MPX Adapter question.

[soundoc03]

I just bought a Sherwood S-3000lll Tuner with a MPX subchassis(model A3MX). I really didn't care that the Tuner looked to be in a really sorry state because I made the deal obstensibly in order to provide a near mint S-2200 with another A3MX. The current Tuner sounds great but there is an intermittent noise coming from one channel that has been traced to the present adapter. Mike Zucaro in SD worked on it and felt it was a bad diode. I subsequently had one of the AK tech guys look at it only to find that the diodes checked out fine.

Well anyway the seller of the S-3000 lll was nice enough to test the tubes for me and I replaced the bad 6AU8A and the 12AZ7. To my surprise the plug and play performance was actually very nice. Perhaps because the local Jazz station is only 30 flat miles away, this surely misalignmed tuner sounded really very decent anyway. I then exchanged the two A3MX adapters and to my disappointment the one that came with the S-3000lll made the S-2200 sound like a tin can whereas either adapter sounded good with the S-3000lll!

What gives here? Must I realign the S-2200 with the working A3MX? I wouldn't think that ailgnment would have such a profound effect on sound quality. Although the adapters are the same model and meant to be used with both of the tuners, they are a little bit different in layout and parts.

I would appreciate any suggestions from those who are familar with this stuff.

 

[spaceman]

I then exchanged the two A3MX adapters and to my disappointment the one that came with the S-3000lll made the S-2200 sound like a tin can whereas either adapter sounded good with the S-3000lll!

At least you know for sure which unit to look at for troubleshooting, so it wasn't a total waste. Maybe you're looking too hard, and need to doublecheck all the simple stuff, like grounding, cables & such? :scratch2: Good luck.

 

[goldear]

I'm glad to hear that you have made a little progress on that unit. Sorry that I couldn't help you more, but only having the MPX adapter on my bench made the trouble-shooting process nearly impossible. But as I pointed out to you, sombody had clearly re-worked the dempahsis on one channel (only) at some point, and that was highly suspect to me...

Anyway, a badly missalligned tuner can sound terrible. But sometimes tuners that are out of allignment actually don't sound too bad. It all depends on which part of the tuner is out a allignment, and how badly misalligned it is.

 

[soundoc03]

When I looked at both MPXs together I saw where someone on the MPX of the S-2200 had actually changed the pin connection to the 12AU7A tube! I resoldered the leads of S-2200 MPX(a) to the same configuration as the leads of the S-3000lll MPX(b)

Now here is where it may get confusing. The S-2200/MPX(a) was professionally aligned before I made the reconnection and prior to GoldenEar advising me to replace the GEA8 tube. The sound quailty, and tuning ability, stereo separation was excellent except for the harsh intermittent crackle that would occur after about 10 mins. The S-3000lll/MPX(b) was definately not aligned and in addition I subsequently replace the 6AU8A and 12AZ7 tubes. The sound was actually very good although the tuning abilitities somewhat limited.

So again, If anyone can really follow this, Both tuners sound fine with the "bad" MPX(a) and both then get the harsh crackles from the one chanel. Reconfiguring the pin connections on the 12AU7A tube did not appear to change anything for that matter.

On the otherhand pairing the S-2200 with the "good" MPX(b) makes it sound like a cheap clock radio. Whereas if I pair it with the unaligned S-3000lll, it sounds great!

I don't have an EE backround and clearly my medical training is of little use here, but nonetheless I am perplexed not to mention very frustrated by this experiment.

 

[spaceman]

I think if I were in your shoes, I'd hook up the ones that sound like a tin can together, for starters, since you'd at least eliminate the crackle. Then double-check all your soldering, especially for any wires on the wrong pin somewhere. I'd definitely be looking at a pic or schematic, to see where everything goes STOCK on that unit, without whatever mod was done, as it might be different from the 3000III. Sorry, I'm no techie either, just offering cheap advice & moral support. :yes:

 

[Brian]

Tuners and MPX circuits are best left to professionals or those with experience. Usually the outcome of amateurs digging into them is they become paperweights. Pack them up and send them both to someone. Sherwood was well known as having in-production run changes. So the fact that the 2 units, though the same model are somewhat different is not surprising. In fact, many a unit will not even match the schematics issued and the company tended not to publish update notices or variation bulletins. What would you expect from a company where the engineers actually controlled it all.

 

[L'wood]

Bob F. from the TIC website is working on a unit that essentially will take any tuner and turn it into one nice sounding piece, it's called the LM4500 MPX. I demo'd the prototype, and though it's no TU-X1, it did compare very favorbly to a number of other tuners. The nice thing is you can take a DX machine like the Kenwood KT-7500 that sounds like crap (to put it bluntly) and when using the adapter you now have a great sounding machine. So any tuner with the proper outputs can use it, and it IS an audio upgrad for almost any tuner out there...you buy/build it once, and can use it with most tuners.

If you're not a member it's easy to join the Yahoo group and here's a post about it from the Yahoo FM Tuner group:

Refer back to message # 35898 for Hank's review of the 1st. proto unit
of the LM4500 MPX unit Bob Fitzgerald is developing. A bit more
reading is in # 25395. Bob is doing a lot of refinement on the
product prior to releasing it but even the first attempt resulted in a
very nice unit. You can find the circuits in files and if your
interest needs more peaking..or peeking..send me an off-line e-mail
and I'll send you photos of the proto unit Hank tested.

L'wood