BELL 3DT, Phono RIAA Odditty

[scaramoucheii]

Hi All

I have a Bell Sound 3DT that I have owned since 1981

It is all stock and still sounds great, but after 30 years of me owning it, and 55 years since it was new, I have decided to upgrade some caps and resistors. I got a hold of the Sam's Photofact and started investigating. I'm listining to Vinyl a lot so decided to do a nice job on the phono section as well and replace all the components in that section, and not just the caps.

Making a list of the components I needed and looking at the schematic to get an idea what to do first, and what components can be replaced together, I noticed that the Left and Right RIAA circuits are different ??

This model also has an eq for the tape head input ( THE original Bell 3D Didn't have a tape input, but this later 50's model did and was renamed the 3DT, Anyway). The tape eq circuit is the same for both channels, and this is implmented with the selector switch, which has a separate wafer that loops in a different feedback loop for each of the tape head and, phono inputs.

TAPE EQ

On the attached schematic the right channel (top) uses C45, R46 and C28 to EQ the tape head, this network is routed to "11" on the selector switch wafer "Sect 2 Rear M4" and the output of the stage, on pin "8" of the selector switch, is routed up through this network when the tape input is selected , this is exactly the same on the opposite channel... simple enough. (C7, R12 and C8 ) NOTE the schematic is drawn differently for each channel, but it is the same.

PHONO RIAA EQ

The phono RIAA network however, right channel (top) uses C26 and R44 in series and this is parralelled with R43 and C27, note that R44 is 240K. This network is connected to pin "12" of the selector switch wafer and the output of the stage is fed back through this network when the "Phono" input is selected on the selector switch. (And the tape EQ network left Open Circuit)

The left channel is ALMOST the same ... using R12 and C8 in series and then paralled with C6 and R13, but note the difference, and extra component.

R13 is 270K (but 240k in the right channel) and R12 the 20k resistor is shunted with a 3000mmf (3000 pf, the schematic is that old that they still use micro micro farad ) capacitor.

So what's up. I have looked into the bottom of the unit and sure enough this is how it is..

So if I replace these components, I'm thinking that I should make both channels identicle, using 1% metal film resistors and 5% poly caps, but what channel to copy ?


Ideas ??

Simon

[scaramoucheii]

So,

Nobody is curious as to why one of the worlds first stereo amplifiers had an asymetrical RIAA circuit ?

And no suggestions on which channel to use as in the rebuild, or if I should change any of the values ?

The phono section currently actualy sounds decent, (Compared to my Modified PAS-3, and my 6dj8 phono stage which I installed into my SCA-35)

[Palustris]

Sams photofacts are known to have occasional errors. An example is the first stage of the Citation V schematic. This could just be another error from Sams.

[scaramoucheii]

Hey Palustris

I'll get back under the hood of the Bell 3DT and double check, it is possible that I was looking at the capacitor shunt on the tape circuit, but I'm usually pretty good at identifying things. But granted the point to point wiring isn't always symetrical either so identifying things isn't as easy as it may appear.

I'll do a more through ID of the components later in the week, but right now I'm using the Bell as my main system while my custom Pre Amp is having all the resistors replaced with 1% PRP's from Parts Connexion

Simon

[scaramoucheii]

Also I'm pretty good at the Ole BBROYGBVGW, but on a 55 year old piece of gear with 49 year old eyes I may have seen a Y when it was a V ???

I'll double check

[scaramoucheii]

Confirmed,

A couple of the RIAA network resistors are different in each channel, but should be the same I would think.

A 240K, (Red, Yellow, Yellow) in one channel

And a 270K (Red, Violet, Yellow) in the other. As seen in the attached photo.

Also the additional capacitor that shunts the 20K resistor in only one channel is there, and visable in the bottom right hand corner of the attached photo.

So, not an error in the Sams PhotoFact. The physical components are they as per the schematic.

SO, as I replace the components in this Vintage Classic, what channel do I use as a model for the new resistors, and why were the 2 channels not identicle to start with ?

[DF96]

The right (top) channel is wrong, as it is missing the HF rolloff which C7 gives the left channel. Unless for some peculiar reason this rolloff is inserted in a later stage. Does the right channel seem rather 'bright' on phono?

This could be a 'Friday afternoon' design - assuming such things happen in the USA?

[scaramoucheii]

Hi DF96

Thanks for the comment, I will add a C7b to opposite channel when I replace the components. But why the different resistor values ?

The sound is fine, The Phono sounds better than the CD player. (As it should )

I was listening for a audible difference, but could not notice anything. Most of the components are 10% and after 55 years, the two channels are most likely not that close to each other anymore anyway.

This was one of the first commercial stereo products, a real gem of a collectors item now, and it is surprising that the engineers would have missed something like this. For some reason it must have been an intentional circuit. The Bell 3D was based on an earlier Mono design. Maybe I should have a look for that schematic and see what the RIAA looks like.

The unit I have is actually a 3DT , a later unit from the late 50's that had a Tape input, and a EQ network for that input. Perhaps I should also look at the schematic for the original 3D and see what it's RIAA looks like.

[Sheldon]

Originally Posted by scaramoucheii

But why the different resistor values ?

Perhaps they were measured and selected to a matched value?

Sheldon

[scaramoucheii]

I don't understand what your saying ?

If they were measured and matched during installation, why is it on the schematic that way ?