Carver MXR-150 "The Receiver" refurb - found a source for the relays!

w1jim

I can fix it but good...
Subscriber
Picked up a Carver MXR-150 "The Receiver" last month from the same guy I got my SX-1980 from. I've been distracted with the Pioneer but finally got around to attending to the Carver.
I've always been a bit suspicious of the Carver gear - maybe it's just that he was so ingenious sometimes that it's hard to believe the hype - but I've never been one to shy away from the Kool Aid so lets give this one a chance.

The Carver utilizes an unusual "Magnetic Field Power Amplifier" which I think is Bob Carver talk for a switching power supply akin to your computer's power supply.
The most noticeable aspect of this is that it doesn't have a large (heavy & expensive) power transformer like other amps.
I'm curious to hear how this compares at 150 watts x 2 with my other receivers in that category - I don't imagine it could have the dynamic headroom and 'punch' of conventional amps but then Caver has been known to be a magician so I'll keep that mind open.

Big problems with these are the output / speaker relays.
They're not up to the task apparently and commonly need to be replaced. Typical symptoms are that you get no audio output or some audio output at high volume levels. The contacts get toasted & pitted. I believe that the relays are configured such that when you select 2 pair of speakers the load is in series versus in parallel which is more typical. Presumably this is to avoid a low impedance load across the outputs. As a result of this I think the load is alway across both relays with either 1 or 2 pair of speakers.

The original relay is part number 621D024 - a 24 volt low current coil and DPDT (2-form "C").
There is a gentleman on ebay selling a pair of these for $20 + shipping but I sourced these from www.mouser.com. The Mouser part number is 677-OMI-SH-224D and the manufacturer's part number is OMI-SH-224D,594. Oh, they're a bit cheaper at $2.46 each - not that cost is an issue with any of us :no:!

Replacing the relays is only a bit tricky as you have to unsolder the 8 legs holding the speaker binding posts to the assembly and then replace the relays. At least there is plenty of room to work in.

Next project will be to replace and slightly relocate some 2-watt resistors with 5-watt versions. These tend to burn the PCB so I'll be getting them up and away a bit more than stock.

For now, some sexy (nude) family pictures.

The pretty face...
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The clear one is the original relay and it's $2.46 replacement...
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Mounted on the PCB...
attachment.php



Reassembled and back into the chassis...
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Next, let's upgrade some power resistors - these boys do get toasty!

The first four are on the CNR-527D "Noise Reduction" PCB.
There are two 62 ohm and two 68 ohm 2-watt resistors. I replaced them with 5-watt parts and spaced them a bit from the board and each other. Leads are in teflon tubing.

Next we have some on the power supply PCB.
As in above, I replaced four 2-watt resistors with 5-watt values and spaced them further from the PCB. Values here are 62, 680, 68 and 820 ohms.

Finally, R635 is a 3300 ohm 1/2 watt resistor - I replaced this with a 1-watt part in accordance with a factory service bulletin. It can be seen to the left of the 5-watt parts.

There were a bunch of heat fatigued solder joints in the vicinity of the power resistors that needed some touch up as well.

Before and after pix are in the attachments.

Adjusted the amp voltages as per the service manual - Now for some listening tests.
 

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Great postings W1jim! This will be my next project (after several Marantz 22xx's). Kudos!

Oh yea... did you do any re-capping while you were in there?
 
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Oh yea... did you do any re-capping while you were in there?
I didn't do any recapping, I've since traded it for some other gear more in line with my collection.
That being said I did a comparison with my Sansui G-9000db at the time (with 2 other friends also listening) and was quite favorably impressed.
 
I wanted to post in this thread to include the part number for the FM tuner trimmer capacitor.

It's 24AA023 at Mouser.com
http://br.mouser.com/Search/ProductDetail.aspx?R=24AA023virtualkey21980000virtualkey24AA023

If you replace the speaker relays, the voltage dropping resistors, and this trimmer, that's all the pattern failure components for these receivers.

