Birch model 75 78 rpm electric phonograph

[radiotvnut]

I just received this old Birch model 75 78 rpm record player as a result of not being able to control myself on ebay. Seriously, 78-only electric record players rarely ever show up; so, I decided to jump on this one. The latches are rusty and the case is dirty; but, I think some cleaning and steel wool will solve that. It uses a two tube (35Z5 and 50L6) amp and the speaker is made by Rola. Someone tacked in another filter cap and there was a yellow Aerovox Dandee electrolytic cap rolling around in the bottom of the cabinet. Since this has no preamp stage, it uses a high output crystal cartridge. I could use a modern cartridge and add a preamp stage; but, since these don't turn up often, I think I'll have the original cartridge rebuilt and keep it original. Besides, this tonearm is so heavy to the point that it may not do well with a current ceramic cartridge. I can't find any information on this unit; but, I'm guessing it to be made between '39-'42.

[radiotvnut]

Upon further inspection, I discovered that this amp is a "hot" chassis design. In later years, there was an isolation capacitor between the chassis and circuit ground. On this model, everything is grounded to the chassis. The cartridge ground terminal connects to the metal cartridge body; which, is attached to the metal tonearm. Depending on the position of the AC plug, that would make the metal tonearm "hot". If someone was in contact with ground and touched the metal tonearm at the same time, the result could be fatal.

Installing a polarized AC cord would be fine, unless the unit was plugged into an outlet that is wired backwards.

So, my option is either to redesign the amp with an isolation cap between the chassis and circuit ground. Or, install an isolation transformer inside the unit. I wanted to keep the design original; but, safety is more important.

I removed the old cartridge and the leads actually solder to the cartridge; which, is rare (especially for a crystal type). Even with the cartridge removed, the tonearm weight is likely still too heavy for a current cartridge and I see no easy way to counterbalance this arm; so, I guess I'll be rebuilding the old cartridge.

[maxhifi]

I always like it when you post these record players. That Rola speaker is really neat.

I installed an isolation transformer in a small portable record player, and no matter where I put it, it induced hum into the amplifier. The isolation cap never seems to work that great either.

A third option is to cut off the plug, and install something weird like a 2 prong jones plug... then make a little aluminum box with an isolation transformer in it, and a jones receptacle. That way you can keep it (mostly) original, and also prevent unsafe operation.

[radiotvnut]

Even back then, I find it hard to believe that they'd release something designed like this. It's just a disaster waiting to happen.

[maxhifi]

It's possible that the person who did that nasty job adding a second filter cap, and left the other one rolling around inside, also bypassed and removed the line isolation cap.

[bob91343]

Isn't there a spring to counterbalance the arm?

One solution to the safety problem is to disconnect the ground from the arm so it floats and then add a high value resistor and perhaps a capacitor between it and the chassis (or tonearm ground). Thus you would have to touch the cartridge body to access the power line.

I agree that there is no reason to keep this original if it's dangerous. On the other hand, do you really intend to use it once the job is done? I wouldn't subject my 78s to such a beast.

[radiotvnut]

No, there is no spring and it looks like there never was one.

Of course, I'll use it; but, I know what records to play on it and what records not to play. I have 78's that are in good condition; but, I have plenty of worthless ones that are well worn. So, playing those on here wouldn't matter.

I need to obtain some sort of scale, just to see how heavy this tonearm tracks.

[maxhifi]

Originally Posted by radiotvnut

I need to obtain some sort of scale, just to see how heavy this tonearm tracks.

Faced with this problem, and not wanting to shell out 100 bucks for quality at a hi-fi store, i went to a 'bad' part of the city, and picked up a small mechanical scale at a sleazy pawn shop for dirt cheap. Drug scales are perfect for measuring tonearm down force, and very cheap.

[bob91343]

You can build one, too. Make a little horizontal lever and counterbalance the stylus force with pennies on the other end. Google to find weight of various coins. You can use a dowel as a fulcrum for best sensitivity.

[Celt]

Originally Posted by radiotvnut

Even back then, I find it hard to believe that they'd release something designed like this. It's just a disaster waiting to happen.

I had two different record players as a kid that were done that way.
They both shocked the pee out of me on more than one occasion.

[dieseljeep]

Originally Posted by Celt

I had two different record players as a kid that were done that way.
They both shocked the pee out of me on more than one occasion.

I think that firm was in business into the sixties. I have a tube that was branded BIRCH, that had a 60's code date. In the unit shown, at least they used a high quality speaker. I always liked Rola speakers. You can see the rest of the assembly was cheap. Using standard staples to hold the wiring in place.

[Reece]

Hard to tell from the underchassis picture, but it looks like the motor is controlled by a separate switch, and the amp off the back of the volume control? I could see why: stop the turntable between records without turning off the amp (and then forget the amp is on and leave it on for days.) I first thought this was a set that put a lower voltage motor in series with the tube heaters, but think this was too early. There must be a wirewound dropper in there for the heaters.

I second the motion to isolate the hot chassis with a cap and high value resistor. Lots of AA5 radios are done that way.

[radiotvnut]

Yes, that is a seperate motor switch. The filament dropping resistor is a flat metal can type resistor that is riveted to the chassis.

[radiotvnut]

I pulled this one back out today and I also have another phonograph that uses this same tonearm and cartridge that both track (or, grind) at around 90 grams. There is no kind of counterweight or counterbalance spring on these tonearms. Even pulling out the original cartridge does not bring the arm down to a low enough pressure for a modern cartridge. The only thing I know to do that might lighten the tonearm is to drill a hole in the rear of the arm and mount a long bolt loaded with large washers to serve as a counterweight; but, that would look tacky. There's not enough room at the rear of the tonearm to conceal any kind of weight; so, I may be stuck with having the original cartridges rebuilt and using them as-is. One of these units belongs to someone and he is not too happy about playing his records on something that tracks at 90 grams. If he decides to have his fixed, I'll advise him to play only well worn/less valuable records on it and not to play anything that he wishes to keep in nice condition.