Today I got a little excited listening to Tool and cranked the ole DH500 up to near concert levels. It wasn't clipping at all and I swear there was plenty of juice in the amp/speakers to go even louder, despite this, about 3 minutes into Chocolate Chip Trip, the left channel lost it's juice, and was a fraction of the volume, and static-filled.
I'm thinking it's possible (maybe probable) that one of the old caps in the unit died on me. That said, I am a complete novice when it comes to electronics testing & repair, but I would like to try diagnosing and fixing the problem myself. I just don't know where to start. Anyone willing to virtually "hold my hand" through the process?
I'm thinking it's possible (maybe probable) that one of the old caps in the unit died on me. That said, I am a complete novice when it comes to electronics testing & repair, but I would like to try diagnosing and fixing the problem myself. I just don't know where to start. Anyone willing to virtually "hold my hand" through the process?
Check the speaker fuses. They are included in the feedback loop and it sounds like one is blown.
Replace only with the same type of fuse.
Replace only with the same type of fuse.
Yup - I actually knew that much, and checked all the fuses, even though I suspect I wouldn't have sound at all out of that channel with a blown fuse. Anyways, they're all good (inside and out).
Last edited:
You would still have sound if the speaker fuse were blown, but much lower in volume.
Does it smell burnt?
Was the fan working? If you suspect bad cap's then you need to measure the rail voltages.
Do you have the manual for the amp? If not, the Hafler website has archives.
Be careful, the rail voltage on these amps are deadly. Make any and all connections of your DVM with the amp off and UNPLUGGED.
Before you do anything, just look inside with it uplugged and look things over. Look for discolored resistors, swollen capacitors, leaking capacitors.
The output transistors probably need new heat sink grease, over the years it dries out and the transistors run hot. This amp does have an overheat protection.
Was the fan working? If you suspect bad cap's then you need to measure the rail voltages.
Do you have the manual for the amp? If not, the Hafler website has archives.
Be careful, the rail voltage on these amps are deadly. Make any and all connections of your DVM with the amp off and UNPLUGGED.
Before you do anything, just look inside with it uplugged and look things over. Look for discolored resistors, swollen capacitors, leaking capacitors.
The output transistors probably need new heat sink grease, over the years it dries out and the transistors run hot. This amp does have an overheat protection.
Unfortunately 99% of electrolytic capacitors go to the dump without ever becoming swollen, discolored, or leaking. What they do at a certain age is go high ESR, which means they won't let the DC electricity out as freely as when they were full of water. You can buy an ESR meter for about $120, but usually it requires taking the cover off and removing the capacitor to measure it. Then you need a datasheet for that capacitor from 20-30 years ago. A much easier measurement can be made with a $30 analog VOM on the speaker terminals. On 50 vac scale, or 20 vac scale for smaller receivers etc, turn up the amp and measure the AC voltage. V=sqrt(P*Z) where P is the power rating of one channel of the amp, and Z is the speaker impedance the amp was rated at. Usually 8 or 4 ohms. V 1/4 or 1/50th of the rated voltage, the amp has dried up railed capacitors. You don't even have to take the cover off to take this reading.
If you take the amp to a shop, they are likely to replace "the one that is bad", gleefully leaving the other 50 or so to fail one by one next month or next year. Then they get to charge you again. The appliance is broken all the time and expensive to repair. Plus, most shops tend to install the cheapest parts, ones rated for 500 or 1000 hours life at temperature. If a shop bids $2 higher than the next guy down the street they will never get the job. You can usually get 8-20 times that rating service life at room temperature but if you listen 8 hours a day you can use the amp 2400 hours in one year. Capacitors now are available in ratings from 3000 hours to 16000 hours service life at 85C or 105C. These only cost 25-50% more than the cheap ones. If you buy a new class D amp or powered speaker to replace this amp, the market has determined 95% are doomed to fail in 3 to 8 years.
If you like the amp, and want 20-30 more years of low trouble life out of it, you can change most or all of the electrolytic capacitors. Mark them with the date on top as you go. If you want to do a minor repair, the 4 or more rail caps and the 2 input caps are likely culprits. Do test the amp functionally every 2 caps or so before proceding, Amateurs make a lot of bad solder joints. Do mark the board for cap polarity before removal, electrolytic caps that are put in backwards pop the tops & leak. Except NP caps like the input cap (sometimes). Measure any metal you touch for <5 v to speaker ground before touching it. Especially the rail caps. Voltage >24 across your heart from one hand to the other can stop it.
Cleaning of heatsinks at this age is an appropriate transistor life extender. Changing 30 year old mica heat washers is a good practice, although I have never had one actually fail. Use heat sink compound on new mica washers should you decide to replace them. I have many transistors that are 50 years old. Volume pots and selector switches are other troublesome parts at a certain age.
Your static problem could be corroded interconnects or bad volume pot, or bad other electrolytic caps. Usually I change all the electrolytic caps before doing actual diagnosis. Removing & replacing is the quickest way to scrape oxide off interconnects. Volume or tone control or bias adjustment pots , you often have to replace them.
Happy maintaining should you decide to follow this path. In the US or Europe you'll have to do the work yourself: labor prices of pros are not competitive with class D trash from the low cost production areas of the world. There are pros that will change every cap and use long life replacement caps, but these people are very rare. Warning, electrolytic caps sold across the counter at shops are usually the short life ones. To get the long life ones you have to order from distributors like farnell, digikey, mouser, RS, or reichelt. I use industrial long life caps from panasonic, niichicon, rubicon, kemet, vishay, not the $$$ "audio" ones.
Last edited:
Eserviceiinfo.com has the manual for this amp. It does have a speaker disconnect relay. Dirty contacts on this relay can cause cessation of sound.
This amp uses output transistors 2sj56 2sk176. These are no longer available. While substitutes could be installed, the distortion might go up from rated .025%.
This amp uses output transistors 2sj56 2sk176. These are no longer available. While substitutes could be installed, the distortion might go up from rated .025%.
Yes that costs less time and money in the long run. Going cheap/doing the job half is only for those that have too much time.
I don't know the amplifier but was interested in them as they seem OK. So a general tip: please use a new relay with hard silver contacts if the old one is worn out. It won't hurt to choose a higher rated part with regards to contact current. Lower coil current would also be nice.
If memory serves the same issue affects the DH-200 which is the same circuit design. The input transistors can fail. These are 2N5401/2N5551 and easy to get.