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 Post subject: Dynakit ST70 Complete restoration Project
PostPosted: Apr Tue 23, 2019 2:06 pm 
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Posts: 154
This is the beginning of what is the second phase of a greater project going back several years, that being the initial salvaging of this amplifier after being found stored in a Florida attic for over 40 years. For this effort a complete video series exists where the amplifier was brought to a working order but left in the original patina also retaining the use of less that reliable parts, which have caused unpredictable issues over the years.

As a primary platform for this restoration, I've opted for the stainless steel chassis from dynakitparts.com along with new pots, switches connectors, screws and so-forth. Further work will be the cosmetic restoration of the transformers as well. An unboxing goes into each part and kit specifically. A smaller secondary unboxing comes later in this episode.

The project itself begins with the removal of the output transformers which i've opted to recolour on the ends of the cables on both sides due to fading. At that point it was safe to cut out of circuit. This process includes cable to the rectifier and the power transformer as well as the selenium rectifier, some of which have two colours. Once all cables are clipped, starting with the power transformer, each are removed one by one. Emphasis for the power transformer starts with the connections on the c-clamps. Output transformers are removed in a similar fashion, noting that the red center tap wire goes all the way to the electrolytic capacitor and needs to be pulled back on one side.

With all transformers removed the covers are unscrewed and separated so that they can be sandblasted and painted. All of the inner transformers appear ok. I take the covers to Jason's bug ranch and put the covers into the sand blaster to clean then which also textures the raw metal for good adhesion. The first painting attempt as a high gloss, didn't go well. too many imperfections in the metal, and dust and wind in the shop. A second attempt after a stripping and re-degreasing came out perfect by allowing for a light texture to come through, almost like a leather or knockdown.

Each of the transformers are masked and sanded with a Dremel for rust removal and painted as well, cleaned at intervals with spirits. During this time, custom threaded rod with acorn nuts are made for the restored transformers to replace the old screws. With all of these pieces, the reassembly of the transformers commences. Paying attention to stamp placement, the output transformers are done first, doing the measurements of the studs for the final cut. With that the covers are tightened down.

The power transformer is assembled last, also ensuring that the cover is on in the correct direction. The rod length must account for the new dampeners so no adjustments will be made at this time. Having completed the transformers, this concludes the first video in this project series.

PART 1
https://www.youtube.com/watch?v=fqVvYbyy8kk




Now in Part 2 of the Dynakit ST70 restoration project, we will begin with the choke as the first component for installation, though I thought it interesting to compare both on the Genrad 1650A impedance bridge to see that the impedance and inductance is.

SPEC
62ohms 1.75h
OLD CHOKE
63ohms 1.50h
NEW CHOKE
64ohms 1.35h

All within acceptable range, the new choke is installed into the chassis, though, the old one is definitely a keeper for other projects. All work is done with the intent on maintaining a scratch free project and with extreme care. The tightening of all screws in this project has the slot of the screws pointing in the same direction.

The two switches followed the choke and interestingly, they were not threaded like the original ones. This was helpful though and allowed the screw heads to maintain a parallel appearance. Following the switches was the assembly and installation of the gold plated input jack board.

Returning to the old chassis, the selenium rectifier, power cable, fuse and stand-off board was removed for transfer. It became apparent that a custom screw for the rectifier would need to be fabricated. As these would be put into the new chassis it was also a good time to introduce the bias pots as well. Tightening in such a manner that the flats would be parallel to the font of the chassis.

The rear jack kit was assembled at this point. This kit did not include the 16 ohm option, instead an internal standoff for 16ohm serving as a tie-off for the feedback connection. These are much better than the screws in the original kit. Upon assembly, these too are mounted. along with the end stand-offs.

Returning from Lowes with a 6/32 screw the fuse socket was installed and the screw was cut as a rod in support of the selenium rectifier, using an acorn nut as the top cap. The standoff board was installed and then I'd assembled some hardware to test the rectifier install. It looked good and was done so that the flat again was parallel to the chassis.

The tube sockets were a little tricky because they are a bit smaller than the hole. So I had to shim it with two turns of paper to center the socket whilst tightening. It seemed to work good though. Mundane is having to repeat this several times. Moving to the power transformer was welcome.

Its challenging though, guiding this heavy thing into the chassis without scratching anything with all of these wires everywhere. Somehow I managed to pull it off. Dampeners and all installed. Immediately I install for regular nuts on the bottom to hold the transformer in for safety. One benefit of the transformer is the ability to flip it to the service position.

