Pair of MK II's, Restoration Project

Hello forum, please call me Tim as it is what everyone else does. I am rather new to this forum, have been lurking for a while though. I am embarking on a slow journey of MK II restorations. I hope I can be helpful to those here and tha this project will be helpful as well.
I hope I am posting this in the right section otherwise please move this for me or let me know where I should post to.
I have been a Dynaco fan since 1969 and decided that the Dynaco would be my ultimate system, and over the years I have collected a lot of vintage audio gear, but not what I really wanted. Retirement helps one focus on the "IT'S Time" aspect, so I bought a pair of MK II's of Ebay to fiddle with. I did worry about the transformers and took a chance on these. Both units have many many issues, but the Iron is good. With that being the case, I now begin the journey of restoring these pups to workable condition.
I go slow, getting old and eyes not what they used to be, on top of that I am in the middle of a move, unpacking state finally, and getting my workbench put together.
Anxious to know what is ahead I opened each up on the kitchen table to get a feel of the conditions inside. So far this is what I have found, Unit #1. Kit build, fairly decent work over all. There is no 25 pF capacitor in this unit (6AN8 pins 2&6), there is no 12 ohm cathode resistor (EL34 pins 1& pins go directly to ground.
As I look over the manual I notice that the image of pictorial 2 actually shows this way of wiring yet the schematic shows them 12 ohm.
As I proceed I like to gather information that gives me a feel for what the amplifier has been through, the history that can be ascertained. I get the impression that this unit was wired by pictorial rather than documentation.
Both transformers DC test good, the power transformer no load is 416-0-418, 6.38 and 5.26 @ 120 Input. The output transformer shows a normal output on 4,8, and 16 ohms with an input of 20 Hz to 20 Khz (HP 400C)input, testing each tap to the center tap. I know that the real test will be with high voltage and Under load.
This seems to be the gentler of the two units and I will refer to this as Unit #1.
The 2nd unit has seen a much harder life, right from the beginning. Again this unit does not have the 25 pF capacitor on the 6AN8, and does have the 12 ohm resistor from the cathodes to ground. On my visual examination I began to find poor solder connections, cold joints. With a little wiggle of the component or wire, the joint would break away and the solder would crumble. Again the transformers tested the same as the first unit, so I will proceed forward using the original transformers.
I was glad to find a post by Bob Latino of the letter Dynaco issued in 1957 for the upgrade to the Mark III circuit. During this restoration my plan is to implement this on both units.
I did test the quad capacitor on both, and as expected one section is shorting. I believe that the quad caps are original in both units, manufacture of Astron. Unit #1 has Ceracaps on the PCB, unit #2has the same except on of the .25 uF has been replaced with a Astron. The tube socket on unit #2 PCB has been replaced with a black body (taller) socket, and the workmanship is rather dismal. I will have to do a lot of trace repairs as well here.
I will get the images I have so far resized and set up ma hosting account as well for the images.

Tim

Tim – Good luck with your project. I restored a pair of MK IIs and the photos are on the Photos section under WLT.

From you description is sounds like you have the early, wider circuits boards. They did not have a place for the 25 pF feedback cap. Mine did not either. Later ones had the narrower MK III style board where you could install the 25pF cap. If you are going to modify the MK IIs to be MK IIIs then you need to decide if you are going to change all of the differences or just some. There is nothing really wrong with a restoration to original specs. Give that some thought. One thing is to determine how good the circuit boards are. Many have lifted traces and are broken. That alone could make you decide to obtain new boards. The earlier, wider boards are not available as new ones (if you find them let me know).

The 12 ohm resister does not change how the amp works much but is vital for measuring the total current thru your output tubes. The DC voltage across it needs to be adjusted for optimum sound and long tube life. Much is written and discussed on this forum so follow their guidance and you will be fine. You need both amps to have the same value resistor.

Plan to re solder all connections and test all parts during your restoration. For my amps I felt I needed to replace all capacitors and any carbon composition resistor that was over 2% from original specs. The results were great. As long as your transformers are good….come up with a good plan and you will be pleased with the results.

