don't go messing with the transformer when it is plugged in if you don't know basic electrical safety, though.

If the transformer is blown, you'll have to replace it rather than repair it.

Thanks, I plan to at least take some measurements to make sure it is performing within specs.

 

[z-adamson]

Lets say I wanted to bypass some features of this preamp.

There are things on this preamp that I have no use for and it seems logical that bypassing may be a way to go. For instance, the tone controls, loudness, all the tape monitor / copy, balance. I feel as though I would have a more reliable preamp if I were to minimize the amount of unused stuff on here.

Does it make sense to do this?

Aux input and volume is all I really need.

 

[jheu02]

I plan to replace ALL aluminum e-caps which as I understand it should cover the lavender problematic caps you speak of.

When it comes to repairing compromised solder joints, what do you do? When you say "best to touch up all of them" are you saying solder over the preexisting solder or remove and resolder?

Yes, replacing all 'lytics will take care of the lavender ones. Are you also replacing the Tantalum caps if there are any? If so, a good replacement for them are film or low-leakage caps...Nichicon's UKL series.

Some go to the extent of desoldering and then resoldering the joints. Too much work for me unless I'm going to test or replace the part in question. All that's really needed is to heat up the existing joint. Using just a touch of fresh solder can help make this easier as it will add a touch of flux so it flows better. Just a quick touch to the joint and pull the iron tip up away from the board along the leg of the component...just like is done when putting new components into the board, only you don't have to trim the excess lead material.

 

[dlucy]

Lets say I wanted to bypass some features of this preamp.

There are things on this preamp that I have no use for and it seems logical that bypassing may be a way to go. For instance, the tone controls, loudness, all the tape monitor / copy, balance. I feel as though I would have a more reliable preamp if I were to minimize the amount of unused stuff on here.

Does it make sense to do this?

Aux input and volume is all I really need.

I think you'll get a lot more answers on this question if you post a new thread with something like "how to bypass time controls and similar on hk725" in the title. That should attract a few more people.

 

[jheu02]

Lets say I wanted to bypass some features of this preamp.

There are things on this preamp that I have no use for and it seems logical that bypassing may be a way to go. For instance, the tone controls, loudness, all the tape monitor / copy, balance. I feel as though I would have a more reliable preamp if I were to minimize the amount of unused stuff on here.

Does it make sense to do this?

Aux input and volume is all I really need.

Not at first. Make sure it's working properly before you start modifying things by bypassing etc. and you do realize that tape inputs, tuner, cd, aux, dat, etc (all except for phono) are line level inputs correct? You say you only need one input????

There are shorting plugs for the others, but I wouldn't solder-bypass the other inputs...you just might want to hook up more than one source at some point. Besides, oftentimes if a switch isn't engaged, it is wffectively being bypassed. Depends on the circuit, but you'll have to look at the schematic to determine that.

 

[z-adamson]

Made some headway this evening.

Got everything apart and went to town with deoxit and boy was it needed. Here it is (see pic). The more I think about it the more I think I will avoid bypassing any components of this preamp.

 

[z-adamson]

I think I may have found the problem with this preamp. There are obvious signs of heat damage to the chassis and looking at the corresponding area on the main pcb it is clear ( I think ) where the heat is coming from. Here are pics. The heat seems to be coming from those metal brackets you see in the pics as this corresponds to the burn mark on the chassis. Additionally, the e-caps that surround the metal brackets are browned only on the area of of the cap body that faces the metal brackets.

Having said that, what are those metal brackets and how do you advise that I address the problem associated with them?

 

[dlucy]

There are obvious signs of heat damage to the chassis and looking at the corresponding area on the main pcb it is clear ( I think ) where the heat is coming from.

...

what are those metal brackets and how do you advise that I address the problem associated with them?

Good catch!

Those metal brackets are heatsinks for the transistors they are attached to. The transistors look like the TO-220 package style and I'd guess they are the voltage regulators for part of the power supply.

They are either the BJT transistors at Q601 & Q602 or the FET transistors at Q603 & Q604.

THAT much heat coming from them means they are likely cooked, but you'll need to remove and check them anyway. The caps near them are also likely to be cooked and dead.

When you replace them, you'll need to figure out why they overheated and prevent that from happening again.

 

[z-adamson]

Good catch!

Those metal brackets are heatsinks for the transistors they are attached to. The transistors look like the TO-220 package style and I'd guess they are the voltage regulators for part of the power supply.

They are either the BJT transistors at Q601 & Q602 or the FET transistors at Q603 & Q604.

THAT much heat coming from them means they are likely cooked, but you'll need to remove and check them anyway. The caps near them are also likely to be cooked and dead.

When you replace them, you'll need to figure out why they overheated and prevent that from happening again.

Looks like q601 and q602 based on the markings on the pcb. So, how do you suggest I determine the cause of failure? What could cause this? I will replace the transistors and caps, but how do i find the problem that caused this?

 

[z-adamson]

PS.....they ARE q601 and q602. Confirmed it with the markings on them and description in the service manual. They are voltage regulators described as "2sc1419 (c) and 2sa775 (c)".

 

[dlucy]

Looks like q601 and q602 based on the markings on the pcb. So, how do you suggest I determine the cause of failure? What could cause this? I will replace the transistors and caps, but how do i find the problem that caused this?

Well, I am not an EE, so most of my advice is guesswork. That being said, I would first guess that the Zener diodes at D601 and or D602 failed. Pull them out and check them with a component tester.

If it wasn't them, then I'd guess one of the resistors between the bridge D603 and the transistors shorted. You can check for that easily with a multimeter.

But I'm just guessing.

 

[dyche01]

The transistors might not have 'overheated,' since they are designed to run hot (hence the heat sinks). It might have been years of normal operation that produced the discoloration in that area. I would worry more about the capacitors sandwiched between them. Marantz built a similar 'capacitor oven' on the power amplifier board of the 1180DC and it was good at cooking electrolytics!