Power transformer wiring begins with the EL34 filaments, BROWN and GREEN. This included their center taps to the double capacitor to ground. Following this will be the connections to the rectifier. Then the primary black wires off of the power transformer to the fuse and switch. Following this, is the single cable to the selenium rectifier.

At this point I go back to the old chassis to remove the star ground hardware and install it in the new chassis. The first connection to it would be the ground off of the power transformer. At this point a 3 prong power cable is introduced and wired in. The ground of this cable also meets the star ground.

With the power testing is conducted with the Heathkit IM-11 VTVM using a fraction of the voltage. Switches are tested and then the ratio's determine that the windings are ok. The center-taps to heaters were measured by resistance across the coil.

Having completed this testing part 2, has come to an end.

PART 2
https://www.youtube.com/watch?v=tN1Z_zNt4hc


Continuing with part 3 of the Dynakit ST70 restoration project, we will get right into the output transformers, though given the mass, and the nice shiny chassis, I cover everything first with masking tape in case of a mishap. All of the wires need be carefully negotiated through and the screws and nuts placed in and gently tightened just enough so the the transformer doesn't shift. Once this is accomplished, there must be an accurate account taken of the location from the end so that it remains parallel before tightening, as well as allowing the other side to be symmetrical. In the desired location, it is torques down. Wash rinse, repeat. Though the second one has the burden of matching the measurements of the first.

Sadly, having caught one wire under the chassis, I got to remove and install the same transformer, twice. I was then able to solder the newly connected transformers to the output jacks and then the connections to the EL34 sockets. The only outstanding transformer connections at this time are the red center taps to the electrolytic capacitor, not yet installed.

Having established a colour code for the circuits within the amp, I had a go first a snipped wires already in the amp from swapped components, this was predominantly the bias circuit, for which yellow was chosen. Green was chosen for Ground. Black for anything after Labor day. Also the grounds off of the output jacks were installed to the star ground.

This is a great time to pull the electrolytic cap from the old chassis, which is new, by the way. and prepare it for the new chassis. This means polish it so it doesn't look so industrial. I just ripped off the sticker and stuck it to the bottom of the power transformer. This Fiasco started at 800 and worked up to 1000 and then to 2000, and the chrome polish sealed the deal. Shiny and new, the capacitor is affixed to the new chassis, taps twisted for a tight and permanent fit.

The cap is immediately grounded to the star ground. and its terminals cleaned up, and its resistors tested. Wiring to the capacitor then begins starting from the rectifier, the HT colour will be Red. This is followed by the coil C-354. The resistors are left on the capacitor, though there are further connections being added later once the circuit board is introduced.

Smoke test time, round two. This time well take several measurements, at full voltage governed by five volts at the tube rectifier filament. This allows the testing of the bias circuit, onward through the individual pots. Also, a further inspection of the AC ripple before and after filtering shows the filter to be working just fine. A final check of the AC output on the tube rectifier socket shows an unladen value of 800 volts. Incredible.

So dropping a 5AR4 in will allow some DC measurement . The unladen rectifier output is 540VDC. This seems like a good stopping point for this Part of the project....


PART 3
https://www.youtube.com/watch?v=nOZ-ujq06yc


Now in part four of the Dynakit ST70 restoration project, I start off by demonstrating how dangerous of a condition i've left this unit in given the fact that nothing is connected to the electrolytic capacitor to drain it off to a load over time. Were talking about well over half an hour and the chassis is still lethal. Food for thought.....

With that out of the way the heaters are brought down from the EL34s to their respective pairs, I choose some nice fabric wire for this task to match the power transformer. After this it was an appropriate time to remove the old PC-3 circuit board from the old chassis by cutting it out and unscrewing it. It really required a cleaning and polishing too, so that was done before continuing. I was careful to leave short ends of the cut wires on that board to keep track of what would be reconnected to this board, I knew with the new bias controls there would be some modifications in the future.

The board was mounted into the new chassis and tightened down. The wiring immediately begins starting from the output transformer for the capacitive feedback. I chose blue for that. Then the bias pots were added to the board. Logically, the bias extended out of the board to the four EL34's though since it goes through 1K resistors on pin5, the cables go to unused pin6 on the EL34 and a new resistor goes to each pin 6 to pin 5.

The high voltage connections come off the electrolytic capacitors in red to the board, also making working on the amp now less dangerous upon shutoff, then the blue wires continue for the capacitive feedback circuit from the board out to one EL34 from each side. After this the main PCB ground is added.

