Mk III Build - little help?

Hullo All,

I recently completed an amp build;  VTA Dynakit Mk III.  Could use some assistance, first time - bone head alert, so please be gentle..
Without the rectifier in, getting filament power, all tubes light up.  With rectifier, uh-oh, mechanical hum and fuse pops within seconds.
According to info here that could mean power trafo miswire..  Any suggestions welcome.  Tried to post photo link this post. Hopefully the link will work to photo.
Maybe those with exp.'d eyes can spot obvious errors... or else recommend where to start to troubleshoot.  Thanks again, for any help
to this novice builder.

K.


Mk III photo

Pic no show ...

Best advice I can give is to go through the build instructions again and carefully (and slowly) review each connection step by step. If you checked them off first go round, just make a slash or something to keep track of progress. Make sure they're where they should be and tight. Look at each connection with a magnifying glass for cold joints and don't be afraid to repoint if there's any doubt. Not unheard of to completely miss soldering a joint too.

If you want to try a short cut ... pay particular attention to the can cap.

Also check for shorts to ground - good time to snip any excess wire to minimize any chance of that.

Double check polarity on caps too.

Not to forget - any unused transformer taps should be tied off and shrink wrapped - those are live even if they're not used.

Hang in there ... you're halfway home ...

PS ... if you do get it working with the rectifier and power banks loaded, do be sure to check all the test voltage points listed in the instructions to make sure you're in spec. Working is one thing ... working right is another.

Thanks, Skitz.

I think the link works - it's just placed at the TOP of the post "MkIII photo", instead of the bottom.
It appears active when I click on it, if others can't I'll try to repost.

Thanks

Thanks, I'm pretty sure the rectifier is ok, pulled from my working ST70.

Hi Dingo ---

I looked at your pictures after saving them off and blowing them up a bit, just before they go fuzzy to my eyes.

Your Power Xfrmer wires look OK to me.  But, to be sure .... there is a Bob Latino Colorized Pictorial of the Original Dynaco Mark III wiring ON THE FORUM. Look for the "Sticky: Free Dynaco tube pictorials" and then select Item #8. Mark III pictorial.

If you can, print it in color and save the color copy. I even took a good color copy to Kinkos or Office Depot and blew it up to the largest size they had. Then I could edit one of the blowups as I worked.  

If you can't print it in color, print it in Black and white and then hand color the wires for the Power and Output Xfrmers that are the same as the original Dynaco Mark III. Just a suggestion.

The colored Output Xfrmer wires that go to the tube sockets and the terminal strip also look good to me.  

I could not tell much about the wiring from the front panel since there was too much shadow for my eyes.  It looks like you have two speaker connections on the panel. The Yellow Output wire goes to the PCB (NFB), the Black Output wire goes to the Speaker Ground connection and also from there to one of the Ground Connections on the PCB.  If you are using 8 ohm speakers the RED Output wire goes to the speaker Hot connection and the Brown or 4 Ohm wire is insulated and tied off. (If you are using 4 Ohm speakers, my last direction is reversed).

The one thing I did see that looked strange to me was .....

(1) you have 3 test point jacks (2 reds and 1 black) for Biasing the two Output tubes.  The Black Jack is connected to the PCB (I assume to a Ground Point) and the 2 Red jacks (Black and Red Wires) go back to one the Twisted Ground Tab of the Quad Cap where they hook up to the 10 Ohm Biasing Resistors which in turn are supposed to connect to Socket Pins 1 and 8.

(2) you should have both resistors tied to the Twisted Tab, the other side of each resistor tied to Pin #1 and then Pin #8.  It looks like you have a Black wire from the socket (Pin ??) back to the Twisted Tab of the Quad Cap.  

(3) I looked at the wiring diagrams that Roy Mottram sent me when I bought his Mk3 VTA Boards and the Black or Common Test Point Jack was wired back to the Twisted Tab (ground) of the Quad Cap, not to a ground solder point on the PCB. (Also, there was no ground wire from the socket back to Twisted Tab).

Do have a look at the Colored Pictorial in the Stickey ... that will tell you whether you have transformer stuff wired correctly.

If you have questions, just ask.

Stew

From the picture, it seems that the 5AR4 socket is mis-wired. Red pair, HV, should be to 4 and 6; yellow, heater, pair should be to 2 ans 7; HV take off to input cap and choke to pin 8, which is cathode. Try this. I believe I'm correct, but I'm also human.
I run Mark 3's, and love them.
Best of luck!
Kevin

Stew,

Thanks for your thoughtful reply.
Yes, I did get the colored pictorials, and as far as I can tell, they correspond for PT/OPT.

As to speaker connects, at the negative post, there is a black wire from the OPT, also a wire which connects to lug 2 of the terminal strip.  The positive lead is connected to the 8 ohm tap, which on my amp is orange, not red.  The yellow wire is connected to NFB at the board and the brown wire is capped.

Absent a diagram, the bias connects are from best assessment of posted pics here and may be wrong.  The red bias terminals are connected to V1 and V2 at pin 8, which is jumped to P1, then to the 10 ohm resistors which tie to the quad cap ground lug.
The black wire you mention comes from a ground lug added at V1, and is per the original Dynakit instructions.  Perhaps I should remove this and route the bias ground directly to the quad cap ground lug?