Thank you for the thoughts WLT & peterh
I will look for and read this thread WLT much appreciated.
Yes on the PCB's being the wider style, and is helpful in determining the age of the units. I ndid consider the restoration to the factory specs of the time, but am choosing to go ahead with the Mods as per the letter Dynaco released as it is in keeping with the original design per the manufacturer. I still struggle with this thought, but as peterh indicated, the selenium rectifier is an item I prefer to do away with for safety and other reasons. I have a manual for the MK III's that shows in the pictorial the use of a silicon diode and a 2 post tie bar in the place of the selenium. Since I am making this change helped me decide to go forth with the complete update. This will include the GZ34, 11.2 ohm cathode resistor and a 12 pF capacitor. I will also change the bias circuit to the 1K--10K Pot---18K to ground, Adding a 50 uF 100 V electrolytic to ground after the silicon diode junction, and reduce the 100 uF capacitor to 50 uF on the wiper of the bias pot. I also am changing the 50 ohm 10 Watt resistor to the 200 mA 1.5H choke. Effectively all changes thaat brought the MK II up to the MK III standard.
I will also be making a few minor changes as well to the power supply, to protect the rectifier and to ease the current surge at power up. I have all the components on hand for all the changes, as well as all new resistors and capacitors. Examining the tube sockets indicated the need to replace them so will change them to the ceramic style. I will probably replace the PCB tube socket as well, and am deciding whether to go with the ceramic there or wait until Dynakitparts has stock on the original style. I would rather (and will) wait to make this change so that I will only put the stress on the PCB once, and not have to redo this again later.
Mention of the copper foils, and the new boards would need to be refitted to mate to this chassis, my thinking at this time is repair the current board(s) as needed. I have the materials to do PCB foil repairs, or at worst case is to work up the art work and etch a replacement board here. There are a lot of available mods that are all good in their own way, but is not within the flavor of what I hope to accomplish here. This may have to be reconsidered later, depending on how the process goes on each unit. If I have to make a change on one, then it has to be mirrored into the the other unit.
I mam still working on the bench, only half set up at this point, and way too many boxes yet to unpack. I will be working on the chassis first, while I still put away parts and assemble the rest of the bench(s). I am still working on (Humor) how to save and re-apply the patina to the chassis.
Spent half the day looking for the cable for the camera, to get a pic or 2 up here. I may have to order a new one since it is special usb type.
Again thank you both for the kind words and helpful thoughts
Tim

It sounds like you have your rebuild plan in place. Complete to MK III specs. OK. I few comments.

The selenium rectifier has to go not mater which path you follow. That and all new capacitors is a must. I like the polystyrene for feedback caps and the PIO caps for coupling and screen grid bypass. Others can be fine as well.

All new resistors. Great. Now which type. Read up on the three major options. Each will sound slightly different.

Now you will be using 6550 or KT 88 as output tubes. Lots of good selections available. Same with the rectifier tube and driver tube. I have tried the socket adapters and used 6U8 and 6GH8 with good success. Teh 6AN8 are just fine as well.

Some of the early chassis had the old style fuse holders that were not very good. Plan to use the MK III style which are just about bullet proof.

If your covers are beat up or rusty the matte black spray paints look very close to original and will really make them look nice. No MK II red labels are available so try to salvage yours and mask them off well.

Hey WLT: Thank you for the insight.
I see we both are on the same page.

The selenium rectifier has to go not mater which path you follow. That and all new capacitors is a must. I like the polystyrene for feedback caps and the PIO caps for coupling and screen grid bypass. Others can be fine as well.
No issue on the selenium of course.
I have played with many quality capacitors and will not skimp on anything in the audio path. I have found that the Cornell Dublier 940C series capacitors have a really nice sound, comparing to many of the upper end high cost ones. I do have both Auricaps and Sonicaps in my parts bins, but do not think in the values necessary for this amp. It is something I am going to play around with it later as well.  Right now I am using the metal film resistors, and will make a decision of what direction to go (Carbon film/ Metal oxide, or original carbon comp), it will depend on the sound once the amps burn in a bit.
I am going with a very long term plan for the restoration, with the most time spent after these are up and running, to learn just how much these smaller changes will make in over all quality. I am heading this direction since I know that my ear is quite different than any one else's ear, part of being human and all that variation being human presents. I also think that the choice of tubes will have an impact on some of these directions as well.
I also have adapters to roll tubes with, and may fiddle with the 6AN8  later on, but the first goal will be the original spec'd tubes and sit back and learn what I hear for a while.
As far as the cages go, I do not think I will be touching them at all. They seem to have been either put away for all the years or they were repainted at some point, but I can not find any signs of being of reworked, so I will probably just store them away. I figure that they had to be repainted though since they are black and not the Dynaco brown. The labels are rather nice as well.
I received my camera cable today, ordered a spare if the other shows up and am setting up an account to store photos. Hoping that later tonight I can start adding the photo's as I know that many a good comment can come from what is seen and not said.
So for now, back to polishing a bit and preparing the bench for the next step.