Now is a good time to finally re-add the clamps that hold all of the cables in place by the transformer, so I stop to do this, since work now moved to the front of the chassis. This work now the front input jacks connects the jacks to the mono/stereo switch as well as the circuit board. These signals on input will be White.

The Bias Balance Control Upgrade, BBCU is the next part of the installation, the special kit i've ordered for this amp. I follow the instructions provided in the provided manual to do so. And remove the rest of the cur cables remaining in the PCB at this time. The only modification to this install is that I tie off all of the grounds to the chassis star ground, and I use my own thicker wire. Again, during socket installation I had to shim with paper to center them.

With the new kit installed, the cables are laced in and this portion of the project is now complete.

PART 4
https://www.youtube.com/watch?v=MKiwA-1fPgA


Concluding our project in part five of the Dynakit ST70 restoration, I point out a terrible mistake I made that could have caused some serious damage. A short to ground from the rectifier through the audio transformers would have not been fun. Having corrected it, i'd decided it might be a good idea to test the resistance across all point of the electrolytic capacitor for good measure.

Now sure there are no shorts that will destroy the amp I use the filament voltages of the EL34 as a reference to see how high the variac will be turned to get the unladen voltage correctly. Then, I turn the bias pots to full tilt to ensure minimum current draw when the tubes go in. As the Variac is introduced the overall current of the amp is monitored and the Variac adjusted to maintain 6.3vac on the EL34 filament pins. With nothing on fire, its a good sign. Also, looking at the rectifier output and shutting the amp we see how quickly the discharge of the electrolytic cap is now.

At this point the phase inverters are introduced into the mix, again adjusting the Variac against the EL34 filaments. The oscilloscope monitors the outputs of the phase inverters to verify operation. Satisfied with this, The EL34's are now introduced into the Amp after shorting plugs are plugged into the inputs and 8 phm 100W bricks are tied off to the output. To test the accuracy of the new system i'm adding my current meter inline to the tubes as well where:

.10 = 10ma
.20 = 20ma etc etc

This was conducted on all four tubes and found to be highly accurate. Knowing I can use the front jack values, biasing could begin. Biasing is the same as the old system except that the contribution on each side must be further balanced by the pot in the front.

I take this time to give the amp with the shorting plugs in a good listen to hear of there is any hum. It is almost non-existent, i'm very pleased with this. I celebrate this by adding the rubber feet to the bottom cover of the amp so that the bottom might be screwer onto the chassis. The bench is then reshuffled for THD testing. At that point I realize I should take out the Hickok 6000 and make sure the EL34 tubes are balanced in each channel, turns out they weren't so I was able to make an improvement before tests even started.

Testing would be conducted at 11.3 volts RMS which is 15W roughly at 8 phms. I'm calling that roughly half power. I do a demo of one given frequency in the amplifier. This method takes a long time, and I lost my good cables to do this, so I get some noise at around 2000hz forming in the cables, but both sides were perfectly consistent, this tells me everything is working just fine.

Seems appropriate that I end both the video and project with a listening portion. I chose some royalty free music from Bensound.com. There will be future videos with, for, and including this amp, but as for the restoration, this is where it ends.

PART 5
https://www.youtube.com/watch?v=7dr4ilFLz4I


Last edited by jcrubin on Aug Mon 12, 2019 1:17 pm, edited 5 times in total.

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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: Apr Wed 24, 2019 5:00 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4528
Location: Sunnyvale CA
Looks pretty good so far! The Dynakitparts chassis is definitely the one I would choose if I was going to do another one (which is *very unlikely).

Whose driver board/modification are you planning on using? In any case, would very strongly recommend the Van Alstine 'Super 70" modifications, particularly the input filter.

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 04, 2019 3:15 am 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
Brett_Buck wrote:
Looks pretty good so far! The Dynakitparts chassis is definitely the one I would choose if I was going to do another one (which is *very unlikely).

Whose driver board/modification are you planning on using? In any case, would very strongly recommend the Van Alstine 'Super 70" modifications, particularly the input filter.

Brett



I didn't actually. I have a modified and upgraded ST70, I decided to instead restore the original Phenalic PC-3 board and go with that in its original glory.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 04, 2019 5:26 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4528
Location: Sunnyvale CA
jcrubin wrote:
Brett_Buck wrote:
Looks pretty good so far! The Dynakitparts chassis is definitely the one I would choose if I was going to do another one (which is *very unlikely).