In any case, I can move the bias ground off the board and route it to a chassis ground at the quad cap.

Much appreciated.

K

Hi Kevin

Thanks for your review.  My Dynakit manual has red OPT connects at 4 and 6 of rectifier (which corresponds), and yellow OPT connects at P2 and P8 of the rectifier (?).  Then 8 also goes to choke and to Quad Cap lug 1.

Is this a P7 connect a rewire that's associated with VTA mod, (or some other miscue on my part)?

Thanks

(the other) Kevin

If you haven't already done the "yellow sheet" diode mod on the rectifier, by all means do so. If nothing else, it gives you a great cheat sheet for the connections.   



If you decide to leave it stock, just slide the red leads left one pin ...

Hi Dingo --- Please check with what Kevin said right after my response.

I looked at the picture again and when I blow it up, it gets too fuzzy for me to make out the actual GZ34/5AR4 Socket pins.

Assuming the tube socket key that guides the tube into the proper position is facing the Quad Cap (3 O'clock position of a clock):

The 2 RED Wires MUST go to Socket P4 and P6 which looks OK in your picture,

The 2 Yellow Wires MUST go to Socket P2 and P8 --- CHECK THIS, it is too fuzzy for me to tell what the Pins are.

One Black Choke wire goes to GZ34 P8 and the second choke wire goes to the quad. (see below)

A wire from GZ34 P8 goes to the Quad Cap Connection where the second choke wire attaches.  

After looking at your Quad Cap connections, I/we need to know what Quad Cap you are using.

Normally with a 80-40-30-20 the following connections are made.
(1) the wire from GZ34 P8 goes to the 40MF connector
(2) the second choke wire goes to the 80MF connector
(3) A jumper wire connects the 30MF and 20MF connectors
(4) A 2200 Ohm Resistor connects between the 80MF and 30MF connectors
(5) The 30MF connector feeds the B+ of the VTA PCB

So --- please check the red & yellow wires on the GZ34 Pins (they must agree with the colored pictorial for the Mark III.

Let us know what Quad Cap you have.

Then we can tell you what the quad wiring must be since not all quad caps are wired the same way.

Suggestion --- get the GZ34 and Quad Cap Wiring correct before making any additional wiring changes.

Stew

Hi Stew,

The Quad reads/connects as follows:

Dynakit:
80 - 2nd choke wire (and red OPT wire)
40 - Connect to P8 rect (and yellow heater)
30 - B+ board connection and Jumper to Lug 3 quad
20 - jumped to 30 mfd (lug 4 quad)

I installed a 3w, 6.8k resistor (per Roy's diagram) between 80 and 30mfd lugs (2+4)

Sound kosher?

Thanks

Kevin

Hi Kevin --- Yes, that wiring sounds OK.

Just Checking --- on my Dynakit 80-40-30-20 @550Volts Quad Cap (which I just looked at)

80 is the Half-Circle
40 is the Square
30 is the Triangle
20 is the Full-Circle

Is yours the same?

What was your comment to Kevin and the ? about P8 and P7 of the GZ34. There is no P7 wiring for the GZ34?? (On the VTA or Original Mk3)

With the rectifier pulled and the Mk3 powered up, have you measured any voltages on the sockets??

There is an original Dynaco Manual for the Mk3 on the Forum or the Website.

There is a table of AC and DC voltages toward the back of the manual that you can check with a multimeter -- CAREFULLY!!! There are some high voltages present.

GZ34 Pins 4 and 6
Tube Filament Voltages
6550/KT88 tube socket Pins driven by the Output Transformer (P3/P4) and the VTA board (P5/P6).
Without the Rectifier there will be meaningless readings at the Quad Cap and VTA B+ and the Cap won't charge up.

You may have to take a soldering iron and reflow your solder connections.

If there are no wiring problems, I don't know what is causing the fuse to blow with the rectifier in.

I guess you could (with the power off) pull the driver tubes from the VTA Board, set the bias pots to their center position then put the power tubes and the rectifier in and with a good fuse power the amp on.

See if the fuse blows. If it does then the problem is not on the VTA Board. If the fuse does not blow, check the voltages at the tube sockets and see what they are as compared to the manual. The rectifier and power tubes will get hot. You should now be able to check the voltages at the Quad Cap Terminals and see what the B+ is to the VTA Board.

That should get you started,
Stew

Pull out the 5AR4 tube and measure pin 5 of both 6550/KT88 tube sockets to make sure you have a negative DC voltage to ground. Touch your black meter lead on the chassis and the red meter lead to pin 5. Adjust the bias pots, you should get -33v to -50v more or less.

Hard to tell on your pictures, is the bias diode positioned correctly with the band on the diode facing the red/black transformer lead. It also looks like you did not solder one side of the diode.

Can you post clearer larger pictures for us to look at?

Sal

Hi Stew,

Yes, those symbols correspond.
I had no connection at P7 of the rect., that was in response to Kevin's post above.

I'll try the voltage checks.

