Hello all - for some reason, I have not posted much here but have lurked for some time. So - a little background on myself to start.
I started hanging out at TV repair shops in the late 60's, early 70's (through my teen years) and then fully employed by a couple in the late 70's. I then moved to electronic security and worked for three different companies engineering the electronic side of large campus and prison security (perimeter, cctv, fire, audio/telephone, etc) until ~1992 when I changed careers to IT - where I remain today.
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So about a month ago, I found a beat up ST-150 locally (Rochester, NY) and decided it would be a fun project to restore and maybe mod a little. I searched the usual places for mod info, and read some interesting threads at various sites - but the one that intrigued me most was the Boak/Jung/Amer Mods published in the early 80's. So I set about trying to find the original articles.
That led me to getting in contact with Walt Jung - and after he discussed with the other authors - I was sent PDF copies of the original articles. Walt has since posted these at his web site (WaltsBlog) and turns out to be a great guy - and his site is full of fantastic information. I now need to find a replacement copy of his Op-Amp Cookbook!
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So I dove in head first to this project. It was sold as non-working. I found the left channel speaker wire broken and the right channel speaker fuse was blown. I fixed/replaced these, did some preliminary testing and found the ST-150 worked fine. The chassis and case were rusty and the front panel was pretty beat up, but it did have the meter option and the plexi meter bezel was in perfect condition. After removing the meter bezel, I was initially disappointed as the right channel meter was missing its pointer. I eventually found it floating in the bottom of the meter case. Turns out, it's simply a small (and delicate) hollow tube that slips over the movement 'stub'. I was able to save it!
Knowing the main components were good (power transformer, amp board, meters, etc) I tore it down.
I did take a lot of pictures and posted them in a quick gallery I set up for projects:
Old Tubes Project Photo Gallery
I replaced all the typical components on the amp board, while doing Stages 1, 2 and 4 from the Mods article. I followed the article pretty closely, but swapped out the recommended cap configuration listed (bypassed the recommened large value/large physical size film and outboard mounting scheme) for C1 and C6 with modern low impedance/high quality electrolytics with small bypass caps also for each.
In place of the original 2N5630/6030 outputs, I used On Semi MJ21193/94 power transistors along with higher quality sockets.
After a lot of checking and wiring validation, power-up was without a hitch. The article recommends 300ma bias current - and I found it just ran too hot. I don't mean temperature specifically, but what I found was that at 300ma - at power-off, the caps discharged (speaker thump) within 1 second. At 150ma, it was right on my target of 3-4 seconds. DC Offset was good for about 1-2mv. I also did a lot of comparison testing with my reference sources/songs and found 150ma was perfect for me - plenty fast and excellent imaging, etc.
Lessons-learned:
To Do:
Things broken/mistakes:
This has been a fun and rewarding project - and while not quite complete, I'm pretty happy with where this is going.
I am completely open to feedback, ideas, recommendations, constructive criticism and your own experiences here.
Thanks - Mark
I started hanging out at TV repair shops in the late 60's, early 70's (through my teen years) and then fully employed by a couple in the late 70's. I then moved to electronic security and worked for three different companies engineering the electronic side of large campus and prison security (perimeter, cctv, fire, audio/telephone, etc) until ~1992 when I changed careers to IT - where I remain today.
===================
So about a month ago, I found a beat up ST-150 locally (Rochester, NY) and decided it would be a fun project to restore and maybe mod a little. I searched the usual places for mod info, and read some interesting threads at various sites - but the one that intrigued me most was the Boak/Jung/Amer Mods published in the early 80's. So I set about trying to find the original articles.
That led me to getting in contact with Walt Jung - and after he discussed with the other authors - I was sent PDF copies of the original articles. Walt has since posted these at his web site (WaltsBlog) and turns out to be a great guy - and his site is full of fantastic information. I now need to find a replacement copy of his Op-Amp Cookbook!
===================
So I dove in head first to this project. It was sold as non-working. I found the left channel speaker wire broken and the right channel speaker fuse was blown. I fixed/replaced these, did some preliminary testing and found the ST-150 worked fine. The chassis and case were rusty and the front panel was pretty beat up, but it did have the meter option and the plexi meter bezel was in perfect condition. After removing the meter bezel, I was initially disappointed as the right channel meter was missing its pointer. I eventually found it floating in the bottom of the meter case. Turns out, it's simply a small (and delicate) hollow tube that slips over the movement 'stub'. I was able to save it!
Knowing the main components were good (power transformer, amp board, meters, etc) I tore it down.
I did take a lot of pictures and posted them in a quick gallery I set up for projects:
Old Tubes Project Photo Gallery
I replaced all the typical components on the amp board, while doing Stages 1, 2 and 4 from the Mods article. I followed the article pretty closely, but swapped out the recommended cap configuration listed (bypassed the recommened large value/large physical size film and outboard mounting scheme) for C1 and C6 with modern low impedance/high quality electrolytics with small bypass caps also for each.
In place of the original 2N5630/6030 outputs, I used On Semi MJ21193/94 power transistors along with higher quality sockets.
