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Old 4th August 2014, 04:00 PM   #1
nuryev is offline nuryev  Australia
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Default Rotel 840bx recap project

Hi all, I am considering recapping a Rotel 840bx. Does anyone have any advice on how to go about this without the risk of destroying a working amplifier?
I have a copy of the service manual but would appreciate if anyone has completed a similar project and can share pics, schematics and or diagrams of improvements if any are possible on this amp.

Thank you.
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Old 5th August 2014, 03:54 PM   #2
nuryev is offline nuryev  Australia
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I am considering recapping a Rotel 840bx that I bought for $200. I liked the sound of this old amp and decided that it might be worth recapping.

I have a quote for 50v 8200uf (as per service manual) as well as price of slit foil caps but 63v 10000uf.
I am not sure of what other components will need to be replaced with a recap.

Any suggestions?
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Old 5th August 2014, 06:22 PM   #3
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Replacing electrolytic capacitors in amplifiers is routine maintenance work - not difficult unless you want to make it so. Most amplifiers will sound a fair bit better when the electrolytics are replaced with even the same part quality because being filled with electrolyte, they do deteriorate with heat and time - like 20-40 years typical and modern grades are generally improved on the standard types used even 20 years ago.

You don't need to replace other types of components unless inspection shows up physical damage of some sort or you tempt fate by playing with the bias control pots. too. These are intended as "set and forget" adjustment pots in domestic amplifiers which sometimes don't survive readjustment. If altered, they could then also need replacing, depending on the stability of the adjustment. The important point about these is to always measure the bias current according to the service manual method before touching anything or you will have no means of re-establishing the original settings. I sometimes find factory settings of even new products to differ from service specifications so note actual measurements.

If you then decide after routine replacements that you want to experiment further with exotic parts, that's a different game that may or may not result in further improved sound. It is often a matter of individual owners showing off their buying choices without reference to proven information or any performance standard.

Finally, it may be normal for pros. to simply go through the amp. replacing everything and powering up normally afterwards. Not always so for the inexperienced - Replace one channel/section at a time so you have a reference and also comparison for your own proof of having done something worthwhile - or not, as the case may be.

Let's trust it all goes fine
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Old 6th August 2014, 02:38 AM   #4
nuryev is offline nuryev  Australia
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Thanks for the encouraging and informative response.

I have the option of replacing caps with standard replacements or using something exotic like the slit foil caps available here: DNM-designed Capacitors

The exciting part is that there is also an option of using the 4TTN caps in place of caps with two leads. If I could access the wiring modifications to achieve this I would like to explore the latter option with anyone who has successfully completed such a project to find out if the mod is worth the trouble (risk!?!)
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Old 7th August 2014, 08:40 AM   #5
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As suggested, rolling exotic capacitors is another game for those with plenty of cash to lavish on their old treasures. 18-45 UKP for some of those Taiwanese exotic caps is a bit rich though. I have seen a few of Supertech's audio types used by Eastern European and Indian DIYs who didn't seem to have paid near as much and weren't all that impressed with their performance - perhaps good counterfeits. They were certainly 4TTN and 2T types, as are DNM's. Not many other specific details that I could be certain of, though.

Naturally, they would be considerably cheaper without DNM's UK freight and retail mark-up pricing which really hikes up the cost of resold Asian goods.
Aluminum electrolytic capacitor -index
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Old 7th August 2014, 10:22 AM   #6
DF96 is offline DF96  England
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To maintain a working amp replace only one or two components at a time, then retest. If you hit trouble you know it must be the last one or two changes which caused it, so you have a smaller fault-tracing problem.
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