About to restore a Harman Kardon 900+

[deanrantala]

I have had this thing for some 15 years or so.

Over the years, I have replaced a couple transistors and did a half-hearted re-cap of the amp boards.

Over the past couple years, one channel on the phono pre-amp went out, static noise became very bad and eventually... one channel completely went out.

So it is time to completely restore this old gem.

The amp service is pretty simple and straight forward. Gonna replace all problematic transistors and do a proper re-cap w/ Nichicon Fine Gold series caps.

Will also re-cap tone board, new transistors as well as the inverter and clamp board. All very straight forward - or I think it will be. Famous last words of a fool.

The first thing I am not sure how I should tackle is repairing the phono stage.

From what I have read here - and according to the service manual, the phono pre-amp is built into the CD-4 decoder board.

Now... I don't want to start a flame war here, but... I am wondering if it is even worth restoring the CD-4 board. Here is my reasoning:
1) Not much seems to ever have been released in CD-4 format.
2) Due to limitations w/ CD-4, the high frequency is cut off around 15k
3) Even when it worked, the noise floor and general quality of the phono stage was less than stellar.
4) I'm more of a stereo 2-channel guy (here comes the flying daggers)...

I purchased an entire spare 900+ (missing one amp board, couple knobs, and some jacks) for a parts unit. So I will likely have anything I would need to repair the CD-4 board, but again.. is it worth it?

Regarding the SQ Matrix board.. not likely I will use this much, but it seems pretty easy to re-cap and replace the transistors on. It even works on normal stereo material from what I understand.

The next major area that I am really going to be out of my comfort zone with is going to be the FM tuner.

The FM section on this one is pretty poor and I noticed a handful of the dreaded 2SC458's. Not to mention all the capacitors that are well over 40 years old. Replacing the components should be easy, but I understand the FM section will need a re-alignment once I change all this. I have NEVER delved into the back art of FM tuner service.

But I HAVE recently purchased a Rigol DS1202Z-E Oscilloscope as well as a Rigol DG1022 function generator. Along with all the normal bench equipment (multimeters, r/c meters, power supplies, etc)... I think I have everything needed for an alignment, no? Research tells me a function generator w/ sweep feature - which I believe the Rigol has. The service manual says "connect FM signal generator".. is this something special that my function generator will not have?

The manual also mentions "pilot signals" and "composite signal" for the MPX alignment. I do not need to understand how this works right this second - that will be a separate post. But I DO want to ensure I have the equipment needed to perform this service.

I live in central Mexico and just ordered the new oscilloscope and function generator - to a US address. I will be back in the 'states for thanksgiving and want to make sure anything else I may need is purchased and waiting for me when I get back to Colorado. Sourcing this type of equipment here in MX can be a challenge.

Finally... I am getting my list together on Mouser. I found a particular member here: 4morhkteck who seems to know this receiver like the back of his hand.

4morhktech - if you are around and read this... any other advice or areas I may need to pay extra attention to or anything else I should replace while I have this thing completely apart?

Taking these 900+ units apart is a true pain in the backside. I don't want to do things more than once.

Each amp will be re-built completely and tested outside the receiver w/ bench power supply, test speaker and source signal. Same for tone board and inverter board. Not practical to do this w/ the tuner boards, so... I want to get this stuff right.

Any help is appreciated!

-Dean

 

[quadsearcher]

Hello!
Flame war, huh?
I view equipment like records, the items will outlive the owners. Unless somebody destroys them, or some part of them. It'd be sad to see something modified because it wasn't worth it to some owner. That said, do you mean to just ignore the phone preamp and demodulator using an external phono preamp instead? No problem with that, someone could still restore it later.

Yes HK stuff can have problematic transistors that get noisy or fail due to the corrosion that turns the leads black or greenish and probably continues inside the device.
Would you use the tuner more than the CD4 feature? There again one could use an external tuner and have the choice of what to use. Since electroytics are the usual suspects they might be changed, I doubt that would cause the need for a full alignment. Any other caps like film/foil, mica, ceramic etc. would only be changed after proper troubleshooting, as those are the cap changes (or even postion changes sometimes) needing full alignment. Transistors in the tuner? I wouldn't change unless having a working tuner in it was very important to me and I was certain the the part(s) changed were the problem. If using modern equivalents instead of new old stock originals, it could make it more difficult for the next owner, and also might need a full alignment or worse, might not be able to be realigned after that. This is not a black art, all this is knowable, but it matters how much time one wants to invest.

Why does the FM section seem "pretty poor"? Is reception bad? Poor sound on FM?
Is there a decent antenna on it when testing?

Does the phono preamp also have C458s? I've changed a bunch of those in HK stuff.

My rule is if I'm not tossing it in the bin right now, and I might want to use it later or give or sell it to someone, I won't make changes that would reduce the value of the item, or are out of my comfort zone and therefore a risk.
I like HK stuff and there is a limited supply. I want to get a 900+ sometime and I hope it is as original as possible. It's part of the fun!

 

[deanrantala]

Good point on preserving the equipment...

Yeah, I have already replaced a few of the C458's over the years on both my HK rigs. For this restoration, I plan to just take an axe to EVERY. LAST. ONE.

When I do restorations on gear like this, I typically go a bit further then just new caps. I usually take each potentiometer apart, clean, lube, any part that may be problematic - replaced.

Now that you have me feeling guilty... lol... yeah.. I will get the CD-4 board at least back to the point that the basic phono pre-amp works. Since I have no CD-4 content, not sure I can test or verify the functionality. Later next year I may look into an extra turtable w/ the proper stylus/cables/etc and a couple test records to truly be able to test it.

FM reception... poor AND trouble pulling in stereo signal. Even when I had my large outdoor pole-mounted antenna w/ 300-ohm wire all the way direct to the receiver. Just never really pulled in the stations like my Sansui/Pioneer or former Marantz gear.

YES.. this receiver is littered w/ C458's everywhere... Phono pre-amp, FM board, phase inverter, clamp board, etc. *sigh*. Fortunately, the KSC1845 replacements are dirt cheap.

The phono pre-amp is integrated into the CD-4 board on this model. Furthermore, the phono pre-amp seems to use a increasingly rare IC for the actual RIAA part: LD3130. I have not yet dug in, so do not know if one of the IC's are bad, but even if so... I got an entire donor unit. Surely I will have 2 good ones.

So fine.. you shamed me into keeping it original

15% of my use for equipment is generally FM. Another 15-25% is LP. The overwhelming majority however is high-bitrate DAC and TIDAL just because it sounds so damn good and is crazy convenient.

Nonetheless, my OCD kicks in and tells me: it has FM, so it should work 100%.

And now that you have publicly shamed me.. I am gonna have to do the same for the CD-4.