ROBYN SB520D

[bluemax]

I mentioned this radio a few weeks ago after finally acquiring one some 30 years after falling in love with it. Glad I didn't wait that long for Mrs. Max! I got some initial input in another thread, but I'm sure to have plenty more.
So let's see if I can get squared away on one trouble spot...namely signal readings. The radio had two apparently bad meters and no lights. So I asked about RF gain and a number of guys say it's not uncommon to have to turn the gain up almost all the way. We can forget about anything to do with the SWR function on this radio. I don't care if I never use it. I bought a few new lamps and a new meter and installed them. (and this time I tried everything prior to soldering) Lights work and the signal meter moves from 0 to 1 only, even yesterday afternoon when my noise level was from 7 to 20 on three radios. The RF power seems to swing ok, at least for now, at about 3 watts DK. I'm not getting any more signal strength by adjusting VR's for RFgain range, Signal meter, or AM RX gain. Now I will mention all I was picking up at the time was noise. I didn't have a quiet background with a strong local signal to test. I will see what tonight's conditions bring.

UPDATED
My noise level continues to be a concern until about 9PM. A few trials over the last few evenings proved to have the same results; somewhat normal RF power reading on TX (compared to the external meter) but only movement from 0 to 1 on signal strength in RX. In all cases I tuned other radios to the same frequency and had signal readings of at least 5 and some as high as 10+ with just a flip of the antenna selector.

I checked frequencies on a few channels against my Ranger at about a mile from home and that seems to be alright.

Any suggestions on where to begin? On the other hand, the radio is quite usable as is, and I don't want to risk fouling it up any further!

[bluemax]

This is a Uniden 858 chassis. Did anyone have similar signal meter issues on another brand radio?

Or am I missing something that SHOULD be obvious. That would be me...

[Sandbagger]

Jun 30, 2017 17:30:24 GMT -5 bluemax said:

This is a Uniden 858 chassis. Did anyone have similar signal meter issues on another brand radio?

Or am I missing something that SHOULD be obvious. That would be me...

Well, I normally run the RF gain at maximum. At that point, the receiver is at max gain, same as if the radio didn't have an RF gain control. At that point, the signal meter should read properly. It's possible that the replacement meter has the wrong resistance, and needs more current to drive it, but I think you said the transmit RF power is working properly, which should rule that out as a suspect. The next order of business is determining whether your receiver is weak, or whether it's simply bad meter drive or AGC issues. Can you hear the local stations with similar clarity and strength as other radios? Or is the receive unusually quiet? Maybe it just needs an alignment. Those 858 chassis radios are not difficult to align, but you do need a signal generator to do it right. VR1 is the S meter adjustment. It should make SOME difference from one extreme to the other. A signal range from 0 to 1 is awfully small, and VR1 should make some difference. If not, maybe the pot is bad.

There are way too many things, beyond the simple checks, that can cause receiver issues, to troubleshoot remotely without more clues. See what you can find, and maybe we can narrow it down from there. It is a 40 year old radio, so all sorts of aging parts issues can come into play.

[bluemax]

I guess talking repairs stuff...OK,  not. Today I had average noise and a fair amount of skip. Some stations were well above S9 on my Royce (which has a very loose meter) and a few were on par with my closest locals. I warmed up the Robyn before switching the antenna. Two items worth reporting...the receive is definitely a little weaker, but I also got swings as high as S5.5. All of the high readings were horrible signals that I couldn't understand, but the radio did record a higher reading. Haven't done anything with the pot for now. I'm going to use it, keep some notes and see if a pattern is evident. Let's be honest, my ears are the only meter that really matters. And I need to find what the correct baseline for a signal really is. Every meter on each radio is different right now.

[2600]

Troubleshooting a symptom of 'weak receive' starts with alignment. Just one mistuned slug can reduce the receiver's sensitivity a lot.

That circuit board has a "gotcha" in the alignment. The noise blanker feeds directly off the antenna, alongside the main receiver's input. L3 is the receiver's antenna-input coil and L1 is the blanker's input coil.

L3 is peaked for best 27 MHz signal level. L1 is peaked for a 23.5 MHz test signal. That's how this blanker works. Noise pulses at 23.5 MHz are pretty well identical to the noise pulses at 27 MHz, and the blanker circuit amplifies the 23.5 MHz noise before using it to 'blank' those pulses from the main receiver a few stages downstream.

Just one problem. With L3 and L1 connected together, they can interact. Tuning one of them can change the setting of the other one.

Another point to aligning the receiver's tuning adjustments is that it's a diagnostic. A tuning slug that doesn't peak properly can point to the circuit fault that's holding back the receiver's sensitivity.

One last thing that alignment can reveal is a defective RF/IF transformer, or 'can'. The tuning slug should always peak slightly below the rim of the hole at the top of the can. A slug that appears to peak with the top of the slug DEAD EVEN with the rim of the hole indicates a failed capacitor inside that can. I'll skip the explanation why for now, but this can sometimes be fixed by soldering a capacitor on the solder side of the pc board to take the place of the failed cap inside the can. Easier than unsoldering all seven pins of the can and finding a suitable replacement.

Weak receive is a report you'll see in a lot of forum posts over the years. I remember one guy who expressed frustration at being told "align the receiver" on several different forums. Thought it was a conspiracy by all those techs, that they were hiding the secret to fixing his radio.

The service manual for this RatShack radio shows the same circuit board used in your Robyn. Has the alignment procedure in it. www.cbtricks.com/radios/realistic/trc_449/index.htm

But it's a diagnostic strategy. If all that's wrong is that someone has 'tweaked' it while watching the wattmeter, alignment may fix the problem without need for a soldering iron. At worst, the behavior of one or more of those adjustments can point to a failed part that's the root of the problem.

73

[bluemax]

Well, I will keep the reference for the future. I think if I were going to attempt my first alignment it might be on an unknown cadaver rather than a long lost love! Good chance that site has info on plenty of other radios I might want to use for a learning curve. Thank you very much. You do, however,  give me too much credit at this point. But Marconi started SOMEWHERE!