I found the info about the trimmer at: http://home.onemain.com/~jvandyke/id8.html

You can speed up the relay replacement by desoldering just one side and bending the posts out of the way.
 
Picked up a Carver MXR-150 "The Receiver" last month from the same guy I got my SX-1980 from. I've been distracted with the Pioneer but finally got around to attending to the Carver.
I've always been a bit suspicious of the Carver gear - maybe it's just that he was so ingenious sometimes that it's hard to believe the hype - but I've never been one to shy away from the Kool Aid so lets give this one a chance.

The Carver utilizes an unusual "Magnetic Field Power Amplifier" which I think is Bob Carver talk for a switching power supply akin to your computer's power supply.
The most noticeable aspect of this is that it doesn't have a large (heavy & expensive) power transformer like other amps.
I'm curious to hear how this compares at 150 watts x 2 with my other receivers in that category - I don't imagine it could have the dynamic headroom and 'punch' of conventional amps but then Caver has been known to be a magician so I'll keep that mind open.

Big problems with these are the output / speaker relays.
They're not up to the task apparently and commonly need to be replaced. Typical symptoms are that you get no audio output or some audio output at high volume levels. The contacts get toasted & pitted. I believe that the relays are configured such that when you select 2 pair of speakers the load is in series versus in parallel which is more typical. Presumably this is to avoid a low impedance load across the outputs. As a result of this I think the load is alway across both relays with either 1 or 2 pair of speakers.

The original relay is part number 621D024 - a 24 volt low current coil and DPDT (2-form "C").
There is a gentleman on ebay selling a pair of these for $20 + shipping but I sourced these from www.mouser.com. The Mouser part number is 677-OMI-SH-224D and the manufacturer's part number is OMI-SH-224D,594. Oh, they're a bit cheaper at $2.46 each - not that cost is an issue with any of us :no:!

Replacing the relays is only a bit tricky as you have to unsolder the 8 legs holding the speaker binding posts to the assembly and then replace the relays. At least there is plenty of room to work in.

Next project will be to replace and slightly relocate some 2-watt resistors with 5-watt versions. These tend to burn the PCB so I'll be getting them up and away a bit more than stock.

For now, some sexy (nude) family pictures.

The pretty face...
attachment.php



The clear one is the original relay and it's $2.46 replacement...
attachment.php



Mounted on the PCB...
attachment.php



Reassembled and back into the chassis...
attachment.php



attachment.php

Picked up a Carver MXR-150 "The Receiver" last month from the same guy I got my SX-1980 from. I've been distracted with the Pioneer but finally got around to attending to the Carver.
I've always been a bit suspicious of the Carver gear - maybe it's just that he was so ingenious sometimes that it's hard to believe the hype - but I've never been one to shy away from the Kool Aid so lets give this one a chance.

The Carver utilizes an unusual "Magnetic Field Power Amplifier" which I think is Bob Carver talk for a switching power supply akin to your computer's power supply.
The most noticeable aspect of this is that it doesn't have a large (heavy & expensive) power transformer like other amps.
I'm curious to hear how this compares at 150 watts x 2 with my other receivers in that category - I don't imagine it could have the dynamic headroom and 'punch' of conventional amps but then Caver has been known to be a magician so I'll keep that mind open.

Big problems with these are the output / speaker relays.
They're not up to the task apparently and commonly need to be replaced. Typical symptoms are that you get no audio output or some audio output at high volume levels. The contacts get toasted & pitted. I believe that the relays are configured such that when you select 2 pair of speakers the load is in series versus in parallel which is more typical. Presumably this is to avoid a low impedance load across the outputs. As a result of this I think the load is alway across both relays with either 1 or 2 pair of speakers.

