New to forum looking to rebuild Dynaco Mk 2s and a 3

Hello,

I own a set of, what I believe are Mk 2s and a third amp which I believe is a Mk 3. I've owned them since sometime around 1972. I bought the MK 2s (along with a set of non Dyna pre-amps - I can't remember the company name) from the guy who built them. I picked up the MK 3 about a year later. He told me that he had built the first one before stereo turntables were available and then the second after stereo turntables were available in order to create a stereo system, which was also when he got the second pre-amp.

I used the Mk 2s and pre-amps as a stereo driving a set of Voice of the Theater style SRO Blow-Out-Proof speakers for quite a few years until the quad cap in one failed and then I put them in storage.

Some time in probably the early 90s, when I was still into HAM radio, I dug them out to rebuild, but life got in the way so they went back in storage, this time in plastic tubs in the basement of a house own but only live in 2 to 3 days a week that I keep for when I am visiting my 95 year old mother. I think there is also a Fisher tube Stereo Receiver in one of the tubs.

I am now, once again, considering rebuilding them, but, to be honest, I have forgotten everything I used to know about circuits capacitors, transistors, etc... so I have spent the last two days trying to re-learn some stuff. I watched a few YouTube videos on rebuilding Mk 3s and think I have a good sense of what needs to be replaced on the bottom, and at minimum the Caps on the top circuit board.  

On the bottom I expect to replace:
- the selenium rectifier with a 1N4007 Diode and a 4.7K Ohm 2 Watt Resistor; with a 100mfd/100v  Cap - - I also will replace the related Ceramic Disc Capacitor, 0.02 µF, 3 kV
- the on-the-top Quad Caps - I want to leave them for the original visual aspect and replace them underneath with modern Caps
- other Caps and resistors

For the boards on the top, I've seen in YouTube videos that I will have to replace the 3 Caps (of course that is depending on which board my units have and I do remember the boards are different/non-matching); but I am also considering a complete upgrade (to a matching set), and am open to any and all suggestions.

I understand the first place to start would be to dig out my amps and do a complete inspection. I'll write an update to this post as soon as I get back to that house and dig them out.

I am not sure about the condition of the pre-amps so I am also open to suggestions on usable alternatives.

I'll also need some suggestions on what equipment I will need. I have a couple of Fluke multi-meters. I have my old soldering irons but I am not sure what shape they are in and if they are usable for circuit board work. I have a monster power supply that weighs like 70 pounds that I used to use on a bench in the basement of my parent's house. I believe it is currently in the garage of the house where the amps are. I have to check that I didn't throw it out in a previous purge, and that it still works. I also have a couple of scopes in the basement of my parent's house which I still own and rent out the upper floors, but I don't have any memory of how to use them, nor know if I would need to in order to rebuild the amps.

OK, that's probably way more than I needed to relate.

Next post should be in a few days or the middle of next week.

Cheers

A bunch of stuff right off:
a) What kind of tooling do you have?
b) Do you have an accurate fine-pitch ammeter and/or the means to measure watts a few watts at a time?
c) Do you own a metered variable auto-transformer (Variac)?
d) Do you have the capacity to test tubes?

There is a lot to be said for cleaning the bejesus out of the amps, tightening all the pin sockets and then testing them very carefully via a metered VAT. The idea is to have a point-of-departure when starting down the road to a complete rebuild. The unit with the blown quad-cap is a give - that will need a new quad-cap and/or cap-board *before* any further tests are made. But the others - whether you intend to replace all the caps or not (and you should do at least this) - may yield useful information via preliminary testing, including the pre-amps. Again, clean everything, tighten tube sockets, and only then do powered testing, with all due respect for the voltages involved.

You will not need scopes at this stage - probably not at all.

You will want an isolation transformer. At B+ at/near 500 VDC, caution is useful. Isolation transformers are not perfect in and of themselves, but they do protect against certain types of issues. and are vastly better than nothing.

Very, very good light.

Eutectic solder (37/63)

Good hand tools

Dental picks in a variety of styles.

Peter W. wrote: A bunch of stuff right off:
a) What kind of tooling do you have?

I have all sorts of good quality tools from various types of pliers to just about everything else I might need, though I may have to hunt them down.  

Peter W. wrote: b) Do you have an accurate fine-pitch ammeter and/or the means to measure watts a few watts at a time?

I believe my Fluke digital meters are good for measuring.

Peter W. wrote: c) Do you own a metered variable auto-transformer (Variac)?  

I think the thing I called a power supply will function in this capacity - that is what I remember using it for back in the 70s, but I have to dig it out, if I still own it, and check it out. Then again it may be a isolation transformer. Like I said, I have to check when I get back there. Probably tomorrow.

Peter W. wrote: d) Do you have the capacity to test tubes?

I used to have two tube testers but I am not sure that I still own them. I sort of remember lending them out. Another item to check when I get back to that house.