Thanks,

PS: Sal, you have sharp eyes! The diode is soldered, (and oriented correctly), it just didn't flow thru. I'll give another shot
just to be sure. Maybe I can post some better photos, these are just phone shots.... Thanks again..

Well, attempted to check voltages without rectifier, and was unable to get any consistent readings.

Though the tubes light up, and get warm, voltages at the pins were a mix of alternating low negative and positive readings,
nothing approaching the voltages specified for the locations.

Fearing the worst.. Maybe smoked the power tranny at start up?

Thanks,

Kevin

Remove all the tubes and measure the power transformer secondary AC voltages going to the 5AR4 tube and the output tubes. Set your meter to AC. Let us know what you measure.

Sal

Tubes removed, (assuming I'm measuring correctly)

OPT Red wires:  120v / 120v
Yellow wires:  4.9v / 0.0v
Green wires:   0.0v / 0.0v

Thanks

Kevin,
I am sending you a PM with my phone number. Call me when you can. Might be easier diagnosing over the phone.

Sal

Hi Kevin --- are you measuring the voltages as follows:

note -- for positive DC voltages, Red Lead on Pin and Black Lead on Ground
      -- for AC voltages, lead color should be the same
      -- for negative DC voltages, Red on Ground and Black on Pin

GZ34 Tube Socket

Yellow Wire - P2 & Ground --- this is a DC voltage, the red or the + lead on P2 and the Black lead on ground, about 450 to 500 vDC

Yellow Wire - P8 & Ground --- this is a DC voltage, the red or the + lead on P2 and the Black lead on ground, about 450 to 500 vDC

Red Wire - P4 & Ground --- this is a AC voltage, the red or the + lead on P4 and the Black lead on ground, about 400 vAC

Red Wire - P6 & Ground --- this is a AC voltage, the red or the + lead on P6 and the Black lead on ground, about 400 vAC

Across P2 & P8 --- this is a AC voltage, filament voltage about 5.0 vAC

Across P4 & P6 --- this is a AC voltage, input voltage about 750 to 800 vAC
(note: some meters do not read AC voltage this high)

IF YOUR READINGS AGREE WITH THESE, YOUR POWER XFRMR IS OK


6550/KT88 Tube Sockets

Green Wires - Across P2 & P7 --- this is a AC voltage, SHOULD BE ABOUT 6 TO 7 vAC

Across P3 & Ground --- this is a DC voltage, SHOULD BE ABOUT 430 to 460 vDC
Across P4 & Ground --- this is a DC voltage, SHOULD BE ABOUT 430 to 460 vDC

Across P5 & Ground --- this is a negative DC voltage, SHOULD BE ABOUT 25 to 55 vDC
Across P6 & Ground --- this is a negative DC voltage, SHOULD BE ABOUT 25 to 55 vDC

Check these voltages to see whether Xfrmrs are OK (there maybe some variation in values)

Stew

Sal wrote:Kevin,
I am sending you a PM with my phone number. Call me when you can. Might be easier diagnosing over the phone.

Sal

Hi Sal, No message yet. I was wrong about one of the connections on the 5AR4 socket, and have re-discovered that the cathode and one leg of the heater are connected to pin 8, and no connection to pin 7. I was thinking of other rectifier tubes, and am sorry for the mistake  . I should have remembered this, as there is a bleep-load of Dyna products around here.
Hope this clarification helps.
Kevin

I'm confused here, I spoke to Kevin (Dingojazz) for about 30 minutes on the phone but stromberg67 stated he did not get my message yet? Are stromberg67 and Dingojazz the same person?

BTW, we didn't find the short over the phone but the fuse blew instantly on power on... Kevin, just one more test if you want to try, once you get more fuses..., disconnect the red and black wire on pin 8 if the 5AR4 tube and see if it pops a fuse again with all the tubes in. You would think the fuse would blow in a in a few seconds and not instantly as the 5AR4 tube has to heat up to ramp up the B+...

Sal

Just a note to give a shout to my friend Sal, who provided some great assistance on this project via phone.

As it turns out, all the tests that we ran through (and they were many) proved to test negative (no fault detected).
Yet, the amp still blows the fuse on start up.  

Looks like I may have to have someone experienced look at it in person.

Thanks again to Sal, Stew and all who lent their time / expertise!!

Kevin

(PS- looks like the Stromberg67 is the "other" Kevin in on this thread..!)

Sal wrote:I'm confused here, I spoke to Kevin (Dingojazz) for about 30 minutes on the phone but stromberg67 stated he did not get my message yet? Are stromberg67 and Dingojazz the same person?

BTW, we didn't find the short over the phone but the fuse blew instantly on power on... Kevin, just one more test if you want to try, once you get more fuses..., disconnect the red and black wire on pin 8 if the 5AR4 tube and see if it pops a fuse again with all the tubes in. You would think the fuse would blow in a in a few seconds and not instantly as the 5AR4 tube has to heat up to ramp up the B+...

Sal

Yup, stromberg67 is also a Kevin. Not too many other Kevin's around when I was a kid, but many now. I'll use my last initial from now on to help differentiate the Kevin's.   
Kevin G.