Whose driver board/modification are you planning on using? In any case, would very strongly recommend the Van Alstine 'Super 70" modifications, particularly the input filter.

Brett



I didn't actually. I have a modified and upgraded ST70, I decided to instead restore the original Phenalic PC-3 board and go with that in its original glory.


Original phenolic PWB? I will light a candle for you....

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sun 05, 2019 2:23 am 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
Brett_Buck wrote:
jcrubin wrote:
Brett_Buck wrote:
Looks pretty good so far! The Dynakitparts chassis is definitely the one I would choose if I was going to do another one (which is *very unlikely).

Whose driver board/modification are you planning on using? In any case, would very strongly recommend the Van Alstine 'Super 70" modifications, particularly the input filter.

Brett



I didn't actually. I have a modified and upgraded ST70, I decided to instead restore the original Phenalic PC-3 board and go with that in its original glory.


Original phenolic PWB? I will light a candle for you....

Brett



Ill admit I do find it a bit funny when folks on an antique radio board allude to "What!!! Your going with original and not new and improved". Happens more than I care to admit...


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Tue 07, 2019 6:32 am 
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Joined: Aug Wed 24, 2011 4:35 am
Posts: 4528
Location: Sunnyvale CA
jcrubin wrote:



Ill admit I do find it a bit funny when folks on an antique radio board allude to "What!!! Your going with original and not new and improved". Happens more than I care to admit...



It will play very nicely. They didn't sell 100,000 of them because they weren't any good. But the stock phenolic boards were the worst of the worst as far as fragility goes. Most of mine ended up with ALL the traces doubled up with bare copper wire, just because it was easier than trying to find and fix them one at a time. So just go carefully!

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Tue 07, 2019 1:41 pm 
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Joined: Aug Sat 01, 2015 8:54 pm
Posts: 154
Brett_Buck wrote:
jcrubin wrote:



Ill admit I do find it a bit funny when folks on an antique radio board allude to "What!!! Your going with original and not new and improved". Happens more than I care to admit...



It will play very nicely. They didn't sell 100,000 of them because they weren't any good. But the stock phenolic boards were the worst of the worst as far as fragility goes. Most of mine ended up with ALL the traces doubled up with bare copper wire, just because it was easier than trying to find and fix them one at a time. So just go carefully!

Brett


The board itself i'd restored separately a few years ago as part of the resurrection project. I believe a few 1% resistors and 2 trace repairs. In this project it was only re-polished and transferred.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Thu 09, 2019 3:37 pm 
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Location: Cortez, Colorado
I restored a ST70 several years ago and went with the stock circuit. The person I bought it from also sent me the stock circuit on a blank new FR4 board. The original phenolic board looked like it was about to turn into a giant carbon comp resistor.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 10, 2019 2:20 am 
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Posts: 4528
Location: Sunnyvale CA
Tin Omen wrote:
I restored a ST70 several years ago and went with the stock circuit. The person I bought it from also sent me the stock circuit on a blank new FR4 board. The original phenolic board looked like it was about to turn into a giant carbon comp resistor.


Precisely. They tend to burn to a crisp around the 7199, and I have seen the socket just hanging from the traces (and the jumpers that were installed when the traces broke). I have also see them where the socket just pulled loose instead of the tube - and then fixed with 1/16 aircraft plywood and JB-Weld. Hence my comment above.

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 10, 2019 3:39 am 
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Joined: Aug Sat 01, 2015 8:54 pm
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Brett_Buck wrote:
Tin Omen wrote:
I restored a ST70 several years ago and went with the stock circuit. The person I bought it from also sent me the stock circuit on a blank new FR4 board. The original phenolic board looked like it was about to turn into a giant carbon comp resistor.


Precisely. They tend to burn to a crisp around the 7199, and I have seen the socket just hanging from the traces (and the jumpers that were installed when the traces broke). I have also see them where the socket just pulled loose instead of the tube - and then fixed with 1/16 aircraft plywood and JB-Weld. Hence my comment above.

