Restoration of ST-70 Finished w OEM Board

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Restoration of ST-70 Finished w OEM Board

Postby joeztech » Fri Sep 19, 2014 12:52 pm

Yesterday I received the last of parts I had on order to finish the Stereo-70 that I have been working on. The power supply was modified by adding radial lead electrolytics under the chassis for the first two sections that were originally in the can. The can is still there and being used, but the four sections were changed by paralleling two filter sections each for the two lowest B+ voltages. The new radials consisted of two 100mFd@350V each stacked and bridged by equalizing resistors. There is one set right after the rectifier tube and another after the choke for equivalent values of about 68mFd @ 600V or better. The OEM circuit board now has all new metal film resistors and polypropylene capacitors. The 10 ohm resistors in the input ground circuitry were all removed and replaced by wire jumpers. The OEM PC board was modified to use 6GH8A tubes as I had 51 of them on hand. New stainless steel chassis, output transformer for one channel and input jacks from DynakitParts.com took care of functional and cosmetic issues. The original chassis was too badly pitted and corroded to satisfy me or the wife. It is now a real piece of eye candy. Of course it sounds great. Bass response is much tighter and solid plus the sound stage, clarity and definition is much better than the 100W per channel solid state amp. that it replaces.

Thanks to everyone here for their help.

Joe
New Chassis w RCA jacks Tubes forum.jpg
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Re: Restoration of ST-70 Finished w OEM Board

Postby EWBrown » Fri Sep 19, 2014 8:47 pm

Don't toss out the old chassis, even the most "leprous" looking chassis can be rehabbed well enough to be used again,
just like the infamous "Gator Poacher" ST-70 chassis shown here:

Image


viewtopic.php?f=6&t=1062&hilit=poacher#p5910
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Re: Restoration of ST-70 Finished w OEM Board

Postby Geek » Fri Sep 19, 2014 10:49 pm

Wow! Great job, Joe! :))
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Re: Restoration of ST-70 Finished w OEM Board

Postby EWBrown » Sat Sep 20, 2014 6:56 am

That was one that Shannon re-habbed, into a 6V6 PP design.
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Re: Restoration of ST-70 Finished w OEM Board

Postby joeztech » Sat Sep 20, 2014 2:32 pm

Thanks for the complements!

Yes, remember seeing that sad looking chassis. Removal of surface rust and primer and some Krylon Satin Nickel paint can get one looking pretty decent. The nice thing about the satin nickel paint is that it is somewhat conductive too. I used some on the chassis of a Philco 40-201 which had a mouse nest on top of the chassis with all the attendant corrosion and chewed into wires. It took lots of parts removal, sanding scrubbing and priming and then the paint. You can see the progress on that one at:
http://s1213.photobucket.com/user/rubal ... t=3&page=1

Doing this Dynakit Stereo-70 was a trip down memory lane for me. I built one in 1964 that I used for many years. It is easy to forget just how great they sounded. The old chassis will be kept for the time being and I would be willing to sell it at some point, though it is not perfect, it does at least look far better than the gator swamp thing example. ;)

I guess now I will get one of the modified PC boards to implement the 3-triode driver/inverter stages used on some more modern versions of the ST-70, although I am not in a hurry to do so. I am still enjoying the sound from this one for now.

Joe
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Re: Restoration of ST-70 Finished w OEM Board

Postby dcriner » Mon Sep 29, 2014 4:59 pm

I really like the looks of it. Personally, I would be tempted to stay with the OEM-type driver board. You could build another ST-70 with whatever hacks tickle you fancy.

What tube brands and types did you go with? The rectifier looks like a fat-boy.

Where did you get the stainless steel transformer covers? How did you do the front labeling? Any thoughts about installing an ST-70 cage over the top of the chassis?

What kind of tube sockets? I built an ST-70 clone with garden-variety tube sockets. I wish I had used ceramics.
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Re: Restoration of ST-70 Finished w OEM Board

Postby joeztech » Tue Sep 30, 2014 5:01 am

I used Duplicolor Instant Chrome on the transformer bells (except not the bottom bell of the power transformer). I wound up buying a new chassis from DynakitParts.com made of polished stainless steel as I just was not satisfied with the OEM chassis - too many pits and rust spots. One new output transformer was also purchased from DynakitParts and I took the bells off the new one and put the Instant Chrome painted old ones on it to retain the stamped A470 nomenclature.

I just finished doing a Van Alstine mod to the input circuit to band-limit the signals coming in. I have ordered a few new polypropylene capacitors to tweak the values there to a .033mFd and 680pFd instead of a .022mFd and .001mFd, but otherwise am satisfied with the sound.

