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Old 15th November 2004, 04:11 AM   #1
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Default buzzing ST-70...plea for help

I'll try to make this short.

Had a stock ST70 with Curcio driver board. Worked great.

Ordered Curcio premium driver board, assembled, but not the premium power supply.

Bad news is that the installation instructions for the premium driver board assume you are rewiring the amp from scratch (total rebuild) and using premium power supply. I had new output sockets so I undertook rebuild approach.

Spent a frustrating week bouncing back and forth between the original ST-70 manual, the Curcio restoration manual, the Curcio cap board manual, and the Curcio premium manual, as well as several emails to Joe Curcio.

Finally thought I had it finished, powered it up and watched a resistor on the premium board connected to C1B flame out. Realized I had misread Joe's email and connected C1B directly to the power primary and not the output primary ends that connect with the choke.

Fixed the wiring, replaced the resistor and fired it up. Both channels buzz. Buzz doesn't become audible until about 10 seconds after power up. You can hear the buzz modulate a little bit as well. Buzz loud enough to hear across the room, louder in one channel than the other. Furthermore, with 10ohm bias resistors, one channel warms up to 1ohm at the lowest bias setting.

Buzz only disappears when I remove all four 6922 driver tubes. Spent this last week doing a back and forth with Joe, checking and rechecking the wiring.

Joe thinks the problem is not the driver board and that I should send the amp to him.

My question is, if it isn't the driver board, what is it? Could it be any of the iron? I guess it has to be the cap board. Anyone have any suggestions before I pack this thing up and send it to Joe?
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Old 15th November 2004, 04:40 AM   #2
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Default Now for my rant

I'd like to consume a few bytes with a rant:

I think this will be my last kit job. From now on I'm building everything from a schematic. My grounded grid pre that I just built came out beautifully. When you don't understand all the subsystems involved and their interaction with one another, it's pretty much impossible to fix yourself.

Right now I'm slogging through B.Rosenblit's and M.Jones' tube design books on which I will base all future tube projects.

Thanks for your time.
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Old 15th November 2004, 11:06 AM   #3
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Sometimes the mistake can be very small and we tend to overlook as such. I have had several problems in the past and I usually set the instructions aside and redrew the schematic from what I saw in front of me and this seems to do the trick. Frank has always told me to draw it out. This is a pain but it does provide some answers in difficult times.

What to do... Check the voltages to see if they are spot on. Check the coupling caps for leakage. Note these readings on your hand drawn schematic for analysis. Remember that we are DIY people that can overcome any challenge before us.

You will find the problem.

I recently bought a used and abused Dynaco Stereo 70 off ebay. The fuse was blown, the bias wouldn't adjust, the bias would go into runaway. I was upset to say the least. I was at the end of my rope and someone offered to purchase the amp from me. This motovated me to fix the driver board so the amp worked properly.
When I had fixed the amp I wasn't satisfied with the sound so I purchased a driver board (VTA S70) and stuffed it. I installed the board and rewired the amp. It sounds great now.

Joe
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Old 15th November 2004, 11:46 AM   #4
SY is offline SY  United States
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I would suspect a cap, given the abuse. There might also be something dead in the regulator. There are two pieces of good news:

1. It's VERY unlikely to be a problem with the iron (that's fatal).

2. My new mantra, courtesy of burnedfingers, "Remember that we are DIY people that can overcome any challenge before us."
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You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
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Old 15th November 2004, 04:51 PM   #5
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I did completely rewire the amp so there is a very good chance I overlooked something but I did juice the driver board's B+ with 360VAC.

The hard part is that I don't really have a wiring reference to ensure that I didn't miss something. All the critical wiring tasks where done with email instruction from Joe which often leaned toward the terse side. I could imagine the hand holding was becoming a bit tiresome so I didn't push him for answers.

I have a scope but I don't really know how to use it. I couldn't sleep at all last night which is odd for me. I read Morgan Jones' fault finding chapter and then got out of bed for some late night probing with the scope.

My DVM reports acceptable voltages but 34mVAC at the right output and 200mVAC at the left output. The buzz is louder on the left.

I managed to make the scope do something vaguely reminiscent of what I've seen on TV.


Click the image to open in full size.
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Click the image to open in full size.
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Old 16th November 2004, 02:25 AM   #6
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burnedfingers, I followed your saga while I was awaiting the delivery of the driver board.

Any direction I think of going seems endless. Should I just start at C1B, replacing parts around the fried resistor? TIP-50, LM317T, zener diode, etc.
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Old 16th November 2004, 02:36 AM   #7
SHiFTY is offline SHiFTY  New Zealand
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You could check for noise on the heater rails. I think the regulator is for the 6.3V? You could try temporarily connecting the preamp heaters to a 6V battery, if the noise goes away thats where it is...

Also check that the preamp tube heaters are not floating. They should be connected to ground or referenced to the B+ somehow. But not both!

One thing I find useful: CAREFULLY tap the components with the PLASTIC end of the screwdriver, sometimes the noise will get louder and then you have found the problem.
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Old 16th November 2004, 04:30 AM   #8
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The regulator is for B+. In my confusion I connected one of the power transformer HV secondary directly the B+ terminal of the driver board so instead of ~450VDC, it received 360 or 720VAC.

Immediately R1 blew. I would wildly guess that Q1 and DZ1 are suspect.
Click the image to open in full size.

The schematic doesn't totally correct but it is the one freely available on Curcio's website.
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Old 16th November 2004, 05:39 AM   #9
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Quote:
Originally posted by ultrachrome
Immediately R1 blew. I would wildly guess that Q1 and DZ1 are suspect.
Click the image to open in full size.
Oops, I meant R2, not R1. Anyway, I'm ordering a new TIP50, zener and regulator.
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Old 16th November 2004, 10:53 AM   #10
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What tubes does this mod use?

Looks like you are on top of things now. Remember the basics first. I have found in my own experience that I often overlook the simple and that is usually where the problem is.

Good Luck!
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