O Learned Denizens,
I'm a Brit living near San Francisco. One of the things I brought with me was a Rega Mira amplifier, circa 2001.
It gradually started losing its mind a few years ago but really fell off a cliff about 3 months ago. It took the form of one or both channels mostly dropping out but occasionally coming back in a very clipped and distorted form.
After asking The Google, I suspected it might be the power amplifier transistors and duly purchased some replacements, which seem readily available.
Finally tonight I got round to lifting off the lid, unscrewing everything, unsoldering the old ones and soldering in the new ones. It all seemed to go without incident, except for the minor problem that the amp will no longer wake up at all - in fact it seems to have blown the 110V -> 240V transformer I have been running it with since I got here. I don't dare try with another one because I don't want to just eat a second one.
So, in no particular order, here are the things I am suspicious of:
1) Cream-coloured tape between transistors and heat sink rail - could have been slightly damaged getting the old transistors to let go of them. I turned all the pieces of tape round 180 degrees about the axis normal to the base of the amp ("yaw") and it all looked like it fitted fine. Could I have introduced a short there?
2) Shared connections with the base pins of the transistor - there are two pairs of transistors which are visually identical but numbered differently. As well as the three legs from each transistor, 1 leg on each transistor appears to "run into" the soldered leg from another component on the board. It seems to be a different leg on each transistor. I didn't add very much solder at all and I don't think I have added any inadvertent connections, but I can't rule it out. Could that give me a short? It feels like it could, but I am struggling to find schematic that lets my (non-electronic) brain make sense of whether the connections I see should or shouldn't be there.
3) Damaged PCB track - one connector of one of the pins came away from the board. I didn't quite realise what I was seeing, thinking it to be scrap solder and as I gently pulled it, it unpeeled the track from the board. It still appears mechanically connected, just not under the varnish/resin/whatever on top of the PCB.
Given the poor availability of 240V power supplies and my own somewhat meagre electronics knowledge, any got any hints? Imagine I'm a mechanical engineer with access to a multimeter; presume nothing...
Thanks in advance,
Damian
I'm a Brit living near San Francisco. One of the things I brought with me was a Rega Mira amplifier, circa 2001.
It gradually started losing its mind a few years ago but really fell off a cliff about 3 months ago. It took the form of one or both channels mostly dropping out but occasionally coming back in a very clipped and distorted form.
After asking The Google, I suspected it might be the power amplifier transistors and duly purchased some replacements, which seem readily available.
Finally tonight I got round to lifting off the lid, unscrewing everything, unsoldering the old ones and soldering in the new ones. It all seemed to go without incident, except for the minor problem that the amp will no longer wake up at all - in fact it seems to have blown the 110V -> 240V transformer I have been running it with since I got here. I don't dare try with another one because I don't want to just eat a second one.
So, in no particular order, here are the things I am suspicious of:
1) Cream-coloured tape between transistors and heat sink rail - could have been slightly damaged getting the old transistors to let go of them. I turned all the pieces of tape round 180 degrees about the axis normal to the base of the amp ("yaw") and it all looked like it fitted fine. Could I have introduced a short there?
2) Shared connections with the base pins of the transistor - there are two pairs of transistors which are visually identical but numbered differently. As well as the three legs from each transistor, 1 leg on each transistor appears to "run into" the soldered leg from another component on the board. It seems to be a different leg on each transistor. I didn't add very much solder at all and I don't think I have added any inadvertent connections, but I can't rule it out. Could that give me a short? It feels like it could, but I am struggling to find schematic that lets my (non-electronic) brain make sense of whether the connections I see should or shouldn't be there.
3) Damaged PCB track - one connector of one of the pins came away from the board. I didn't quite realise what I was seeing, thinking it to be scrap solder and as I gently pulled it, it unpeeled the track from the board. It still appears mechanically connected, just not under the varnish/resin/whatever on top of the PCB.
Given the poor availability of 240V power supplies and my own somewhat meagre electronics knowledge, any got any hints? Imagine I'm a mechanical engineer with access to a multimeter; presume nothing...