Brett


I dont see why though, the 7199 is not a monster of a tube, and for this tube, the stock socket is substantial, I believe a 3W heater. The only resistors rated over 1/4 watt are not in and around that socket either to my recollection. Most of the voltage dropping heat generation is done at the electrolytic capacitor if any, off board.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 10, 2019 3:55 am 
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I did a vta 70 board with mine, completely tore down, hammer tone painted the chassis, painted the transformers,and built a new bank of power supply caps, it too was sitting for about 40 years,my wife bought this one for 10 bucks about 25 years and I let it sit on a shelf the whole time until recently.Using Russian 7581A tubes.I felt guilty letting it rot on the shelf so finally fixed it.I don't need all the power it has (LaScala's ask for very little power) It sounds very nice,and I think the 6L6 type tubes heater draw is a little easier on the power transformer.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 10, 2019 4:16 am 
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Joined: Aug Sat 01, 2015 8:54 pm
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9050lx wrote:
I did a vta 70 board with mine, completely tore down, hammer tone painted the chassis, painted the transformers,and built a new bank of power supply caps, it too was sitting for about 40 years,my wife bought this one for 10 bucks about 25 years and I let it sit on a shelf the whole time until recently.Using Russian 7581A tubes.I felt guilty letting it rot on the shelf so finally fixed it.I don't need all the power it has (LaScala's ask for very little power) It sounds very nice,and I think the 6L6 type tubes heater draw is a little easier on the power transformer.


10$ jesus... Imagine if she bought 50. You could sell them and retire to a private island...


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 11, 2019 3:55 am 
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Even had the original mullard el34s and a 5ar4. My wife could trip over a bar of gold.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 11, 2019 5:04 am 
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Posts: 4528
Location: Sunnyvale CA
jcrubin wrote:
Brett_Buck wrote:
Tin Omen wrote:
I restored a ST70 several years ago and went with the stock circuit. The person I bought it from also sent me the stock circuit on a blank new FR4 board. The original phenolic board looked like it was about to turn into a giant carbon comp resistor.


Precisely. They tend to burn to a crisp around the 7199, and I have seen the socket just hanging from the traces (and the jumpers that were installed when the traces broke). I have also see them where the socket just pulled loose instead of the tube - and then fixed with 1/16 aircraft plywood and JB-Weld. Hence my comment above.

Brett


I dont see why though, the 7199 is not a monster of a tube, and for this tube, the stock socket is substantial, I believe a 3W heater. The only resistors rated over 1/4 watt are not in and around that socket either to my recollection. Most of the voltage dropping heat generation is done at the electrolytic capacitor if any, off board.



This is from another thread (not mine), note blackened PWB:
Attachment:
crude6.jpg
crude6.jpg [ 78.05 KiB | Viewed 1257 times ]

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Sat 11, 2019 12:26 pm 
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Quote:

This is from another thread (not mine), note blackened PWB:
Attachment:
crude6.jpg

Brett


You can see on the bottom of the board is a coating, which off to the left under point 10, some scratches have been made, easily done with a finger nail. Any amount of heat can darken this original coating, which I believe was there to cover the traces. This is not an overall indication of the board health itself. The 3W heaters over time could transfer heat through those sockets to darken those areas, if you look at the bottom perimeter you can see where this coating does not exist on this particular board. im sure the board and traces are just fine though. The top of the boards do not have this coating, none that I have seen. I would not be surprised if it was paraffin based.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Mon 13, 2019 1:21 pm 
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Part 2 has been added to the original post


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Thu 16, 2019 12:38 am 
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Maybe I missed it in there but why did you replace the choke if the old one was ok?


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Thu 16, 2019 1:17 am 
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enelson14 wrote:
Maybe I missed it in there but why did you replace the choke if the old one was ok?



Correct, The old choke did test ok. From the original work from 4 years ago after finding it in the attic, half of the wax from the choke had already melted onto the bottom cover. It didn't see viable as a long term solution.


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Thu 16, 2019 4:06 am 
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Location: Sunnyvale CA
jcrubin wrote:
Quote:

This is from another thread (not mine), note blackened PWB:
Attachment:
crude6.jpg

Brett


You can see on the bottom of the board is a coating, which off to the left under point 10, some scratches have been made, easily done with a finger nail. Any amount of heat can darken this original coating, which I believe was there to cover the traces. This is not an overall indication of the board health itself. T


Later, when it crumbles to granulated carbon, is that an indication?

These boards are absolutely notorious for failing in this manner, the picture I posted is just an early stage of the process.

It's a free country, of course, do as you wish.

Brett


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 Post subject: Re: Dynakit ST70 Complete restoration Project
PostPosted: May Fri 17, 2019 12:00 am 
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Location: Bay City Mi
Not to steal the thread... but what are these amps worth? I found one sitting in storage for years... have not tried it... audio nuts tell me these go for anywhere from 300 to 1000.... is that true?... What make them so special?
Thanks


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