Grunow model 580 only tracking on half of tuning cap - Printable Version
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+--- Thread: Grunow model 580 only tracking on half of tuning cap (/showthread.php?tid=22842)
Grunow model 580 only tracking on half of tuning cap - NostalgiaRadioTime - 09-21-2021
Schematic Here
Did the usual recap, changed the out of tolerance resistors and have it playing strong. However despite my efforts to do an alignment (and even prior to the 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.
RE: Grunow model 580 only tracking on half of tuning cap - mikethedruid - 09-21-2021
Check the oscillator section of the tuning condenser to see if there are any shorts that happen at a certain point in the meshing. You can do this by hooking a multimeter set to ohms across the section, and very slowly tuning across the whole range. It should register 0 ohms across the whole range. If it does not, you will be able to find where the short is occurring and then carefully bend the condenser plates back into position so they don't short. while you are at it, check ALL the sections of the tuning condenser.
RE: Grunow model 580 only tracking on half of tuning cap - NostalgiaRadioTime - 09-21-2021
Thanks, Mike. I gave the tuning capacitor the once over and the fins look 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.
And this is the tuning capacitor with the radio playing beautifully at 1470 AM the strong station here in West Bend.
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
This one has me stumped.
RE: Grunow model 580 only tracking on half of tuning cap - mikethedruid - 09-21-2021
The voltages are a little off, but that can just be due to line voltage and to resistor drift. Looking at the pictures of the condenser positions for 620kc and 1470kc they seem a little "unmeshed" for both frequencies, but it is very difficult for me to say since I cannot see the dial scale. I would try a complete alignment, following the instructions given in Riders for that model. http://www.nostalgiaair.org/pagesbymodel/372/M0008372.pdf The notes on the alignment instructions say the proper IF frequency should be stamped on the back of the chassis. It also gives instructions how to check it if it isn't, basically, feed first one, and then the other frequency into the receiver through a 200mmf condenser with the signal from the signal generator on the antenna terminal and the ground to the ground terminal. The correct frequency should give you a signal through. Just read and follow the instructions. I do find it odd that you say the 2 short wave bands work fine and the frequencies are where they should be on the dial. I also noticed that the schematic uses the letter "M" for 1000 ohms, like modern schematics use "K" . It uses "Meg" for megohm, so don't let that confuse you. They did that on some old schematics. It confused me the first time I ran into it.
RE: Grunow model 580 only tracking on half of tuning cap - NostalgiaRadioTime - 09-21-2021
That's what I'm going to try next is re-doing the alignment once I have the dial reassembled. The string was broken and so I had to take the whole face and gear mechanism off to get to it to restring. Once I have that on, and the face on and the dial pointers I can do a more accurate alignment and see if the trimmer isn't just turned in (or out) several turns too many. I had the tuner off to replace the grommets underneath and that could have messed up the alignment because I had to unsolder the wires from it. Yes I caught the "M" for 1000 ohms instead of the more common "K". Luckily all of the resistors I replaced still had vibrant colors so I could double verify from the schematic verses what the actual resistor code read. You're right though, years back when I first discovered that it was confusing to me as well! Will keep you posted once I get the dial restrung....next to coil rewinding my least favorite part of radio restoring!
RE: Grunow model 580 only tracking on half of tuning cap - mikethedruid - 09-21-2021
Your broken dial cord reminds me of the Crosley I am currently working on. I had to replace the dial cord too. At first it kept coming off the big pulley attached to the condensers, then it broke after I managed to get it back. I had to take the whole dial apart to get it back together. Oh well, it gave me a chance to clean the inside of the dial well. You can read about those adventures here: https://philcoradio.com/phorum/showthread.php?tid=22817 which is an ongoing thread. I am just waiting now for some nice weather to build the cabinet. I've been busy the past couple of days drawing the blueprints and thinking it out.
RE: Grunow model 580 only tracking on half of tuning cap - NostalgiaRadioTime - 09-26-2021
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 was the electrolytics were replaced. 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. Lesson learned....never assume anything with regards to what appears to be untouched. It can and will kick your....
RE: Grunow model 580 only tracking on half of tuning cap - mikethedruid - 09-26-2021
Congratulations, Greg ! It's always nice when we figure out what is at the root of a problem. Good lesson from this. Assume nothing. As we go through the radio, check each piece we remove against the schematic. You are not the first to have found some odd things which looked original. Glad it all worked out.
RE: Grunow model 580 only tracking on half of tuning cap - John Bartley - 09-26-2021
Nice work! Persistence sometimes pays off and yes, people can do strange things sometimes. This reminded me of a radio I was given 20+ years ago. It had been my great-uncle's and he had taken it to a radio repair shop decades before (a Crosley Mohawk) and when it came back it didn't work, so he put it away and ignored it rather than go back. I spent hours .... many hours trying to find out why the voltages wouldn't measure correctly even though ALL the parts were in their correct places and in good shape.
Turns out some *$%#@ had clipped out the wire that was the B- return ........ it just wasn't there .... period. I wired it in and that radio played like a champ!
Good for you !