The original relay is part number 621D024 - a 24 volt low current coil and DPDT (2-form "C").
There is a gentleman on ebay selling a pair of these for $20 + shipping but I sourced these from www.mouser.com. The Mouser part number is 677-OMI-SH-224D and the manufacturer's part number is OMI-SH-224D,594. Oh, they're a bit cheaper at $2.46 each - not that cost is an issue with any of us :no:!

Replacing the relays is only a bit tricky as you have to unsolder the 8 legs holding the speaker binding posts to the assembly and then replace the relays. At least there is plenty of room to work in.

Next project will be to replace and slightly relocate some 2-watt resistors with 5-watt versions. These tend to burn the PCB so I'll be getting them up and away a bit more than stock.

For now, some sexy (nude) family pictures.

The pretty face...
attachment.php



The clear one is the original relay and it's $2.46 replacement...
attachment.php



Mounted on the PCB...
attachment.php



Reassembled and back into the chassis...
attachment.php



attachment.php
Picked up a Carver MXR-150 "The Receiver" last month from the same guy I got my SX-1980 from. I've been distracted with the Pioneer but finally got around to attending to the Carver.
I've always been a bit suspicious of the Carver gear - maybe it's just that he was so ingenious sometimes that it's hard to believe the hype - but I've never been one to shy away from the Kool Aid so lets give this one a chance.

The Carver utilizes an unusual "Magnetic Field Power Amplifier" which I think is Bob Carver talk for a switching power supply akin to your computer's power supply.
The most noticeable aspect of this is that it doesn't have a large (heavy & expensive) power transformer like other amps.
I'm curious to hear how this compares at 150 watts x 2 with my other receivers in that category - I don't imagine it could have the dynamic headroom and 'punch' of conventional amps but then Caver has been known to be a magician so I'll keep that mind open.

Big problems with these are the output / speaker relays.
They're not up to the task apparently and commonly need to be replaced. Typical symptoms are that you get no audio output or some audio output at high volume levels. The contacts get toasted & pitted. I believe that the relays are configured such that when you select 2 pair of speakers the load is in series versus in parallel which is more typical. Presumably this is to avoid a low impedance load across the outputs. As a result of this I think the load is alway across both relays with either 1 or 2 pair of speakers.

The original relay is part number 621D024 - a 24 volt low current coil and DPDT (2-form "C").
There is a gentleman on ebay selling a pair of these for $20 + shipping but I sourced these from www.mouser.com. The Mouser part number is 677-OMI-SH-224D and the manufacturer's part number is OMI-SH-224D,594. Oh, they're a bit cheaper at $2.46 each - not that cost is an issue with any of us :no:!

Replacing the relays is only a bit tricky as you have to unsolder the 8 legs holding the speaker binding posts to the assembly and then replace the relays. At least there is plenty of room to work in.

Next project will be to replace and slightly relocate some 2-watt resistors with 5-watt versions. These tend to burn the PCB so I'll be getting them up and away a bit more than stock.

For now, some sexy (nude) family pictures.

The pretty face...
attachment.php



The clear one is the original relay and it's $2.46 replacement...
attachment.php



Mounted on the PCB...
attachment.php



Reassembled and back into the chassis...
attachment.php



attachment.php
You don't imagine it has the dynamic headroom of conventional amps and you're right it has way more Bob found that to recreate the sound of a scissor snap you momentarily need over 2000 watts and while most amplifiers are incapable of this feat for enen a millisecond his m1.5t amp can for about a millisecond (more modest peak demands for longer) now that's headroom, How can his designs produce such huge albeit brief power being so light ? He does not use heavy transformers and huge capacitors he instead uses a triac on the power supply driven by the music demands to supply all the power needed for faithful sound reproduction .Also when most class A-B amplifiers idle at half power so that a 200W amp draws 100W with no input his design only draws power when the input signal demands hence much smaller heatsinks for the output transistors running much cooler as such he is able to put a 2700W amplifier inside of his Sunfire 10 inch subwoofer which is the sonic equivalent of three 15 inch subs. Magician? I guess it would seem that way until you readup on how he does it.
 
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