Rega Mira 2000 (clamshell): It mutes with a BANG!!

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Uncle Ben

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Joined 2012
#1
Hi there!

One of my amps is (still) a Rega Mira 2000, connected to Chario Syntar 513. It's a nice match since I've changed the op-amps, but...
A couple of weeks ago I suddenly noticed a BANG and the amp was muted. Changing the source doesn't have any effect. Switching the amp off and on works for a while, but it's not a cure. I cannot recognize any rule, how or why this effect happens.
I've put the Rega via pre-out to a power amp: same effect (it mutes with a BANG!)

Has anybody any idea??? Can anybody tell me, how I can replace the thermal-pads without destroying the transistors?? I would like to replace the relays..

Thanx!
 
#2
Perhaps the problem is just a rail disappearing to the opamps in the preamp. Hard to say really... you are going to have to get the top off and measure basics (supplies, opamp offsets etc) when it is in the faulty state.

Can't see any problem replacing thermal pads. The transistors (you mean normal output type devices such as T0247, T03 package etc) are physically tough. Why replace the pads though ?
 
#3
Uncle Ben said:
..... I've changed the op-amps, but...
A couple of weeks ago I suddenly noticed a BANG and the amp was muted. Changing the source doesn't have any effect. Switching the amp off and on works for a while, but it's not a cure......
Click to expand...
http://www.diyaudio.com/forums/atta...7-strange-crackle-my-rega-mira-mirci0ma13.pdf
The above link is to a schematic from this thread: http://www.diyaudio.com/forums/solid-state/232880-strange-crackle-my-rega-mira.html Many problems are caused by mods like changing components to newer, bigger, better, faster etc, ones without knowing the consequences. Just what substitutions did you make and why would you want to replace the thermal pads on the power transistors unless they had failed?

The only sensible way to replace the pads is to unbolt, desolder with solder wicking braid or a desoldering pump that actually works(!) and remove the TO3P transistors first, avoiding bending the leads unnecessarily. Then either remove the mica washers and the old thermal grease or otherwise, just the silicone rubber pads. Clean up all contacting faces thoroughly, and start again with fresh materials (use the correct type of thermal grease with mica). Don't resolder until in final position and clamped down.
 
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#4
Thank you for reply!

Maybe I damaged a rail by soldering on the bottom side of the amp when I replaced the op-amps. It was a rather tricky job without seperating the PCB from the bottom plate. My plan was to remove the transistors+heatpads without desolder anything. "glueing" transistors directly to the bottom plate without leaving any space is a cheap and not very service friendly decision.

@Ian Finch: after the replacement of the op-amps the Mira sounded much better! I did this first on my tinker-around-amp "MF Synthesis".
 
#5
Are you saying that the original assembly was by glueing the transistors directly to the baseplate? I don't think so but often, self adhesive silpads are used to aid assembly and these will probably remain "stuck" when you later try to remove the transistors.

In any case assuming these are silicone pads, whether plain or self-adhesive, they always need replacement after dissassembly. Don't tempt fate by re-using old pads as they need to conform perfectly to their matching surfaces to meet their rating and you can be sure with small box amplifiers, that they will run hot even at half power.
 
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#6
add a series resistor to the speaker. try a 1k so that you still hear the "bang" as the amp goes into mute.
Also add an 8r0 dummy load.

Now try to get the amp to mute.

While still powered up measure the output offset while in mute.

You will need to remove the lid so that you have access to the output muting relays.
You may find that the input to one, or both relays, has gone to a supply rail voltage.
 
#7
Had a look at the sch post3
the power amp side:
Is the VAS transistor (tr23 & 27) correct? it shows a 2n5401 !
Should it be a 2n5551 NPN type?

PR2, the Vbe multiplier adjuster, is on the wrong side of the transistor.
If it goes open circuit the output stage takes a massive pulse of current and is likely to blow up.
If tr3 is actualy a PNP type, then the pr2 location is correct.
Check the marking on the transistor to identify type.
 
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#9
Here are pics of the Mira
An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.


The amp has the nice feature that you can use it as an integrated amp, as a pre-amp or as a power-amp. So I connected the Mira with my Sugden Pre to check it, but the issue has reached the next level: Mira is totally mute now. The relay doesn't click anymore, but les transistores still produce nearly the same heat as before.

@AndrewT: Thanks for your detailed help. I will have a closer look at the board asap.
 
#10
The relay is to protect the speakers from DC at the output terminals. This is caused by failed components or shorts in the amplifier, such as in the output transistors or an assembly fault. When the relay will not close at all, the protection circuit is most likely doing its job correctly and protecting your speakers. Remove them and any input signal.

Now, you need to test the amplifier outputs before they connect to the relay, so that you know if any DC voltage is present there. If you measure more than about 100mV with respect to output ground, you have a serious problem that may have been caused by replacing the opamps but now has possibly resulted in output transistor failure.

The loud bang suggests to me that the problem was caused by the modifications you performed but now, I think that has caused more serious damage. You must check the fundamental operating conditions of any amplifier by measurement of voltages like DC offset after any modifications. It is too late now but at least check what you have.

Your images, BTW, do not open.
 
#11
AndrewT said:
....Is the VAS transistor (tr23 & 27) correct? it shows a 2n5401....Should it be a 2n5551 NPN type?
Click to expand...
I think you are looking at TR25,28 which are shown to the left side of their part numbers and are actually BD139. TR23,27 are also to the left of their part numbers and seem to be correctly shown as 2N5401.

It's Lender's push-pull VAS discussed by Self in APAD6 p209 ff. Apparently, someone at Rega was desperate to be different.
 
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