[Joeztech]

Here is the Dynakit Stereo-70 that I acquired recently. I am in the process of bringing it back to life. I spent time cleaning the chassis and removing a Rotron fan that had been added to the right side of the top cover, tightened the contacts in the output tube sockets, replaced the selenium bias rectifier with a 1N4006 silicon rectifier. Inspection showed that one output transformer was bad. A new transformer is on order from Heyboer Transformer Co. I have a selection of new resistors on order from Antique Electronic Supply. I have some film type capacitors on hand already to replace those. Bfrohwein gave me some used 6CA7s to use for a test. I just finished making circuit trace cuts and jumper wire connections to convert the OEM board to use 6GH8 tubes yesterday. I did a recheck to make sure I got the connections right. The cathode, signal grid and plate are the connections of the pentode section of the driver/inverter stage that change connection. I intend to order a power supply board from Joe Curcio to replace the can electrolytic with individual electrolytics. The rectifier tube could be changed to silicon rectifiers, but for now I think I will keep the tube.

Joe
   
   

[Phlogiston]

You are lucky that the plating on the chassis is in good shape.

Are you going to run it with the cover off?

[Joeztech]

Phlogiston;

Yes, I like the looks of the unit without the top cover. The chassis does have a lot of small pits here and there and the bottom cover that wraps up over the left and right sides has even more plus some rust on the left side. However, if I want to, I can get an 18 gauge stainless steel new chassis with bottom cover for about $125 and after I get this unit going and checked out I might just go with a new chassis and use a star grounding system. I like to keep a watch on the output tubes just in case something begins to go wrong. I was lucky that the chassis cleaned and polished up as well as it did.

I just finished installing new 0.1mFd @ 630VDC polypropylene caps and also the .05mFd caps were replaced by .047mFd @ 630VDC polypropylene caps.

Joe

[codefox1]

I think the chassis looks fine, but you might want to take the bells off the original transformers, strip and repaint them.

[Joeztech]

Codefox1;

Yes, that will be one of the steps of the restoration. Also the leaves of the transformers need to be brushed with a wire brush and repainted too. There is still a lot to be done. I just sent another order to Mouser Electronics for some additional resistors and some electrolytics for the power supply. I plan to use tubular electrolytics under the chassis and will probably leave the existing can where it is. The existing electrolyic began at a relatively low 360VDC at the filter section the output of the choke connects to and gradually increased to about 390VDC over about 10 minutes. I do not trust the existing can electrolytic.

As to what finish to put on the transformer bells, I might go with a different finish than the black normally used. I am considering using some of the satin nickel I used on the Philco 40-201 I restored. Alternatively there is an "almost chrome" paint available that would make the transformer bells look almost like the nickel plating on the chassis.

I plan to make some detailed impedance measurements on the two transformers before I install a new transformer. It will be interesting to see how much different the two measure in that way.

Joe

[Phlogiston]

AES does sell a replacement for the original cap, though the specs on it are not that great. There are several sites that sell improved versions of that filter cap as well.

[gregb]

Hey Joe:
Let us know how that replacement transformer sounds in the ST70. I have an original and a new manufacture kit and they both sound good but they do sound different. I am curious to know what difference if any you will be able to notice with a new one and an original on the same chassis.

Gregb

[Joeztech]

Gregb;

Will do.

Yesterday I tried using an external audio signal generator to drive the Heathkit IB-2A impedance bridge, but found that at frequencies below about 600Hz the output of the generator was insufficient to drive the bridge to levels where it could be calibrated following the operating instructions of the bridge. I will have to use an amplifier to build up the signal to a higher level before I can complete those tests.

Joe
I spent some time today cleaning the bottom cover of the amp and put some clear plastic feet on it that will raise it about 3/8 of an inch above the surface it rests upon. This part of the outer sheet metal is pretty badly pitted and there are some spots of rust showing, but overall it still looks far better than it did to begin with.

