Linn LP12 Valhalla PS repair and Zener Diode Mod

Hi all,
I've recently discovered HifiHaven...

Reading all of those pretty cool posts made me feel a little ashamed for merely lurking and not contributing. loafing is one of my superpowers.

In the spirit of giving back, I'll...recycle another post I put up on another website. A time-honored technique employed by students of untold generations, and every consultant I've every known.

I have a Linn LP12/Valhalla that I've owned for 35+ years. As has been documented in other forums, the early Linn Valhalla boards are prone to inrush surge damage, usually taking out the fuse, R1 and bridge rectifier BR1. The original installed Valhalla board, shown below in photo #1, finally blew R1. Literally. As you can see, the inrushing power surge blew the top right off of it.

The sharp-eyed amongst you will also spot that this board is pre-Zener diode modification. In later Valhallas with the modification, the two zoners and resistor that make up the design improvement will be located in the lower part of the photo, between R38 and R3 in this photo.

#1 Valhalla blown R1 #1.jpg

Photo #2 shows the damage done when R1 blew apart. The ventilation holes show the heat scorching from R2.
#2 Valhalla empty R1 BR1 R2.jpg

Others have commented online about the 2.5W R2/R3 resistors tending to overheat, age prematurely, and drift in value, which seriously affects the PS' stability and overall performance. As you can see, these guys throw off a lot of heat, and over time, the exterior coatings are showing damage. Here you can see the damage done to the smaller R1 a bit more clearly. I'll be replacing both of these resistors.
#3 Valhalla blown R1 and R2.jpg

Conventional wisdom says to replace the wimpy 'lil rectifier with a beefier, more stable unit. Linn did. So I guess I'll replace that, too.

In the Linn forums, many opine that all of these caps must be replaced because they are "tired" and "worn out" and will cause great harm to...(fill in the blank). Hmmmm. I'm a little leery of those claims. Perhaps it's my limited experience working on these boards, but I see an awful lot of electronics from the late 60's, 70's, 80's (like this Valhalla), and 90's...that are working just fine, whose electrolytics are completely intact and functioning within spec.

Actually, I'm leery of most opinions expressed in the Linn forums. I never drank the Ivor T. Kool-Aid. But that's an exploration best left for other forums or post.

At any rate, I'll do the repair and zener upgrade, and see what happens. If the old gal works, then, "yay!" If other problems crop up, then I'll cross that bridge when I get to it. And, if it turns out that caps are blown/blowing, then I'll happily eat crow and replace as many of those unreliable f***'g parts as it takes...

To recap, no pun intended, I'll be replacing:
R1 (40 ohm resistor) with an Antherm inrush current limiter, PN SG240
BR1 with a 6 amp 1000 volt Vishay bridge rectifier, GBPC610-E4/51
R2/R3 with 7W 15K 5% Vishay wirewound resistor, (see photo below for PN)

I'll also be adding the Zener Diode modification. This includes the following parts:
2 each: Vishay Zener diodes, 68Volt, 0.5 W
1 each: Vishay thin film resistor, 0.6W 12K 1%

I sourced all of these parts from Mouser electronics here in the United States. I don't remember what I paid for the entire lot, but it was around $7 or so. With USPS shipping, I paid a whopping $12. Mouser ships quickly, so I had my parts within 2-3 days of placing my order.

In order of work, then...

Starting with a new bridge rectifier, the mighty 6A 1000v Vishay. Surely it can handle things with a tad more aplomb.
#4 Valhalla BR1 soldering into place.jpg
#5 Valhalla BR1 part label.jpg

The new inrush limiter, Antherm SG240, soldered in place:
#6 Valhalla SG240 Soldering in place.jpg
#7 Valhalla SG240 part with label.jpg

And now, the 7W resistors. They can handle the heat better, plus they're a whole lot prettier. Yeah, I said that. Prettier.
#8 Valhalla 7W reisistors with label.jpg

Of course, caught up in my soldering frenzy, I forgot to stop and photograph the resistors luxuriating in their new board...but such is life. You'll get a chance to see these forest green beauties later...

