Hi,
My sons ran my Crown PS-400 on the beach (Regatta) when a sudden downpour occurred. The amp was playing hard driving two large Klipsh near clipping. Needless to say, two outputs shorted and lit one Klipsh loudspeaker on fire!
The original MJ15150 outputs are NLA so I installed two Motorola MJ15022's as replacements which had comparatively low hfe (20) vs (100) for all the other outputs. While I was in there, I found all the small electrolytic caps to be bad (way under value ie: 22uf would measure 2uf). I replaced all the caps. The relay and 10 amp fuse also failed. Upon power-up, the DC offsets were a little off and so was the bias. after I reset the offsets and bias I decided to give my new to me HP8903A analyizer a go. I put a 8 ohm dummy load on it and here is what I got measuring distortion. The amp sounds good, and I set the DC offsets to 5mv as in the service manual, although, I could change them to zero. The bias is set to 430mv as in the manual also. My question is, what would cause the repaired channel to have more distortion and should I be concerned?
CH-1 original un-blown CH-2 Repaired
400hz @ 56v p/p = .016% .026%
1Khz @ 56v p/p = .018% .028%
10Khz @56v P/P =.075% .100%
Gary
My sons ran my Crown PS-400 on the beach (Regatta) when a sudden downpour occurred. The amp was playing hard driving two large Klipsh near clipping. Needless to say, two outputs shorted and lit one Klipsh loudspeaker on fire!
The original MJ15150 outputs are NLA so I installed two Motorola MJ15022's as replacements which had comparatively low hfe (20) vs (100) for all the other outputs. While I was in there, I found all the small electrolytic caps to be bad (way under value ie: 22uf would measure 2uf). I replaced all the caps. The relay and 10 amp fuse also failed. Upon power-up, the DC offsets were a little off and so was the bias. after I reset the offsets and bias I decided to give my new to me HP8903A analyizer a go. I put a 8 ohm dummy load on it and here is what I got measuring distortion. The amp sounds good, and I set the DC offsets to 5mv as in the service manual, although, I could change them to zero. The bias is set to 430mv as in the manual also. My question is, what would cause the repaired channel to have more distortion and should I be concerned?
CH-1 original un-blown CH-2 Repaired
400hz @ 56v p/p = .016% .026%
1Khz @ 56v p/p = .018% .028%
10Khz @56v P/P =.075% .100%
Gary
undoubtedly the difference in HFE is the cause of the increased distortion and it's a grey area as to whether it's audible or will in the long term lead to thermal instability due to the increase in distortion...as to whether it's a cause for concern, that an open ended question that hopefully others may weigh in on.
Hi,
This is the first time I've tried to use this distortion analyzer and don't know much on using it yet. The measurements are all off like 10x when compared to the Crown specs. When I hooked it up, I tried connecting my scope to the "monitor out" on the analyzer and just got noise. I should have stopped there and sorted that out but instead hooked the scope to the speaker terminals. Also, my analyzer and Amp were plugged into different outlets that I think are on the same circuit. I did not enable the high or low filters either, or try switch to float the grounds. So, the numbers may be THD plus noise. I know the monitor out works as it was fine during the HP verification tests where you loop its output to the input. Seems I will need to retest. Although, the numbers I did get do show the increase in distortion.
Gary