Tim

After your last response I have a few more comments

Capacitors – It sounds like you have a plan to try a few options as you go. I am doing that by putting different type caps in the different Dynaco amps I own. Your way is probably better as you can really compare better.

Resistors – The three options I meant were carbon composition, carbon film and metal film. Most consider the metal oxide to not be up to the sound quality of the first three. If you look in the photos section for my MK II, MK III and Mk IV you will see the carbon comps in the MK II and Kiwame carbon films in the other two. When I rebuilt the MK IIIs in 1990 I used metal film. I removed them in the 2018 rebuild and put in those Kiwame resistors. I like these better. The metal films are the tightest tolerance, are plentiful and very reasonable cost. The carbon films are close in tolerance but the 2 watt Kiwame are more money.

Covers – Great news on the condition of yours. Many do not get that lucky. The original MK IIs were all black covers. It was the MK III that made the switch to classic brown. Sometime around 1972/3 they changed the MK IIIs to black for the rest of the production. I purchased my MK IIIs new in 1974. At least that is what I have found. If you find out differently let me know.

Finally it can become time for pictures. It is amazing how things get packed into who knows where or what box when you have to move. I ordered anew cable for the camera since it has a special usb connector to it and none of the standard USB plugs I have would fit. So I now can put pictures with my posts.
I think these units are really good shape for 64 year old amplifiers. I see that they will need a really hefty cleaning and buff shine to rid the 64 years of oxidation. I will have to address the age spots the chassis's have so a bit more than a buff shine is required. So with out further adieu....
Unit #1





And Unit #2




Note the chassis on this unit has 3 extra holes on the back, something I will have to address.

I will post more photos later, as I get to know the forum and what is preferred as to how many and what method is best to post images.

Tim

Great amps for your project. Both have the fuse holders that should be replaced. The MK III style holder has a screw that will go thru the chassis to hold it down. You will need to remove a transformer to do so. IThe transformer will need to come out to mount the choke anyway. Both are in pretty good shape with minimal corrosion. These are cadmium plated not painted. When you try to restore the surface be careful not to go to deep to quick. They will never be perfect but should clean up nicely.

Amp 2 - Once you remove the non standard parts from the back panel the holes can be filled with plugs from Lowes or Home Depot. It is good that t they are on the back. One of mine has holes in the front face from a previous owner mounting a non standard choke.

Looking forward to seeing your progress.

Thank you for the reply and input WLT.
Yes on both accounts of remove transformer to mount the choke and fuse holder. I will have to remove everything to clean the chassis up anyway. I know I can get the hole plugs, and may try doing a repair  (attempt) to see if I can fill the holes somehow. I am considering brass washers and brazing them into the holes, then prep the chassis for labeling, and guild over the hole area. I am not sure of the labeling over the gilding yet. Experimenting will be necessary.

Unit #1: Chassis

I am getting some advice on the bottom plate from the supervisor here.
I am glad to hear you confirm my suspicions that the chassis are cad plated, it was what I figured for these.

On removal of components I found another expected issue, in that the cloth leads from the transformers will need to be replaced, and sure enough it is something that is required. Any small movement on the leads you can hear the internal coating crack and break. I will not use compromised wires here so the replacements are a must. I will keep with the era and use cloth cover vinyl wire rated at 600 Volts for these.

Power Tran now onto output tran

I am adding rubber grommets to the bell housings for additional protection. After 64 years with out them and no issues I realize that it is not necessary, except for peace of mind.

More to come....