At this point, my repairs are made, and I should reinstall the board, power it up...if it will at all...and check voltages at the motor connection, making adjustments as necessary. But, no, I'm not. Careless mook that I am, I'm going to proceed immediately to the fabled Zener diode modification.

So let's take a look at the Linn LP12 setup manual. I downloaded a 1998 copy from Vinyl Engine. (Thank you, gentle Vinyl Engine people who upload your vast store houses of manuals, catalogues, schematics and other ephemera. You all do the Lord's work.) You can find the Zener diode modification on page 47.

Looks like a poor man's square wave generator to me. Okay, I didn't come up with that one. See:

First, mark the board where the mounting holes are to be drilled. You can see, faintly, the traces showing through from the other side. Before I drill anything, I double check the board, front and back, to make sure that I've correctly identified the right traces to connect with. Now, this board is not particularly complicated, but it's always good practice to measure twice and cut once. Sweet Jesus, how many times have I not followed that advice and made life more difficult for myself?!? Haven't we all?
#9 Valhalla marking board for drilling.jpg

Before I use a twist drill bit, though, I'm going to borrow a common work practice from our machinist brethren, and use another tool to start the boring process. In this case, it's a conical diamond bit. A twist bit wants to walk away from the initial point of the surface it's asked to engage. Since I want my through hole to exit the other side of the board centered on the trace, it's a good idea to take this precautionary step. The diamond bit can be pushed quickly into the somewhat soft surface of this board, with less wandering, cutting a perfect little pilot divot for the conventional twist bit which follows to complete the drilling operation.
#10 Valhalla board with conical drill bit.jpg

The holes now marked, then piloted using the conical bit (not completely necessary), are shown below. I've quickly and carefully scraped the area around the drilled hole to produce a suitably clean surface for soldering. Notice that the copper trace around the hole is clean. I've deburred it using the same conical bit. Doing so abraids away the somewhat fragile, ragged copper trace which has lifted during drilling. Do so, reduces the chance of it somehow tearing and lifting during the work remaining. Also...sigh, one of the holes isn't exactly centered. I tried. Honestly, I tried. Next time...
#11 Valhalla board traces scaped for zener mod.jpg

The parts:
#12 Valhalla zeners with part label.jpg
#13 Valhalla zener mod resistor with label.jpg

The manual says that the zeners are rated at 63 volts. The schematic says 68 volts. I flipped a coin and 68 volts won. However, if you know something I don't and honestly believe that another value is the way to go, then let me know. Not Linn sure, but 100% sure based on reliable, referenceable, sane sources...

Before soldering any of these parts into the circuit board, I’ll take a vernier caliper and measure the distance between the center of the actual holes I drilled, and lock this measurement down. I'll then manually form the parts together on a third hand vise, and use the caliper as a "go-no go" gauge. Once the parts are physically formatted the way I want them to be and test OK, I solder them together as a single module. I then solder the Zener mod module in place.
#14 Valhalla zener mod in place.jpg

So there, that's everything I've set out to do in this post. Take a look at the business end of the completed board:
#15 Valhalla board completed with all repairs and zener mod.jpg

Look at it. Now, honestly, aren't those green 7W resistors are totally sexy, or what? The photos don't show it, I know, but they really are gorgeous. I may have to order a hundred pounds of 'em and make a show-stopping helmet and breast plate out of them. I'll be the envy of all who see me.

With parts in hand, this job took me about an hour. That's because I'm slow, and I talk a lot. In fact, it took me longer to figure out which parts to order than it did to do the actual work.

This Valhalla, once reinstalled, worked just fine. In fact, it worked significantly better. For shizzle, it did, my homies. The motor ran noticeably quieter. Musical pitch was noticeably more stable. I'll live with this for awhile, to make sure that what I think I'm perceiving is actually so, before I offer any more opinions.

I hope this helps...

If I can't find my preferred Ohmite Brown Devils, those green Vishay wirewounds are my first runner up.