Tube AM, dead, only 910Khz and noise... Can we fix it?

[elnaldo]

Well , after the cleaning, and tweaking the antenna coil, and oscillator coil a bit, I managed to tune up to 1190KHz, but all the time full of whistles and noises.

Is it time to play with the IF coils ??? It seems nothing improves moving the antenna coil and oscillator coil further. I've got the best I can get from those.

Closing the tuning capacitor, more noise, like a thunder, quite loud, no stations down there...

Another thing. If I turn ON my desktop computer, located at 5 or 6 feet from the tube tuner, the noise increases and cover all the reception. Moving my arm to use the mouse, changes the noises.

 

[elnaldo]

Thank's! I'll read there later today.

THINGS ARE IMPROVING very fast!!!

About that background noise: is almost gone, connecting the antenna this way: I did a "loop" with the wire, and attached one end to ground (chassis). Now it's almost immune to the computer noise, but I lose weaker stations. VERY LISTENABLE.

The problem changed to this: I can tune from 710KHz, nothing under that, up to 1190 (actually one more station after that , I don't have the number)

I´ve read the antenna coil manages one end, and the trimmer capacitor the other. I've noted if I adjust the trimmer for better reception on hi feq, I lose volume on lower Hz stations. If I tune the trimmer for low freqs, I start to lose the hi freqs...

I see I'm close to a very acceptable result... What can I try from here?

Edited: Answering my own questions, for future reference, I've found this site that states:
"DO NOT touch the ferrite on the high part of the band, or the trimming capacitor on the low part of the band!"
http://www.hard-core-dx.com/nordicdx/antenna/loop/amloop.html

So, now the problem is the shift in the reception: With all the tuning capacitor OPEN, the highest freq I get is near 1190KHz. And, going down, the lowest is 710KHz, at aprox 3/4 of the tuning capacitor. There is still some more capacitor to close.

 

[elnaldo]

EDITED: What you read in the schematic is: Those are the trimmers but you are reading the SW and AM circuits. The function selector is that 3 dots after the antenna wire. L1 and L2 are Antenna coil (L1 AM, L2 SW) and AM oscillator coil is L3 at the bottom. Trimmer for oscillator is C3.

Tuning capacitor is C1 and C7.

I'll try to lower the tuning, I'm getting 1190KHz in place of the 1600 range. I've noted if I move the oscillator coil core, the tuning moves, and I can follow the tune with the tuning capacitor. I'll move it left in the scale, slowly, to have the 1190 moved, perhaps the rest of the station appears at the right of the scale. Then, adjust the lower freqs again, and so, adjust both ends.

 

[elnaldo]

OK, si the best I can get, is 1220 KHz, opening all the tuning cap, so I guess I need less capacitance to get higher stations...

Can I do that???

 

[gadget73]

if you have a capacitor tester, it may be worth checking C2 to see if it has drifted. If its off considerably, it will throw things off

 

[elnaldo]

At the minimum capacity (tuning cap open) I get 1220 KHz.

So, decreasing C2 is what I need to raise the oscillator freq.. I can do that. Actually, C2 is that white "pipe", I don't know what kind of cap is, but I don't trust it. It has some color bands, not very visible.

On the other end, tuning cap CLOSED, I get only noise... The lowest station is 710MHz when the cap is like 75% closed.

Do this match with the "too large C2" diagnose? Could it be a bad IF freq? (too high or too low?)

What I don't know, is the ACTUAL IF, since I've played with the IF coils to get the best reception, but I think it's close to factory adjust.

I've moved L3 (oscillator coil) for best reception. I've moved it all the way up and down, to see if other freqs appear, but not. Then, C3, the oscillator trimmer, is SO sensitive, just touching it moves the reception. But it's OK now. Playing with the antenna wire, the stations I get are very clear and loud, I'd say perfect for a (1968) 46 years old tuner.

I've cleaned the tuning cap with isopropylic alcohol (spray).

 

[elnaldo]

I was reading about how to change the IF frequency, tewaking the IF coils , something like tuning a station on the low part (600KHz), move the dial a bit without losing the station, adjust the IF coils for best reception at this position, the adjust the oscillator again at top of the scale, move to 600KHz again, and repeat adjusting both ends. It should shift the tuned freqs...

Or, inject a signal from another tuner, with a shielded wire, from each IF stage into this one, and adjust with that the IF coils...

I'll measure that C2 and then try one of this methods. Injecting the signal seems more reliable, but harder to do.

 

[elnaldo]

I have a digital tuner with a loop antenna. Should I put that antenna near the oscillator of the tube tuner? should I hear the station tuned in the old radio through the digital tuner, but at a different freq?

