PA 060 bench testing

Today after tubing an ST70 I had picked up recently, and run previously, with the same tube compliment as before, the pa060 brown wires smoked after about ten seconds . I was at the same turning the bias down a bit, though i know it wasn't turned up too high, same tubes.
The transformer does not have cloth leads and has a thicker stack, similar to Triode Elect's and has been removed.
I've checked for shorts internally as described in another post I saw here,and will be gutting the amp, but want to verify the PA060 condition.

Three questions.
1. Can I test the voltages using the values given in the ST70 manual, ground to iron? I have all bare leads.
2. other than a chafing short, perhaps in the bell or where the wires come into the chassis,
which is my suspicion, could there be some other cause ?I see no wires touching in the rest of the wiring.
3. Can I do the same continuity checks for the output transformers?

The amp was been modded or a kit built by the previous owner, including a new Dynakitpart 7199 board ,with all modern parts, new bias caps , selenium rectifier replaced etc.
Thanks,
Steve

Before removing the transformer, did you try to see what would happen with all those "new" tubes removed?

I could not try the amp with tubes removed.
The brown wires insulation were cooked up into the bell. That's the extent of the damage.
I rechecked all tubes and they check the same.It's a set I keep just for such an occasion.
The guy I got it from said it "worked ,then quit".He assumed the bias point was 1.56 and was able to "get it that high" but the bias resistor had been changed to bias at 1v. Then , he said "a tube turned white". That's the condition I got it.
The 22k resistor on the quad cap had blown. It had been wired between the 1 and 3 positions.I made that correction and the amp worked like a charm for the rest of the night.

I'm thinking you have a problem in the amplifier that has caused that overload to the transformer and which may well have cooked it. But if you put in a new transformer, the same thing will happen.

What bothers me is that the brown wires on a Stereo 70 transformer are FILAMENT wires. That's strange unless there is a parts fault, a wiring fault, or frayed wiring, or an unintended short circuit someplace. Most failures occur in the B+ circuit (red wires, rectifier, choke and filter capacitors)

In general when this happens:

1. Especially if you do not have a means of monitoring the current the amp is pulling from the AC outlet, check the fuse and make sure it is of correct rating; not too big. Too many times, someone replaces a blown fuse with a 20 amp fuse that he dug out of the fuse box on his 1962 Ford Fairlane with a dented hood (that has been sitting behind the barn for forty years and now has mice in it). That 20 amp fuse will provide NO protection.

2. Make sure power is off, pull the output tubes and see what happens when power is turned back on. If the other tubes light up and warm up, that's an indication that there is a fault in one or more output tubes. If it pulls too much current and gets too hot, turn it off and pull the rectifier tube, then try again. If that cures it, the problem is either in the rectifier tube or the quad capacitor, or perhaps the quad cap has shorted and took the rectifier tube with it. If it still gets too hot, turn it off and pull the driver/inverter tubes, then try again. If it still gets too hot, shut it down and investigate the associated wiring in the chassis, which amounts to only what is on the circuit board and the bias circuit.

Finally, if nothing else checks out, unsolder the wires that are getting too hot and prevent them from touching each other. There is now NOTHING that should cause those wires to get too hot and burn up when the power is turned on; but if the transformer itself gets hot under this condition, it is probably shot or its internal wiring has chafed and is shorted.

The fuse is a 2A slow blow. Considering it had at least 1 wiring problem, unknown history and a nasty paint job,
it got a fast track to rebuild.It's already been gutted and paint stripped .
I see no shorts in the transformer wiring.Just the burnt insulation back to where it meets the windings. Their coating is pristine. I guess I'll hope for the best and rewire it and guess there was a nick in the insulation that caused the problem.
Once its back together i can do the checks and throw a clamp-meter on it.
I was hoping to see if I can testy the voltages on the bench if that and the output transformers before I got started.
Steve

Thanks Bob ,
I'll do that and post my results.
Steve

Bob,
Checks were made according to how the amp was wired.
This transformer does not use the normal colors,
For r/y, y
For br/y, or
For gn/y , purple
For r/bk, gy
All the voltages were good except the last two. I'm seeing 35vac and 36vac.
Trash I assume.

Any idea whose transformer this might be?

Is there any testing for the output transformers?

The output transformers are also miscolored.
For w/bu, and w/gn pair w and gy

Thanks,
Steve

Steve,
Lead colors indicate this transformer was made by the infamous
HandWound transformer people. Do a search on these guys who are no
longer in business...for good reasons.

Kevin @ Dynakit

Kevin,

Would that have been Dave's HandWound Transformers? He used to sell on ebay. I bought a power transformer and two outputs from him back in '01 and they are still in the box, part of a still to be completed project. He was extremely slow at shipping, taking over three months to get them to me. I haven't heard much good about his transformers, but I may try them somewhere in the future.

Tony

Thanks Bob ,

After this I'll have to agree with you.
I appreciate your help.
Steve

The thread at

http://www.diyaudio.com/forums/tubes-valves/13932-handwound-transformers-com-3.html

...mentions "Lucas Electronics (AKA Handwound Transformers)" in Pennsylvania and has nothing good to say about them:

"For awhile his secretary 'Jane' was telling me Dave was out of town. Then his best winder was sick for weeks, then his laminations were back ordered. Twice he promised they would be sent, "this week"!! All the while Roy Mottram(tubes4hifi)got his orders processed and received(same things I was ordering)so I know it's all bull!! I'm convinced this Dave Lucas has no conscience or intentions of honoring our contract concerning my x-formers."

Hi,
The business name of "Handwound Transformers" was a bit misleading
since these audio transformers were actually machine wound.
If I recall correctly ...most if not all of their Dynaco clones
were bobbin wound. I suspect these were made in China which accounts
for their low price as compared to other vendors at the time.
This guy Lucas had a history of poor customer service and unethical
business tactics. I know first hand as I purchased some transformers
from this guy....Took nearly (6) months to get my order after many
emails & voice mail messages. Fortunately, he is no longer in business.

Kevin @ Dynakit

To repair this amp,I've used a trans from a set I was hoping to use on a full restoration for myself.
Since I have everyone's attention and a blown transformer ,please excuse me for posting this ad:

If anyone out there has a used but good condition PA060 for sale I would appreciate it.
Cloth leads please. Fair price will be paid.

Thanks,
Steve

Bob Latino wrote:Steve,


Across the two RED wires > 700 - 750 AC
Across the two WHITE wires > 5 to 6 volts AC
Across the two GREEN wires > 6.0 to 7.0 volts AC
Across the two BROWN wires > 6.0 to 7.0 volts AC
From the RED/BLACK to the RED/YELLOW wire 50 to 60 VAC
From the GREEN/YELLOW to the RED/YELLOW > A low voltage (4 volts or less)
From the BROWN/YELLOW to the RED/YELLOW > A low voltage (4 volts or less)

Bob

Sorry to revive such an old post but in the chart above it says to measure from green/yellow to red/yellow and also to measure from brown/yellow to red/yellow but isn't the red/yellow wire on an entirely different winding?  Isn't it open with respect to the brown/yellow and green/yellow taps?  I'm going by the schematic of the PA 060 on Dynakitparts.com website.