The output tubes are 6CA7/EL34 JJ tubes and the rectifier came with the unit when I bought it. The normal 5AR4/GZ34 nomenclature of the rectifier is completely missing. The only alphanumeric characters I see on it are: f32 and under that 84H5 - no manufacturer lettering is on it either. After I modified the OEM board for using 6GH8A tubes I went through the 51 of them I had on hand and found 7 that tested good and then chose the best of that group. The left channel unit is an Amperex and the right one is an RCA.

The coupling capacitors were changed to .68mFd with an additional .1mFd in parallel. The larger ones were mounted under the board. I have some new 1mFd ploypropylene flat plate capacitors on order from Mouser. These are axial lead types, yellow in color with epoxy filled ends and as long as they fit will be mounted on top of the board. Right now the bypass caps for the screen grid of the pentode drivers are .047mFd, but I plan to change those to ,1mFd.

Listening tests show that the sound stage is very good and I can hear details in the music that I could not hear with the Onkyo M282 that the ST-70 replaced. My FM tuner is a Kenwood KT-7500 which does better than any other tuner I have tried at my location. I have one local station broadcasting at 100.7mHz only 4.5 miles from my house while I listen to a station at 101.1mHz that is distant. The Kenwood brings in the distant station in full quieting in narrow bandwidth while other tuners I have just don't quite get there. Tube tuners just don't have enough selectivity to do the job. Listening to good FM broadcasts I can also hear details in music that were masked with the Onkyo M282, so it will go to the barn for utility use. I use an old Winegard FM only antenna that dates back to the early 1970s at the house and another one at the barn that was built using parts from an old TV antenna to duplicate the design of the old Winegard. The local station is on the back side of the antenna while the desired distant station is at the front end.

I listened to a good jazz session broadcast from the Lincoln Center which had lots of quickly moving bass notes and it comes through properly with well defined notes and no muddiness or hang-over. Listening to some pipe organ music shows that only the very lowest notes are somewhat lacking, hence the reason to change the input filter caps to .033mFd and 680pFd. That should extend the bass and treble just enough to suit me. The speakers are AR 2Ax units that I bought new in 1964. Those had to have new polypropylene caps in the crossover networks along with L-pads to replace the old corroded rheostats and new HiVi tweeters to replace popped tweeters. They sound as good as new now. The woofers are the old cloth surround types that had to have the phenolic rings re-glued to the baskets, but were otherwise OK.

I am considering replacing the existing bias controls with new 50k ohm ones and use four instead of only two. The cathode resistors of the output tubes will be changed to 10 ohms each and measuring points will be brought to the front octal sockets for set-up. The resistor network in the negative bias control circuit will be changed to establish the right adjustment range for the bias at the grids of the output tubes. With 10 ohm resistors in the cathode the current reading and adjustment of each tube will be easier to evaluate and output tubes will not have to be matched pairs. Two of the bias controls will be installed in the original bias control positions (for the rear output tubes) and two will be mounted behind the front octal preamp connection sockets to take care of the front output tubes. Those will be accessed through the center hole of the octal sockets. I can mount the controls on a small piece of aluminum held by standoffs secured behind the octal sockets.

Joe
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Re: Restoration of ST-70 Finished w OEM Board

Postby dcriner » Tue Sep 30, 2014 9:36 am

Thanks - good information.

My FM tuner is a Kenwood KT-7500 which does better than any other tuner I have tried at my location.

I use an older Kenwood KT-2001 (solid state), which works fine.

FM music, particularly classical, is much too compressed for my taste - ppp sounds like p and fff sounds like f. Therefore, I use an expander (dbx 1BX-III), which helps immensely. Vinyl and CDs normally aren't compressed. I think that FM stations' target audience is car radios with competing background noise from traffic and road noise. Used expanders often show up on eBay. I complained to a classical FM station in Chicago, and the broadcast engineer told me it was outside his control - the DJs adjust the compression, possibly to achieve a minimum dynamic range as shown on their instrumentation.
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Re: Restoration of ST-70 Finished w OEM Board

Postby Auricle » Tue Sep 30, 2014 2:38 pm

VERY nice !

I used Duplicolor Instant Chrome on the transformer bells ...


That turned out very nicely. Please let us know how well the paint tolerates the heat.

I will be building a Budgie SE in the near future, and so I am looking for alternatives to cover the hideous Edcor blue end bells.

