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 Post subject: Radiola 18 no sound
PostPosted: May Fri 24, 2019 4:09 am 
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Joined: Jul Wed 19, 2017 3:33 am
Posts: 67
Location: Houston, TX
This radiola 18 that just produces static. I’ve fixed/replaced the following so far.
• I replaced he output choke with the primary of a universal output transformer and am using a 101a loudspeaker.
• I replaced the capacitors in the SPU because they were in the same box as the bad choke.
• The grid bias resistor, R2 and R3, and the B+ dropping resistor, R1, as labeled in the nostalgia air article)
• I replaced the concentrated coil in the 3rd RF transformer. I used an IF coil out of a junker radio of similar DC resistance (26ohms) and physical size. I tried to make sure it was installed into the RF transformer with the same winding direction as the original.
• Volume pot was cleaned and tracks well.
• Sandpaper and DeoxIT tube pins and sockets.
After these repairs, I turned on the radio and I could only hear quiet static. The static does seem to respond with the tuning dial. The B+ on all the plates look good; 170V on the 71, 30V on the 27, and 130V on the 26's. The filaments all light up bright and the filament voltages are normal. The voltage of the grid with respect to the filament on the 26's were -6V. I compared this to a 2nd working Radiola 18 and it has a grid to filament voltage of -12V. After this I replaced the filament filters and B+ filters in the receiver section thinking it might help. It didn’t. The audio section is loud and clear if I inject audio in the grid of the 26 after the detector. If I inject RF through the pot I can hear it through the loudspeaker. Then I scope the plates of the first three 26 tubes. There is not much gain from stage to stage. Here’s where it gets interesting. I swapped the PSU of the repair unit with the one from the working Radiola 18, and it worked! So I think, ah ha! There is something wrong with the grid bias circuit in the SPU. I followed all the leads going from center tapped R6 resistor. And they look like they are going where they should be which makes sense because the original cable harness looks fairly untouched. I’m attaching an illustration of how that bias circuit is physically wired up. The ground is in green. It goes from Lug 9 to where C2 and C3 used to tie to ground (the new ones are on a grounded terminal strip next to the filter reactors). Those lead wires are now soldered together. The lead then goes to the center tap of R5 and then to where R2 and R3 are grounded. The red lead is what connects the center tap of R6 to the high side of R3. I think the orange lead is the shielded ground wire for the power transformer. I disconnected all the leads going to R6 to get a good resistance reading on it and the 1.5V filament winding. They both look good. I started at it until I went cross eyed trying to find a mistake, but it looks fine to me. Anyway, I still think the problem is the low grid-filament voltage, but I can’t figure out what’s causing it. Any ideas of what I should check?


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 Post subject: Re: Radiola 18 no sound
PostPosted: May Fri 24, 2019 1:05 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 1081
Location: St. Louis, MO, USA
Do you have a good antenna and ground? I didn’t see in your post that you tested the tubes. Good plate/filament/grid voltages don’t compensate for a bad tube. Another test you can do is touch the antenna lead to the grids/plates of the RF tubes and see if you get stations. Work backward from the detector to the first RF tube. This sometimes works better than using a signal generator.

Dennis


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 Post subject: Re: Radiola 18 no sound
PostPosted: May Fri 24, 2019 4:00 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9337
Location: Beaver Falls, PA. USA
If the radio does work with the other power supply unit, you're halfway there! Compare the wiring and components of both units, and you will find the problem.

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Tim KA3JRT


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 Post subject: Re: Radiola 18 no sound
PostPosted: May Tue 28, 2019 4:23 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 30
Whats the grid voltage respect to ground? In these radios I believe the grid is close to ground potential and and the filaments/ cathodes are positive with respect to the grid.
What are the filament voltages in the RF section?


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 Post subject: Re: Radiola 18 no sound
PostPosted: Jun Thu 06, 2019 3:09 pm 
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Joined: Jul Wed 19, 2017 3:33 am
Posts: 67
Location: Houston, TX
I figured out one of the problems was that the original grid/filament bias resistor for the 26 tubes was actually good. I had tacked in a replacement on top of it. This cut the resistance in half due to the parallel resistors which in turn lowered the gird/filament bias. I removed the replacement resistor and the grid/filament bias jumped to -9.5V. I can pick up about 4 stations. It still seems a bit deaf. What is really strange is that half way through the dial the radio goes silent. Not even noise. This is on the lower half of the dial as the the tuning condenser becomes more meshed. Does that sound like the rotor/stator plates are shorting?
Also, I'm grounding the pot of radio to the ground lug of my house. The tubes all test great. The AC filament voltages at the tubes are all what they should be 5V, 2.25V and 1.5V. The grid to ground bias is close to zero for all the tubes.


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 Post subject: Re: Radiola 18 no sound
PostPosted: Jun Fri 07, 2019 9:06 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 9337
Location: Beaver Falls, PA. USA
Check the capacitor plates carefully; some manufacturers used plating on them, which can peel off and short them out. An ohmmeter check of the cap section, with the coil disconnected, will tell if it has a short..

The rotor and stator plates cannot touch. If one of them is bent, carefully straighten it.

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Tim KA3JRT


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 Post subject: Re: Radiola 18 no sound
PostPosted: Jun Mon 17, 2019 5:26 pm 
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Joined: Jul Wed 19, 2017 3:33 am
Posts: 67
Location: Houston, TX
It turned out that this radio had not 1 but 2 broken concentrated coils. The 2nd one in the RF chain I had replaced with secondary of an IF can. The first had already been been removed and replaced with some kind of washer. The two lead going to the original coil were just shorted together on the washer. I did not take it apart to see exactly what was going on. I just ordered a salvaged RF coil assembly off eBay and replaced the whole thing. That brought the radio back to life. Restoration complete.


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