TRC-448 repairs

Got tired of working on this thing, so I thought I would write about it instead.

On the bench is a Realistic TRC-448. It has problems. But I'm working my way through them, slowly.

First problem, power cord. Easy enough fix, just pull out the remains of the old one and put the new one in. Simple, right? Nope. Followed the "original" wiring as it was in the radio and the new one... trips the power supply overcurrent protection? Well, that's not a good thing. Start looking for shorts and not finding any. Happened to look in the right place and noticed the little line from the power cord hole to the circle with a "+" in it hidden under some components. Just so happens the black wire is in that hole, just like the old cord. Yep, I followed a bad example. It does explain why they cut the power cord next to the back of the chassis, though. Swapped the wires around and did a quick power test, lamps came on and the channel display showed me a number. Good enough for now.

Next up, volume control shaft bent. This actually was an easy fix, since I had a parts donor chassis that I had previously stolen the Plessy chip out of to work on a Console V. Stole the volume/rfgain/on-off control and swapped them out. Works like a charm.

Next up, Q218 was blown apart. Probably not going to work very well in that condition. Stole one from the parts chassis.

Meter appeared to be stuck. Yep, parts chassis again.

Found the nylon screws broken on Q2, Q229, Q235, and Q236. Raid my spare screws and nuts and add little plastic insulators for the tab so they don't short out. Now I need more spare screws and nuts.

Pulled some glue out that had been left when a previous owner recapped it.

Found that the two 1 uF electrolytics on the mode switch weren't put back on correctly during the recap. Fixed that.

Now I was finally ready to see if it did anything. Yeah, it does stuff, just not what it's supposed to.

Found that the LSB LED was constantly on. Another look at the mode switch showed two points wired together that really shouldn't be. Removed the solder and little chunk of wire between them, so that works as it should now.

Found that I could kind of receive something, but only when flexing the PLL board. Which isn't really ideal. Tried looking for any obvious broken joints, none to be found. Start working my way through the board, reflowing a little at a time and rechecking. Managed to find a disconnected component lead buried under the bodge resistor on the back of the board, but that wasn't it. Finally tracked it down to one of the channel selector connections. I'm guessing the one that kicks in an "out of lock" signal when you're between channels. Now receives without flexing the board.

Which brings us up to current. Radio is receiving a signal as indicated by the meter, but there's no AM RX audio. There is RX audio in sideband. More concerning is that in all modes there's an audio frequency oscillation that kicks in when the volume gets above a certain point. In sideband it mixes with the incoming 1K tone on the AM signal I'm injecting and it sounds kind of nasty. Not that it sounds good in AM.

I haven't even tried to transmit yet. I'm sure that will be an adventure as well.

Anyways, feel free to ignore this thread. Or mock my meager skills. Whatever.

Found the no AM RX audio problem. Someone had removed the solder from the pins of C272. Left the legs bent so it was hanging in the board. Soldered the leads and AM RX audio works again.

Now to try and track down where the internal oscillation is coming from.

AudioShockwav said:

Looks kinda like you are moving forward, meager skills aside, good job so far on a radio that sounds like it's had a rough life already......

73
Jeff

Click to expand...

I'm guessing it was recapped by someone doing it for the first time, if the lead lengths on the electrolytics are any indication. Probably in an attempt to fix something. Then got golden screwdrivered when the problem didn't go away. After that came the power cord f-up.

Still trying to pin down the source of the internal oscillation. Went through voltages to see if they're good. Only found one that was suspect, and only because it was a little high. Still within a volt. I was looking to see if anything was on that shouldn't be, like when the mode switch was miswired at the start of this.

Tried going through the receiver alignment portion of the program. Found that the AGC won't quite adjust to the spec of 1.4V at TP208. VR207 is cranked all the way over to one side, which makes me think there might be problem there. The AGC circuit seems to be doing it's job, though. Or at least the receive side of the meter works and that's driven off the AGC.

Then the XYL came into the room and wanted to take a walk while the football game she's watching was in halftime. I did what any man would do at that point. I looked her square in the eye and said, "Let me get my shoes on."

brandon7861 said:

Edit: Im an idiot and now see where you said you got AM rx working

I remember reading that self-oscillation in an IF stage causes the AGC to max out. Since the AGC and meter are tied together, does your meter peg when the oscillations occur, or does it stay the same?

Click to expand...

Meter doesn't move when the oscillation occurs.

brandon7861 said:

You probably already saw this in the service manual, and I know it doesn't fit perfectly, but there is this:
View attachment 74354
I'm sure everyone is tired of me talking about these, but a Carlson Superprobe would probably work good here.

I was sitting at my desk last night wondering if a small loop on a common base amp with a small detector diode after it would pick up enough of the audio oscillation for a PC mic input to detect, but I was too tired to find out.

Click to expand...

I hadn't seen that since I was using Sam's CB-251. I've now downloaded the actual service manual. If nothing else, it will make locating test points and controls much faster.

Haven't written this thing off just yet. Just haven't had a heck of a lot of time for it.

For giggles, tried a transmit test today. Low power in AM and USB, LSB seems dead. No modulation in AM, USB seems to work in that respect but with weird artifacts in the audio. Although that could be feedback from my monitor radio.

RX audio oscillation still present. Whatever is making this possible seems to be feeding through to the RF side, as the oscillation does appear to be on the input to the AM detector. So I guess I get to try and trace it back to see where it enters the IF chain, if I can. Really hoping it's not a mode switch problem, but with everything else this radio has been through...

Thanks again, Brandon, for the link to the service manual. Made it easy to locate parts I wanted to probe. Should come in handy in the future.

Been poking around in the guts of this thing a bit this evening. I no longer think that this is coming through the IF circuits. I can see the oscillation at the detectors for SSB and AM, but that seems to be feedback from where they connect up with the AF amp.

I'm tempted to yank C239 and see if the oscillation goes away altogether, or if it's only on one side of that cap.

If anything I would guess it's probably Q232 and/or C330/C331. Just because that configuration of parts looks more like an oscillator than anything else in the audio chain. So I may and up swapping those with known questionable parts from one of the donor chassis.

But not tonight.