Joe

[Joeztech]

I have posted pictures of the restoration at: http://s1213.photobucket.com/user/rubal1...t=2&page=1
I did remove and paint the transformer bells. In the last pictures you can see the results of the painting effort. I used the "almost chrome" product from Krylon. The power transformer bell showed considerable rust on the inside. I sanded that to clean metal and then sprayed the inside with clear Krylon lacquer to protect the interior surface. The exterior of all the bells had to be sanded down to clean metal and then repainted. Most of the components on the PC board have been changed to new parts using metal film or carbon film resistors plus polypropylene capacitors. The PC board was modified to use 6GH8A driver/inverter tubes. I have replaced the AC fuse socket and installed a fresh 3A fuse. The AC switch was repaired. The Rotron fan was removed as was the top cover. A new output transformer is being made by Heyboer Transformer Co. Next step will be to replace the first two sections of the electrolytic can capacitor with new radial units under the chassis on terminal strips. two 100mFd @ 350VDC units X 2 will be stacked with equalizing resistors across them to bring capacitance to about 68mFd at over 600VDC both before and after the B+ choke. I will take some pictures of that parts install and details later.

Joe

[Ron Ramirez]

Joe, please tell us more about this Krylon "almost chrome" spray paint. Exactly what is the name on the can? I like it...I might want to try it out on some small parts sometime.

[Pillo69]

In exchange for a selenium rectifier 1N4007, you can adjust the Bias ??. I remember having to change the resistor values ​​pin terminal 7's for a proper fit.

I kept the original PCI, I placed a new PCI with orange capacitors and metal film resistors.

[Image: http://s27.postimg.org/9k26p24hf/DSC_0178_1.jpg]

I also put a diode between the pin's of the grinding valves for longer life.

[Image: http://s11.postimg.org/reuo63byb/Tube_Re...de_Mod.jpg]

[Joeztech]

Ron and Pillo69;

I made a mistake when I called the Krylon product "almost chrome" it is instead their "Premium Original Chrome" #1010 or on the bar code label 24504 01010. There is also a Dupli-Color product #CS101 their bar code 26916 33101 which is identical in color and final finish. The Krylon product comes in at least an 8oz. can while the Dupli-Color comes in an 11oz. can.
One tip on re-using spray paint, I have found that the newer cans no longer provide for being able to clear the spray nozzle on the can. So I started putting a small dab of heavy grease over the opening that seals in the volatile parts of the paint and prevents the nozzle from stopping up. To use it again, shake as usual and wipe away the grease, then do a spot spray to open the nozzle full again, then proceed to paint in the usual way. The newer cans have a feature that allows the user to spray items holding the can upside down and thus you can no longer clear the nozzle by doing so, hence my trick of sealing the nozzle with heavy grease.

Joe

[Joeztech]

I received word this morning that the new output transformer from Heyboer Transformer Co. is on its way and should arrive tomorrow. I also ordered a new chassis from Dynakitparts.com and it should arrive some time next week. I have some new SS slotted pan head 6-32 screws on order which are also being shipped. I need to get some new octal tube sockets on order. When I finish the unit will be somewhat like jacking up a hood ornament and sliding a new car underneath. However, I want the unit to look like new and this will be the easiest way to get to that state. I will not have to worry about stainless steel hardware corroding like the OEM nickel plated hardware and chassis.

Joe

[Joeztech]

Today I received the Heyboer Dynaco 470, NTS-8093 transformer. Boy am I irritated! This is not at all what I expected. I expected a duplicate for the Dynaco A470 complete with primary center tap, two screen grid leads and two plate leads - a true ultra-linear transformer. Instead what I see is a unit that has only a primary with center tap and two plate leads - a Williamson type transformer. I have sent them an e-mail explaining that I cannot use this as it is not an ultra-linear transformer. Yes, I could install it and get it to work, but it would never be what Dynaco intended for the design and I will not compromise on that. This will have to go back and I hope they will be willing to give me credit back on my account. The other aspect is that I could have already had an off-the-shelf Triode Electronics transformer that would have been correct. This is costing me not only money (shipping to return) but time as well.

So the moral to my story is do not order a replacement output transformer from Heyboer thinking you are getting a transformer that duplicates the Dynaco design. It will not be the same design as the original.

Joe

[Ron Ramirez]

Joe

Did you specify screen taps when you ordered it?

I have had three transformers (all power transformers, never an audio...yet) wound by Heyboer. I quickly found that I needed to give them every detail I could possibly think of (physical measurements of the transformer and complete details of every winding), and never assume anything.

I hope you can get things worked out with them.