The PHILCO Phorum
AM transmitter issues - Printable Version

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AM transmitter issues - oldschoolphotoguy - 04-15-2023

Hi there,

First, let me start by saying I'm definitely more of a collector and not strong on the engineering side of things. I currently use a Spitfire AM transmitter which worked well when I was feeding it with a cheap radio/phone combo with adjustable volume. I'm now upgrading to a Garrard RC-88/4 turntable and preamp. Unfortunately, it sounds like the preamp is overpowering the transmitter even at the lowest gain setting (which is 39db). Just getting alot of distortion even after playing with various frequencies and ATU switch settings. Are there any other settings on this transmitter I can play with?

Another option is I have an SSTran AMT-3000. That has gain adjustment of its own, which would probably solve my problem. But the unit stopped working a few years back, which is why I bought the Spitfire. On the AMT-3000, it lights up but does not send out a signal. (I've also played with frequencies and settings on this unit, and it does have an antenna wire plugged in.) 

Anyone who has any thoughts on either dealing with the distortion on the Spitfire or getting the AMT-3000 playing again, would be greatly appreciated. I'm trying to avoid buying a third transmitter.

Thanks,
Steve

RE: AM transmitter issues - vinzer - 04-15-2023

you use a pot between source and the transmitter.

RE: AM transmitter issues - MrFixr55 - 04-16-2023

+1 on what Vinzer said. However, check the carrtridge. Most "phono" preamps, especially those little boxes with no controls) are for magnetic cartridges. If the RC/88 has a crystal or ceramic cartridge in it, it will overdrive a magnetic preamp. Plug the preamp that the RC88 is feeding into the "line level" input of an amp, to confirm that it works. If the Spitfire has a "line level" input and the phono and preamp are functional, distortion should not be an issue. Otherwise, get a 100K audio taper pot, connect the output of the preamp between the 2 outer legs (ensure the leg closest to minimum is the common), and the transmitter between the center (wiper) contact and the common leg.

Hopefully this works.

Re the SSTRAN, I understand that they stopped making them but still support them. If possible, tune the SSTRAN to an OCCUPIED channel, turn it off and tune that channel on the radio. Power up the SSTRAN. If no difference is heard, the transmitter is dead. I don't know how many stages that there are to the transmitter, but either the oscillator, RF amp or power is the issue. If the unit was run without an antenna or a poorly matched antenna, the output transistor may have blown (although these are only 100mW units and I hope that the output transistor was a little heftier than that). Do you have a schematic? Post it, and we all will see if we can help. I don't have the SSTRAN (I have a Vector kit and a Talking House) but several other Phriends on the Phorum do have them.

RE: AM transmitter issues - Radioroslyn - 04-16-2023

Here you go John have fun!

https://ia800706.us.archive.org/0/items/Transmitters-Antennas/TX%20TRANSMITTERS/AM%20TRANSMITTER%20-%20SSTRAN%20AMT3000.pdf

Steve, What John is saying is that maybe Q4 and 5 are bad from having a mismatched antenna. That would give you the symptom that you described. It's important that it sees a proper load across the antenna.

GL

RE: AM transmitter issues - radiorich - 04-16-2023

Hello Terry,
I guess I have been lucky with my SSTRAN the one thing I found makes a big deal is to have a good earth ground mine has it's own 8ft grounding rod also Peaking it .

I feed mine from a Radioshack 4 way switcher with Pioneer Analog tuner ,Pioneer Cd player and I can also stream audio .

Sincerely Richard

RE: AM transmitter issues - oldschoolphotoguy - 04-21-2023

Thank you all so much for your advice.

The antenna issue on the SSTran is a possibility. It has a RCA plug for the antenna, and I think I used the one from the Spitfire when I tested it. The original antenna wire for it may be long gone since this unit was in storage for awhile, so I'm not sure how to test it if using it without an antenna can cause the same issue. It may be a moot point if it's most likely dead, so I might need to just focus on the Spitfire.

Thanks again for your help,

Steve