I have an early DH-500 which has worked fine all these years until recently. I bought the Qual com driver boards and have them halfway assembled, then all of a sudden the working DH500 is in protection mode and won't release. Took it to a local shop and the guy tells me he checked everything on the PC9a board and can't find anything wrong - his solution was that "amps have gotten so much better in 40 years you should trash this and by a new Krell from me". So i ordered the rebuild kit for the PC9 from Ed at Qualcom and installed first the caps (no change), then the diodes (still no change), and finally the transistor- this time the relay kicked on and beautiful music plays (for good measure i replaced the IC as well). However, now the relay kicks on as soon as the power is switched on, and releases when power goes off. This makes a loud pop (which this amp never had before), and i worry that it may not be protecting the amp properly. I recorded the voltages at the test points for the PC9a and will try to post it here.
I'd like to have the amp running correctly before I install the new driver boards but seem stuck on this protection circuit. I saw an old thread about similar issues but it's archived now. Any/all help appreciated! I
I'd like to have the amp running correctly before I install the new driver boards but seem stuck on this protection circuit. I saw an old thread about similar issues but it's archived now. Any/all help appreciated! I
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I would check the timing capacitor. In fact, check them all in the protection circuit, or just replace them. Do not increase any capacitance values, higher voltage ratings are more than fine. Try to get ones where you don't have to bend the leads.
Some older amplifiers are so well designed that modern ones are sometimes not as good. I just repaired a Hafler DH200 that performs much better than many new designs. Yes, I hand matched some parts, but there was no need to change the circuit. Here is an example of the one I just did. Here is also a little Sansui AU-517 I did.
Performance is not an issue here.
Some older amplifiers are so well designed that modern ones are sometimes not as good. I just repaired a Hafler DH200 that performs much better than many new designs. Yes, I hand matched some parts, but there was no need to change the circuit. Here is an example of the one I just did. Here is also a little Sansui AU-517 I did.
Performance is not an issue here.
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I did replace the two electrolytics. The other caps are poly and i didn't think likely they would fail - not sure if they are the timing caps you refer to - C101, 102 & 103 on the schematic i attached.
C104 is the timing cap, C105 powers the relay (do not increase this value). If those are fine, check Q101 for shorts or being leaky. The last resort would be IC101. Make certain the two rail voltages are within the ratings of the IC. If either zener diode is open (solder joints maybe), the voltage from the 90 volt rails might be above what the IC can handle. Check the basics. Rail voltages, Q101 and the two capacitors C104 and C105.
C101 through C103 filter the speaker DC detect lines. If they go open, normal music would trip protection. These are not your problem for this fault.
C101 through C103 filter the speaker DC detect lines. If they go open, normal music would trip protection. These are not your problem for this fault.