Get a car battery charger. Old fashioned 12 vdc e frame transformer with 2 rectifiers, nothing with electronic sales gimmick devices. Put a 25 v electrolytic cap across it. That is an 18v peak power supply. Solder a 47k resistor to one lead. Put the amps scale of your DVM in series with it. Hook up a negative probe, get some clip leads. That is a leakage tester that finds more problems than the 2v compliance 2megohm scale of your DVM. I found some things with it, including toasted film caps and output transistors that were "okay" by the double diode test but had the habit of exploding when I put 75 VDC on them. When testing naked transistors, short b to e before putting that much voltage on them.
I held the rail voltage down on my amp from +-85 with a room heater in series with the power lead, but if I held it down too much with a 100 W light bulb, the op amp supply wouldn't reach +-15v. the room heater did keep the silicon from blowing absolutely across the room through the steel TO3 cases.
I held the rail voltage down on my amp from +-85 with a room heater in series with the power lead, but if I held it down too much with a 100 W light bulb, the op amp supply wouldn't reach +-15v. the room heater did keep the silicon from blowing absolutely across the room through the steel TO3 cases.
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OK, at last, music without Kaboom! All drivers mounted very carefully with new mica washers & new shoulder washers under the screws, onto heatsinks that had been carefully rubbed down with fine emery. Tested, tested and tested again for isolation. The same with new power devices in the damaged channel.
And now, for some long promised pictures:
http://www.flickr.com/photos/16059325@N03/8173815187/
And now, for some long promised pictures:
http://www.flickr.com/photos/16059325@N03/8173815187/
Just one thing left, to let it soak in and set the biasing and offset as per the manual.
My listening test was rudimentary, plugged into my cellphone playing from Pandora, and driving a couple of bookshelf speakers in the garage. That was enough to tell me it was operating cleanly, and I could hear the quality limitations of the Pandora feed. Tomorrow I'll finish reassembling the case, check those transistors one last time for mounting shorts, and give it a proper test.
Cheers,
Ed
My listening test was rudimentary, plugged into my cellphone playing from Pandora, and driving a couple of bookshelf speakers in the garage. That was enough to tell me it was operating cleanly, and I could hear the quality limitations of the Pandora feed. Tomorrow I'll finish reassembling the case, check those transistors one last time for mounting shorts, and give it a proper test.
Cheers,
Ed
Congratulations. I love the look of the milled slot knobs, the blue inset on the brushed anodized front panel. My working dynakit equipment is the plastic knob brown paint front panel, very plebian like the price I paid for them. The St120 is black on black.
The copper wrapped transformer is no slouch, either.
The copper wrapped transformer is no slouch, either.
Yes, it is a rather handsome bit of kit, and seemingly well looked after. There's no wear on the lettering, just a little dirt that was removed easily with careful cleaning. The knobs were treated to a scrub with dish soap and a toothbrush to clean out the grooves to return the sparkle. It will look great once back together. Then it's time to think about putting the amp to work again 
Thanks again to all for comments and advice
Ed
Thanks again to all for comments and advice
Ed
Congrats with this beautiful amp !
I own one, build it in the 80’s ? and still love it . Had to replace the power transistors and drivers a couple of times after heavy partying ( it’s not beer proof)
One warning , don’t place it on top of one off your speakers as I did with my Tannoy Berkeley’s .
The giant transformer of the stereo 400 demagnetised one of my HPD385 speaker magnets over the years . I only found out after 2 recones of my left speaker and had the speaker to be remagnetised .
Cheers ,
Rens
I own one, build it in the 80’s ? and still love it . Had to replace the power transistors and drivers a couple of times after heavy partying ( it’s not beer proof
)One warning , don’t place it on top of one off your speakers as I did with my Tannoy Berkeley’s .
The giant transformer of the stereo 400 demagnetised one of my HPD385 speaker magnets over the years . I only found out after 2 recones of my left speaker and had the speaker to be remagnetised .
Cheers ,
Rens
OK, it's time to sign off this thread I'd not had time to complete reassembly of the case last week, plus I wanted to let it sit for a while, then test again just in case trouble made a return visit. All was well when I powered up yesterday, so I checked the bias and set the DC offset of the left channel and put everything back together bar the last cover. Again I fed the amp from my phone, and let it play for a good 1/2 hour at decent volume to "soak test". No problems, no offset drift, no overheating, just music, so I replaced the cover and celebrated the satisfactory completion of the job.
It's been fun, even given the odd frustration. Thanks to all who offered advice and support
Ed
I'd not had time to complete reassembly of the case last week, plus I wanted to let it sit for a while, then test again just in case trouble made a return visit. All was well when I powered up yesterday, so I checked the bias and set the DC offset of the left channel and put everything back together bar the last cover. Again I fed the amp from my phone, and let it play for a good 1/2 hour at decent volume to "soak test". No problems, no offset drift, no overheating, just music, so I replaced the cover and celebrated the satisfactory completion of the job.It's been fun, even given the odd frustration. Thanks to all who offered advice and support
Ed
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