HK 730 DC offsets 5-8 VDC left channel

Discussion in 'Solid State' started by cobohnert, Jun 10, 2018.

  1. cobohnert

    cobohnert Member

    Messages:
    86
    I just recently bought this thing on Craigslist for $20 plus a couple hours windshield time. It looked a little rough, was very dirty inside and was missing pre/main jumpers and a fuse between transformer 2 and the sub rectifier board. I blew the inside out and lightly brushed everything off with a small soft paintbrush and replaced what was missing and then checked my DC offsets and was 4-5 mv on one channel and 8 mv on the other channel so I put some speakers to it.

    Everything was good for a week until today. Fired it up and let it sit a few minutes and hit speaker 1 and with no source playing I heard a noise like a mix between a humming and static coming from the left channel and pretty quickly turned it off. I pulled it out and checked the DC at the speaker terminals and the right channel was where it was last week but now the left channel is at 5V DC on start up and then climbs to 8 and then settles back down around 5-6V.

    I'm pretty ignorant with electronics so this may be out of my league and I won't be out much if it is but does anyone have an idea what might have happened and is there anything someone could walk me through to try and check and possibly repair this thing? Any and all help is greatly appreciated in advance.
     
    Last edited: Jun 11, 2018

     

    Please register to disable this ad.

  2. Tom B

    Tom B AK Subscriber Subscriber

    Messages:
    1,346
    Location:
    Bay Ridge, Brooklyn New York
    As this is an old unit, some component has gone bad. It could be as simple as one of the 4 main filter caps going bad (4700uf @50v), or one of the small signal transistors failing on the driver board. You can check for bad connections on the power supply and driver circuit boards, but I think it is component failure.
     
    cobohnert likes this.
  3. ivandezande

    ivandezande Super Member

    Messages:
    1,514
    Location:
    Twin Cities
    I just restored one of these. Deoxit the trimmer pots and the connector where the amp board meets the power supply, those are known to be bad. Check the transistors on that amp channel while you're in there. Really should have the trimmers replaced and the thing recapped.
     
    cobohnert likes this.
  4. xrayspex77

    xrayspex77 AK Subscriber Subscriber

    Messages:
    716
    Location:
    Toronto
    Do you have a Multimeter? Can you Solder? If yes then you can fix it.

    I had a similar problem with mine.

    I replaced Q402,Q404, Q406 and Q408, Q412, with the Subs suggested in other threads. I also replaced all the Electrolytics on the L Amp Board. This solved the high DC on mine
     
    cobohnert likes this.
  5. cobohnert

    cobohnert Member

    Messages:
    86
    Thanks for the suggestions! This unit is a little (considerably) rough around the edges so I won't stick a bunch of money in this unit by having someone else do the work and I've never replaced any parts on an actual circuit board before but I guess as this thing is useless right now what's the harm huh? Or not? I own a nice soldering iron but only have 1 tip for it and a desoldering tool that looks like a large pen with a vacuum plunger . Not sure if I'd need different size soldering tips to do this job. I have some generic solder with rosin core and some rat shack soldering flux but do I need a specific solder for a job like this?

    When you say deoxit the trimmer pots do you mean the pots for volume, bass, treble and push bottom switches for contour, hi cut etc? If so I've done that once and used some faderlube after but could do it again if need be. Also how would I check the transistors? Do the boards need to be removed from the case? What about the transistors from the board? Seems like I've seen some components can't be checked while on the board for reasons beyond my lack of electronic knowledge.

    Sorry I know this is a lot of questions but if I don't ask I don't learn. Even if it's learning I'm way too green to attempt to tackle a task like this.
     
  6. cobohnert

    cobohnert Member

    Messages:
    86
    You ask if I know how to solder. The answer is I have soldered wires together but never attempted to solder on a board. From my reading on this site removing components without destroying foil traces can be a task maybe not suited for beginners so maybe I'm not ready for this? And yes I do own a nice fluke multimeter which is a more capable tool than the operator who will be using it.

    But you give some good information in your post and believe me it is appreciated. I wish my skills could equal the desire I have to work on electronics like this.
     

     

    Please register to disable this ad.

  7. xrayspex77

    xrayspex77 AK Subscriber Subscriber

    Messages:
    716
    Location:
    Toronto
    The 730 is easy to work on, the whole board comes out, but go buy a $10 DVD Player from a thrift store and practise removing components and then replacing them.
     
