Realistic TRC-457 stock, freq issue

[Hawkeye351]

Got a Realistic TRC-457 on the desk today. This radio belongs to a long time friend of mine. He uses this radio every once in a while. He wanted me to check it out cause he's noticing some issues as of lately. He wants no mods installed, he wants it stock.

Upon evaluation:
Transmit frequency is off by 28, frequency counter shows 27.2022mhz when keyed in AM.
USB is way off, both receive and xmit.
LSB is great, slightly off frequency on both receive and xmit.
Transmit power is great, AM is 3w key swinging up to 16w peak and swinging forward up to 6w in average. SSB (both) is around 18w peak and around 12w average.
Receive is good.
Someone has changed out the 1307 final for a 1969. Still has the original driver of 1306.
Some has also already changed out the Tantalum cap located at C179 for an electrolytic cap of same value.
I see no mods nowhere, no tinkering around D21/22, no jumping of the filters, no broadband mod, nothing as far as mods, not even a clarifier mod.

Issue:
After alignment, which went fine except for USB. I noticed am transmit frequency was off by 28 but LSB transmit frequency was spot on, as was LSB receive frequency with my monitor radio keyed up.

USB:
When flipping from USB to LSB, there is no change in frequency. USB shows the LSB frequency. Both transmit and receive frequency is way off, can't tune in USB transmit at all, and you have to turn clarifier fully clockwise just to be able to understand a USB receive signal.

Any tips, advice?
Gonna swap out a few caps and then get back with you all.

But until then, what would cause all the frequency issues and USB to be so far off?

 

[nomadradio]

Drifted crystal. If a trimmer cap goes all the way to one extreme and still won't reach the desired frequency, this is a clue.

Did you check the two carrier crystals, X1 and X2? If they are off, the channel frequency will be as well.

73

 

[loosecannon]

also, if you are measuring at TP8, this chassis drops down 2.5khz on AM TX.

so if you measured 35.005 on receive, you'd measure 35.0025 on AM TX.
LC

 

[SuperLid]

The crystals are way out as told already here.

Good luck with it, great radio, one of the best!

 

[Hawkeye351]

Got all the crystals checked. All are well within range.

7.8025 = flashing back and forth from 7.8024 to 7.8025.
7.7975 = flashing back and forth from 7.7974 to 7.7975.

11.2858 = flashing back and forth from 11.2857 to 11.2858.
The other 2 synthesizer crystals do the same, flashing from right frequency to a hair lower, just a hair, still well within range.

I tested the 10.240 crystal also. Flashes from 10.2399 to 10.240.

AM receive frequency is dead on with service manual.
AM transmit is off 2.8k.
LSB receive frequency is dead on with service manual.
LSB transmit frequency is spot on (zero beat) to monitor radio.
USB receive frequency is actually on 27.20247.
USB transmit frequency is actually on 27.20247.

Haven't checked or replaced any caps yet.

How I tested crystals:
1. Remove crystal from radio.
2. Set signal generator to crystals frequency at 5v.
3. Connect red probe of generator to one leg of crystal.
4. Connect red probe of frequency counter to other pin of crystal.
5. Connect ground of both probes together.
6. Frequency displays on frequency counter.

I swept across frequencies of each crystal to check the ranges. All of the crystals have good range. The 7.8025 crystal will drop off once you get below 7.800 on the generator, and will drop off once you go above 7.8070.

 

[Hawkeye351]

Update, not fixed yet though:

I have another radio sitting here I've already went through that gave no problems at all. It's the same chassis, President Washington 858.

I yanked all the crystals out of both this bad 457 and the Washington. I put all the 457 crystals in the Washington, and all the Washington crystals in the 457.

No change in 457 issues. Still same issues with Washington crystals in the 457.

But, the 457 crystals worked perfectly in the Washington.

Replaced 17 more electrolytic caps also, still no change.

All the oscillator CT's adjust to specs with no issues now. But, AM transmit is still off (receive is spot on).
LSB receive and transmit is spot on with clarifier centered.
USB receive and transmit frequencies are still way out of whack.

