Refried Power Supply?

[Looneytunes]

Hello everyone,

I’m a NewB. This is my first thread, and I need a little help.

Being a NewB, I feel the need to explain why I’m here. I’ve always wanted to build an awesome stereo, and after toying with the idea for literally years, I decided why not?

I’m going to start simply with a SE line stage. I was totally inspired by both Bryan’s and Mark’s 12B4A threads, so that’s where I’m going to start. For the past 6 to 8 weeks, I have been reading, following your threads, collecting parts, and asking the occasional (really) stupid question. Btw, thanks for the great answers! I really do learn a lot from them.

To get me started, I decided to buy a regulated power supply, instead of building one. I bought an old Heathkit PS-3. Here is my problem... I bought it on ebay.

It was advertised as working, and amazingly it was. I measured B+ from ~0 to ~470V, and the 6.3V AC heater supply was right on the money (both measurements with no load.) I pulled the cover off to take a look, and the wires to the 6.3V terminals were toast. The insulation was completely burned off. You could also see where one of the 6.3V leads had burned through the wire leading to the B+ terminal. (I did not think to measure the 6.3V AC output for DC.) The transformer also appears to have leaked some soft wax.

My first thought was to remove the transformer, check out the damage, and (probably) fix it. But to put safety first, I thought that I should ask the gurus. I like saving $50 as much as the next guy, but I don’t want to be electrocuted for it.

So after all this, here are my questions.
First, do you think this is repairable? My main concern is safety. I can remove the transformer and (probably) insulate the leads, but I don’t know what the wax means. I really don’t want the B+ somehow being shorted to the (all metal) chassis.
Second, if this is salvageable, would this power supply eliminate the need for an isolation transformer since the B+ is floating? I understand that an isolation transformer does offer some protection from shock, and I would like that extra insurance.
And last, are there any improvements worth making? A fuse has already been added between the main and the transformer. It also seems to me that the chassis should be grounded. Yes? I am also measuring 17 mV ripple (120 Hz) at 250V. This increases to about 27mV as I move toward the minimum and maximum B+ ratings. Could I do better? Can anyone make any suggestions to possibly clean this up a bit? I’m thinking, I could add another RC stage to the filter supplying the voltage reference by moving some of the resistance around. I do plan to replace the caps as well as the damaged parts. Not to mention, the wiring is a rat’s nest. Also, all of the resistors are carbon (except for one wire wound). Should some be replaced with metal film?

Any comments would be very much appreciated, especially those related to safety and damage assessment.

Once again, thanks to all for such a great forum.

Looney

[Looneytunes]

Here is a picture of the transformer. Notice the brown "wax" at its base.

[Looneytunes]

And finally, here is the schematic.

Looney

[SHiFTY]

Yeah looks like it got a bit overheated! I have seen some where the sealing wax has leaked out like that due to heat. However if it still works, it's probably salvageable. Transformers either work or they don't, there generally is no inbetween!

I would definitely re-insulate any places where the wires are bare. That big wrapped bit of black insulation tape has got to go! Use heatshrink tubing at the very least. See if the wires look ok where they come out of the transformer.

In terms of safety, I would examine the capacitors for signs of swelling or leaking (discharge them with a resistor first!) and replace the 0.1uF cap as it is probably gone. Replace the power supply caps if you are going to use it long term, they could be dead. Also some of the resistors may have drifted in value and need replacing.

With regards to safety, just make sure that the chassis is connected to safety ground, and all wires insulated, you should be fine.

[SY]

You'll want to pull the transformer off the chassis, remove the end bells, see where the burnt lead wires are attached, then replace 'em with new ones.

Yes, three wire power cord, ground lead bolted down tight to the chassis.

[anatech]

You'll want to pull the transformer off the chassis, remove the end bells, see where the burnt lead wires are attached, then replace 'em with new ones.

Yes! What he said.

I have one of these myself. I really like it. Believe it or not, the caps are still good. I really ought to replace them at some point <lazy>.

-Chris

[Looneytunes]

Thanks everyone,
It looks like this will be my first “project”. I need to practice point to point soldering anyway.

A solid chassis ground will be my first modification.

SHiFTY,
I will definitely replace all of the capacitors. Thank for the tip on the 0.1 uF. I assumed that this was a film cap, and wouldn’t degrade too much. I’ll check all of the resistors too.

SY,
I will definitely take your advice, remove the end bells, and replace the entire burned wire.

Chris,
I was also amazed that the capacitors were still good, but I’m going to replace them anyway, just as a precaution.

I am glad you like the PS. It’s nice to know that it will be worth the trouble. Have you ever measured the ripple on the B+? I see ~17 to ~27 mV, but I don’t know what I should expect.

Thanks again,
Looney

[anatech]

Hi Looney,
Yes, when I first got it. I'll bet it improves after I service it - whenever that happens. Probably not until something goes wrong with it, just being honest with myself here.

-Chris

[Giaime]

Hello Loneytoones,

I've seen that you've disabled the email function, so I cannot write in private to you.

I wanted to ask if I could use your photos (that you've posted in this thread) in an article on tube amp repairing that maybe will be published on an italian hi-fi webzine. Can you give me the permission?

[Looneytunes]

Hi Giame,

Sure, feel free to use the photos however you wish.

I am trying to change my account settings to allow other members to contact me. Until them, I will send you the original photos in an e-mail with my e-mail address. Also, I plan to take apart the transformer sometime this week. If you (or anyone) are interested, I can send you (or post) pictures of the inside.

Looney