Tim

Amidst all the activities of moving and such, I have been trying to blunder ahead with this project. So along with waiting to get camera cables n such I have been forging on, just have to get caught up on here now. So tonight I will try to get a bit up to speed, at least on the cabinet process.
I wound up having to go down to 400 emery on both, due to both chassis were bent some, around the transformer region, maybe dropped at one time. So I had to do a bit of truing up. I was not able to return things to the factory state, but for these units it will do fine. Flat surface at least can be polished. I will have a 1 inch x ¼ inch surface dimple that will be visible from under the transformer. Even at 400 both still retain this small dimple line. If I had opted to go back to 200 or possibly 120 maybe this could have been removed, but I chose to stop at this point as this is small compared to what the total removal would end up looking like. I am erring on the side of caution.
Unit #1


I ran through the grits 400-5000 before switching to polishing. I was curious if I could obtain a mirror or semi mirror finish with the grain of the steel being so predominate. I will let the pictures tell the rest of the story.

and Unit#2




Unit #2
Hole plugs… I did try to braze brass washers into the holes, and for as well as it went I was not happy with the results, I do not like what looks like patch work. Hole plugs begin to look more favorable to other options. I did some digging about to see what hardware was unpacked, and found my hole plugs, all the wrong sizes, and happened across a 1/4-20 blind bolt. So I played with this option and as I continued to dig, ran across a couple 1/4-20 steel countersunk flat head bolts and the bell went off on what to try. I carefully countersunk the openings of the holes, testing the bolt heads in the holes and was able to go for a flush fit without too much countersinking. I liked this patch job much better than the washers and decided to fill the slot in the head and buff that out. With a bit more work on the slot I think this will look... ok. I can still opt for another plug this way. I will leave it to the “Jury is out” for now.


Unit#1
I decided to try no label the first unit, to see how well I could do at this point in my life. Believing it went well, until I took the bug eyes off and saw things from a distance. What I used to do is not available to me now, since my eyes and hands could not keep a straight line doing the transfer method. So I will have to make a template, and have heat transfer I will try next. I also had tried a silver foil label method, but visually it did not look good. I can use the computer to make the transfer and this way I will know it fits better and remains straight. One of the things that impacts the label is the C—8--16 terminal strip, and since the transformers are the A-431 type, I want to use the mounting screw for the C and wire the strip for a 4–8--16 and a thumb screw cor common on the left mounting screw. This way I do not make any changes to the chassis and utilize the current hardware as well.



In my next post I will go through the process of the PCB tear down and repairs….

Impressive. The polishing came out much better than I thought it could. Nice job.

I now have mine out and playing. I will wait to see your final products before I decide how far to go with mine. Mine have more corrosion on both the chassis and transformers than yours did. Still they could be real nice. I am thinking yours will be top notch when completed. Keep us posted.

My circuit boards are in rough shape - working but not the best. I would like to have new replacement boards and restuff them. I do not ever find these wider boards on line. Maybe developing my own is what I would have to do. I am looking forward to seeing what you do with yours.

I decided to finish up the #2 unit cabinet with the exception of the labeling. I still have find my materials for that, in a box yet somewhere.


I am satisfied with the final results of the overall chassis, even with a bit of fog around the edges and 2 small spots from the dent and a warp. Most of what will be visible with parts though are fairly clear. Them hole plugs came out ok actually. I wound up using a flat head blind bolt and buffing it out to sort of blend in with the chassis.


Once I finish with labeling I will do a clear coat over the chassis to extend the life of the polish. I will mask off mechanical areas, screw holes and transformer feet, to insure a good electrical contact though. I noticed on the chassis before polishing that there were many areas that had some arcing between parts mechanically affixed. This is normal but can be improved on with conductive grease or in my case I will use conductive mat. HA HA when I find that too.
I sent you a PM WLT, and thank you I appreciate your comments.

Tim

I have been a miss at keeping up with things here, and slowly getting back to working on the amps. I have had a few personal interference's and still have a few more that will slow the project down.
Since my last post I have not had any polish time, and should get back  to that this week.
I also ordered new quad caps (550V/600V) and thanks to Kevin making sure that they were delivered into my hands rather than being a package stolen from the entryway. I have been reflecting on the PC Boards and considering whether or not to repair or replace them, and searching for dark brown PC board material has hit brick walls. I have been able to get brown, but is the lighter brown in color.
Over the next few days I will be installing the back bench finally in the shop room. That will help a whole heap doing this project.

Delay's delay's delay's
That is always the way when you have something you wish to work on.
I was not able to spend any time this last week on the restoration.
I see yet a few more days of delay with things to deal with.
NOT Bored!

At least the second bench is coming together, and will keep the mess at bay...?really??
The good part is all the thinking and planning I do when unable to be productive.
So with that we will see! what the rest of the week brings.