Thanks in advance,
Damian
The exact model/version helps but there are no free service manuals out there to refer to for the Mira 2000, if that's the version you have. There is a PCB only schematic posted here: http://www.diyaudio.com/forums/atta...a-circa-year-2000-help-mira-2000-main-pcb.pdf
The transformer could have been fitted with dual 120V primary windings - series wired for 240V, parallel for 120V. According to pics though, the AC supply leads are a cable, sealed into the transformer's film wrapping, meaning it can't be rewired, probably because the primary was rated 240V only.
I'd think you could buy a local US supply Antek model or better whose voltages and VA rating would suit anyway. A 60W/8R audio power output rating suggests 2 x 28VAC secondary windings and at least 160VA rating - a very common transformer if you found it simple to replace and the dimensions fitted the available space. This might suit: http://www.antekinc.com/content/AS-2228.pdf
Bear in mind that you also need to read what the connections mean and how to wire up a toroidal transformer to get them correct - first time! In this case, you cant just copy the mains wiring.
Here's some broad guidelines to amateur audio repairs. It is aimed at DIY builds but the topics and principles are clear enough for commercial products too, if you search them: Troubleshooting & Repair Guide
Note the guidelines (9.0) for removing parts there.
As you haven't referred to any guidelines and seem to be "winging it" for a repair, I think you are in for more trouble here - tearing tracks from PCBs is really careless even if you have little skill. Common sense says get practical help regardless, before further damage occurs, if you want to have a working amplifier. My concern is that if you continue to solve your problems by kludging around the board, it will be destined for the bin , not the hi-fi shelf, since audio repairers would refuse to touch it and you may not have the resources to keep replacing the damaged before it even shows promise.
Don't forget to use a lightbulb limiter or commonly, a dim bulb tester to power up after any repairs, or it's goodbye all over again. This video details all - a 60W only incandescent bulb (a halogen variety is ok) will suit, as this isn't a current-hungry tube guitar amp. https://www.youtube.com/watch?v=wRFRwOnLsZI
The transformer could have been fitted with dual 120V primary windings - series wired for 240V, parallel for 120V. According to pics though, the AC supply leads are a cable, sealed into the transformer's film wrapping, meaning it can't be rewired, probably because the primary was rated 240V only.
I'd think you could buy a local US supply Antek model or better whose voltages and VA rating would suit anyway. A 60W/8R audio power output rating suggests 2 x 28VAC secondary windings and at least 160VA rating - a very common transformer if you found it simple to replace and the dimensions fitted the available space. This might suit: http://www.antekinc.com/content/AS-2228.pdf
Bear in mind that you also need to read what the connections mean and how to wire up a toroidal transformer to get them correct - first time! In this case, you cant just copy the mains wiring.
Here's some broad guidelines to amateur audio repairs. It is aimed at DIY builds but the topics and principles are clear enough for commercial products too, if you search them: Troubleshooting & Repair Guide
Note the guidelines (9.0) for removing parts there.
As you haven't referred to any guidelines and seem to be "winging it" for a repair, I think you are in for more trouble here - tearing tracks from PCBs is really careless even if you have little skill. Common sense says get practical help regardless, before further damage occurs, if you want to have a working amplifier. My concern is that if you continue to solve your problems by kludging around the board, it will be destined for the bin , not the hi-fi shelf, since audio repairers would refuse to touch it and you may not have the resources to keep replacing the damaged before it even shows promise.
Don't forget to use a lightbulb limiter or commonly, a dim bulb tester to power up after any repairs, or it's goodbye all over again. This video details all - a 60W only incandescent bulb (a halogen variety is ok) will suit, as this isn't a current-hungry tube guitar amp. https://www.youtube.com/watch?v=wRFRwOnLsZI
For one, the power transistors are about the least likely source of weird intermittent problems. Under normal conditions they are running a long way from their max ratings and thus their lifespan tends to be a very long one (barring any unfortunate events regarding their loading). Jumping in to replace these was ill-advised, something you are probably gleaning my now.
The 'tape' is a thermally conductive insulator. These are designed to be used once, then discarded and replaced with new should the device have to be removed. I can't tell you what to use as a replacement since I don't know what type of power transistor we're talking about here (TO-3, TO-3P, TO-264, MT-200...)
