Harman Kardon HK 725 Preamplifier - Repairs (Ep. 46)

[@syl1101-1]

Thanks for your no-frills repair videos, they certainly have a very enjoyable 'feel-at-home' atmosphere. It's nice to see knowledgeable and still humble people. As for the resistor tone controls my 1ct, i agree with Asriazh, plus better sound. They say a (reasonable quality) fixed resistor always sounds better than a pot (-> stepped resistor attenuators).

[@TheThatoneguy12121]

I know this video is old, but if you are in your search for obscure hardware that old units use, McMaster Carr is a great place to find it. They literally have everything. I was able to find replacement flange nuts that held down the cross overs in my onkyo speakers from the 70s that were JIS (Japanese Industrial Standard) and fit perfectly. Hope this helps.

[@iropmovs]

Thank you for the informative guide. I have a 725 here and the volume squeaks just the same, so it will be a general problem. As for the bass and treble buttons, if they had used relays and touch buttons, or a touch display, it would have made more sense, but they didn't have that available at the time, so they chose buttons that have a guaranteed lifespan of 5 years or more they didn't care. Besides, it was fashionable in Japan at that time, just look at e.g. Optonica products.

[@pdx1jmh]

Love the video. What cleaners and lubrication products do you use? Do they work well? Thank you for putting these videos out.

[@Asriazh]

First of all, nice job, fixing that preamp *thumbs up* Removing and cleaning those switches looked like the most tedious and mindnumbing job ever, or at least one of them, heh. I can guess though why they used switches instead of a pot. Pots are not very accurate, especially when dealing with a stereo signal and keeping things equal on both channels. With buttons you can use a fixed resistor value, where you can change the resistor if it drifts too far. I actually like that idea. More precision.

[@TrevorsBench]

Thanks. Cleaning those switches had to be one of the most tedious jobs ever but I was able to take breaks which made it more bearable. At first I thought the switches for tone controls was gimmicky but after I tested them and saw how well they worked it kinda made sense. Still, I would rather have a continuously variable tone control than having just a few steps to choose from

[@georgeharrisonfan789]

Hey, I’ve got this same piece. I’m having an issue where the right channel has been occasionally dropping out, and now has fully gone out. Any clue as to why that would be or how I could remedy the issue?

[@TrevorsBench]

Remember I'm only guessing here but it's likely the relay on the output that has dirty/corroded contacts. I have the same problem on this preamp and I plan on making a video to address this issue

[@johnpeterson99]

Any trick to get the volume knob off? I feel like I'm going to break the darn thing.

[@TrevorsBench]

It should just pull straight off unless somebody glued it?

[@johnpeterson99]

@@TrevorsBench That's what I was thinking might have happened but I just kept pulling and prying with a plastic spudger and then the metallic cover popped off and the plastic cover was still attached. The silver facia (parts shown in the service manual) was held on with two sided tape. There is spring collar around the plastic female end of the knob that seems to provide tension to keep the knob from getting loose over time. I did manage to get it off so I could get some deoxit in the pot. Really helped with the scraping noise that I was also getting from mine.

[@Ltcarlton]

Any chance you remember what bulbs you used for the switches?

[@CneyTMS]

I would like to know as well. But i first have to figure out why I see 36 volts across from the bulbs. The 1kohm 1 watt resistors are within spec and the manual gives + and - 6V on the power lines for the bulbs