 

[gadget73]

you normally just hear a dead carrier. Basically it will hiss a little, but it will quiet down the general static.

http://youtu.be/AuLP31cLBMQ

will give you a pretty good idea of what you'll hear.


I'd really advise not monkeying with the IF strip without knowing for sure that its messed up. I'd be far more inclined to verify the values of the small value caps in the oscillator circuit first. I've had a lot more cases of those drifting off value than a never-touched IF strip being grossly out of whack. Chances are extremely good that it worked properly when new, and if it hasn't been touched, chances are still good that it will work correctly set as it is.

 

[Trio]

First, if the ECH81 is weak, it may not oscillate. Has this tube been checked? Gadget is correct about C2- if you do not have a tester, you can swap in a new silver mica 470 picofarad capacitor. The original was a silver Mica as well, and while they are usually reliable, that construction method that was used for those tubulars can sometimes leave them in less desirable condition.

C3 should move your frequency of the oscillator up or down, so if you have a station in the vicinity of 1600Khz, open the tuning gang and adjust C3.

C5 trims the antenna to the tuner.

At this point, adjust C5 for maximum signal. Then see if you can adjust the station you can recieve (910?) to the correct position on the dial. Once you are there, adjust the IF's to maximum signal. This will make the reciever very sensitive in that part of the band. Then see how the other stations fall on the dial. If the other 2 are close to correct locations, the issue becomes more related to the condition of the ECH81. And the 6AU6 to a lesser extent. If the signal of the one station is strong and clear, the 6AV6 can be assumed to be functional.

If you have voltage at pins 6 and 8 of the ECH81, and pin 5 of the 6AU6, the IF's can be considered okay. If you do not have voltage at all 3 of those pins, there is potentially an issue of an open winding in T1 or T2. If you have a test oscillator, you can attach the output of the oscillator to the fixed blades of the oscillator section of the tuning gang. Just by looking at the IF's, they could be anywhere from 440 to 475KHZ. These can usually be peaked at any of the frequencies in between, but try 455 first and see how that works. If the stations are too spread out, then peak to 465Khz and see how they line up. If still too far, try 470 Khz.

It is unlikely, but it is still slightly possible that the IF is 240 to 270Khz, but if you were able to peak them at the higher frequencies, it is a higher frequency IF.

Once the IF's are peaked, weak reception is going to be one of the resistors out of range if the tubes are satisfactory. R1, R4, R10, R14, R7 are the likely culprits.

 

[gadget73]

I have run into marginal oscillator tubes that worked at only part of the band. I've also had that caused by low voltages, usually from bad resistors in the B+ circuit. Its usually more of a problem on FM, but its not impossible to have that on AM.

 

[elnaldo]

All your appreciations are right, I can receive. very clear, some stations, 710, 790, 910, 1030, 1190, 1220, and I've tried to move the LO to lower those 1220 in the dial, with limited success, just a little, but if I go further, I lose the reception completely.

I'll proceed to test the suspect parts, including the capacitors and the LO freq with a second tuner. (very helpful that youtube video, now I know how it sounds like)

I could check the voltages at the tubes too, but when it works, it sounds good, nothing seems to be wrong in those stations.

Anyway, I'll take a break on this unit, since It took much longer than I thought, and I have 2 more units waiting, I'll diagnose those first, at least to answer to the owners, and then continue with this old radio (it's my cousin´s console, it was unused for many years, so she can wait a bit ...)

I'll post back the results in a couple of days... THANK'S VERY MUCH , I'm learning and understanding a lot about the hetherodyne principle.

 

[amptramp]

There is a possibility that something in the radio that should not be oscillating is in fact oscillating such as the IF or mixer stage. If the point where reception stops is accompanied by an increase in AVC voltage as measured at C12 suddenly goes more negative, then something is oscillating. This could be caused by open bypass capacitors or a bad suppressor connection on the IF tube. This is possible for an older set like this.

 

[Trio]

The OP reports strong reception within a reduced range on the MW band. Are you suggesting that the ECH81 is dropping out of oscillation outside this range?

Yes. It could be component drift, it could be a weak tube. If any of those resistors drifted up in resistance, they would have the same effect as increasing a "grid Leak" resistor value in an old battery set like a Freshmann Masterpiece- increased selectivity with sometimes limited band spread at the expense of sensitivity. Where a lower resistance reduces selectivity but increases sensitivity.

I got the impression that the OP could not get the LO to beat with anything higher than 1220 kHz and send it though the IF, even with C1 and C3 set to minimum capacitance.