As far as the rest, the only way I'd see that it makes sense for you to continue is if you really don't care about the eventual outcome with this piece. Any other mindset suggests that your best bet is to find someone with access to service information on that particular unit (which I can't find) and who you trust to get the job done. It already sounds like you've done some real damage here and in order to prevent more of the same, I suggest that you stop now and start looking for a professional.
The 'tape' is a thermally conductive insulator. These are designed to be used once, then discarded and replaced with new should the device have to be removed. I can't tell you what to use as a replacement since I don't know what type of power transistor we're talking about here (TO-3, TO-3P, TO-264, MT-200...)
As far as the rest, the only way I'd see that it makes sense for you to continue is if you really don't care about the eventual outcome with this piece. Any other mindset suggests that your best bet is to find someone with access to service information on that particular unit (which I can't find) and who you trust to get the job done. It already sounds like you've done some real damage here and in order to prevent more of the same, I suggest that you stop now and start looking for a professional.
I've scanned the ron elliott amp repair article and it seems to be mostly tuned to repairing amps you built yourself. The problems of a new untried amp, and a previously working geriatric amp, are mostly very different. Read part 5 & 6 first about meter use first.
Output transistors only go if the wiring was ****ed up (think bar band on stage in the dark with multipurpose 1/4 phone plugs) the fan if any, stopped, or the speaker shorted the windings after the rubber surround fell apart. Or it was designed by idiots selling on e-bay.
Mostly geriatric amps that distort get 1. oxide on internal connectors that are brass or tin plate, a 8-10 year problem
2. Worn out or oxide polluted controls like volume pot wipers, (most common) mono/stereo or source switches, etc
3. dried up e-caps that affect frequencies differently, or tantalum caps that drop out occasionally or have noises like popcorn or something. Occasionally really old e-caps or tantalums short out. 5 to 25 year problem depending on the expected life of the sealant use to build the e-caps. TV's and PC capacitors can **** out in 3 years. E-caps used in industrial motor drives like AB or TB Woods can run 24/7 in a 150 deg enviroment for 15 years, and 30 years or more at room temperature.
Bad connections or pot wipers, you exercise, remove & replace connection, spray off with contact cleaner (warning flammable from the lumberyard, and the non-flammable bromide formula from the electrical supply dissolves plastic) Volume pots frequently have to be replaced, i've replaced mine twice in the preamp I bought in 1970.
E-caps, I look at the calender, listen to the sound, and shotgun replace them, starting at the power supply rail, next the input caps, then eventually all of them. Some people measure them with an ESR meter, but I find they perform worse in cold rooms in winter, or heated up by vigorous use, so a single temperature measurement IMHO is fairly useless. The calander + bad sound is my guide. I replace with >3000 hour service life caps for 1500 uf up, 10000 hr for 22 uf to 1200 uf, and plastic film or CPO ceramic for 10 uf & under.
You definitely need to buy replacements for the heat pads your messed up. Scrape the old ones off with a wood chisel. Don't leave any burrs on the heat sink. See the Arvid catalog from newark(farnell) mouser or digikey. One lifted land is unfortunate but not a tragedy, you can bridge it with some wire.
Soldering skill is important. Cheap irons are ****. I use a Weller WP35 with 3/16" wide chisel tip. those weller 70 W workstations are even faster. Use solder wick (braid) or vacuum tool to remove old solder.
Finding the problem with a scope or AC scale of an ANALOG VOM is important. Digital DVM miss transients, they average over 4 seconds usually. Also the most DVMs produce random numbers on music. Put .047 to .47 uf cap on ground lead of analog VOM, to keep DC from showing up on the AC scale. VOM with 2 v and 20 v ac scales will outlast any $100 scope you buy, mine is from 1986. No e-caps to fail.
Notice on analog VOM, pointer will kick with the beat of rock music, from a radio, whereas a high steady pointer might be RF oscillation. 390 pf blocking cap fingers the RF oscillation for sure, music won't go through that.
First thing find the problem source with the VOM or scope. Check after input switches and pots, collector of first transistor or output of first op amp, collector of voltage amplifier stage, interstage resistor (both ends required, its a current check) after the drivers if you didn't find the ittermittant sound before that. Then focus on the stage before the problem occurs.
