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

I've tested a bit more... It seems the lower I can "cancel" is 1250 at the digital tuner ...

If I tune the digital to 1140, for example, the Ranser has no effect on the digital, moving the tuner capacitor all the way up or down, the carrier never appears at the digital tuner...

I was trying to see if I could reach 600KHz on the tube radio. But noting happens on the digital.

And I am a tuner guy.

This is the functioning of the ECH81 as used here:

First off, the ECH81 is a 2 section tube, it consists of a triode and a type of pentagrid converter.

As it is used in the Ranser, the triode section is the oscillator, and it's plate signal is tied to a grid of the converter section and injects the oscillated frequency of the IF in at the third grid section, where the input from the tuning section is injected in via the first grid, these two signals are mixed, (this tube is the first detector as well just by the action of that first grid in the circuit.) Plus voltage gain does occur from this tube due to the interaction of the cathode, plate, screens (grids 4 and 2) and cathode screen (grid 5).

R2 is critical to sensitivity, if too high, very selective but not sensitive tuning resuslts. If the ECH81 needs a critically tuned circuit to operate- it will have a narrow range of functionality meaning you can only tune a portion of the band.

Now Elnaldo's determination of an IF at 480 Khz is somewhat useful, but because of the limitations of a digital tuner in the manner suggested/used, suggests the IF's may not be peaked. 480Khz was used as an IF, but it may be the result of one of the IF's being right and the other being broad. In the manner I outlined to use an analog tuner or radio, much more useful information would have been revealed. I have been working on this stuff since the 1970's, so I am speaking from experience- not simply some inflated theory akin to the ZPE crowd sometimes.

Now, the trimmer C3 and oscillator trimmer C5 affect the entire band, they do not "pull up" or pull down the range of tuning as incorrectly theorized if the tube is sound and the rest of the circuit is functional.

The symptoms up to now are supporting entirely what I stated 2 pages back- A problematic ECH81. If R2 is right on value or within tolerances, and the indicated/mentioned capacitors have been checked- the only possibilites are an issue with an IF transformer- (but voltages present at the plates of the ECH81 would eliminate an IF transformer from being burned out.) or the tube itself is the problem, Which again- mentioned 2 pages back. It is very possible that tube still tests okay- but it just does not work in that specific tuner- therefore swap the tube for another ECH81, or rewire the socket and circuit to accept a 6BE6 if that is more readily available.

So- If the few stations that Elnaldo can recieve are correctly lined up the dial- he needs to replace the ECH81- There is no other option. Setting up a signal generator at 480Khz and injecting that signal into the stationary oscilator plates and peak the IFs is the only other real action available, also mentioned 2 pages back- but that will not counteract a tube that is defective in this specific radio.

Without testing the tube, as I sense that is not an option right now and a replacement is not available. There is nothing more to be done and really nothing more to be discussed.

If R2 is right on value or within tolerances, and the indicated/mentioned capacitors have been checked- the only possibilites are an issue with an IF transformer- (but voltages present at the plates of the ECH81 would eliminate an IF transformer from being burned out.) or the tube itself is the problem

Click to expand...

Thank's. I'll re-read every suggestion, check all this, and proceed.

I think that for sure some cap in the antenna section is out of specs, since turning the tuning cap all the way up, doesn't reach 1500 or 1600kHz, the 1220 I get is approx at the1500 on the dial, what (to me) means I have too high capacitance, what means lower frequency. I understand (after all this thread) that I need less capacitance (or less inductance) to raise the frequency.

I'll check all the mentioned components against schematic values, and if needed, rebuild the whole section and align "by ear", what gave me some result (I started the thread with just one station, full of noise, and now I have an usable 790 to 1220 band)

if i could throw that far i would throw a ECH81 your way ..am sure i have 1 or 2 new ones here .

The stations I get, seems at the right dial point at 790, but shifting, so the last one (1220) is almost at the end. I was told that this point to the antenna section. A local engineer show me a graph with some lines and angles, and what could be happening in theory. But, to him, the antenna section is not OK, since at the end of the dial, tuning cap fully open, I get approx 1300kHz. And to him, the low freqs I lose, could be some "non linearity" or "miss tracking" of the antenna and LO sections, that go parallel (aligned) up to a point, but start to lose that IF difference.

But anyway, I need to check-replace all the caps involved. I'll do it the next days. I have enough theory to understand a bit better what to expect.

