The end result is to be able to use the existing field speakers with the new FM box, which has inputs on it for the mic, cd player, and the sound head. It has two outputs: one for an antenna, the other to go to the field amp thru the Radio Shack mixer/amp. The problem started shortly after initial installation. When the mic is used the sound is garbled on both FM and field speakers and is what I describe as a "warble", although the soundhead signal was fine thru the FM and the field speakers during films. During troubleshooting by different people, the problem degraded to the point that the FM box had to be disconnected to get decent sound out of the field speakers. I understand the FM box is made by Bill Robinson from Kentucky, if anyone is familiar with his unit. It has been tested by him recently and is working properly. I think the problem may be in the wiring so I want to consider replacing the wiring segments.

All suggestions and guidance is greatly appreciated.

Thanks in advance,
Joe

<<<The end result is to be able to use the existing field speakers with the new FM box, which has inputs on it for the mic, cd player, and the sound head. It has two outputs: one for an antenna, the other to go to the field amp thru the Radio Shackmixer/amp.>>>

It could be that your new "all-in-one" FM transmitter has a built-in preamp, and you may be "pre-amping" your field speaker signal twice.

The Radio Shack mixer/preamp has a built-in preamp that will boost the signal that's already been boosted by the FM box.

I have a similar setup in my basement. I've got a Simplex XL riding atop an RCA-9030 soundhead. I went from the solar cell outputs of the soundhead to the "phono inputs" of a Numark DJ Mixer with sheilded cable. The "phono inputs" have a built-in pre-amp to boost the signal of ceramic turntable cartridges, and work very well with the solar cell.
My mixer has 3 stereo "IN" channels with inputs for 6 different devices, and three stereo "output" channels.
I went directly from the "Master" output of the mixer to a Ramsey FM-25 transmitter. I had a slight "hum" in the FM signal, but I put a ferrite core around the 12volt power supply lines going to the transmitter, and "rack mounted" the transmitter in a one space metal chasis from Par Metal Products, www.par-metal.com
and it eliminated the hum completely.
When my drive-in is completed, I can go from the "Record Output" of the mixer to the field speaker amp, and use the "Booth Monitor Output" to drive the amp for the ceiling mounted speakers in the concession stand.
The other two input channels on the mixer can be used for a CD Player and tape deck. My mixer also has 2 microphone inputs, that I'll use for the microphones.

I used all sheilded stranded cable if that makes any difference.

Hope this helps.....

Suspect that the mike impedence may not match the impedence expected by the FM box.
A new mike is a cheap fix. Get one that either has a switchable impedence or the proper impedence to match the box. A new mike cord is a nice touch as well.

I don't quite understand why Radio Snack had to get involved? It sounds like a kludge someone added to fix the mike problem?

Low level signal is usually twisted pair, shielded and grounded at the preamp/amp. Wires such as mike cords that will flex alot are stranded- primarily to keep metal fatigue and breaks from causing an intermittent connections or loss of sound. Hard wired stuff like soundheads can be either stranded or solid wire. Size can be small, since there is very little current. The key is proper twisting and the shield.

The Radio Shack mixer/amp is currently used to condition the signal before it's fed to the tube amps for the field speakers. I understand that a higher level signal is required for the old amps we have (don't know their model number right now). The new FM box has 2 outputs, one for the antenna, and one to feed the field speakers but if I'm told everything correctly, it's level needs to be boosted for the old tube amps to use it.

The new FM box has the proper input so that the sound head can connect direct to it. The mic in use was a new one supposedly compatible with the FM box.

In a previous life the Radio Shack mixer/amp was used to connect the old mic, the cd player, the phonograph, and the Smart 2B-DIT processor to the tube amps for the field speakers. I understand it's currently the only thing that has the proper output levels for the field amps, although that doesn't seem right to me. Still collecting info.

Thanks for the input so far.

"This speaker buzzes."

I just found out that the real reason the Radio Shack amp was there was because the two field amps that drive the field at 70 watts required a 15 watt input to operate. The Radio Shack box takes the signal and boosts it to 15 amps before it goes to the field amps. When the original RCA preamps failed (primary and standby) a few years ago, they were disconnected and the Mod2B-DIT and Radio Shack amp were substituted for them. The original sound system was described as a "complete" indoor theatre sound system, whatever that means. Still working on model numbers.

John,

Earlier today I picked up some 22guage stranded 2-wire shielded cable just in case I needed it. Looks like I guessed right.
Ran into an acquantance of yours at Keysville drive-in recently, Nathan Fulmer. I was down there taking pictures and he mentioned you were doing some install work. Hope it all works out.

It's beginning to look like the "all-in-one' FM transmitter that replaced the Mod 2b-DIT just isn't compatible with something in the booth. I expect we'll eventually reinstall the MOD-2B and go back to a Ramsey FM transmitter if we can't find any wiring problem.

That is a 70 VOLT output, not 70 watts. A 15 WATT amp doesn't put out 15 amps either.
I'm hoping those were just slips of the keyboard.

I'm inclined to bet on something crapping out in the power amps. An amp of that vintage could have leaky caps, or any number of problems. If that is the case, new PA amp might be a simpler (and ultimately cheaper) solution.

FWIW, My first scratchbuilt amplifier was a nice 6L6 15 watt jobber, straight out of the RCA tube manual, with a power transformer that would sink a ship. Durned thing would probably power your drive-in field on its own.

No slip of the keyboard. From the tech that originally installed it, the Radio Shack amp is rated at 55 watts and it's used to output 15 watts to drive the two RCA tube field amps. Each field amp puts out 70 watts ON a 70 volt level. His exact words. From there the transformer in each speaker post drops the level to 1/2 watt for use by 2 1/4 watt speakers.

I don't doubt that the amps could have a bad cap or two considering their age, but they worked fine prior to the recent changes in the booth equipment. If they now have a problem that's causing the humm/hiss it may be just a coincidence, but I'm still guessing on a wiring problem (bad/no ground somewhere or a bad shield.

We already had a discussion about a new amp and all agree that it should have been done several years ago. Right now the budget for sound equiupment is 0.

The solar cell output goes to either the new FM box (which has its own internal pre-amp) OR the mic input of the Radio Shack amp if the FM box is disconnected, not both. I thought of that already..... When the new FM box is connected, its supposed to output a signal that gets amplified by the Radio Shack amp to 15 watts, then sent to the field amps. We're beginning to think there's an rf leak somewhere that's causing problems. All this worked fine initially, but on the first day of real use after testing the mic attached to the fm mic input wouldn't work right during intermission. It worked fine before the first show. We're also thinking about a heat related problem but haven't found one yet. We got a new mic but it didn't fix the problem. So we're currently running only on the Radio Shack box and the field amps and I'm told there's a slight hum in the speakers. I'll know more on Thursday, I hope...

Joe