This amp is now trash in its current condition, I'd carry on to the end instead of worrying about it. When I did my first SS amp before internet was invented, I ended up with 6 W out because i didn't understand the limiter circuit. It was so burned I couldn't tell if the transistor was NPN or PNP. This amp sounds in better shape than that.
Have fun.
Output transistors only go if the wiring was ****ed up (think bar band on stage in the dark with multipurpose 1/4 phone plugs) the fan if any, stopped, or the speaker shorted the windings after the rubber surround fell apart. Or it was designed by idiots selling on e-bay.
Mostly geriatric amps that distort get 1. oxide on internal connectors that are brass or tin plate, a 8-10 year problem
2. Worn out or oxide polluted controls like volume pot wipers, (most common) mono/stereo or source switches, etc
3. dried up e-caps that affect frequencies differently, or tantalum caps that drop out occasionally or have noises like popcorn or something. Occasionally really old e-caps or tantalums short out. 5 to 25 year problem depending on the expected life of the sealant use to build the e-caps. TV's and PC capacitors can **** out in 3 years. E-caps used in industrial motor drives like AB or TB Woods can run 24/7 in a 150 deg enviroment for 15 years, and 30 years or more at room temperature.
Bad connections or pot wipers, you exercise, remove & replace connection, spray off with contact cleaner (warning flammable from the lumberyard, and the non-flammable bromide formula from the electrical supply dissolves plastic) Volume pots frequently have to be replaced, i've replaced mine twice in the preamp I bought in 1970.
E-caps, I look at the calender, listen to the sound, and shotgun replace them, starting at the power supply rail, next the input caps, then eventually all of them. Some people measure them with an ESR meter, but I find they perform worse in cold rooms in winter, or heated up by vigorous use, so a single temperature measurement IMHO is fairly useless. The calander + bad sound is my guide. I replace with >3000 hour service life caps for 1500 uf up, 10000 hr for 22 uf to 1200 uf, and plastic film or CPO ceramic for 10 uf & under.
You definitely need to buy replacements for the heat pads your messed up. Scrape the old ones off with a wood chisel. Don't leave any burrs on the heat sink. See the Arvid catalog from newark(farnell) mouser or digikey. One lifted land is unfortunate but not a tragedy, you can bridge it with some wire.
Soldering skill is important. Cheap irons are ****. I use a Weller WP35 with 3/16" wide chisel tip. those weller 70 W workstations are even faster. Use solder wick (braid) or vacuum tool to remove old solder.
Finding the problem with a scope or AC scale of an ANALOG VOM is important. Digital DVM miss transients, they average over 4 seconds usually. Also the most DVMs produce random numbers on music. Put .047 to .47 uf cap on ground lead of analog VOM, to keep DC from showing up on the AC scale. VOM with 2 v and 20 v ac scales will outlast any $100 scope you buy, mine is from 1986. No e-caps to fail.
Notice on analog VOM, pointer will kick with the beat of rock music, from a radio, whereas a high steady pointer might be RF oscillation. 390 pf blocking cap fingers the RF oscillation for sure, music won't go through that.
First thing find the problem source with the VOM or scope. Check after input switches and pots, collector of first transistor or output of first op amp, collector of voltage amplifier stage, interstage resistor (both ends required, its a current check) after the drivers if you didn't find the ittermittant sound before that. Then focus on the stage before the problem occurs.
This amp is now trash in its current condition, I'd carry on to the end instead of worrying about it. When I did my first SS amp before internet was invented, I ended up with 6 W out because i didn't understand the limiter circuit. It was so burned I couldn't tell if the transistor was NPN or PNP. This amp sounds in better shape than that.
Have fun.
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Thanks for the replies.
I've had 16 years from the amp and although I like it, I'm not wed to it - so if I do kill it I don't mind. As far as I am concerned I am attempting to revive a dying man. The worst that can possibly happen is that he dies anyway.
There is no way I am taking it to someone who will suck air over his/her teeth, charge me hours of labour and then tell me I was stupid for trying to fix it and should have bought a new one anyway. America regards people who take things (cars, amplifiers, etc) to such people as fair game and is unsympathetic to people who can't help themselves. I'm seeing this as a relatively inconsequential game, in which I may learn something.
