HMV 5a soundbox - adjustment/disassembly

[frenchmarky]

Just got a real sharp-looking HMV 5a just to play around with and compare on my Victrola 10-50. It is in super nice condition and can't see anything physically amiss and it sounds pretty good as is, but I was wondering about checking the pivots to see that they are 'dialed in' in their adjustment. I've totally rebuilt three Victor 5a's, but have never touched an HMV before.
I don't want to do anything to it if there is a good risk of damaging it while taking off the backplate, etc.
Is there an established procedure for adjusting these pivots? Like, does the backplate need to be removed so that the needlebar/diaphragm can be checked for the right amount of drag?
Any other tips or links regarding adjusting this soundbox are greatly appreciated, thank you sirs!

Marky

[1926CredenzaOwner]

!

[OrthoFan]

The #5a and 5b sound boxes use the same type of pivot used in the earlier Victor/HMV #4.

Here's a cutaway illustration from the old Nipperhead page:

Needle Bar Pivot.JPG (29.67 KiB) Viewed 3295 times

Each lug, which fits inside the "collar" on either side of the needle bar, is held in place by tension created between the nut, on the needle bar end, and the end-cap screw, on the other. (As noted, the lugs, themselves, are NOT threaded, but slide in and out of the collars once the nuts and end-cap screws are removed.)

The simplest way, I've found, to adjust a loose fit is to spray a few drops of penetrating oil to pivot areas, and then, loosen BOTH end cap screws a few turns. Then, apply equal pressure to the screws to push both lugs gently toward the center (needle bar). While still applying pressure, tighten the nuts until they are firm, and then, carefully tighten each end cap screw.

While the needle bar should be as free as possible to move the diaphragm in and out, there should be no movement, whatsoever, in the opposite--sideways-- direction. (That leads to buzzing and blasting on some notes.)

If the needle-bar seems to want to position itself off center, check to see if the sound box casing is slightly deformed, due to pot metal swelling, or poor workmanship at the factory.

As for removing the back plate, changing out the gaskets, etc., I'd leave that to the experts if the sound box shows any signs of pot metal deterioration. I've heard horror stories of seemingly good condition back plates cracking in half after they've been removed. As it is, I believe that both the 5a & 5b sound boxes were fitted with felt gaskets, so there should not be any of the issues with "too hard" or "frozen" gaskets. I've known a few collectors who've put a few drops of oil around the parameter of the diaphragm to make the original felt gaskets more air tight, but don't know if that really works or not.

I guess I should add that if the pot metal around the area of the needle bar pivots seems at all distorted--crazed, cracked, etc. -- proceed with extreme caution before making any adjustments to the pivot lugs.

[Steve]

I'd agree with the article above although I would add that 9 out of 10 5a/5b's I've owned have had swollen back-plates making their removal nearly impossible. I have been forced to discard a few good diaphragms in the name of removing the back-plate! The pot metal expands and literally grows into the front brass bezzel, thus distorting it and putting unwanted pressure on the stylus bar mounting. This alone might cause the stylus bar to 'freeze' in its current position, preventing it from working properly.

If you are lucky enough to remove the back-plate, I would suggest filing down the inner raised rim of it (not seen when soundbox is rebuilt) that should sit comfortably inside the bezzel. More often that not these are binding very tightly due to pot metal expansion. I use a hand file and gradually work my way around the lip until it looks smooth and equally filed down around the circumference. If you're a perfectionist, you might want to repaint that strip of bare metal before re-assembling the unit, even though you'll never see it!

[frenchmarky]

I was able to get the backplate off since it is only held with 4 screws, but the needlebar seems skewed to one side too much, where it nearly touching the surrounding area and a lot of rusty gunk has accumulated between them. I got the needlebar nuts loose, but the pivot lugs don't want to budge.

I'm going to try soaking the whole pivot area of the box in some penetrating oil and see if it will loosen things up. I'll need to get those lugs loose in order to re-center the needlebar. I'm now seeing that it actually is a lot like the #4 box. Other than that it's in good shape including the felt ring for the diaphragm.

[frenchmarky]

I was able to get the pivot posts loose with a little oil and some prodding, so should be no problem re-centering the needlebar now (was skewed too far to one side and rubbing on the surrounding metal.) Need to reflow the solder on the diaphragm and I should have it buttoned back up in a few days. I do like the way you can take lift the needlebar-diaphragm assembly completely out much more easily than the Victor version - just pulling out the two pivots and removing both front and back plates.

