Repair of Myst G-Ohm power amplifier

[Firebottle]

For those unfamiliar with the Myst it is a 3 box unit, PSU plus L and R monoblocks.

It is an old design based on the Hitachi FET amplifier circuit http://www.angelfire.com/sd/paulkemble/sound7g.html

The power supply runs at plus and minus 50V so the amps should be good for 80W into 8 ohm speakers, possibly slightly more.

Here is what they look like, on test after repair/fettling:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040928.jpg

The Dulci valve amp to the right is just being used as a passive volume control :mental:


Previous modifications have been made to the Myst, notably changing all the capacitors apart from the main two smoothing caps in the power supply.
Plaited Kimber cable has also been fitted for the dc cable from the psu, with XLR connectors replacing the original 3 pin Belling power connectors.

It appears that a 'replace all' approach was taken, possibly without an understanding of the circuit function of each capacitor, as the timing and decoupling capacitors for the output protection circuit have also been replaced with Elna along with the more important signal line capacitors.

Multiple Kimber foil caps have been fitted across all decoupling electrolytics as well.


The actual fault with the set up was a broken connection on one of the replacement 'Zero Volt' cables on the 0V speaker terminal.
Also apparent were poor soldered joints where power supply wiring had been replaced.

This is the broken joint before and after fitting an extra solder tag to enable secure connection:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040917.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040925.jpg

This was how the 0V speaker terminal looked once I had managed to extract it before the repair:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040924.jpg


Next are the before and after shots for the various connections in the power supply:

Mains input socket,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040916.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040920.jpg

Mains on/off switch,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040914.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040919.jpg

Rectifier,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040913.jpg

Smoothing capacitor,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040915.jpg


The dc output wiring was attached directly to the rectifier rather than to the smoothing capacitors so was altered.

DC wiring after re-routing,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040923.jpg

It's quite difficult to make multiple wire connections to one solder tag look tidy but I did my best ;)

Whilst the solder tags were being re-done for the capacitors, I took the opportunity to measure the values to see how they were fairing.

For 15000uF capacitors of unknown vintage they measured very well, particularly as the measuring frequency of my meter is 120Hz and not 100Hz.

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040921.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040922.jpg


Where the replacement plaited leads entered the amp enclosures the original cable clamps had been removed, so I fitted a couple of cable ties to secure the rubber sleeves in the correct place:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040926.jpg



Here is the internal shot of one of the power amps:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040927.jpg

The supply dropper resistor for the output protection IC was quite under rated and had turned dark brown, so was replaced with the two resistors seen standing up on the board. Another resistor is slightly discoloured so will be replaced as well.


Testing of the amps showed the output offsets to be low at below 20mV. The quiescent current for each amplifier was adjusted to approx 100mA as per the recommendation on the above circuit, so the first couple of watts output will be class A :thumbsup:

There is only one more concern about the Myst in that the dc connectors look very 'blackened' as if they have been used as disconnect devices.
It has been agreed that these will be changed for new ones.

Blackened connector sockets:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040918.jpg


It's now up and running sounding very sweet with plenty of grunt :eek: If it were mine it would be a keeper..............

Cheers,
Alan

[Ali Tait]

Nice job Alan.

[Oldpinkman]

That is a great little amp. I have spent 25 years hoping to find one in a boot fair - well, find one at all really

[jandl100]

Wow - :respect:

Fab job, Alan!! - I can hardly wait to get it back. :yay:

80 watts per, that's more than I was expecting for some reason.

[Beobloke]

Didn't realise you had one of these, Jerry, although obviously I'm not too surprised! ;) Lovely bit of kit. Do you have the preamp as well?

[jandl100]

Hi Adam - it's a recent acquisition in a swap deal.
It didn't work when it arrived, so Alan / Firebottle kindly offered to sort it for me. :thumbsup:

So I haven't heard it yet! - but very excited about doing so when it wends its way back to me.

No, I've not got the pre-amp module.

[anthonyTD]

Quite Shocking Alan,
I see quite a lot of this kind of soldering in Guitars!
For those unfamiliar with the Myst it is a 3 box unit, PSU plus L and R monoblocks.

