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 Post subject: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 6:02 am 
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Location: Tucson, AZ
Hello all,
Well, I've almost got my RCA 128 done, but the shortwave bands are still dead. It's been recapped, (restuffed caps) tubes checked, and alignment complete. The AM broadcast band performs like a champ with a short wire hooked up to the ant terminal, but I only pick up one or two stations (China) on the 41 and 49 meter bands, when the bands are crowded (checked with other radios). The alignment process went pretty smooth, the signal (from the generator) went through the radio just fine. Is the switch dirty or faulty? It's been sprayed several times with tuner cleaner and deoxit. Any ideas to check would be greatly appreciated. Thanks!


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 8:01 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
I think you need a long wire antenna for the short wave bands.

The oscillator tube may not be working properly on the short wave bands because of the higher frequency; try a new one.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 12:04 pm 
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Location: Victoria, Australia
I agree on better antenna. Due to sunspot activity recently some bands have gone "dead" for several days.

Falling over at high frequency is typical behaviour for a Pentagrid. Tired tube and / or low voltages due to high resistors. Tubes like 6J8 love voltage, better slightly high than low with them.

Marc


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 1:45 pm 
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Location: Dandridge, TN 37725
In addition to the suggested longer antenna AND GROUND, checking resistors and voltages, and trying another converter tube, another problem is that you may have peaked the short wave trimmers on the IMAGE frequency - in other words, the wrong peak. This can greatly affect sensitivity. You can check by peaking the trimmers per the alignment instructions and then looking for the image signal about 1mHz (or 2 times the IF frequency) below on the dial (you may have to increase the signal generator output). If not found, the alignment may be off. Go back to the original dial frequency and signal generator frequency and try to find ANOTHER peak. Then recheck for the image.

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Dave McClellan, W6SQV(/4)
http://mcclellans.com/RadiosPage2015.htm


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 3:31 pm 
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Location: Cedarville OH USA
The 128 is usually a killer on SW reception. I was going to suggest checking coil continuity, but it sounds like you are getting a signal through. Could definitely be a weak 6A7. Also check for alingment on an image-I've done that myself. You will need more antenna for SW.

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Yes, it plays. No, there was no FM stereo in 1932. Yes, some people still enjoy AM radio.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 6:00 pm 
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Location: Tucson, AZ
All good suggestions above!!!! I tried a longer wire and it definitely boosted the performance! But their are still large sections of bands B and C that are dead. When I do receive stations, there is a large amount of drift and distortion. So the suggestion that the 6A7 is weak or bad is legit. What is a "good" reading on a 6A7??? The two that I have read about 1200/ 400 and 1200/ 300 on a Hickok 800. I bought a "NOS" 6A7 on ebay and it didn't test any better than the old one that was in the radio to begin with. I also suspect dirty tube sockets....going to clean those tonight, any suggestions?
Thanks again all!
Eric


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 7:24 pm 
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ericschlepp wrote:
All good suggestions above!!!! I tried a longer wire and it definitely boosted the performance! But their are still large sections of bands B and C that are dead. When I do receive stations, there is a large amount of drift and distortion. So the suggestion that the 6A7 is weak or bad is legit. What is a "good" reading on a 6A7??? The two that I have read about 1200/ 400 and 1200/ 300 on a Hickok 800. I bought a "NOS" 6A7 on ebay and it didn't test any better than the old one that was in the radio to begin with. I also suspect dirty tube sockets....going to clean those tonight, any suggestions?
Thanks again all!
Eric


Yes, use Deoxit and pipe cleaners, and blast underside of socket too. Grime and dust creates partially conductive paths that create loss, especially at higher frequencies. This should be done with all the sockets. Also, while you're down there consider replacing the carbon resistors in the front end. We all know they have drifted up, but how far? And do the values change when the set heats up? Cost's you a buck and an hour or less of your time.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 7:36 pm 
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Location: Amherst, WI
I'm watching this thread with great interest, having just completed a capandectomy on my 128. The radio displays strong performance on the BC band but, on both SW bands, sensitivity seems strong in the middle of the band and low at both edges. Could this be an alignment issue?

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Amherst, WI


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 7:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4504
Location: St. Louis, MO, USA
It's quite normal, for any given antenna, that there will be certain shortwave bands with good reception and certain others that are almost dead. RCA published the chart below for one of their receivers to illustrate the point. It doesn't necessarily apply exactly to all receivers though.
There are antenna tuning units (ATU) that can be built so that the good/bad areas can be shifted.
Some are quite sophisticated, but a simple kit that works with vintage radios is the Morgan Antenna Tuner:
http://www.radiodaze.com/product/838.aspx
Image

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Terry Davidson


Last edited by Oldbear on Mar Tue 06, 2012 7:40 pm, edited 1 time in total.

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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 7:38 pm 
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I restored this very radio also and I've never felt like it performs that well on SW either. Likewise it too only picks up stations more on one side of the dial. Something to consider is that there simply aren't as many stations out there anymore on the SW band. Also- you need a super-long length of wire to pick up much of anything anyway. With mine I ran a wire all the way outside to the clothes line. Probably 30-40 feet and even then the performance was marginally better.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Tue 06, 2012 8:56 pm 
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Location: Silver Spring MD, USA
These are all good suggestions - keep at it and you'll find the issue(s).

I would think the 49 meter band should be loaded with Latin American stations in the evening at your location. I get a lot on the East Coast. You definitely should be getting more than just a couple of stations.

