Model 80 Compact alignment - Printable Version

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+--- Thread: Model 80 Compact alignment (/showthread.php?tid=6923)

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RE: Model 80 Compact alignment - Ron Ramirez - 06-26-2013

Definition of ham fisted: clumsy, awkward, inept.

For what it's worth, I recall having an 80 Jr. on my bench once that refused to work correctly until I substituted another IF transformer from either an 80 or 84 (don't remember which one right now). The original had good continuity. I suspect moisture issues. Anyway, once replaced, the set came to life and aligned properly.

All right, have a nice day.

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RE: Model 80 Compact alignment - Mondial - 06-26-2013

I think Brenda has a good point regarding possible overloading of the IF when injecting a 460KHz signal. You may be injecting 230 KHz but the IF could be actually tuned to the second harmonic and is resonating at 460 KHz.

When you looked at the resonance peak, you had the frequency counter on the output of the generator, but not on the output of the IF. Can you connect the counter to the plate circuit of the detector and see what is the frequency found there when injecting 260 KHz into the grid of the converter? Alternately you could just connect the scope across the IF transformer and measure the period one complete cycle of the IF waveform at resonance. Taking the reciprocal of the period in uS will give the actual resonant frequency in MHz.

What type of signal generator are you using and what is the level in microvolts of the injected signal? Some generators have an asymmetrical waveform rich in harmonics, which also may cause a counter to read at a submultiple of the actual frequency. When doing the alignment what are you using as an indicator to measure peak signal levels?

I think it is highly unlikely that the IF is actually tuned to anything else than 460 KHz. This is the difference frequency that the antenna and oscillator coils were designed to maintain across the broadcast band tuning range. With a 260 KHz IF (and difference freq), the tracking would be way off and the sensitivity would vary greatly across the band. The dial calibration would also be way off.

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RE: Model 80 Compact alignment - Radioroslyn - 06-26-2013

I had a 84 that had a similar problem. Couldn't get a sharp peak when adj the IF. I ended up removing the IF coils from the can and gave good looking over but didn't find anything that was a problem. While I had it apart I change out the grid leak resistor and resolder all or the internal connections. Worked fine after that. A good alignment is key for good performance on these sets!
Terry

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RE: Model 80 Compact alignment - Ron Ramirez - 06-26-2013

Terry - bingo! Thanks much for mentioning that. Yes, the 4 meg resistor inside the IF can is almost always bad, and must be replaced. Improved operation can be had by changing that resistor to 2 meg.

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RE: Model 80 Compact alignment - keithw - 06-26-2013

Hello all,

Found the problem. I have been using my Heathkit SG-8. Terribly convenient but one thing I have never liked about it was the harmonic rich output waveform. Did as suggested and scoped the IF and calculated the frequency. Looks like my SG-8 has more 460 khz in it at 230 khz than it does at 460 khz. I am very happy to have sorted this out and thank all of you for your patience. I need to dig out my other signal generator, a Philco 077. It is a lot bigger but has a much nicer wave form. A little more period correct also.

In any case, more by accident than intent, my IF is peaked at 460 khz.

keithw

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RE: Model 80 Compact alignment - barshnik - 06-26-2013

Excellent, Keith,

I know that was bugging you, glad it is sorted out, lets get is back in the cabinet! Oh, can either use the grill cloth that was in it (don't care for it much), lets wait and see what the supposedly 'period correct' cloth looks like, it should be here tomorrow or Friday.

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RE: Model 80 Compact alignment - keithw - 06-27-2013

It was bugging me quite a lot. So, moved the SG-8 out of the way and fired up the recently repaired and sorted Philco 077 signal generator. As I mentioned, this is what this radio would have been alligned with back in the day. It has a very nice clean output wave form unlike the raspy mess of the SG-8. The allignment went very smoothly and everything did what was expected. I checked the sensitivity across the band and there is a slight drop off at the very bottom around 550 khz.

What is the best way to get to that 4 meg resistor in the IF can? The can is rivited to the chassis and I am not thrilled about drilling them out. What are the symptoms of it being bad?

On the grill cloth. Let's wait until the new cloth arrives. The stuff in it is awfull.

keithw

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RE: Model 80 Compact alignment - Ron Ramirez - 06-27-2013

The IF transformer is held in by one bolt, under the chassis. Remove the bolt and the transformer can be pulled out from underneath. The resistor in question is on the very top of the transformer.

I have found the best way to replace the resistor is to clip the leads right at the resistor body and then attach the new resistor to the stubs. I tried unsoldering one of those resistors from the transformer once...and ruined the transformer.

If your 80 now works to your liking, it is probably best to leave the resistor alone. In my experience, replacing it with a 2 meg resistor really helped the set's sensitivity.

Oh, and congratulations on finding the problem!

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RE: Model 80 Compact alignment - morzh - 06-27-2013

Actually the grid leak resistor in series with the coil (parallel to the cap) is the same as just connecting that same resistor from the grid directly to the chassis.
So you do not even have to disassemble the transformer - you could try to simply throw the resistor to the same chassis point where the IF transformer's centertap connects.
And if you want to try 2Meg i/o 4Meg (provided your 4 Meg is OK) - throw another 3-4 Meg resistor from the Grid to the Chassis.

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RE: Model 80 Compact alignment - keithw - 06-27-2013

I am not unhappy with how it is working, although I am sure that I will not be able to resist digging into it since it does not involve drilling out those rivits. We were also planning to do the preformance mod involving the tickler on the front end to see how it works. This thing is about ready to be put to bed.

keithw

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RE: Model 80 Compact alignment - barshnik - 06-27-2013

Was over at Keith's this afternoon. With the 4' loop, it'll pick up anything in the area of NV UT AZ at night - it is really working well. With a 5' wire, it'll pick up anything in town. It sounds great, don't know what more I could expect. We'll post pics when it is back in the cabinet. Lots of history goes with this set, and it works well despite with folk say about the 'cheap' 4 tuber.

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RE: Model 80 Compact alignment - codefox1 - 06-28-2013

Crunch, use old times heat sink or roach clip, if you still have one, and replace that suspected or guilty component without causing further problems.

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RE: Model 80 Compact alignment - Radioroslyn - 06-29-2013

I've always found the performance of the four tube sets to be quite good. The later models have a pentagrid mixer which is a bit better than the audyne job. With a good ant and gnd will work as good or better than a AA5 set.
Terry

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RE: Model 80 Compact alignment - keithw - 07-16-2013

Back from my road trips and figured I should finish this up. Dug out the grid leak resistor. It measured 4.6 meg rather than the 4 meg it is supposed to. I added a parallel 4.7 meg resistor bringing it down to about 2.3 meg. This did seem to improve the sensitivity.

There are a couple of other performance mods that John dug up that I will try before putting this one to bed.

keithw

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