It is an old design based on the Hitachi FET amplifier circuit http://www.angelfire.com/sd/paulkemble/sound7g.html

The power supply runs at plus and minus 50V so the amps should be good for 80W into 8 ohm speakers, possibly slightly more.

Here is what they look like, on test after repair/fettling:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040928.jpg

The Dulci valve amp to the right is just being used as a passive volume control :mental:


Previous modifications have been made to the Myst, notably changing all the capacitors apart from the main two smoothing caps in the power supply.
Plaited Kimber cable has also been fitted for the dc cable from the psu, with XLR connectors replacing the original 3 pin Belling power connectors.

It appears that a 'replace all' approach was taken, possibly without an understanding of the circuit function of each capacitor, as the timing and decoupling capacitors for the output protection circuit have also been replaced with Elna along with the more important signal line capacitors.

Multiple Kimber foil caps have been fitted across all decoupling electrolytics as well.


The actual fault with the set up was a broken connection on one of the replacement 'Zero Volt' cables on the 0V speaker terminal.
Also apparent were poor soldered joints where power supply wiring had been replaced.

This is the broken joint before and after fitting an extra solder tag to enable secure connection:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040917.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040925.jpg

This was how the 0V speaker terminal looked once I had managed to extract it before the repair:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040924.jpg


Next are the before and after shots for the various connections in the power supply:

Mains input socket,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040916.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040920.jpg

Mains on/off switch,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040914.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040919.jpg

Rectifier,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040913.jpg

Smoothing capacitor,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040915.jpg


The dc output wiring was attached directly to the rectifier rather than to the smoothing capacitors so was altered.

DC wiring after re-routing,
http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040923.jpg

It's quite difficult to make multiple wire connections to one solder tag look tidy but I did my best ;)

Whilst the solder tags were being re-done for the capacitors, I took the opportunity to measure the values to see how they were fairing.

For 15000uF capacitors of unknown vintage they measured very well, particularly as the measuring frequency of my meter is 120Hz and not 100Hz.

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040921.jpghttp://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040922.jpg


Where the replacement plaited leads entered the amp enclosures the original cable clamps had been removed, so I fitted a couple of cable ties to secure the rubber sleeves in the correct place:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040926.jpg



Here is the internal shot of one of the power amps:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040927.jpg

The supply dropper resistor for the output protection IC was quite under rated and had turned dark brown, so was replaced with the two resistors seen standing up on the board. Another resistor is slightly discoloured so will be replaced as well.


Testing of the amps showed the output offsets to be low at below 20mV. The quiescent current for each amplifier was adjusted to approx 100mA as per the recommendation on the above circuit, so the first couple of watts output will be class A :thumbsup:

There is only one more concern about the Myst in that the dc connectors look very 'blackened' as if they have been used as disconnect devices.
It has been agreed that these will be changed for new ones.

Blackened connector sockets:

http://i601.photobucket.com/albums/tt97/Paramotorpilot/Myst%20G-ohm%20repair/P1040918.jpg


It's now up and running sounding very sweet with plenty of grunt :eek: If it were mine it would be a keeper..............

Cheers,
Alan

[Gordon Steadman]

Good to see that Alan is still 'at it'

This means he will live forever of course.


As you know....old solderers never die:doh:

Not calling you old really Alan but you aren't in the first flush of youth after all:cool:

[anthonyTD]

Incase of any miss-understanding, i meant the soldering before Alan touched it! :)
Good to see another piece of kit resurrected.
A...

[Firebottle]

Well folks, for a piece of kit resurrected I am seriously, seriously impressed :eek:

I linked it up driven with my new KIN line/phono amp (first one completed) and boy does it perform. Startling clarity and depth of image, another big step up from the Firebottle 2 :D

Jerry are you sure you want this amp back :eyebrows:

[jandl100]

You bet your yellow trousers I do ;)

[struth]

Good to hear you got it all worked out to your satisfaction Jerry. Had a feeling it would potentially be special....will it halt the box swapping though;)

[jandl100]

Werll, we'll see what I think of it once it arrives at Jerry Mansions. I have some very fine amplification already. :)

Not sure I want to swap my boxswapping anyway - I have far too much fun doing it. :yay:
My old Parasound A21 lasted 18 months before the itch grew too much, mayhap the Myst will last longer.

[struth]

nah you'll hate it mate...best sell it now :eyebrows: