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    Here we go...novice checks voltage Mk III

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    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Here we go...novice checks voltage Mk III

    Post by wildiowa on Fri Jun 22, 2012 8:49 pm

    Ok, I've done enough amateur work on two other Dynas and ready to graduate into checking voltages on my inoperable MkIII. It is old old old, like from the 50's or early 60's and looks a bit like the spaghetti build you guys have been goofing on lately. I have the voltage chart from the manual but want to double check a couple of things...this is the first time I've done this so please have patience....

    - Pin 2 and 7 of the KT88 are both 6.3v AC, not DC
    - Pin 3 and 4, 475v DC
    - Can I measure pins 5 and 6 (-55v DC) with my digital multimeter or do I in fact need a "vacuum tube volt meter"...also, will the meter register with a visual "-" when i test negative DC voltage?
    - Pin 2 of the rectifier tube is 490v DC, correct? not AC...
    - Pins 4 and 6 of the rectifier tube are 430v AC...holy crap thats a lot of voltage
    - Pins 4 and 5 of the 6AN8 should be 6.3v DC. Only other measurement on that tube is the 1.0v DC off pin 9
    - Eyelets 4, 5, and 6 are pretty high voltages and are measured in DC

    If you guys could clarify or confirm my assumptions I'm off and running...will report any variations or results and see what the next step is. Thanks. IA

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Here we go...novice checks voltage Mk III

    Post by Bob Latino on Sat Jun 23, 2012 9:40 am

    Hi,

    You do have the correct voltages mentioned in your post. You can use a digital voltmeter to measure the -55 VDC off pins 5 and 6 of each output tube. The exact measurement that you get on pins 5 and 6 will depend on the setting of the bias potentiometer. Set the bias pot to the approximate center of its rotation when you measure that -55 VDC on pins 5 and 6. Both tubes should measure between -50 and -60 VDC on both pins 5 and 6 of each output tube.

    The other thing to consider is that Dynaco used 117 VAC line voltage as the basis to measure voltages. In 1957 when the Mark III made its debut, 117 volts was the standard AC line voltage in most parts of the country. If you operate the amp on today's 120 VAC line voltage, then the voltage readings you get may be higher than what Dynaco specified in their amp assembly manual.

    Bob

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Here we go...novice checks voltage Mk III

    Post by wildiowa on Sat Jun 23, 2012 12:18 pm

    Thanks Bob...I am the last home at the end of a rural line. My multimeter cycles every two seconds and a read at the wall outlet this morning fluctuates between 121 and 123vac. Quite a difference between the old 110v or 117v line voltage and appliances from years past.

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Here we go...novice checks voltage Mk III

    Post by wildiowa on Mon Jun 25, 2012 2:00 pm

    Have tried twice to power the Mark III up to check voltages but fuse has blown both times before I can even get that far. Not immediately, however. Seems to be powering up and tubes begin to glow then after 10-15 sec. or so fuse blows with a visible "zap" in the fuse glass. Tried a 5 amp last time and still failed. Have replaced the board caps and bias caps but not the quad cap. Tubes tested OK when I started messing with this but have had a couple failures since then. I have a speaker connected when all this happened but no input. Thoughts? Thanks....

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Here we go...novice checks voltage Mk III

    Post by Bob Latino on Mon Jun 25, 2012 2:12 pm

    Try a new or known to be good 5AR4/GZ34 recifier in the amp and see if that cures the problem. Just because the rectifier tube "lights up" doesn't mean that it is good ..

    As a general rule ...
    1. If a fuse blows instantly when you turn the amp on either you have a wiring error or the power transformer is bad.
    2. If a fuse blows after 10 to 30 seconds either the rectifier tube is bad or the quad cap or some other high voltage component is bad.

    Bob

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Here we go...novice checks voltage Mk III

    Post by hawaii.ken on Mon Jun 25, 2012 2:43 pm

    IMHO the original quad cap should always be replaced first.

    So should the selenium rectifier.

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Here we go...novice checks voltage Mk III

    Post by wildiowa on Mon Jun 25, 2012 3:44 pm

    Thanks to all...the rectifier tube is new as the one there was broken...changed over to the diode, but have not yet done the quad cap. I know it is like the most important thing to start with but avoided it because it is one of the higher cost components and thought I could limp through but appears that is the next step. To do it right I suppose I should I get the higher voltage one from Dynakit parts. Is it worth the extra $$$...or is there an equivalent? Man this thing is an old old timer, very early model. Thanks again...

    Bob Latino
    Admin

    Posts : 2411
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Here we go...novice checks voltage Mk III

    Post by Bob Latino on Mon Jun 25, 2012 4:23 pm

    Yes - On a Mark III which runs a B+ between 475 - 500 volts you should use the higher volt rated (550 VDC) quad cap that Dynakitparts sells.

    Bob

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Here we go...novice checks voltage Mk III

    Post by anbitet66 on Mon Jun 25, 2012 8:22 pm

    Since no one else said this, I will:

    Shocked Never use a higher rated fuse like you did! Shocked

    You are asking to fry something much more expensive than the fuse. I'd disconnect the quad cap and try again with the proper rated fuse to see if it holds this time. My guess is the filter cap is shot.

    Actually a new rectifier tube, quad cap, and diode for the bias circuit are the best starting points. That will most likely get the amp up and running enough to see what else may be needed (like a recap, new resistors, weak tubes, etc..).

    hawaii.ken

    Posts : 157
    Join date : 2012-01-31

    Re: Here we go...novice checks voltage Mk III

    Post by hawaii.ken on Mon Jun 25, 2012 10:48 pm

    wildiowa wrote:the rectifier tube is new as the one there was broken...changed over to the diode
    What do you mean when you say "changed over to the diode"? Replacing the tube rectifier with solid state diodes is not recommended, if you did this there would be an extra 20-40 volts at the quad cap.

    wildiowa wrote:To do it right I suppose I should I get the higher voltage one from Dynakit parts. Is it worth the extra $$$...or is there an equivalent?
    It's safer to use the higher rated quad cap from Dynakitparts because your line voltage is higher than 120v. I don't recommend using NOS quad caps even if you can find them.

    There are alternatives to replacing the quad cap such as: http://www.classicvalve.ca/dynaco.html#MK-IIIP

    wildiowa

    Posts : 126
    Join date : 2012-03-19

    Re: Here we go...novice checks voltage Mk III

    Post by wildiowa on Mon Jun 25, 2012 11:36 pm

    Yes I was referring to the selenium rectifier mod and have replaced it with the diode. I have another Mark III in which a guy replaced the rectifier tube with a diode setup and I went back to stock. I will spring for the better cap from Dynakitparts as it will handle the higher voltage. Thanks!

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