OMG 😱, look at the schematics first before digging into those Crowns.
Discontinued Amplifier Products
You really need an experienced audio tech. to hold your hand with those unless you can read and understand how they work. They are not that difficult in a relative sense to other more recent designs but that assumes you know what you are looking for, can take necessary precautions and make the basic tests. I would think about your capabilities and experience before diving in, flags flying and the last post playing in your ears.
Don't forget - you need full matched sets of power transistors and you can't measure those from Ebay or electronics retail parts on your DMM or just swap the burnt ones with any hope of restoring the amp. to original reliability performance. That's why people are only too glad when you agree to repair the stuff. Professional repairs are not at all cheap when the usual problems of blown outputs turn up. New Class D replacement amps can work out cheaper so there are now a lot of unrepaired and even working class AB amps out there for cheap.😱
I didn't know they needed full matched sets? Oh well the guy who gave them to me said to have a look and see if I could do anything with them.I'm so glad you told me this before I started throwing money at them.Honestly Ian I would have been up **** creek if you hadn't have filled me in on all this.😱I checked out BGaudio but they must have taken the Doug Self APADH 4th ED down coz I couldn't find it? Ill ask the local library to order it in along with the others you suggested.They've already asked me to compile a list of interesting books to order.Thanks again mate,you've been a big help to me and saved me from a lot of needless frustration. 🙂OMG 😱, look at the schematics first before digging into those Crowns.
Discontinued Amplifier Products
You really need an experienced audio tech. to hold your hand with those unless you can read and understand how they work. They are not that difficult in a relative sense to other more recent designs but that assumes you know what you are looking for, can take necessary precautions and make the basic tests. I would think about your capabilities and experience before diving in, flags flying and the last post playing in your ears.
Don't forget - you need full matched sets of power transistors and you can't measure those from Ebay or electronics retail parts on your DMM or just swap the burnt ones with any hope of restoring the amp. to original reliability performance. That's why people are only too glad when you agree to repair the stuff. Professional repairs are not at all cheap when the usual problems of blown outputs turn up. New Class D replacement amps can work out cheaper so there are now a lot of unrepaired and even working class AB amps out there for cheap.😱
Current sharing is less a problem with only 3 pairs in each channel. You only need 3 of any type to match together there over the 2 channels. Many repairs are done cheap by using only the part number as a spec. and replacing only the dead parts but the reliability will be much reduced. The unit will fail again, and sooner, due to second breakdown damage in the remaining transistors. I think that is what happened - and several times over with the J700.
The Crowns use fairly sophisticated limiting circuits to squeeze the most power safely from fewer parts. You don't really have much option there but to restore it fully with each repair so that it works as designed.
As said, a manufacturer probably just fits the parts as they come - getting adequate, very convenient and cheap matches by the transistor manufacturing batch/wafer/die sequence. Unless you have a fancy test jig or curve tracer, you'll find it very hard to get near the original product's reliability otherwise, assuming it really was a good product.
For service though, you have no way of knowing more than the batch number which is useless when your parts are 20 years newer and made in another Fab. The better solution, when there are mixed failures, is to budget for the replacement of the full set from whatever minimum quantity you need to buy to do that. If that's too expensive, tell the client the options left and what the prognosis will be. Parts are cheaper than labour and having to do it all over again though.
I should mention that not everyone needs a proper working PA amp - maybe just a big and cheap amp. that they hope to use for parties that Pop's old 30W stereo amp can't cope with. Perhaps another patch job will be good enough there if precautions such as keeping speaker impedance high are taken. 😉
The Crowns use fairly sophisticated limiting circuits to squeeze the most power safely from fewer parts. You don't really have much option there but to restore it fully with each repair so that it works as designed.
As said, a manufacturer probably just fits the parts as they come - getting adequate, very convenient and cheap matches by the transistor manufacturing batch/wafer/die sequence. Unless you have a fancy test jig or curve tracer, you'll find it very hard to get near the original product's reliability otherwise, assuming it really was a good product.
For service though, you have no way of knowing more than the batch number which is useless when your parts are 20 years newer and made in another Fab. The better solution, when there are mixed failures, is to budget for the replacement of the full set from whatever minimum quantity you need to buy to do that. If that's too expensive, tell the client the options left and what the prognosis will be. Parts are cheaper than labour and having to do it all over again though.
I should mention that not everyone needs a proper working PA amp - maybe just a big and cheap amp. that they hope to use for parties that Pop's old 30W stereo amp can't cope with. Perhaps another patch job will be good enough there if precautions such as keeping speaker impedance high are taken. 😉
I think it failed on one side because a PNP out-put transistor was used instead of an NPN? The other-side failed because one of the input transistors went Kaput.I found out that the previous work was done at the local repair shop.that's the reason I started learning how to fix amplifiers myself because it was hopeless taking anything there.They don't seem to know how to fix things at the component level,they will just change boards?
Jands amplifiers
Hi,
Sorry to revive a year-old thread, but if the op is anything like me, the nonworking amplifiers are still sitting in a corner somewhere...
The 920 was designed by Doug Ford, who also had a lot to do with repairing design flaws in the J700 and J1000. He makes periodic appearances in the "EEVblog" videos on YouTube, and it might be possible to reach out to him through the associated forum.
He SEEMS to be friendly to the diy world, based on his appearance in the videos. Might be a good lead for info on these amps.
Hi,
Sorry to revive a year-old thread, but if the op is anything like me, the nonworking amplifiers are still sitting in a corner somewhere...
The 920 was designed by Doug Ford, who also had a lot to do with repairing design flaws in the J700 and J1000. He makes periodic appearances in the "EEVblog" videos on YouTube, and it might be possible to reach out to him through the associated forum.
He SEEMS to be friendly to the diy world, based on his appearance in the videos. Might be a good lead for info on these amps.
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