Harman Kardon 330B, High DC-Offset

[danrclem]

I bought this receiver today as is and when I got it home I hooked it up to my DBT and everything is fine there. I then checked the DC-Offset. One channel is near zero but the other is .6 V. I've done some reading and I'm taking it that since this is cap-coupled the DC-Offset has no adjustment pots and I'll have to change the capacitor to fix this ailment. Is this correct? If it is I'll change them both while I'm in there. The seller said that it had dirty pots so I know I'll have to clean them. Is there any other preventive maintenance that needs to be done to this receiver?

 

[avionic]

Cap coupled . You must have a load connected to the output to measure the DC offset. Speaker or dummy load.

 

[Leestereo]

Did you have speakers hooked up when you checked the DC offset? If not, the DC measured is just with 1.5kohm resistor (R921/R922) as a load; it will decrease with a proper speaker load.

 

[danrclem]

I didn't have the speakers hooked up. I'll try that and see what it reads. A cap coupled amp is still read at the speaker terminals isn't it. If I test a direct coupled amp do I have to have speakers hooked up to it too?

I doubt if it makes any difference but it's a series B-1 made in Japan.

Thanks guys.

 

[avionic]

If I test a direct coupled amp do I have to have speakers hooked up to it too?

No..

 

[avionic]

A cap coupled amp is still read at the speaker terminals isn't it.

Yes

 

[danrclem]

Thanks guys. I haven't had a chance to do it yet but will this evening when I have time.

 

[blhagstrom]

HK 330B restoration.

Clean the controls and switches, especially the switches.

Recap it.

Replace all the 2SC458 transistors.

Check bias, adjust as needed.

Enjoy.

Cap coupled amps with no offset adjusters are good to go.

Cap coupled amps with adjusters need an o-scope to adjust and that just sets things to the mid point (quiescent point). Unless you plan on running the amp near the clipping points ( full power ) forget about it.

 

[danrclem]

I got a chance to hook some speakers up to it and the left channel that was reading .6 v is now 1-5 mv. The other channel read 0. I hooked up a cd player and the right channel that read 0 doesn't work at all. I didn't check to see if anything was coming out of the preamp so I put it up on a shelf and I'm going to try and work on it this winter when I have more time.

Just by listening to the one channel I can understand why this little receiver has a big following. Thanks to all.

 

[dlucy]

I then checked the DC-Offset. One channel is near zero but the other is .6 V. I've done some reading and I'm taking it that since this is cap-coupled the DC-Offset has no adjustment pots and I'll have to change the capacitor to fix this ailment. Is this correct?

In the HK 330B there are two trimpots per channel on the power amp board. The adjustment procedure, from the service manual, is as follows:



As @blhagstrom pointed out, you should:

• Replace all the electrplytic caps especially the two output-coupling caps on the power amp board (since they are a big part of the sound). When doing so, use low-leakage caps like the Nichicon KL series for those in the audio path, use Nichicon PW series for the power supplies, use the same capacitance value (especially for the output-coupling caps) and feel free to bump up to the next voltage rating when buying replacements (e.g. for a 47 uF 50 V cap you might replace it with a 47 uf 63 V one). There are two axial electrolytic caps on the preamp / control amp board right in front of the power amp board and you can replace those axial caps with cheaper radial ones. All these electrolytic caps will degrade in performance over time and may have already failed.
• 
  
• Replace the 2SC458 small signal transistors with KSC1845 transistors. The 458s are know to go bad or go noisy.
• 
  
• Deoxit all the switches, pots and jacks. This includes the trimpots on the power amp and FM boards. Carefully note the current position of the trimpots before you deoxit and work them back and forth a few dozen times. You'll want to return all the trimposts on the FM and MPX boards to their original positions.
• 
  
• Consider replacing the two trimpots per channel on the power amp board. These are open-air trimpots, they can oxidize over time and if they short anywhere in their adjustment, they could blow your output transistors.
• 
  
• Consider pulling the output tranistors out, cleaning them, putting them back in with fresh thermacote and a mica insulator.

 

[dlucy]

On the preamp (control amp) board you can:

Replace the axial caps with radial caps, either electrolytic or film (I'm using WIMA films in this photo) at the yellow arrows.
Replace the 2SC458 transistors (at the green arrows) with KSC1845 transistors (just pay attention to the pinouts).
Replace the 1 uF 50 V electrolytic caps (at the red arrows) with 1 uF film caps.



Replace the trimpots on the main amp board (yellow arrows) and the output-coupling caps (at the green arrows):



And if you listen to vinyl, you may want to rebuild your phono board because it is cheap, easy, and improves the sound:

 

[danrclem]

Thanks for the detailed info Doug. I'll refer back to this when it comes time to work on it.

 

[Zeromort]

Hello
Can't i used Nichicon Muse 1uf ?

 

[dlucy]

Hello
Can't i used Nichicon Muse 1uf ?

Yes, any capacitor of value 1 uF will "work" here. That said:

1. A film cap will have zero D.C. leakage, so it is best. A metalized plastic cap (e.g. Metalized polypropylene, metalized polyester, metalized PET (what?)) works best in an audio path.

2. An electrolytic _with low leakage_ would be the next-best choice. As far as I know the Nichicon KL series is the only one with 1/3 the leakage of normal electrolytic caps.

3. A normal electrolytic would be next

4. A ceramic cap using the C0G NP0 dielectric would be next

5. and 6. (stretching my limited knowledge here) other-dielectric ceramic caps and tantalum caps will work, they just don't pass audio as colorlessly as others (?)

No doubt you will encounter "audio grade" or exotic caps sold for audio applications. I have zero experience with those and there is AFAIK zero quantitative data on how they are better than others, so you'll have to use your own best judgement there.

 

[Zeromort]

Thanks I change and use film cap.Sorry for my English.

 

[dlucy]

Thanks I change and use film cap.Sorry for my English.

You're doing great!

 

[Zeromort]

Can’t I change the 2sc458 for BC456?