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Harman Kardon 730 Updates/Upgrades

JMiP

JMiP

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I just recently purchased a long sought after Harman Kardon 730 and ran across this thread by @dyche01 regarding the upgrading of the phono stage. The thread focuses on upgrading some caps and a few resistors, but I'm not clear on the value of the resistors he used. For instance, the schematic for that particular board tells us that the resistors labeled R735 and R736 are a value of 680 and that's it. I'm guessing that it's 680ohms, but is there a value for watts or volts that I should know about?

He also mentions, "...After some routine cleaning and maintenance, which included upgrading the power supply and sub-rectifier capacitors..." (emphasis mine) Do you know what those upgrades could be?

Any other updates and upgrades that I can make on this unit? Are there any known problematic transistors and the like that I should replace?

Thanks!
 
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Found my Technical Manual for the unit and I see a note that states "Unless otherwise specified, resistors are 1/4 watt." I can tell by looking at the higher value resistors that the ones to replace on the equalizer/phono board are just 680Ω.

I'm looking through the transistor list and so far, I'm not seeing anything listed in this thread that should worry me. But I'm probably overlooking the bad ones in the manual. My eyes have gone a little cross.

I have some other questions, though, but I'll save them for if anyone chimes in here.
 
JMiP said:
For instance, the schematic for that particular board tells us that the resistors labeled R735 and R736 are a value of 680 and that's it. I'm guessing that it's 680ohms, but is there a value for watts or volts that I should know about?
Click to expand...

While not ALWAYS the case, it is common for resistors to be labeled as nnn and nnn K and nnn M on schematics with the understanding “M” means megaohm, “K” means kiloohm, and no letter means just ohms. There is often a mention, near the edges of the schematic or in the parts list in the service manual, that sets the expectations for the schematic labels. A typical statement is “all resistors in ohms unless otherwise specified” meaning they will put a “K” or a “M” on and resistor value that isn’t just ohms.

Similarly, there is usually a statement about wattage for the resistors, again often near one edge of the schematic or in the parts list. “All resistors 1/4 watt unless otherwise specified” is the one I see most frequently. For audio gear, it would be very rare to have a voltage potential of more than 200 V, so 1/4 watt is the most common, assumed base. In the power supply and for the emitter resistors of the output transistors you’ll often see 1 W, or 2 W or even 5 W. Anything higher than that and you should double check what you’re seeing. While it is possible to need more wattage than that in a resistor, e.g. the 10 W emitter resistors in a Tandberg 2080 stereo receiver power amp board, it is uncommon (in my hobbyist-only experience).

You can always safely bump up in wattage when you order replacement resistors. When I’m searching for a replacement, I search by resistance value and my favorite vendor’s favorite resistor family... and don’t specify wattage... so I can see all the available options and prices. Sometimes a 1 W resistor (of a selected resistance value) is cheaper than a 1/4 watt resistor.

There are a few places where you may see a 1/8 watt resistor or a “fusible” resistor specified on the schematic or parts list. If these cases is it important to stick to the specified wattage.
 
Thanks, @dlucy. Are you aware of any problematic transistors in the 730 that I should keep my eye on?

I bought a equlaizer/phono board for this unit and look forward to modding it the way that @dyche01 described in his thread. I may need someone's help in putting together a parts list that I can easily read and use as a purchasing guide.
 
I just did a recap and restore to a 730 two weeks ago, so I have a complete list of parts. I can start a new thread tonight and share that list.
 
rudie818 said:
Any updates, trials, testimony to the sound after the mod? I’d like to try it as well.
Click to expand...
I had a problematic HK730 that after a full recap was working beautifully for a few weeks then the orig issue of the left channel going quiet came back. Did quite a bit of troubleshooting and eventually replaced all the resistors on the eq board and double checked the traces for continuity. Haven’t had any problems since and all the old resistors were within spec so not exactly sure
 
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