The transformer I popped is external to the amplifier. It's a 110V to 240V step up transformer, rated at 100 Watts. So I probably haven't hurt the amplifier internal transformer - as far as I can tell it never got energised because there was never any response to the power switch - not even a flicker.
On that note, I ended up trying the other identical external transformer I have, but with only the cable - that is to say with the other end of the lead in free space. There was a large flash and that transformer died too. They aren't expensive but are mail order, which introduces an irritating delay.
On that basis, I can conclude that I can't conclude anything; I have no actual proof that the amplifier *doesn't* work, only some likelihood of either a short in the cable (seems unlikely to me, but not impossible) or a flaky design of international plug orifice causing me to contact the live prong of my UK plug into the internal earth connector inside the external transformer as I wiggled the plug into it while it was plugged into the socket. Or "outlet", as they call them here. I flake out these transformers regularly, which is why I usually have a spare to hand. They seem pretty flimsy to me - and yes, I look at the power rating what I plug into them. They just seem to go like lightbulbs, randomly and for no discernible reason. At least they are cheap. Anyway, lesson learned - plug in the UK plug to the transformer, then the transformer to the wall.
I take on board that the power transistors was a somewhat random place to start. But what the hell, I started there.
I'm also grateful for the advice about the tape. I will look into finding some of that at work, where there are lots of electronic people. Well, not actual electronic people. But people who work on electronics. Or else I will buy some.
The advice about the analogue meter vs digital is pretty golden, I think. I will also sort myself out a light bulb. And the advice about what fails when feels like a lot of experience distilled down beautifully. I am also very grateful for that.
One thing is for sure - it will definitely be fun, as I have nothing to lose!
I don't get much spare time, so progress will be sporadic, but I will report back as I go. Who knows, I may be listening to my Linn again before the decade is out!
I've had 16 years from the amp and although I like it, I'm not wed to it - so if I do kill it I don't mind. As far as I am concerned I am attempting to revive a dying man. The worst that can possibly happen is that he dies anyway.
There is no way I am taking it to someone who will suck air over his/her teeth, charge me hours of labour and then tell me I was stupid for trying to fix it and should have bought a new one anyway. America regards people who take things (cars, amplifiers, etc) to such people as fair game and is unsympathetic to people who can't help themselves. I'm seeing this as a relatively inconsequential game, in which I may learn something.
The transformer I popped is external to the amplifier. It's a 110V to 240V step up transformer, rated at 100 Watts. So I probably haven't hurt the amplifier internal transformer - as far as I can tell it never got energised because there was never any response to the power switch - not even a flicker.
On that note, I ended up trying the other identical external transformer I have, but with only the cable - that is to say with the other end of the lead in free space. There was a large flash and that transformer died too. They aren't expensive but are mail order, which introduces an irritating delay.
On that basis, I can conclude that I can't conclude anything; I have no actual proof that the amplifier *doesn't* work, only some likelihood of either a short in the cable (seems unlikely to me, but not impossible) or a flaky design of international plug orifice causing me to contact the live prong of my UK plug into the internal earth connector inside the external transformer as I wiggled the plug into it while it was plugged into the socket. Or "outlet", as they call them here. I flake out these transformers regularly, which is why I usually have a spare to hand. They seem pretty flimsy to me - and yes, I look at the power rating what I plug into them. They just seem to go like lightbulbs, randomly and for no discernible reason. At least they are cheap. Anyway, lesson learned - plug in the UK plug to the transformer, then the transformer to the wall.
I take on board that the power transistors was a somewhat random place to start. But what the hell, I started there.
I'm also grateful for the advice about the tape. I will look into finding some of that at work, where there are lots of electronic people. Well, not actual electronic people. But people who work on electronics. Or else I will buy some.
The advice about the analogue meter vs digital is pretty golden, I think. I will also sort myself out a light bulb. And the advice about what fails when feels like a lot of experience distilled down beautifully. I am also very grateful for that.
One thing is for sure - it will definitely be fun, as I have nothing to lose!
I don't get much spare time, so progress will be sporadic, but I will report back as I go. Who knows, I may be listening to my Linn again before the decade is out!
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