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Old 12th September 2011, 09:19 PM   #1
ODougbo is offline ODougbo  United States
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Default Question about Rotel RB-880 amplifier

My old amplifier seems to work much better as it warms up, problem is that can take about an hour.

The amp is old 1984-1987, but never driven hard (typ. 8 ohm), and also sat on a shelf now and then over the years.

The RB-880 is an impressive looking model, 100 watts per channel and a large heavy chassis (40+lbs) the toroidal transformer is huge. The 100 watt rating is very conservative.

What come be the problem, power transistors?

Last edited by ODougbo; 12th September 2011 at 09:37 PM.
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Old 12th September 2011, 09:56 PM   #2
jaycee is offline jaycee  United Kingdom
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Capacitors. Replace them all, especially the big main supply capacitors. They have an average life of about 20-25 years, less inside something that generates heat
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Old 12th September 2011, 10:45 PM   #3
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Quote:
Originally Posted by jaycee View Post
Capacitors. Replace them all, especially the big main supply capacitors. They have an average life of about 20-25 years, less inside something that generates heat
I definitely second this! Those large capacitors might be expensive but after 20-25 years, they are definitely worth replacing!

You can test your capacitors by testing them with an ohm meter. If they start to charge when you apply a large current, you will know that they are at least working partially. The difficult thing is that it might be hard to tell how well they are working with this method.
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Old 13th September 2011, 12:19 AM   #4
ODougbo is offline ODougbo  United States
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Thank fellows!

The other thing I noticed is how long it took to warm up (to the touch) hours would go by and it was only slightly warm, It wasn’t like that when it was “new”, so I guess it wasn’t working very hard.

Capacitors I never had to replace any, not the large ones anyway, but good with fixes in general. They look like they solder them and use glue, will the glue break away?

Will I be able to find exact replacements, thru Rotle maybe? Is there an alterative place to buy them? I bet finding the right parts is half the project.
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Old 13th September 2011, 01:01 AM   #5
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my old arcam d290 takes about 20 minutes to sound right , i guess thatīs normal for this model , 1 hour i think itīs too much , they are right you should check the capacitors
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Old 13th September 2011, 02:04 AM   #6
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I seriously doubt that the C005/C006 22,000uF/80V main filters are the problem. You can replace them now and scratch your head later. The big filters are expensive. Grab you DVM and set it to AC volts, measure the ripple. Tell me what you read.

I would look into the output bias first. I takes time to stabilize the driver, outputs and the Vbe multiplier due to the thermal mass of the heatsink. Transistors ( especially those that get very warm during operation) and the trimmer usesdto set the bias age and drift over time.

Note that the RB-880 uses a regulated power supply for the power amp and you might confuse the power amp section from the regulator section. The regulator section uses bigger Sanken LAPT MT-200 transistors. The amp needs to have the +/-50V regulator output checked and adjusted accordingly after it warms up.

There are 4 aluminum electrolytic capacitors in the power amplifier section that will directly affect the audio.AC bypass C613/C614 (100uf/6.3V) and C615/C616 (10uF/50V). I would look into these one first after confirming the bias and regulator output. The rest of the aluminum electrolytics are in the power supply circuitry.

Do more digging before blindly replacing parts.Inspect the amplifier board for bad solder joints.
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Old 13th September 2011, 09:13 AM   #7
ODougbo is offline ODougbo  United States
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Oh boy, that’s over my head as far as technical knowledge. I did a simple switch last night; I swapped out two channels with a RMB-1095 and used the RB 880 to run the rear channels. Now the bigger amplifier is running the two small subs; the system hasn’t sound that good in a long time.
I can take the 880 to a friends who works on amps, but he’s moving and doesn’t have much time, at least we can put it on a scope. I will take your notes along ~ Thanks!!
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Old 13th September 2011, 11:41 AM   #8
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in a few words ...

if main filter caps are faulty or partially faulty or old that will not effect much of performance of the amplifier unless power demands exceed 20% ...under this level should be no problem .

suggestion to replace weels in a car that warms up too much ....obviously wrong approach

the only post that makes sense is the post made by the Hitachi nut .....

plus that big caps are expensive to replace for not obvious reasons...
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