And I know what you mean about the Victor 5As, I've had a couple that were literally impossible to remove the back plate without destroying it, and hope that the front half doesn't end up getting cracked in half in the process. Some of them might as well have been SUPERGLUED together! Even after busting up the plate to sacrifice it, even the pieces didn't want to come out. Had much better luck with 5Bs, many times the back ring twists off without any effort at all as if it had just been put together yesterday, wish they all did that. Just opened up one today like this. Guess it's the age, the purity of the pot metal used at the time and the conditions the soundbox endured.

Thanks for the info everybody!
Frenchy

[frenchmarky]

The base of the needlebar was rubbing against the base plate even before the pivots were tightened, so I dremeled a little bit of the inner side of the needlebar to more round it off. Resoldered the bar to the diaphragm, screwed everything back together and it sounds great. Both felt rings were in perfect shape as well so I was able to just reuse them.
Has a bit different tone than my Victor boxes, but then even they don't all sound alike. Nice bass and range. I might use this box on my vv-210, since it has a nickle tonearm and they match.
Now I can get to rebuilding that Victor 5B that I picked on ebay for 10 bucks. I have one extra box already. I'm starting to wonder if fixing these boxes can become just as addicting as collecting records and Victrolas! I figure who knows, the more of them you have to choose from, the better the BEST one of them is going to sound, right? : )
Thanks again.
Frenchy

[gramophoneshane]

Was the needlebar bent or something?
There should have been plenty of clearance around the needlebar & the reproducer body. Everything the needlebar attaches to, or passes above, is made of brass, so there should be no problems with the metal changing shape over time & closing clearances. The brass bezel will usually crack across one of the rear screw holes if the potmetal back has swollen, before it will actually bend the thick solid brass found around the needlebar mounts.
I suppose it's also possible that one of the needlebar mounting tubes, attached to the bezel, could have received a hard knock at some stage, causing it roll a little towards the back & closing the clearance.
I also reuse the original felt gaskets on these. I've tried new replacements & haven't noticed any difference at all in sound quality. It was also suggested by someone on another board some years ago, that adding grease or vasoline to the felt would make an airtight seal around the diaphragm, but after trying it, again I couldn't notice any audible improvement.
I have thought of experimenting, by using traditional rubber gaskets instead of the felt, but somehow I think if rubber gaskets improved the sound quality &/or volume, HMV would have used them to begin with.

I have to ask- how do you find the HMV compared to the Victor, on records from the 40s & 50s?

I've often heard people say the Victor can distort a little on later records, due to the increased frequency range used to record them, but I've never noticed this problem with the 5a/b's I've had.
Even rock-n-roll discs seem to play back without distortion, provided the record itself is not worn & causing the distortion.
Apparently the Victor box has a greater volume output, so perhaps that accentuates any record problems, making them more apparent, or it could simply be the frequencies over-ride the mechanics of the Victor box?

[frenchmarky]

The needlebar and the whole case don't seem to have any swelling or damage, just the little flat spot at the base of the needlebar was too close to the surrounding case metal for comfort, especially if the needlebar wasn't perfectly centered. So rather than try to tweak the needlebar I just did a smidgeon of shaving there and rounded it off.

Actually I did put a few drops of oil onto the felt rings to seal it up better, after that it tested out totally leak-free. On the two few Victor 5's I rebuilt, I cut out my own gaskets using that really thin white sheet-foam that people pack things in, really works great. Plus this foam does have a little bit of 'give' to it so I don't have to tighten the back ring down nearly so hard to get a full seal. It is nice because while it is foam, both sides are sealed so air cannot just leak through it.

As for sound, I've now have a Victor 5a, three 5b's, and now this HMV 5a. Maybe my 50-year-old ears are getting creaky, but after tuning them all up with new ball bearings and gaskets etc., if I had my eyes closed there is no way I'd be able to tell which one was on there, or even which model! : \ All good and loud though. I'll see if I can dig up my newest 40s-50s records and give them a test however, I only have a few records that 'new'.

[frenchmarky]

The only one of the three soundboxes I restored this week that I don't like the sound of is the Victor 5a. The HMV 5a and Victor 5b both sound great, the Victor being more strident. But the Vic 5a just has a totally acoustic sound to it, very tinny like I'm not even playing an orthophonic record. Seems to have good movement of the parts compared to the other boxes and it's sealed, but...

One of the two little prongs on the end of the needlebar had broken off when I was trying to straighten and reposition them and the remaining one is most likely weak, but I managed to get it soldered in. Darnit I guess I'm gonna have to swap in another needlebar, so I'll have to knock the pivot out of the old one to get it out of there, right? I thought perhaps there was some other magic way to get the needlebar out without having to do that but sure doesn't look like it! Hmm maybe I'll just see if I can do a little work on that end with the iron so there is more of the bar end soldered on besides just that one weak prong, will report back if this works and has an effect.