Just replacing those caps increased the audio in transmit and made that stock realistic mic sound like a beefy power mic. Amazing what a few bad caps in the audio chain can make a radio sound.

Still stumped on this 457.
AM receive frequency is spot on, but AM transmit is still off.
LSB receive and transmit are both spot on.
USB receive and transmit are both way off.

All Oscillator CT's adjusted smoothly to the service manual specs.

Any tips/advice guys/gals?

 

[TM86]

I think what's stumping you may be stumping a few others as well. I know it's got me perplexed. AM TX and USB TX/RX both rely on the 7.8025 MHz crystal being on frequency, so that's where I'd start looking. But if you've already looked at that and the oscillator is spot on, then I'm not sure where to go with that other than checking VCO frequencies in each mode (and RX/TX) and see if they match up with what the Sam's says they should be. If your oscillators and VCO are both spot on then simple math says everything should be working and there's something else that's been introduced to the equation that should not be there.

 

[Hawkeye351]

Yea, I came to the 7.8025 conclusion myself last night.

I figured:

USB - 34.9875−27.185 equals 7.8025.

AM - 34.985−27.185 equals 7.8

LSB - 34.9825−27.185 equals 7.7975

But all those are spot on with service manual.

The 34.9875 tuned in fine.
The 34.9850 tuned in fine.
The 34.9825 tuned in fine.

The 7.8025 tuned in fine.
The 7.7975 tuned in fine.

10.240 crystal checks out as 10.2399.

I'm stumped, lol...

 

[Hawkeye351]

I just now used my monitor radio set on USB to find out what frequency the 457 is receiving on and Transmitting on.

Both radios set to channel 20 (27.205) and on USB: 457 clarifier centered.
457 receives at 27.20180.
457 transmits at 27.20358.
And that's with all the oscillators spot on with service manual.

LSB is spot on frequency with monitor radio, receive and transmit with 457 clarifier centered.

AM receive is spot on, AM transmit is off by 2.8k still.

It has to be something around that 7.8025 crystal, cause only AM transmit along with USB receive and transmit are acting up.

 

[guitar_199]

I'm dragging up late on this one....

Have you done any "nosing around" in the area of FET7? That is the XMIT Mixer.

Just to verify that the inputs are good, that the voltages present indicate a proper bias condition, AND if the output of that mixer is on frequency??

Just an idea....

 

[Hawkeye351]

I haven't, but I will.

Thank you

 

[guitar_199]

I haven't, but I will.

Thank you

I am actually doing my day job at the moment... but I just had a thought that compelled me to come back here and "blather" some more. None of this may make any sense.... I may be way out of my league here.........

But....

You say that your transmit is off by about 2.8K.

It just hit me that 2.8K is pretty close to the value of the offset used for SSB on these things. Usually, you have a fixed 7.8 Mhz for AM and then plus or minus offsets for UPPER and LOWER sideband.

I have not been able to really do a "detailed" look... but .... following the inputs back from the XMIT MIXER they go up into the area of the 7.8 Mhz filter and the 7.8 Mhz IF.... and there is a bit of "diode switching" going on up there. Some of it turned on by voltages that are XMT...... some of it turned on by voltages that are XMT-SSB. It kind of looks like they are using that "filter/IF" area to condition the 7.8 Mhz and maybe even SELECT the one that goes down to FET7 to be mixed with the VCO.

What hit me was..... "what if something is wrong in the 'filter/IF' area.... and the WRONG 7.8 Mhz source is being turned on? Like.... what if... one of the sideband oscillators was being sent down to FET7 INSTEAD of the fixed 7.8 Mhz??? Your AM XMT signal would be "OFF" by one of the offsets.

I would check that 7.8 Mhz input to FET7n as close as I could. The oscillators and offsets may be "dead on the money". BUT if that diode switching is hosed..... the WRONG ONE could be getting routed down to FET 7 causing this AM offset...... BUT it could ALSO be causing problems in the associated sideband too.

If I recall.. you seemed to indicate that LSB was looking better than USB.... so there could be a switching problem involving the USB 7.8Mhz source and it's switching.