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You say :
"On the bottom I expect to replace:
- the selenium rectifier with a 1N4007 Diode and a 4.7K Ohm 2 Watt Resistor; with a 100mfd/100v Cap - - I also will replace the related Ceramic Disc Capacitor, 0.02 µF, 3 kV
- the on-the-top Quad Caps - I want to leave them for the original visual aspect and replace them underneath with modern Caps
- other Caps and resistors "
The selenium rect replacement is avail as a kit from dynakitparts. They also have new cancaps
which will save you from a mess with axial caps below.

I do not see what the ceramic cap should be replaced. Same with a lot of other things that
arn't broken. Du a survey first ( power up, check voltages against the manual, listen for hum
and noice, and if possible, using a dummy load, see if it will deliver full power.
Inspect powercord ( you should probably replace by safety reasons), inspect power swich for
mechanical stability ( it should not break power when moved unless it is pushed to the off position)

.

OK, I got in about an hour ago. I dug out the amps, and a bunch of others that I had forgotten about. I took some pictures -

This is "MK2-#2" :


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Same amp "MK2-#2"


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One more (bottom view) of "MK2-#2"

BEFORE CLEANUP
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AFTER INITIAL CLEANUP
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MK2-#1:

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"MK2-#1"

BEFORE CLEANUP
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AFTER INITIAL CLEANUP
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"MK2-#1" circuit board

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The two preamps I have are "Scott 121-C Dynaural Equalized Preamplifier"

I also have a Scott 299 Stereo Amplifier and a Fisher X-100; a Scott Type 299 Stereo amplifier.

I do have a tube tester - it is a Precision Apparatus series 945-G


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My MK3:

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The MK3's circuit board

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and the bottom view of the MK3:

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Great projects and well worth doing. Both of your MK IIs have been modified so keeping them looking original is already not an option. A similar but slightly different goal/rebuild criteria is reasonable.

I have rebuilt both MK II and MK IIIs. Comments on the MK IIs follow. I agree with PeterCapo in that you have an early MK II (wider board) and a later MK II (narrower board). If you look at the published MK II schematic you will find a 25 pF cap on the board. The later narrower board will have the position for it but the earlier wider board will not.

It appears your amps have had at least one coupling cap replaced in each amp, a fuse added to the face of one, and it looks like the selenium rectifiers have been replaced with a diode. Hard to tell from the small pics about anything else.

It would be the easiest to rebuild both MK IIs to match each other. Once you study the MK II schematic you will see many differences with the MK III. The stock MK II had one 100 uF cap in the bias supply not two.

Replacing the four section electrolytic cap, the bias cap, both coupling caps and the screen grid bypass cap are essential for safety, durability and sound quality. I have tested the feedback caps on these old amps and find they are out of spec many times. I like to replace them but they are not as critical as the ones I listed above. Testing as many resisters (in circuit) as you can may weed out a few of them to be replaced as well. I would replace all parts in there respective positions and not try to mount underneath. I recommend purchasing the 550VDC AuthenticCap four section cap due to the higher B+ voltage at turn on from the 5U4 rectifier. The MK III use the GZ34 so they turn on slowly and the 525 VDC rated caps are fine.

Your MK III is a later version with the green board but a non stock cap is installed underneath. Not sure why but maybe to add capacitance to the power supply. Enjoy the rebuild and let us know how they turn out. Back to original they should sound great.

.

I agree with your assessment. I have not found other differences to date.

I have the earlier wider circuit boards on my MK IIs. I have 120/121 Volt AC house voltage. At turn on I can measure 540 -550 VDC on the B+ prior to the output tubes conducting. I had turn on arcing problems with the CP 525 VDC rated quad caps and had to replace them with the 550 VDC version. No problems after that.

Even my early version amps have the 10K ohm bias pot. I have not found the 5K versions but they must exist. Even though very early I am not sure why both of mine are 10K.

The feedback caps are very durable and many leave them in for aesthetics etc. My acceptable tolerance may be different than others. I found some as high as 45% off with many in the 20-25% range. To far off for me. When I rebuilt my amps I used 2% max which meant replacement of them. I think tight tolerance makes a difference. They were probably never within 2% even when they left the factory. Still 5-10% is reasonable but 45% is way off.

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WLT wrote:... it looks like the selenium rectifiers have been replaced with a diode.

Yes, they obviously have, though I am not happy with the quality of the work and will need to test the components.

WLT wrote: Replacing the four section electrolytic cap, the bias cap, both coupling caps and the screen grid bypass cap are essential for safety, durability and sound quality.

That will be what I look into doing. I also want to get both four section caps to match, as they do not presently. I can see this will take some further investigation into what someone did to replace the one.

WLT wrote: Testing as many resisters (in circuit) as you can may weed out a few of them to be replaced as well.

I'll start running a meter through the wiring and document everything before I do anything else.

WLT wrote: Your MK III is a later version with the green board ....

