Hello Everyone,
I just recently installed the SDS Capacitor replacement board on my two Dynaco MKIIIs.
These amps were bone stock, factory assembled units.

After installing the SDS boards, I fired them up, and attempted to bias.
What I found was that I could not get the bias voltage (measured at the stock test point) down to 1.56, about 2.4 is the lowest I could get it.

So I started checking voltages. What I found is that I'm only getting 372 volts DC at pin 8 of the Rectifier tube. The PT is putting 425 Volts AC onto pins 4/6 of the rectifier. Even if I disconnect the wire going from pin 8 of the Recto to the SDS cap board, I still only get about 390 VDC.

Bad rectifier, you say? I have 4 GZ34 rectos and all give me the same reading.

I must be missing something simple, because, like a moron, I did the mod to TWO amps at the same time, not firing one up and leaving the other for reference. Both amps behave exactly the same, so I 'm thinking I missed something simple...

Before me messing with them, they both worked fine, biased to 1.56 V, The Recto was out-putting around 500 Volts(seeing that is what scared me into replaceing the Cap can.) Everything was cool. Let me see if I can upload some pics of the boards inatalled into the chassis.

Thanks for any help.
Mike

thanks for the reply Bob, sorry to not get back sooner, I was testing some things out to make sure I'm not missing something.

Yeah, I know, that sounds impossible.

I get the full 5.3VAC on the rectifier heaters.

this last week, I was testing these rectifier tubes out, usinf a guitar amp that uses the GZ34 Rectifier. That amp (fender deluxe clone from Weber) puts out 282VAC (measured from pin to chassis) off the Power Transformer. I tried all four rectifiers, and also a Weber Coper Cap rectifier, and they all worked, putting between 355 and 385 volts CD out onto pin 8 of the rectifier.

I checked and recorded all the voltages:
PT:
Red leads = 425VAC measured from pin to chassis.
Yellow = 5.6 VAC measure across pins
Green = 6.5 VAC measured across the pins
Red Black Bias = 56 VAC and near 80 VDC after teh Selenium Rectifier.

This is really making me scratch my head.
I'm going to check it out some more tonight.

One thing that seems weird to me, and may provide a clue:

On the dynacos, I get no reading if I measure across pins 4/6 on the rectifier tube. If I measure from one pin to the chassis, I get the 425 VAC that's expected. On yellow and green heaters, If I measure from pin to pin, I get the full 5.4 or 6.5 VAC, ie the full swing. If I measure pin to ground, I get half voltage, 2.6 or 3.2 VAC.

Now, I think this is mostly normal, but is it weird that I can't read a voltage across pins 4/6, but only from one pin to ground?

Reason I ask this is because on the Fender amp I was comparing, If I measure the PT output pin to pin, I get 565 VAC, the full swing, but if I measure pin 4 or pin 6 to ground, I get half swing, or 282 volts.

THANKS!!! I know this must be something we can figure out.

Hi Bob, I'm home for lunch and I checked it out.

My meter says right on the front, 650V max. I never noticed, I'm bad apparently.
The meter looks like it's maxing out or malfunctioning when trying to read that large voltage.
It dosen't read "0.00" or anything like that. I'd take some hunting to check the meter against another, or procure another. Maybe I should just buy a fluke...

I know that the PTs worked right before my SDS board install. They biased properly, I had 500 plus volts on pin 8 of the rectifier. It sgotta be something I did, cause both amps are doing the same thing.

I'm going to desolder the lead of pin 8 on both amps and check this stuff again.
Perhaps my meter is crapping out at 525 V that the recto is putting out and giving me a bad reading.

But, if my meter is innacurate at large voltages, is must be better down low, like with the heaters and the bias voltages, and anything under 400 or so, it reads like I'd expect.

Which makes the fact that I can't get the bias voltage below 2.4V at the test point even weirder.