I hope this "drivel" makes sense.... and maybe gives you a fresh idea.

Bob

 

[Hawkeye351]

Yes it makes sense, and I will check that.
As far as LSB, it's perfect, no work needed on LSB. Receive and transmit are dead on frequency with clarifier centered. LSB zero beats to my yaesu 950 perfectly on frequency.

 

[guitar_199]

I haven't, but I will.

Thank you

I am hoping that the attached sectional may be of some help. In that last post I was speaking of "switching the 7.8Mhz that winds up going to the XMIT MIXER".

This drawing is an attempt to give you an idea of what I have stuck in my head.

The BLUE colored path is the Balanced Modulator coming up to the 7.8 MHz crystal filter and "borrowing it" to cut off the opposite sideband.

Notice that ... CKT TRC 12 is a 9.31 v RCV ONLY voltage. It serves to bias D4 on in RCV to pass the incoming RF to the filter. But this supply goes OFF in XMT so D4 is shut off.

Next CKT TRC 20 is a 9.22 v XMIT SSB ONLY voltage. IT serves to bias D5 and D6 on. D5 being on will help pull any incoming RF down to ground. D6 being on will allow the Balanced Modulator output to go into the 7.8 MHz filter.

Now, lets go to the far end. The BROWN lead (CKT TRC 63). This drops down to the diode that is controlled by CKT TRC 17... which is 8.28 v in ANY XMT MODE. So.... when you are in ANY XMT mode (AM or SSB) the output of the 7.8 Mhz transformer is routed straight down to the 7.8 Mhz input of the XMIT MIXER. WHATEVER we feed down there... gets mixed with the VCO to result in the final transmitted frequency. So .... WHAT ARE WE FEEDING DOWN there.....?????

IN the middle.... TR6 and TR7 appear to make some sort of Double Pole switch arrangement. They BOTH have collector voltage all the time.... from CKT TRC 3 an 8 voltish all the time supply.

TR6 appears to get it's bias from the ALC circuit.... but it's INPUT.... comes from the 7.8 Mhz filter... which... in SSB XMT... is coming from the Balanced Modulator. So... in LSB or USB transmit...... the "wanted sideband" (7.8 Mhz plus or minus the offset!) gets pushed through the filter, through TR6 which then goes over to the 7.8 Mhz transformer and winds up being shoved down the brown wire to the XMIT mixer... where you get the VCO mixed with the "shifted 7.8 Mhz" and.... OUT IT GOES towards the final.

Now AM I haven't proven yet... but AM is the ORANGE wire case. CKT TRC 19 is a 8.27 v AM XMT ONLY voltage. So when you transmit in AM CKT TRC 19 comes on and biases TR7 on. (In the meantime, since we are NOT in SSB ...TR6 will NOT HAVE ANY INPUT from the filter because it's diode D6 SHOULD BE UNBIASED and not passing any signal!). I think that TR7 input comes from a fixed 7.8 Mhz signal that is pushed through TR7, into the same 7.8 Mhz transformer... and down the brown wire to the mixer.

So what this looks like to me is...... (to sum it up!).....

THe brown wire.... carries SOMETHING down to the XMIT MIXER to be summed with the VCO to give us our 27 Mhz output.

IN SSB .... it SHOULD BE the BLUE path...... which is the "selected/shifted sideband" coming up from the balanced modulator. TR7 is biased off so nothing should be coming through it. So in SSB (either one) the BLUE should be pushing to the BROWN.

IN AM ..... it SHOULD BE the ORANGE path.... which I believe to be a fixed 7.8 MHz should be pushing to the BROWN.

My theory is.... what if something is wrong here... causing the wrong signal to be gated to the BROWN wire???? The problem COULD be HERE in this diagram....... BUT.... it could also be down in the carrier oscillator area where the crystals and adjustments for the USB/LSB offsets are located. It just seems to me that if the wrong frequency were switched to the wrong place.... you could wind up with the wrong signal being piped down the brown wire.

I MAY BE WRONG..... if I am.... I will accept the laughter and catcalls !!!!!