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Crawls Backward When Alarmed: Guitars, amps, vintage radios and more.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 12:16 am 
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Location: Victoria, Australia
Looking at the set as one who has had nothing to do with one of this model. If the tuning gang sweeps the whole BC band without issue then the tuning gang is unlikely to be the issue. Albiet that it will benefit from being clean and the set from a good antenna.

Aside from the tube (deal with first), bearing in mind 6A7 / 6A8 is best on BC and looses sensitivity especially around 20 metres. Which is why it was superceeded by 6J8 later on in SW sets, or often was preceeded by a pre-amp in earlier sets. The issue has to be within the switching and circuits of the SW coils. RF feeding back due to bad sheilding will de-sensitise a set & I have had & seen it happen, often with "hiss" so a stage oscillating cannot be overlooked.

I have had sets where several RF coils were open, stopping the oscillator, or causing it going supersonic. Some sets where there is a padder and trimmer on the oscillator can be cranky. A Midwest was the one I had issues with recently (as above). Over a period of days the middle of one band was dead, 6M was totally dead and not only on it (Hams were having problems). eg WWVH at 10 & 12MHz was missing for a week. Interesting is that this was in a way consitent with this set, however the middle or part of one band would be missing but when the switch was changed there would be stations at the same point on the dial. That's typical of sunspots.

Fortunately I have communications reciever (50m longwire antenna) that I can use to see if stations are in fact missing within a range and an oscilloscope that can see if the modulation within the tube is dying as the frequency increases..

The same basic alignment rules apply (unless colpitts) as setting the BC band. Oscillator low end, aerial trimmer/s high end and frequency specified in the alignment data (if provided).

Marc


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 5:16 am 
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Joined: Jan Sun 03, 2010 4:46 am
Posts: 259
Location: Tucson, AZ
I will clean the tube sockets, test the 6A7 again, and realign the set paying special attention to the first peak. I know I may not have hit the first peak initially because I did not find the image up the dial. Below is a pic of the setup including my grundig satellit 750 as a sanity-check receiver and frequency counter for the triplett sig gen. If you all don't have a satellite 750 yet, you should! Great little radios!


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 5:18 am 
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Location: Tucson, AZ
codefox wrote:
ericschlepp wrote:
All good suggestions above!!!! I tried a longer wire and it definitely boosted the performance! But their are still large sections of bands B and C that are dead. When I do receive stations, there is a large amount of drift and distortion. So the suggestion that the 6A7 is weak or bad is legit. What is a "good" reading on a 6A7??? The two that I have read about 1200/ 400 and 1200/ 300 on a Hickok 800. I bought a "NOS" 6A7 on ebay and it didn't test any better than the old one that was in the radio to begin with. I also suspect dirty tube sockets....going to clean those tonight, any suggestions?
Thanks again all!
Eric


Yes, use Deoxit and pipe cleaners, and blast underside of socket too. Grime and dust creates partially conductive paths that create loss, especially at higher frequencies. This should be done with all the sockets. Also, while you're down there consider replacing the carbon resistors in the front end. We all know they have drifted up, but how far? And do the values change when the set heats up? Cost's you a buck and an hour or less of your time.




What do you mean by "front end"? Like the big power resistors (30Kohm?)


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 5:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 18544
Location: Albuquerque, NM 87123
IMO, tubes like 6A7 and 6A8 should be tested by substitution with known NEW tubes, and you should have a few on hand. They are known to be problematic, and it is quite possible to have one that will function well in one receiver, and poorly in another.

If you find one of them that performs badly, keep it; it may work in another radio. Unless they are dead or have practically no emission, they should be kept and marked as weak. Also, test the radio for several hours to make sure it keeps working well.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 6:46 am 
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Location: Tucson, AZ
THIS JUST IN!

I realigned the radio and the SW bands came to life! I WAS indeed hitting the second peaks on both B and C oscillators! DAMMIT! I didn't realize how far you had to back the screws out in order to find the true first peak! Once I found the true first peaks, I was able to find their images at 4240 KC and 17,080 KC respectively, and repeating ever so often down the scale! MAN, this radio IS a good SW performer! With only a ten foot piece of wire! :) YAY!

Note to other 128 restorers! After recapping and cleaning (this set was very very dirty) the alignment was WAY WAY OFF! You will think you did something very wrong, as I did, but realignment will bring it back to life. My B+ voltage is still 20 volts low which I don't understand why, but the set is sensitive, and CRANKS with an awesome 41 output tube!


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Wed 07, 2012 9:29 am 
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Contary to what I was told by someone, when sorting out an Australian HMV "that it should be alright" . In a rare statement on the schematic was a line pointing out to the effect, that if you changed parts within the RF section of the radio, it should be re-aligned.

I was not at all surprised when I checked it, to find, as predicted by the manufacturer and as I expected: It was well out of alignment.

Therein lies a lesson.

Marc


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Thu 08, 2012 12:14 am 
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Some of the carbon resistors are up to 20% over the schematic value (the 30K power resistor reads 35K+). I'm thinking the combination of all these increased value resistors is whats limiting my B+ voltage. R7 reads 1490 ohm, should be 1100, etc.


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 Post subject: Re: RCA 128 dead shortwave bands
PostPosted: Mar Thu 08, 2012 3:38 am 
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As I do work commercially, I am probably a lot more ruthless, however as I toss out the capacitors I check the resistors. Resistors with yellow bands, det audio plate resistors and resistors around 6V6 & family have a very high attrition rate. By doing this, despite the school of one bit at time, a lot of grief & time is saved, by not having to go back & do re-work due to faults that you should have eliminated in the first pass..

While there will be disagreement, I will toss out any resistor that is 10% or more out of spec. Rework also costs time.

Marc


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