Regards,
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Re: Restoration of ST-70 Finished w OEM Board

Postby joeztech » Tue Sep 30, 2014 2:46 pm

I am lucky to have WRR FM to listen to from Dallas. It is primarily a classical music station, but as you note most all FM stations use some compression. However WRR is not as bad as some. Their audio signal level varies much more than other stations in the area. Their audio is notably quieter and lower level than others and the dynamic range is greater. They also broadcast a HD1 signal with the same program as the analog signal and it can achieve CD sound dynamics. Most all LPs I have seen do incorporate some degree of compression in order to provide for closer spacing of the groove and extend play time. I do have one Command record of pipe organ music that was made back in the 1960s using 35mm tape and special Scully cutting heads to maximize the dynamic range of the recording. Unfortunately, I never did find a needle/stylus assembly and cartridge that would track the highest velocity portions of the recording without break-up and distortion. I tried cartridges that tracked as low as 0.5 grams and some that went higher say to 3 grams. I tried conical, elliptical and Shibata styli and never found a combination that would do the task. Fortunately CDs came along and after some initial problems in the early years the quality of audio now available on good recordings is better than any of my LPs. Of course, we are always subject to the expertise of the recording engineer. Some do much a better job than others.

In a pinch WRR FM can be streamed over the internet, but I prefer to listen using my FM tuner I may look for an audio expander to use. Thanks for the recommendation.

Joe
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Re: Restoration of ST-70 Finished w OEM Board

Postby joeztech » Tue Sep 30, 2014 2:56 pm

Auricle;

I first tried Krylon's Original Chrome paint, but found that the surface after curing was rather soft and would smudge with even the touch of finger tips. The Duplicolor Instant Chrome has a tougher finish when cured - perhaps because it was made for automobile service conditions. I think for long term protection, it may be good to put a top coat of clear lacquer over them to help them stay in good shape.

I am operating the amp in open air without the cover. I like to see the tubes in operation. The open air operation should help keep excessive exposure to heat away from the transformer bells. We have no young children in the house, just an older cat who does not do a lot of jumping about so I don't have to worry about him getting shocked by the B+ present on the PC board. I installed equalization resistors across stacked B+ electrolytic capacitors in the power supply and when the unit is turned off the electrolytics quickly discharge. If it should turn out that the cat does ever hop up on the top of the stereo cabinet and get shocked I might buy a protective cover to go over the components on the PC board. I could fabricate a clear plexiglass cover and mount it using standoffs with cutouts for the tubes. That way the components would still be visible. For now, I think I will leave it as is.

Joe
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Re: Restoration of ST-70 Finished w OEM Board

Postby Auricle » Wed Oct 01, 2014 1:32 am

Hi Joe,

The plexiglass cover for the PCB sound good - I hope you fabricate this "pre-need".

If it should turn out that the cat does ever hop up on the top of the stereo cabinet and get shocked...


I fear this may happen only once.

I may follow your lead to keep my cat away from my ST-70 and lose the metal cover. I like the look of glowing tubes - it's nearly as comforting as a fireplace, and nearly as warm ;-) Unfortunately, cats are attracted to warmth.

Best,
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Re: Restoration of ST-70 Finished w OEM Board

Postby cheehpogi » Fri Mar 13, 2015 6:38 am

Nice build, can you snap some pictures of the under chassis? Thanks.
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Re: Restoration of ST-70 Finished w OEM Board

Postby dcriner » Sat Mar 14, 2015 6:10 pm

Bass response is much tighter and solid plus the sound stage, clarity and definition is much better

Please help me. What is "tighter and solid" bass response? What is the definition of "sound stage"? Can "clarity and definition" be quantifiably measured or described?
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Re: Restoration of ST-70 Finished w OEM Board

Postby joeztech » Sat Mar 14, 2015 6:32 pm

Doug;

When I say the bass is tighter, I mean that contrary to the original design which had tubby, muddy sounding or boomy bass, it now sounds correct when compared to a live performance. It starts and stops when it is supposed to and does not impart any distortion to the sound. As to sound stage, it has an open clarity that allows one to locate the physical position of voices and instruments, perhaps best described as a holographic sound. All I can say is you have to hear it to experience it. If you have the chance to listen to some live performances in auditoriums, concert halls etc. then come home and hear the same or similar music on your home system, you should be able tell where performers are located. In my system I use two sets of bookshelf speakers, both 8 ohm units. Two of those are AR-2Ax speakers and the others are custom speakers using similar acoustic suspension 10in. woofers, 3in mid-range and 1.25in silk dome tweeters. The AR-2Ax speakers did have to be repaired a couple of years ago. I used recommended parts mentioned on the Classic Speakers Page and was very happy with the results.

The ST-70 is connected to both sets of speakers in parallel and tied to the 4 ohm output terminals of the amplifier.

Joe
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