Author Topic: Bought a faulty Rotel RA1520 where to begin  (Read 1862 times)

Offline Tzs503gp

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #30 on: February 14, 2018, 01:24:58 PM »
Safely in the hands of Nilam. Will report when he sends me the diagnosis


Offline thenoizeguy

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #31 on: February 14, 2018, 01:56:26 PM »
I hope you told him about the bias pots ?.
Those things need micro adjustments at best. 90 degrees is a disaster.
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Offline Tzs503gp

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #32 on: February 14, 2018, 02:02:00 PM »
We had a conversation about those. His take on them is more of a ďballparkĒ concept. I have adjusted those pots on other amps, and my experience would agree with Nilamís. You adjust one channel, then move to the other, and when back at the first, the mV reading has wandered. Apparently it wanders quite a bit due to temperature etc.

Offline fdlsys

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #33 on: February 14, 2018, 02:09:01 PM »
... His take on them is more of a ďballparkĒ concept...
In class-A you can afford the "ballpark" because you have to have lots of heatsink to begin with.
In class-AB, no. On single-turn trimmer ports you quickly go from mV to 10s of mV or from 10s to 100s. Disaster.

Bias is only set once the heatskinks are at nominal temperature.
Further problem with heatsink sizing, AB class of operation and biasing is that temperature is entirely a function of speaker impedance and how loudly you play over longer periods of time.

So, no thumbsucks and ballparks. Adjust it to what it says in the service manual. Less will progressively sound like poo, more ... you stand a chance of meltdown.
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Offline Hi-Phibian

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #34 on: February 14, 2018, 03:30:30 PM »
I donít think Nilamís ballpark spans 280 degrees of trimmer rotation.  Some
Rotel Amplifiers were biased very high often not aiding longevity.  On those one can back off a little but generally itís within a fairly tight range.

Just glad he knows they may be left high.   







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Offline Ampdog

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #35 on: February 14, 2018, 04:02:28 PM »
I hope you told him about the bias pots ?.
Those things need micro adjustments at best. 90 degrees is a disaster.

A bit of tutoring:
Bias adjustment is there to set the collector currtent; not too low otherwise cross-over distortion, not too high otherwise meltdown or worse. The exact current value is a question of design, not outright sound quality.

[Before some soil their pants because of that statement: yes, sound quality in the end and depending on what you are listening to, but not as in immediately audible; NFB sees to that. Immediate sonic effects will only be caused when the bias point is grossly exceeded, higher or lower.]

There are some amplifiers where a mere touch to a bias pot causes substantial shift in current. That denotes either poor design, a poor quality trimpot, or both. Poor design as in that the designer was incapable of determining the correct necessary spread and simply installed a pot x10 what was necessary to cover all bases. After setting I usually tap the pots lightly, noticing whether the current jumps about as a result.

For designers/builders: Use at least a cermet pot, or (more expensive) a true multiturn (heli) pot for convenient adjustment as well as maintaining the set value. 
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline marantz123

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #36 on: February 15, 2018, 08:50:03 AM »
Have had good results with Bourns 4xxx series of cermets but need to be careful as wiper orientation is not always the same..even though it looks the same...may be opposite to the original...learnt to set it up out of cct in comparison to the original and still check again in cct before power up.  :facepalm:  :flyswat:

Offline charles

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #37 on: February 15, 2018, 11:52:30 AM »
 bit of tutoring:
''Bias adjustment is there to set the collector currtent; not too low otherwise cross-over distortion, not too high otherwise meltdown or worse. The exact current value is a question of design, not outright sound quality.

[Before some soil their pants because of that statement: yes, sound quality in the end and depending on what you are listening to, but not as in immediately audible; NFB sees to that. Immediate sonic effects will only be caused when the bias point is grossly exceeded, higher or lower.]

There are some amplifiers where a mere touch to a bias pot causes substantial shift in current. That denotes either poor design, a poor quality trimpot, or both. Poor design as in that the designer was incapable of determining the correct necessary spread and simply installed a pot x10 what was necessary to cover all bases. After setting I usually tap the pots lightly, noticing whether the current jumps about as a result.

For designers/builders: Use at least a cermet pot, or (more expensive) a true multiturn (heli) pot for convenient adjustment as well as maintaining the set value. "

Agree


Offline Tzs503gp

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #38 on: February 16, 2018, 02:04:17 PM »
Back from Nilam. A small issue in the mute circuit. Replaced a resistor and a transistor and Robertís your mothers brother.
Kickass sound from 60watt. Makes my VR2s sing. Iím surprised at its speed in bass. Itís no 200watt monster mind, it cannot compete with my RB-1582, for control, at high volume.
Iíd love to keep it, but the wife has taken a dislike to it- money spent and all.
Iím still listening to it, but check the Ďfor salesí soon.

Thanks for the chat.

Offline PaganMcLoud

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #39 on: February 18, 2018, 10:08:11 AM »
Have you guys lost the plot, or have I?
Could it be that replies to other threads are somehow landing in this one?
Hi-Phibian and PaganMcleodís jokes were surely meant for Joke if the day.
JonnyPís comment surely belongs to another thread?
Hi-phiibian's post,  was more parable than joke, and still very much on-topic I feel.  The meaning and lesson will be different for each of us but I think it's essence is:  We allow ourselves to become so "driven" or "consumed" by what we think we "want" that we reject all else, even when it can satisfy our "need."   The 2 men wanted water so badly that they refused jelly, custard, cream.  All of which would have sated their thirst, as all of these contain water. 

*Mine was not as deep a parable.  It was purely for comic relief.    ;)
From the Dalai Lama:  Cables in allegory, are the "umbilical cord" or thread between life and afterlife. The quality of such life depends on the "source" of the energy, and not on the stitch or the thread...

Offline Hi-Phibian

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #40 on: February 18, 2018, 10:10:52 AM »
Buy a unit thatís faulty.  Donít get refund but get 50% back.  Obviously you wanted it.  Get it repaired.
Sell it.  Why not get 100% refund rather.  Bizarre.   







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Offline seanjammy

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #41 on: February 18, 2018, 10:16:11 AM »
Buy a unit thatís faulty.  Donít get refund but get 50% back.  Obviously you wanted it.  Get it repaired.
Sell it.  Why not get 100% refund rather.  Bizarre.   







[Anyone who thinks that wealth is measured in money is blind and narrow minded.  Wealth is obviously measured in turntables and records.]

I admire your patience in participating in this fiasco. Head hurting stuff here.

Offline Hi-Phibian

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #42 on: February 18, 2018, 10:25:07 AM »
I admire your patience in participating in this fiasco. Head hurting stuff here.


lol
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Offline Tzs503gp

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #43 on: February 18, 2018, 11:56:17 AM »
Still smiling- love you guys!

Offline TimbaLand

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Re: Bought a faulty Rotel RA1520 where to begin
« Reply #44 on: February 18, 2018, 03:37:18 PM »
Hi Tz

A few years ago I bought a Rotel RMB1095 for a ridiculous amount of R5000. It was apparently in storage for a while. I got home, connected it and I think I saw some bit of smoke but what Iím certain of is that it didnít play. The owner offered to fully refund me but o took my gamble and had it fixed by the agents. I also changed the faceplate from black to silver and all costs came to R2500. I had a fully functioning monster power amp for far less than it was going for in the market. Iím glad it worked out for u
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