I understand what Elnaldo was saying, but he has clarified that he loses recption as he tries to lower the LO to bring stations to line up on dial- sometimes this can be corrosion on the tuning gang bearings, a single tuning blade shorting across, or faulty tube. This was not entirely clear in the earlier posts

The OP reports strong reception within a reduced MW range. There's probably continuity in those windings. I think the uncertainty regarding the IF has to do with the compatibility of its "tweaked" passband with the range of the LO. Knowing what that range is may help us interpret what the OP has already reported.

[/quote]
Not always- limited tuning is also an indication of capacitive coupling between stages and not inductive coupling- especially when they cannot be shifted on the dial. The voltage check is the only reliable way to know that there is current flow in that part of the tuner stage. The actual voltage on those plates is not as critical as just their being present above millivolt to a few volts range (anything above 40 volts is usually good- on station or off).

What he can do is use that second radio to see where the nulls appear on that second radio while tuning the reciever he is working on to determine where the IF's are set currently on the set he is working on- imperceptable nulls, broad swaths of null or 4 nulls indicate both the IF's are off.

By leaving that second radio on one station at 1200Khz while tuning the original being worked on through through it's dial range- (opposite what you were suggesting based on skimming.) You can determine approximately where the ideal IF peak should be for the radio being worked on by the difference between 1200 and the lower approximate frequency of the radio being worked on when it nulls the 1200Hz signal. There should also be one above that point as well by an equal amount. So if it nulls at say where 740 should fall, the IF is approximately 460 Khz- If it nulls at 920, the Current IF is about 280Khz.

(And I have used what you are suggesting to recieve CW signals on a radio that did not have a BFO installed, and it does work, but you do not need to change the dial on the second radio until you wanted to fine tune, and even then it was only to fine tune the audio.)

But regardless, that involved process with 2 radios is only useful for determining An IF frequency- nothing more, and depending how carried out will determine one or the other's IF frequency, and not much more than that.

If the radio were in front of me, I would look for bent or loose plates on the rotor of the tuning gang- only able to be spotted when they just begin to short.
1)
I have had one that there was one blade that would shift when it was at one specific spot in rotation, but the shift was imperceptable with simple casual visual checks. It was secure when moved side to side towards or against the ends, but it was not secure vertically (up and down) and the clearances were close enough that you did not see the one end drop away to short out on the rack below. The blade would partly rotate out of the slot on the shaft.
2)
I had another that there is a corroded spot on the tuning gang shaft where the shaft loses it's contact to ground through the ball bearings sometimes, and in the same spot on the tuning dial, the tuning went away completely. Initially with that set, the tuning was all over the place and the needle could not be set to reliably to indicate the frequencies/stations, plus it initially only had a narrow range of tuning comparable to what Elnaldo is looking at. And Deoxit only has limited effectiveness on heavy corrosion. I still have this set, and the only true repair is to take apart the tuning gang and replace those bearings.

But this would be done after the tubes were tubes were tested and maybe a couple swapped to see if the tube was selective in it's response, as I have seen that more than once.

 

[elnaldo]

WOW, this is getting interesting !!!

I've tested the tunig cap this way: connected my multimeter, in capacitance mode. Turned all the way up and down, capacity moves smooth, no shorts. Then, changed to continuity test mode, and turned all the way up and down, nothing, no "beep" or "light", so I believe the cap is nor shorted.

Then, I'll do the "2nd receiver test", both ways, tune a station on the tube radio, try to pick the carrier , and the other way, tune the portable at 1200, and move the tube radio until get the carrier signal on the portable.

The voltage check is the only reliable way to know that there is current flow in that part of the tuner stage. The actual voltage on those plates is not as critical as just their being present above millivolt to a few volts range (anything above 40 volts is usually good- on station or off).

I'd like to check those voltages. Where should I measure? I think I'm missing something in the translation ...

 

[Trio]

Are we basically using the LO of the radio he's working on to "jam" reception of the 1200 kHz station by the other radio?

Yes, you will also have null points above and below at the images; created by the IF of the radio being worked on- in this case the tube radio. Tune the tube radio to see if you get nulls above and below that 1200 Khz on the second radio after identifying that the second radio gets nulled by the first radio when it is tuned to 1200 and determine the IF frequency of the first radio by the null points created as you tune the first radio (the one being worked on) up and down the dial carefully because the image nulls are not as strong(using the dial indicated frequency of the first radio assuming some accuracy of the tuning dial) At worst, it indicates the IF's are at least functioning, and an approximate idea of what the IF frequency should be.

The second radio allows you to demonstrate and characterize LO operation across the tuning range. The difference between LO and station frequency tells you about the current tuned state of the IF. These are good data points and neither involve estimating frequencies from dial position.

This is assuming the oscillator section of the radio being worked on is fully functional- which has not been fully determined yet.

