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 Post subject: Grunow model 580 half there
PostPosted: Sep Tue 21, 2021 3:46 am 
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Location: West Bend, WI
http://www.nostalgiaair.org/pagesbymode ... 008372.pdf

Did the usual recap, changed the out of tolerance resistors and have it playing strong. However despite my efforts to do an alignment, the set only receives from the halfway point of the tuning capacitor to the high end. Meaning when the tuner is open to around where 900 - 950 Khz should be, I receive 620 strong. I receive the rest of the broadcast band from that point up (1470 is right where it should be). When I go below the 620 mark, I can get 540 fairly close to where it should be (just a bit below 620) but then from the half open point to totally meshed I just get static that gradually increases in volume the closer to fully meshed it goes. I can move 1470 up and down with the A5 trimmer, but the A7 trimmer for the lower end does nothing. No increase in signal strength no matter how many turns I move it in or out.

I tried known good 6A7, 75, and 6D6 tubes to no avail. I didn't do the local OSC test with another radio yet but my thought is the OSC must be working somewhat for me to receive all the stations I usually do, just that they only appear from the half way point to fully open on the TC. I've never experienced this phenomenon before and am at a loss as to what to try next. Perhaps some of the very talented and knowledgeable folks here can peruse the schematic and give me an idea of what to zero in on. Or maybe I'm just not understanding the alignment procedure correctly.


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 Post subject: Re: Grunow model 580 half there
PostPosted: Sep Tue 21, 2021 9:47 pm 
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Posts: 270
Location: West Bend, WI
I gave the tuning capacitor the once over and it looks nice and straight and don't appear to be binding anywhere. I'll check it with the multimeter but there is no discernible "click" or "pop" or crackle as you tune that would indicate one "fin" was touching another. Just so it makes more sense, this is the tuning capacitor with the radio playing beautifully at 620 AM, the strongest Milwaukee station some 40 miles away.

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And this is the tuning capacitor with the radio playing beautifully at 1470 AM the strong station here in West Bend.

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That one appears to be correct. And I get all of the local stations in between 620 and 1470....just that it's on half of the rotation of the tuning cap. I'm beginning to wonder, since this is a 3 band radio and only a two gang tuner if they somehow isolated part of the broadcast band for the police band? The Shortwave band has full range of motion, meaning stations can be heard all across the rotation from fully meshed to fully open.

I had the Grunow set at 620 AM and tried the local OSC test with another AM radio and could not find the signal at either 465 (or 490) above on the test radio.

I did take some voltage readings on the 3 tubes involved in the RF section:

6A7
Pin 2 Plate should be 245v, I had 266
Pin 3 SG should be 85v, I had 83
Pin 4 G2 should be 190v, I had 206
Pin 5 Cathode should be 4.5v, I had 3.3
Grid cap read (negative) -1.9v

6D6
Pin 2 Plate should be 230v, I had 269
Pin 3 SG should be 80v, I had 83
Pin 4 G2 & Pin 5 cathode, shows 2.5v combined. I had 1.3 volts on each pin 4 & 5

75
Pin 2 Plate should be 90v, I had 95
Pin 5 Cathode should be 1v, I had 1v

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 Post subject: Re: Grunow model 580 half there
PostPosted: Sep Wed 22, 2021 1:32 pm 
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Location: Near Brandon, Iowa
The antenna input tuning circuit works by resonating a fixed coil with a variable capacitor... this should allow predictable tuning across the entire band. If part of the tuning range is dead or discontinuous, that would point to a defect in one of the resonating elements: a partially-shorted coil, a bad trimmer cap, or a tuning cap which shorts out in some positions.

Unfortunately, a defect in the local oscillator tuning circuit can cause similar-seeming effects: if the oscillator is dead or off-frequency at certain tuning cap positions, it will cause the converter output to be invalid (good signal mixed with junk gives junk out) at those points, OK at others.

I think that you can parse this out by using an oscilloscope to monitor the local oscillator frequency change as the tuning cap is swept through the entire band. The change should be a smooth progression with no dropouts or dramatic frequency jumps. If you observe correct operation of the LO, it can be ruled out as the source of your tuning trouble and you can concentrate on the antenna-signal tuning circuit.


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 Post subject: Re: Grunow model 580 half there
PostPosted: Sep Wed 22, 2021 2:48 pm 
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Location: Beaver Falls, PA. USA
If you have another 6A7 on hand, try replacing it; they often get weak, in the oscillator section.

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 Post subject: Re: Grunow model 580 half there
PostPosted: Sep Sun 26, 2021 5:17 am 
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Joined: Apr Sun 02, 2017 4:18 pm
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Location: West Bend, WI
Finally, after days of tracing, I have it functioning as it should. I've never seen a set as messed up as this one was, yet all the paper caps looked factory. They even had the factory numbers which corresponded to the schematic stamped on them. The only thing that "appeared" molested were the electrolytics were obvious replacements. But someone decades ago majorly shuffled components around the volume and tone controls and basically it wasn't connected right to the 2nd IF. As soon as I got it wired correctly, with the proper caps and resistors where they should be, the trimmer that wasn't doing anything worked normal, and I had the full range on the tuner.

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