  8. cobohnert

    cobohnert Member

    Messages:
    86
    Better yet I work at a scrap metal business and we get electronic boards in all the time and I can take some home and pracrice on them. Time is my biggest issue with a 3 year old and an 8 month pregnant wife but I will take your advice and practice on some junk boards and see where it takes me.
     
  9. xrayspex77

    xrayspex77 AK Subscriber Subscriber

    Messages:
    716
    Location:
    Toronto
    There are a ton of youtube videos on how to Solder that I used to teach myself, if I can do it....
     
  10. petehall347

    petehall347 the brandy coffee man Subscriber

    Messages:
    26,853
    Location:
    uk.. the middle bit
    building a kit is good for practising .
     
  11. ivandezande

    ivandezande Super Member

    Messages:
    1,514
    Location:
    Twin Cities
    Dang sounds like you have the tools, you should be good with that stuff! Nah I mean the trimmers to control the bias on the amp boards. You can leave the transistors in, you just need to do a diode check on the base pin to the emitter and collector pins. Polarity reverses depending whether it's NPN or PNP. Google is your friend if you have more questions on that.
     

     

    Please register to disable this ad.

  12. lico

    lico Super Member

    Messages:
    2,102
    You are being convinced to practice. In some areas, there are tech shops that might fix it for $60 flat fee.

    You paid $20 - how much is it worth learning? Guys here will tell you it's easy but chances are it is not that easy, it might have multiple problems. Good luck.
     
  13. cobohnert

    cobohnert Member

    Messages:
    86
    Thank you very much for the info! Yes I should know how to use the tools I have and I will one day... Hopefully

    I have been busy the last few years remodeling my house, which was a learn as i go kind of deal, and just finished learning how to put siding on and finished that job just in time for the baby to arrive so now maybe when she's 3 or 4 years old I will have time to get more proficient with the soldering iron and multimeter lol.

    But I will not be defeated by this thing so easily and will throw some time at it as soon as I can. I own a much nicer HK 430 which was the reason I had been on the lookout for the 730 and bought some outdoor speakers once I found this until and had been enjoying them for a week or so until this unfortunate turn of events.
     
    ivandezande likes this.
  14. cobohnert

    cobohnert Member

    Messages:
    86
    I live in a small town an hour south of St. Louis, Missouri. No shops anywhere near my town but an hour drive to STL is no big deal if I could find someone to actually fix it for a fee that small. Options to consider for sure.

    If by any chance someone sees this and knows anyone around my area that works on these and more importantly trusts their work, please let me know.
     
  15. OMGCat!

    OMGCat! AK Subscriber Subscriber

    Messages:
    2,883
    Location:
    El Sereno, CA
    Sounds like some good tips so far. I'll add another on the "practice soldering before going into something you care about". Hopefully you'll get some free time to get some work done because it sounds like you've got your hands full. I find repair work to be relaxing and a good way to forget about the day so beware that it might become addictive.
     
  16. EvanF

    EvanF AK Member

    Messages:
    288
    Location:
    Kirkwood, Missouri
    I work for 18/hr, I can get you 10+ references. Probably no further from you than St. Louis city is.
     

     

    Please register to disable this ad.

  17. cobohnert

    cobohnert Member

    Messages:
    86
    I sent you a PM. Thank you for reaching out to me.
     
  18. Hajidub

    Hajidub Chihuahua/Pug = Chug Subscriber

    Messages:
    5,089
    Location:
    Colorado Springs, CO
    What a deal! BTW, when you find time I would still recommend soldering/desoldering with throw away boards.
     
    cobohnert likes this.
  19. cobohnert

    cobohnert Member

    Messages:
    86
    I absolutely plan on doing exactly that. The interest is there to play around with this stuff. Just need a few more hours in the day.
     
  20. EvanF

    EvanF AK Member

    Messages:
    288
    Location:
    Kirkwood, Missouri
    Alright, it looks like I will be working on this amp from here on out. Anyone interested in how this repair is going to go can stay here on this thread, developments will come. Any hurdles I run into with parts I will inquire here. If needed, I'll start a new one purely for parts and troubleshooting, probably focusing on the former if it comes to that. I will update on the repair process, pictures might have to wait though.
     

Share This Page