I haven't decided what to do with this unit yet. Originally I used it as the driver for a center channel but never really found a proper speaker (I also used it to drive a set of horns placed on top of the speakers). I am not even thinking speakers yet. I originally had three sets: the previously mentioned Voice-of-the-theater ones, a set of custom setup bookshelf cabinets, and a set of custom setup 3 way ones, but we had a leaking pipe (when no one had come to this house for a few months), and one of the 3 ways was destroyed.

WLT wrote: Back to original they should sound great.

I am not sure yet if "back to original" is exactly where I want to go.

Numbers on bias pots (if I am reading them correctly):
Mk3 = 1377230 - this would make it a late July of 1972 production
Mk2-#1 - larger board = 137649 - would that mean it was produced in 56 or 66?
Mk2-#2 - smaller board = the pot has been replaced and I can't see any discernable numbers

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After some consideration, I recognize that some of my notes documented here are incorrect. When I cut-n-pasted them over the formatting got messed up and I edited online without referencing my original document. I definitely got some of the edits wrong. I will review this and make corrections at a later date.

I spent most of the day working through the two MK2s making notes and checking that the circuits looked correct. I also removed some stuff, like that large Sprague Cap as well as some resistors and wires that weren't connected at both ends or were connected to the external pre-amp plug that were not required (I documented what I removed just in case I later find a reason why something was there).

Here are my notes on the components:

Mk2 #2 – smaller board amp
• On Off Switch
       o Broken
• No Capacitor [0.02 µF] on P782 leads – instead lead connects directly to the two different P782 cables
• Cable from P782 to rear (input/stepped up voltage) tube
       o needs shrink wrap
• Bias Pot
        •     NOT original
       o Left and right leads reversed from Mk3 setup
       o Capacitor
        •     47uf 100v
        •     Considering replacing with a 56uf 400v Cap (positive to ground)
• Selenium rectifier replacement components
       o Capacitor
        •     P782 lead to A431 Ground and Speaker ground is on middle lead and should be on right lead to match the other amp (after Capacitor is installed)
       o From right lead of Bias Pot to ground resistor –
        •     Color code = Brown Black Red Gold
        •     reads on meter as 989 (1k)
       o Capacitor
        •     47uf 100v
o Diode
        •     Properly orientated - Silver towards (P728) power supply


• On 4 Cap –
      o P782 lead to terminal on the input/stepped up voltage tube receptacle (same terminal that the ceramic choke connects to) to terminal on 4 Cap – circuit looks complete
      o Cap to Pin #5 on board and A431 lead
      o Blue Lt Blue Red = reads on meter as 789 (I believe tis should be a 6.8k ohm Resistor
• Resistor on speaker terminals – Blue Lt Blue Brown = reads on meter as 0.8
• Choke – 50 Oh 10 w = 52.6 (discolored)

• Tube 1 receptacle
      o Resistor – Brown Black Red Silver = reads on meter as 1877
• Tube 2  receptacle
      o Resistor – Brown Black Red Silver = reads on meter as 1684


Mk2 #1 – larger board amp
• No Capacitor [0.02 µF] on P782 leads – instead lead connects two P782 cables together then to ground
• Bias Pot
      o Left and right leads reversed from Mk3 setup
      o Capacitor
        •     47uf 100v - this Cap is not showing decrease in ohms on meter reading like the others do
• Selenium rectifier replacement components
      o Capacitor
        •     P782 lead to A431 Ground and Speaker ground is on middle lead and should be on right lead to match the other amp (after Capacitor is installed)
      o Resistor from right lead of Bias Pot to ground
        •     Color code = Brown Black Red Gold = Meter reads at 986
      o Capacitor
        •     47uf 100v
      o Diode
        •     Properly orientated - Silver towards (P728) power supply


• On 4 Cap –
      o P782 lead to terminal on the input/stepped up voltage tube receptacle (same terminal that the choke connects to) to terminal on 4 Cap – Circuit seems to be missing the wire between the input/stepped up voltage tube receptacle (same terminal that the choke connects to) and the 4 Cap terminal and the 4 Cap doesn’t have the extra terminal to connect it to as this is the amp with the replaced Can
      o Resistor & Capacitor to Pin #5 on board and A431 lead
             • Resistor - Blue White Red Silver = on meter reads as 7.18
            • Capacitor - 450v 40mf
• Tube 1 ground - tube terminal to Bias test lead
          o 12 ohm
          o In video this is - 11.2 ohm - 1 watt resistor
• On speaker terminals – Resistor - Blue Lt Blue Brown = on meter reads as 0.7
• Choke – 50 Oh 10 w = 52.6 (not discolored)
• Cap from Bias to Tube 1 – not decreasing through meter
• On Tube 1
          o Resistor – Brown Black Red Silver = on meter reads as 1260
• On Tube 2
           o Resistor – Brown Black Red Silver = on meter reads as 1472