advice on Harman Kardon A 300 integrated amplifier

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advice on Harman Kardon A 300 integrated amplifier

Post by Fred Longworth on Tue Mar 20, 2012 1:16 am

Hello All,

This A300 has apparently been worked on and "modded" by three other technicians. Their work was of about average quality, however, one of them disconnected the phono preamp. Since the filaments of the 12AX7's in the phono preamp were used as the collective cathode resistor for the quad of 7408 output tubes, he subbed a 150 ohm resistor. My first task was to remove the 150 ohm resistor and reattach the filament line. I also replaced four film caps that he had removed from the phono preamp section.

My question concerns the schematic. The owner provided me with a good-quality schematic, however, the print does not agree with the machine. For example, the RIAA equalization network on the actual set uses different R/C networks from the schematic values. Also, the grid resistor for the second stage of the phono preamps is 6.8m in the schematic, but only 330k in the actual machine. This is an enormous difference. (NOTE that in the schematic the phono preamps are 12AX7's, labeled V1 and V2.) Additionally, the voltages on both plates of the second stages are very much lower (65v) than the schematic I have indicates (140v), and this discrepancy is too large to ignore.

This suggests that HK had at least two variants of the A 300. It would be nice to get a schematic that actually corresponds to the machine! If anyone has had to wrestle with the phono preamp section on the A 300, and has turned up anything unusual, please let me know.

Fred
Owner of Classic Audio Repair in San Diego


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Re: advice on Harman Kardon A 300 integrated amplifier

Post by willy3486 on Tue Mar 20, 2012 7:59 am

I just rebuilt one last November. The best I remember it looked exactly like this one at this page.

http://www.angelfire.com/electronic/funwithtubes/Rst_Harman-Kardon_A-300.html

That page has a partial schematic. There is another schematic out there at
http://shermanr.web.prw.net/A300%20SCHEMATIC.jpg
I was able to get it going with minimal problems. It had a great sound. If I remember correctly I thought someone had changed stuff but It may have been due to the diagrams being different on those two pages. I can't remember if they were different but just that I was thinking they got me confused at first. It was a nice sounding amp
Hope that helps.

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Re: advice on Harman Kardon A 300 integrated amplifier

Post by Fred Longworth on Tue Mar 20, 2012 1:08 pm

Willy, the first link you provided -- which goes to "Restoring a Harman Kardon A 300 -- is the same as the close-but-no-cigar schematic I have, however, the second link goes to a schematic that looks just like what I have. I need to figure out some way to blow it up a bit. Thanks!!!

The "modders" in my instance were of the straight wire with gain school. You know, disconnect all the switches on the frontpanel except for the selector. Clip the coax going to several RCA jacks on the rear panel in order to remove a subtle veil from the sound. Replace all the resistors with 1% metal films. I wish they'd taken a course in how to solder at a professional level.

Fred

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Re: advice on Harman Kardon A 300 integrated amplifier

Post by Fred Longworth on Wed Apr 04, 2012 2:16 am

I've restored the phono section, and returned the unit to "stock" by allowing the filament line for the phono section to be the cathode resistor for the four 7408's. I've replaced all four 7408's, the 12AU6 phase-splitter, both phono tubes (12AX7), and one additional 12AX7. I've cleared up some shoddy wiring. Now, there's a persistent low-level hum that appears to be originating in the 12AX7 that follows the volume/balance/tone controls and precedes the phase-splitter. Initial observation is that the turkey who worked on this before used ordinary hookup wire to carry the signals in this area, rather than shielded cable, and he routed this hookup wire with little or no concern about how close the wire was to hum sources. So, the next step is to replace the hookup wire with good-quality shielded cable. If the hum remains after this, I'm going to hunt for a ground loop.

Fred
owner
Classic Audio Repair

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