I've tested the tunig cap this way: connected my multimeter, in capacitance mode. Turned all the way up and down, capacity moves smooth, no shorts. Then, changed to continuity test mode, and turned all the way up and down, nothing, no "beep" or "light", so I believe the cap is nor shorted.

At this time that is indication it is working as it should.

Then, I'll do the "2nd receiver test", both ways, tune a station on the tube radio, try to pick the carrier , and the other way, tune the portable at 1200, and move the tube radio until get the carrier signal on the portable.

The three pins I mentioned earlier of the EH81 (pins 6 and 8) and the 6BA6 (pin 5) are just the plate connections- these should have some voltage above 40 volts. If you have a measured potential, the windings of the IF's at least should be okay as these plates get their voltage potential through those windings.

Amptramp mentions the other capacitors to check, and they may need removal to test them accurately C12, C11, C15. C9 could be suspect (open). And check C38 to make sure it is not shorted.

It shows on the schematic inside of each IF transformer can, 2 capacitors- most likely ceramic. Check these for merit. Since it is slug tuned, those might be 100pf each (they would be the same value, but that value can vary), but they should be marked as well.

 

[elnaldo]

I'll do this test perhaps whithin 24 hs (I'm having a hard time fixing an SL1700 TT)

I hope to get the 2nd "Image Jamming" and be certain about the IF . Then, check voltages and capacitors.

 

[Trio]

Take your time, I'm in no hurry, it's a great project. :thmbsp:

I can't recommend the 2nd "Image Jamming" method because the role of the Ranser's IF is unclear, and the digital tuner probably has a significant amount of image rejection built into its circuitry.

No, the IF of the Ranser is very clear- it is just a Super hetrodyne reciever- no mystery. the role of the IF's is perfectly clear. What is not clear is what the Ranser's IF is supposed to be at for a frequency.

Let's assume for the sake of discussion:


1) The image rejection capability of the digital tuner gets overwhelmed by the Ranser's LO.

Use an analog tuner. A cheap battery radio is a wonderful test instrument.

2) The MW station at 1200 kHz gets jammed by the Ranser's LO when the LO is at 1200 kHz.

correct

3) The MW station at 1200 kHz gets jammed by the Ranser's LO when the LO is at 2110 kHz.

If the digital tuner is being tuned to that point- that would be indicating the IF frequency of the digital tuner, not to mention it is way off- however the IF can also develop an image, but weaker than the seconday image in intensity. It is possible the Ranser could recieve that image if set to Short Wave.

If the Ranser is being tuned to that point- the IF of the Ranser is way off. The IF image for a 455 IF is going to null the second reciever at 1655 or 755 if the IF's are peaked correctly (if the IF of 455 is the correct IF frequency for the Rancer). At the image frequency, if you are close but not on the frequency- you will get an audible oscillation to occur- it nulls at the IF. This is how you can get CW reception on a Short Wave radio that does not have a "BFO". Go test it for yourself.

The image issue is also one reason why 180, 220, 240 and 262 are no longer used as IF frequencies in home audio. you can have multiple images at times across the dial. You can also get images at the station fundamental, so a station at 800Khz can image at 1600 if there is not a local station strong enough to override that signal.

You do mention the primary drawback of using a digital reciever as the second receiver- Image rejection.

Another problem with digitals is that newer units use a microprocessor to the signal detection and processing. Sometimes the IF is hard coded into the processor and shielding of the processor greatly reduces any RF re-radiation. And some just use the crystal frequency to establish bandwidth as well as processor clocking.

Now you're able to estimate that the IF of your shiny new digital tuner is 455 kHz. You've also learned that the Ranser's LO can oscillate up to 2110 kHz which should be plenty of range to heterodyne a MW station broadcasting on 1600 kHz.

But that requires the that the Ranser has a functional oscillator- if the tube is selective in the range of where it will oscillate- it will not function- and you learn next to nothing other than the Local Oscillator is not working at the upper portion of the tunign dial.

 

[gadget73]

This is how you can get CW reception on a Short Wave radio that does not have a "BFO".

I have done this. I've also used a signal generator loose coupled into the IF strip as a BFO. Its a bit clunky, but it works very effectively.

 

[elnaldo]

OK, I have something>: When the Ranser (tube radio) is tuned to 790, it cancels the digital tuner tuned at 1270. That gives me 480KHz difference... (actually 1269, so 479KHz)

I did it the other way, tuned the digital a bit lower, to 1250, and started to move the tube dial a bit down, and that whistle appears on the digital, cancelling the reception.

If I tune both to the same freq, I hear both tuners perfectly.

So the LO seems to be working, at 480KHz.

What whould you do next ? measure voltages, measure capacitors (desoldering...) , or leave it alone ???