But Ijust couldn't help passing this thought on.....

Bob

 

[TM86]

I am hoping that the attached sectional may be of some help. In that last post I was speaking of "switching the 7.8Mhz that winds up going to the XMIT MIXER".

This drawing is an attempt to give you an idea of what I have stuck in my head.

The BLUE colored path is the Balanced Modulator coming up to the 7.8 MHz crystal filter and "borrowing it" to cut off the opposite sideband.

Notice that ... CKT TRC 12 is a 9.31 v RCV ONLY voltage. It serves to bias D4 on in RCV to pass the incoming RF to the filter. But this supply goes OFF in XMT so D4 is shut off.

Next CKT TRC 20 is a 9.22 v XMIT SSB ONLY voltage. IT serves to bias D5 and D6 on. D5 being on will help pull any incoming RF down to ground. D6 being on will allow the Balanced Modulator output to go into the 7.8 MHz filter.

Now, lets go to the far end. The BROWN lead (CKT TRC 63). This drops down to the diode that is controlled by CKT TRC 17... which is 8.28 v in ANY XMT MODE. So.... when you are in ANY XMT mode (AM or SSB) the output of the 7.8 Mhz transformer is routed straight down to the 7.8 Mhz input of the XMIT MIXER. WHATEVER we feed down there... gets mixed with the VCO to result in the final transmitted frequency. So .... WHAT ARE WE FEEDING DOWN there.....?????

IN the middle.... TR6 and TR7 appear to make some sort of Double Pole switch arrangement. They BOTH have collector voltage all the time.... from CKT TRC 3 an 8 voltish all the time supply.

TR6 appears to get it's bias from the ALC circuit.... but it's INPUT.... comes from the 7.8 Mhz filter... which... in SSB XMT... is coming from the Balanced Modulator. So... in LSB or USB transmit...... the "wanted sideband" (7.8 Mhz plus or minus the offset!) gets pushed through the filter, through TR6 which then goes over to the 7.8 Mhz transformer and winds up being shoved down the brown wire to the XMIT mixer... where you get the VCO mixed with the "shifted 7.8 Mhz" and.... OUT IT GOES towards the final.

Now AM I haven't proven yet... but AM is the ORANGE wire case. CKT TRC 19 is a 8.27 v AM XMT ONLY voltage. So when you transmit in AM CKT TRC 19 comes on and biases TR7 on. (In the meantime, since we are NOT in SSB ...TR6 will NOT HAVE ANY INPUT from the filter because it's diode D6 SHOULD BE UNBIASED and not passing any signal!). I think that TR7 input comes from a fixed 7.8 Mhz signal that is pushed through TR7, into the same 7.8 Mhz transformer... and down the brown wire to the mixer.

So what this looks like to me is...... (to sum it up!).....

THe brown wire.... carries SOMETHING down to the XMIT MIXER to be summed with the VCO to give us our 27 Mhz output.

IN SSB .... it SHOULD BE the BLUE path...... which is the "selected/shifted sideband" coming up from the balanced modulator. TR7 is biased off so nothing should be coming through it. So in SSB (either one) the BLUE should be pushing to the BROWN.

IN AM ..... it SHOULD BE the ORANGE path.... which I believe to be a fixed 7.8 MHz should be pushing to the BROWN.

My theory is.... what if something is wrong here... causing the wrong signal to be gated to the BROWN wire???? The problem COULD be HERE in this diagram....... BUT.... it could also be down in the carrier oscillator area where the crystals and adjustments for the USB/LSB offsets are located. It just seems to me that if the wrong frequency were switched to the wrong place.... you could wind up with the wrong signal being piped down the brown wire.

I MAY BE WRONG..... if I am.... I will accept the laughter and catcalls !!!!!

But Ijust couldn't help passing this thought on.....

Bob

Thanks for posting this. I thought this was how the signals were routed but you confirmed it for me with the colored paths. And I just now fixed a 449 with a bad L39. (Not the same 449 I had thread on a few weeks ago.)

We now return this thread to fixing the 457 Hakweye351 is working on.