Obviously, I need to check these high voltages (825V from the PT and 525 Volts on pin 8 ) with a good meter. Gimme some time and I'll do that and get back.
Thanks for the help, please post if you have any thoughts.

Can I send you a few pics to check out? I tried posting them, but coundn't.
Thanks

Mike

Hi Bob,
OK, I procured a Fluke 175 this weekend.

The readings with the Fluke are all the same as with the Radio Skack DDM, with the exception that I can now read the AC vlotage on the primarys of the PT, it reads 850VAC.

As before, EAch individual primary reads about 425 VAC (terminal to chassis), the heaters on the rectofier socket give 5.3 VAC terminal to terminal.

Today, at lunch, I unsoldered the lead off of therectifier tube (to be clear, there are 4 wires now hooked up to the rectifier tube, 2 red primaries, and 2 yellow heaters, all from the PT)

I get 412 VCD on pin 8 of the rectifier. I tried two rectifiers, and got the same voltage.

On the other amp, I get 390 VAC from Primary to chassis, about 775 VAC across the primaries, and about 395 VCD on pin 8 of the rectifier, again, without the rectifier connected to the rest of the circuit.

This makes no sense. You probably think I'm crazy. I'm going to work on posting some pics on Photobucket or something.

I'm also going to buy a new rectifier to test.

Thanks for the help
Mike

Here are a few pics. they show:

the Amp before I touched it (factory wired)
The amp with the SDS board mounted, but no wired moved
The amp with the SDS board installed and connected
The old Cap Can as it stands now, unhooked.

All I did was move the leads off of the Cap Can, and sent then to the SDS board,
Namely, I moved 2 choke wires, 1 Rectifier output wire, 1 red PT center tap wire, the wire from point 5, and the wire from point 6 on the circuit board. That's it.

Here are some pics: http://s1237.photobucket.com/albums/ff479/Mikeyv1231/

Mike --

I realize that this is an old thread, but did you ever solve the problem?

I ask because I think I have a similar problem with a "sort-of-stock" Mark 3 and a SDS Cap Board that I just put into an old Mark 3 that I recently acquired.

By "sort-of-stock" I mean the driver board was replaced by me with a modern version of the old board and the Quad Cap and its extra dangling electrolytic caps were replaced by an SDS Cap Board (also by me). Oh, I also replaced the selenium rectifier and the bias caps with a 1N4005 diode and 2 new 100MF/160v electrolytic caps.

I have done all of this before on other Mark 3s and not seen any problems. Maybe I was lucky??

If you solved your problem please let us know!!

Stew

NICE!! It's good to see this come up.

No, I haven't fixed them yet, they're just sitting, collcting dust, and I'm listening to my back-up Onkyo, sounds terrible.
So are you saying you have the same problem? Where you cant get full rectified voltage out of the rectifier? I'd love to begin talking about this again, because I dont know how much more Onkyo I can take.

Hi Mike -

Yes, I definitely have some sort of a rectifier voltage problem on the Mark III. On pins 4 and 6(red Wires), I am seeing the nominal 425v to ground, but on on Pins 2 and 8 by (Yellow Wires) themselves I am getting almost no reading at all. Sometimes the tube heaters are functional and sometime the tubes are dark.

On Friday, I had tubes glowing plus low voltages on 2 and 8 and also had very low sound coming out of the speaker I had hooked up.

Monday morning I had the same conditions, but in the afternoon I had dark tubes, no voltage on 2 and 8 and no sound. I am starting to think I may have a power transformer problem also, since the Tranny wires are real faded and brittle??

If it wasn't for Bob's Colored Wire Pictorial of the Mark III, I would have a very hard time telling what wire is what color.

Oh well, tomorrow I will change out the 45+ year old power switch and start working myself downstream and see what happens.

Stew

Mike --

Just to let you know, I changed out the power switch and now have glowing tubes and the very low sound from the speaker. The glowing tube condition seems to be permanent, unlike the case with the old switch.

That is all I had time to do. I will try to get back to it next week.

Stew