After a lot of checking and wiring validation, power-up was without a hitch. The article recommends 300ma bias current - and I found it just ran too hot. I don't mean temperature specifically, but what I found was that at 300ma - at power-off, the caps discharged (speaker thump) within 1 second. At 150ma, it was right on my target of 3-4 seconds. DC Offset was good for about 1-2mv. I also did a lot of comparison testing with my reference sources/songs and found 150ma was perfect for me - plenty fast and excellent imaging, etc.
Lessons-learned:
- Don't feel guily about modding an amp like this
- Use the best quality components you can afford the first time
- Slow down (see number 2 above)
- Ask for advice even for basic questions
- BG Micro and others are your best parts friends
- Check your 400ma meter settings before doing load testing (see #3 above)
To Do:
- Restore front panel
- Replace RCA input jacks with better quality parts
- Restore very rusty cover panel
- Replace wood side panels
- Regulated PS??
Things broken/mistakes:
- 440ma fuse in my Fluke - I left on ma scale across B+ and proceeded to do some load testing - which drew over an amp... and well...
- My Tek 365 scope gave up on me (lost its trace) mid-testing
This has been a fun and rewarding project - and while not quite complete, I'm pretty happy with where this is going.
I am completely open to feedback, ideas, recommendations, constructive criticism and your own experiences here.
Thanks - Mark
Congratulation. Dynaco power transformers are first quality, IMHO.
I didn't download the mod file since I don't own one. but I found my predecessor ST120 model melted solder when I tried to use it at PA level power for four hours. The Dynaco power rating was very much a "preheat FTC required time, test to power FTC required time" rating.
I've installed auxillary heat sinks above the washer on the output transistors, and also installed a pair of fans blowing on them. The fans and wiring are external to the case, with the fans blowing through the cover. 12 VDC fans salvaged from PCAT power supplies, driven by a 9 vDC wall transformer.
Another modification I've done is reduce bias current when the heat sinks are hot. I hooked up a 150k PTC thermister to the heat sink, connected between drain and gate of a 2n7000 FET. The Source of the fet is connected via 150 k resistor to the gate. The fet is wired across a bias component, so that when it turns on hot, it shorts out the bias component. In the stock PC36 schematic, I would try shorting out P2 the potentiometer to see if bias current goes down when you do that. If so, that is the component you want shorted out hot. Using this, I have been able to use the amp up to 18 hours per day in home use without overheating.
The stock PS regulator doesn't work properly on my unit, but it doesn't affect the sound IMHO. It only cuts out when there is overcurrent.
I did install fuses between the storage capacitors and the the power transistors, and a led to light up when it was blown. I walk around the amp some so the possibility of speaker wires shorted against frame is more than zero.
If your speakers are worth more than the amp (mine are) the Michael Bean speaker disconnect FETs are a useful modification for this split supply amp. You connect two low resistance enhancement NFETs source to source, and put a pair in each speaker hot. You drive the gates of both fets with a photovoltaic optoisolator like a IR PVI5013, or Panasonic APV1122. You drive the optoisolator with 10 ma from a circuit that shuts off current when a DC on speaker detector triggers.
I have trouble keeping old scopes working, too. Until I figure out a way to get the glued in boards out of mine to replace the electrolytic caps, I'm using a Simpson 260 VOM to read AC. With a .047 cap in series with the probe, the 2 VAC and 20 VAC scales are quite useful.
Enjoy the music.
I didn't download the mod file since I don't own one. but I found my predecessor ST120 model melted solder when I tried to use it at PA level power for four hours. The Dynaco power rating was very much a "preheat FTC required time, test to power FTC required time" rating.
I've installed auxillary heat sinks above the washer on the output transistors, and also installed a pair of fans blowing on them. The fans and wiring are external to the case, with the fans blowing through the cover. 12 VDC fans salvaged from PCAT power supplies, driven by a 9 vDC wall transformer.
Another modification I've done is reduce bias current when the heat sinks are hot. I hooked up a 150k PTC thermister to the heat sink, connected between drain and gate of a 2n7000 FET. The Source of the fet is connected via 150 k resistor to the gate. The fet is wired across a bias component, so that when it turns on hot, it shorts out the bias component. In the stock PC36 schematic, I would try shorting out P2 the potentiometer to see if bias current goes down when you do that. If so, that is the component you want shorted out hot. Using this, I have been able to use the amp up to 18 hours per day in home use without overheating.
The stock PS regulator doesn't work properly on my unit, but it doesn't affect the sound IMHO. It only cuts out when there is overcurrent.
I did install fuses between the storage capacitors and the the power transistors, and a led to light up when it was blown. I walk around the amp some so the possibility of speaker wires shorted against frame is more than zero.
If your speakers are worth more than the amp (mine are) the Michael Bean speaker disconnect FETs are a useful modification for this split supply amp. You connect two low resistance enhancement NFETs source to source, and put a pair in each speaker hot. You drive the gates of both fets with a photovoltaic optoisolator like a IR PVI5013, or Panasonic APV1122. You drive the optoisolator with 10 ma from a circuit that shuts off current when a DC on speaker detector triggers.
I have trouble keeping old scopes working, too. Until I figure out a way to get the glued in boards out of mine to replace the electrolytic caps, I'm using a Simpson 260 VOM to read AC. With a .047 cap in series with the probe, the 2 VAC and 20 VAC scales are quite useful.
Enjoy the music.
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