I have been working on a TRF receiver that I posted about in another thread. Figured on starting new thread as the old one was a bit long.

Here's the schematic. The local/distant switch is not present as I doubt it will work. Also the antenna preamp is not completed yet with the antenna connecting to the primary of the first RF transformer.



I hav a problem of low sensitivity using two TRF stages.

I took some measurements last night. I do not have a proper scope probe so I just used a regular wire which could have affected measurements somewhat.

550 KHZ signal at tuning cap of first TRF stage





550 KHz signal at ptimary of second RF transformer





2 MHz signal at tuning cap of first TRF stage





2 MHz signal at primary of second RF transformer





Any ideas as to what the problem could be?

I wonder if maybe I need to use a choke in the source circuit of the FET and just capacitor couple the RF transformers like in the following schematic?



I tried to connect a 6800 uF cap to the 12 volt power wires rto the amp board (where main 12 volt power for whole receiver is connected) and got a low frequency oscillation. I then tried the same capacitor from the regulator output to ground and got oscillation. I am thinking there may be some coupling in the power supply as when I turn up the volume control nearly all the way with no signal being received I get a higher frequency oscillation.

Would it be a good idea to insert a resistor in series with the 12 volt power wire to each regulator and then use a filter cap after the resistor to further isolate each stage's B+?

I used a resistor and capacitor on each regulator to decouple them from each other and it seemed to work. The signal is better and the radio actually works decently now. I got better signal when I connected the ground of the RF generator to the chassis. Once I did that I noticed the waveform was badly distorted on the primary on the second RF transformer so I reduced the voltage to the FET and once I got to around 1.2 Vdc from the regulator (low as it will go) the signal became a pretty nice looking sinewave. I will update the schematic again with what is hopefully the last changes that need to be made. I still plan on using the antenna preamp if only to isolate the first RF transformer from the antenna given the antenna length does affect the TRF stage somewhat.

I still have a bad squeal when I set the AGC control to where it is at optimum. I suspect it is the fact that I used unshielded wire from the audio transformer to the volume control and amp along with using unshielded wire for the AGC control. The wiring around the transformer is twisted together and is also where the AGC control connects. I suspect that the high impedance and gain of the MK-484 is causing some feedback. I will replace that wiring with shielded wire and see if that helps cure the problem.

Does the MK494 act as a power detector?

Here's the info on the IC http://www.datasheetcatalog.org/datashe ... /MK484.pdf

Thanks for the .pdf link. They seem to only have the whole pocket radio kit listed now.

You're welcome.

I used an AK model 40 radio as the chassis and to supply the variable capacitors and RF transformers simply because I have one good working AK-40 and this one was a spare for parts.

I used shielded wire to all connections on the volume control. I used one of the 10K trimpots for the AGC control and mounted it on the PC board with the voltage regulator for the MK-484. I then used shielded wire to connect the pot to the circuit. Now I have virtually no unwanted oscillations.

All I have left to do is finish the antenna preamp and install it which should make up for any gain lost by adding the second TRF circuit. I wonder if I should have put a resistor in series with the first tuned coil cap circuit and the FET to reduce the signal which would have probably allowed me to run more B+ on the FET. I still have yet to figure out why if the FET is run above 1.5 Vdc it badly distorts the waveform. I should have built the FET circuit per the schematic that was given me for the antenna preamp and just capacitor coupled the RF transformer. That would have probably worked better.

I fed a signal from my two tube 6GY6 transmitter which is being fed by my computer tuning Winamp with the stereo tools plugin and the received audio (ignoring the typical RFI that is usually present in the background on the AM band) is nearly identical in quality to the signal fed into the transmitter.

I have found that with the speaker I used (12" Oxford) I need no tone controls to reduce the treble given the speaker does not reproduce the treble very good if at all above maybe 6KHz. Basically I cannot hear the 10KHz whistle (common to some AM radios) at all.

I have another question.

Not sure exactly how the AGC works with the MK-484, but if it uses a change in voltage that is not only internal to the unit would it be possible to apply that voltage to the FET to vary its gain as well?

Looks like I will have to do a frequency response test on the amplifier used as bass response is rather pathetic at best. Could be anything from the MK-484, the tra,sformer, the amp, the speaker or the box the speaker is mounted in.

I m compairing the sound to how good my RCA K-80 console sounds playing the same station being received from my 6GY6 two tube broadcaster.

I doubt I will get the exact sound, but I do feel the receiver should have a bit more bass then what it does. More than likely if I built a proper wood box the bass would improve. Also could just be that 12" Oxford speaker. Not exactly what its original use was.

I have a couple 15" Magnavox speakers, but they were used in a solid state console so probably are not as efficient as the Oxford is.

I have an observation, a problem and another update to the schematic.

I have observed that the AGC control does affect bass response.

I do have a problem. I cannot seem to get the first TRF stage to work right far as the FET is concerned. I noticed yesterday afternoon I had fed a signal to the antenna input and was getting signal on the primary of the second RF transformer. I was not getting anything from the speaker so I checked and the power supply was turned off. Not sure what is causing that. Also I have a problem with the FET in that if I increase the B+ voltage to the FET above 1.2 Vdc the waveform on the primary of the RF transformer distorts pretty badly. I honestly think I need to put a RF choke in series with the FET and just capacitor couple the RF transforemr to the FET.