It's funny, but I didn't even try the PC scope, because it works through the audio input, so I assumed it's limited to the audio bandwidth... I'll give it a try, at least as a freq counter... I don't think it will get that freq though.

elnaldo said:

The stations I get, seems at the right dial point at 790, but shifting, so the last one (1220) is almost at the end. I was told that this point to the antenna section. A local engineer show me a graph with some lines and angles, and what could be happening in theory. But, to him, the antenna section is not OK, since at the end of the dial, tuning cap fully open, I get approx 1300kHz. And to him, the low freqs I lose, could be some "non linearity" or "miss tracking" of the antenna and LO sections, that go parallel (aligned) up to a point, but start to lose that IF difference.

But anyway, I need to check-replace all the caps involved. I'll do it the next days. I have enough theory to understand a bit better what to expect.

It's funny, but I didn't even try the PC scope, because it works through the audio input, so I assumed it's limited to the audio bandwidth... I'll give it a try, at least as a freq counter... I don't think it will get that freq though.

Click to expand...

The thing to do with minimal test gear at his point, because the non-linearity of the tuning error is as apparent as it is- which was not mentioned previously, but is very important to diagnosis, the fastest and most effective way to get it working correctly without needing to know any IF frequency is this: Tune to a station as high as you can presently reach with the dial, peak the IF's for the strongest signal. The broad tuning is a symptom of the first IF being off, and it can safely be assumed the other is off as well.

Peak the IF's, check the linearity of what you are able to recieve, and then adjust the oscillator trimmer to bring the station into agreement with what you have for stations, and check for accuracy. Make adjustments to the IF's and oscillator and sometimes the antenna trimer

By doing it this way- you do not need to know the IF frequency at all, and no test equipment is needed- you are simply peaking the IF's to match what you are able to recieve to what you have on the dial and adjusting accordingly. In most cases it takes about 15 minutes and it is done.

It is faster, and is one of the first posts I made on this and while purists may cringe- I have put numerous radios into service by doing it this way with no problems.

Actually, this is the way I've managed to tune some stations...

But when I try to tune the oscillator, to move the station to the right position on the dial, it seems it has a limit. moving it further, I lose the stations and no way to tune them lower in the dial.

I'll replace the caps and resistors, and do it again. I suspect some cap is out of specs and something is not aligned or not as linear as it was designed. It's easier to put new components than desolder the old ones , measure them, and resolder again.

If you peak the IF's for strongest reception first, then you can make adjustments to the oscillator if needed. If you need to reduce capacitance because the screw needs to come all the way out- remove the screw and bend the metal tab so it looks like (. Or with a reference- instead of "ll" you have "l(" This will reduce the capacitance of the trimmer.

By peaking the IF's first, you make them less broad in their range and you bring them together by doing so. If as an example, one is low and one is high- both are broad in their range of what they pass, and make the range of tuning on the dial very broad for very few frequencies.

Once they are peaked, you can make slight adjustments to the antenna trimmer and oscillator trimmer to bring the stations into their correct location. The key is to keep trying to tune to a higher station as you go. If you lose tuning altogether, try a different ECH81 as yours may be too selective in operating range. The old one may function in other sets- just not this one.

As you move stations on the dial into position, go back and check the IF peaking and readjust as needed as you progress. Normally it only takes 15 minutes, but sometimes you need to go back through the peaking steps as you get things closer many times and take longer. When done with this process once you are done, the IF's will be at their correct frequency, and it can be identified again with the second tuner and written down on the schematic you have for future reference.

All of the parts I mentioned previous should be checked for merit and value, and if they are not correct- yes, replace them. Were it mine, I would reflow all of the soldered connections in the tuning section just to make sure they were good, and all ground connections to the tuning gang metal body were good and secure.

I think we have a guilty:

R4, what feeds the 83V to ECH81, should be 33Kohm (actually in my unit, there is a 22K resistor), and it measures 760K !!!! So I have 6V instead of 83 V !!!!!!!! I'm measuring 177V across the resistor.

The caps seems to be all within specs, actually very close, I think less than 5%. The only one a bit lower is a coupling cap, C11, measures 0.0017uF instead of 0.0022, but it's a 20% less...

I'd fault that resistor a whole lot more than the capacitor. The 20% tolerance tells me its not super critical.

Well, replacing the resistor, I've got 100V at the tube, that solved INSTANTLY the low freqs problem, I started to receive down to 530kHz...

With a minimal tweaking, I'm getting from 530 to 1440 at the top end !!!

I can even tune a weak Tango station on top of that 1440kHz, what the digital tuner can't !! (Edited, it's 1550 what I'm tuning at the top end Perfectly clear, much better than in the digital tuner)

I'll keep adjusting...

BTW, all noises and whistles disappeared when the tube received the right voltage.

Edited: It's official: 530kHz to 1550kHz, full closed to fully open , perfect reception at both ends.

Awesome, nice work! :thmbsp:

Thank's to all of you who taught me all this about the "heterodyne"...

Now, if I could tune some short wave station.. that would be GREAT !!!