Here's an updated schematic. I think I got everything correct, but I am not sure. The MK-484 circuit is still a bit touchy on setting its operating voltage and AGC having to find the right combination that produces no unwanted oscillation while working properly.



I am thinking about using a different amplifier as the current amp might be causing a problem as well. I do have a Bogen CHS-60A amplifier that I could restore and use with the circuit. I would then not need the transformer as the input if too low in level for the line in could go right to the microphone input and the amplifier could also power the receiver. I don't know if the higher input impedance of the Bogen amp would help the MK-484 work better or not.

I suggest adding RF bypass capacitors from the power supply connection side of the rf transformers to ground preferably physically located close to the coils but alternatively at the output pin of the voltage regulators. A value of .1 uF should work fine. Short lead length is somewhat important, an inch total will not matter but avoid running the ground wire side of the capacitor from one side of the chassis to the other if you can avoid it.

The problem is without the bypass caps, the rf current flowing through the coil has no place to go - remember it is trying to get to ground to complete the circuit- other than the output pin of the voltage regulator and the output impedance of the LM317 voltage regulators is easily a hundred Ohms or more at the AM broadcast radio frequencies.

bruce kj3z


PS

he MK-484 circuit is still a bit touchy on setting its operating voltage and AGC having to find the right combination that produces no unwanted oscillation while working properly.

I am not familiar with the MK-484 but I think it is very likely this circuit needs fairly goof rf construction techniques to work well. Point to point wiring is likely to produce disappointing results. Suggest you use a piece of un-etched pcb clad material as a continuous ground plane and do proper RF construction. Google Dead Bug construction for an example. Contact me I think I have a couple pieces of clad I could send to you if you wnat to make use of them...

Oh ok I'll try the bypass capacitors.

Main problem right now is not being able to get gain out of the FET stage without the stage making the audio sound bad. I suspect that maybe there is some sort of feedback through the power supply given I have no bypass caps.

I have yet to build the antenna preamp stage.

Looking at the schematic I should have known that I needed bypass caps.

To avoid most of the point to point wiring problems I built the first TRF stage on a piece of perfboard while the MK-484 circuit was built on an octal tube socket with shielded wiring going to the transformer along with shielded wiring going to and from the AGC control which is mounted on the voltage regulator board. Aside from the lack of gain (may actually be losing some gain) from the FET circuit it all works pretty good.

The two circuits are mounted very close to the coils which are mounted very close to the tuning capacitors keeping wiring very short and everything is grounded to the closest chassis ground points near each circuit.

I don't have access to the updated schematic right now, but will post it when I can upload it to photobucket.

I still have yet to build the antenna preamp which may not actually be needed now. If I do build it I'll make it a buffer so that the antenna does not affect the tuned circuit any.

I'll list the changes to the schematic.

Added a bypass cap between the output of each voltage regulator and ground.

Had also mislabeled the source and drain on the schematic.

While adding the bypass caps in the circuit I decided to check the wiring of the JFET and found I had the gate and source reversed. I wired the JFET properly and now varying the B+ to the JFET affects gain like it should.

I noticed that with the bypass cap installed on the B+ line (connected to the junction of the regulator and 1K resistor) feeding the MK-484 the voltage and AGC controls are easier to set for proper operation with less chance of feedback.

The JFET stage works real nicely now and I have the gain on it set to just before the point the MK-484 gets overloaded when receiving my two tube broadcaster with just a short wire connected to the TRF radio.

I may try and experiment some to see if I can modify things to get better control over the gain. Maybe I can tap the voltage from the AGC control of the MK-484 and feed it to the JFET gate to control its gain too. Would definitely make the receiver a bit better on the real strong stations as it overloads with a long antenna connected to it while trying to receive my two tube broadcaster.

I do notice that without an antenna and with the top cover on the AK-40 cabinet I can still receive my two tube broadcaster pretty decently.

Not sure if it is the speaker (Oxford 12"), speaker box (from late 50's Magnavox stereo phono) or a lack of bass response of the circuit, but I cannot seem to get the bass like my RCA K-80 console produces. I suspect that I will need to build a proper open back speaker enclosure for the speaker as the Magnavox uses a thin board of some type where the speaker and grille cloth are mounted to. I suspect that plywood will be ok to use? I will more than likely make it the same size as the Magnavox speaker box. Reason I suspect the box is because my RCA K-80 is an all wood cabinet and without any bass boost in the console's amplifier it gets nice warm full sounding bass.

I did a frequency response test by feeding an AF generator (app on my andriod phone which works very well) into my two tube broadcaster (used that because it has a VU meter so that I could ensure the audio voltage was the same level at all frequencies used) and the voltage on the output of the transformer driven by the MK-484 output didn't vary much at all. Now I need to do the same test today only this time monitoring the output of the amplifier to see how much if any the voltage changes as I go lower in frequency. If the amp is indeed at fault I will either try to make the current amplifier have a flat low fewquency response or if I cannot I will simply get another amplifier that has a flat low frequency response.