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 Post subject: Re: RCA 9-INT-9 Restoration Step 1, Step 2
PostPosted: Nov Fri 09, 2018 7:58 pm 
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My tubes came in, so I did a smoke test this morning, with good news and bad news. First, the good news: great audio! Just what you would expect from a German radio. And my new EM34 works.

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RCAg22.jpg
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The bad news: Completely deaf on AM, SW1, and SW2. On FM, I am getting the top half of the FM band, but it's coming in on the lower half of the FM dial scale :shock: 107.3 MHz comes in at about 98 MHz, with 95.3 MHz coming in just above 88 MHz.

I will probably see if I can figure out what's going on with the AM first, then work on the FM. I cleaned the piano switches earlier, but there could still be an issue. The 6AJ8 AM mixer and oscillator is a NOS tube that tested OK, and I don't have another to subsitute. I think all the other tubes share functions with FM.

I know someone has been in the radio in the past as the selenium rectifier was replaced. I wonder if someone screwed with the FM section or alignment? Any thoughts on what could throw the FM band by 10 MHz?

While I was waiting for the tubes to come in, I worked on the cabinet. The cabinet was dirty, but otherwise in very good condition. There was the usual fine crazing of the original nitrocellulose lacquer finish. There were no scrapes or scuffs in the wood, just wear along the top edges of the radio, exposing raw wood.

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RCAg19.jpg
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I masked off the front grille and the two tweeter grilles on either side, then cleaned the cabinet with turpentine and 0000 steel wool. I used Minwax Special Walnut stain to re-stain all the exposed edges, then gave the entire cabinet a light wipe-down with stain. I let that dry for a couple days, then gave the cabinet a light misting of spar urethane. I don't have a good photo yet as the radio is still apart, but I am happy with how it turned out, with very little work.

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RCAg21.jpg
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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 12:31 am 
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Ed...I'd check for issues around the ECH81 triode section. This is the local oscillator for AM & SW. If you have a signal generator, see if you can push a modulated 460kHz signal through, applied to pin 2, the grid of the ECH81 mixer section. Check for correct voltages around the oscillator. As for FM, confirm first that the tuning cap is actually properly aligned to the dial indicator. Otherwise being that far off is indicative of a failed cap in the front end oscillator circuit. That's an unusual front end arrangement for a German set, as most use a 6AQ8 instead of a 12AT7.

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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 12:51 am 
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Jim, thanks so much for the suggestions. I followed the dial stringing diagram carefully, so I don't think the indicator is off, but I will double check everything and check out the FM section. And I do have a signal generator and will start trouble shooting the AM RF section as well.

I will scan the schematic and add a link later this evening for anyone who wants to look at it.

I will post an update when I have some news, but I welcome all suggestions!

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Last edited by Ed Morris on Nov Sat 10, 2018 12:56 am, edited 1 time in total.

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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 12:52 am 
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Location: Clearwater, KS
I kept the new Amperex EM34 eye tube I had for mine. It cost me almost as much as the radio. I need to get rid of it.


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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 1:16 am 
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Ed:

Great thread...keep up the good work!

BR,

Terry


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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 5:57 pm 
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I was able to get the AM section working this morning.

The problem turned out to be the dropping resistor I placed between the rectifier bridge and and the filter caps. The dropping resistor reduced the source voltage from 300 volts to 286 volts. At the plate of the 6AJ8 mixer (pin 6), I was seeing 268 volts where there should have been 285 volts. At pin 8, the plate of the 6AJ8 oscillator, I measured 117 volts, instead of the expected 125 volts.

Back in 2011, I restored a Zenith 835 high fidelity AM/FM set that did the same thing after a recap: FM, no AM. The problem was low voltage at the oscillator after installing a silicon diode and dropping resistor. I learned then that some oscillators may not oscillate if the voltage is off by very much.

http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=177802

I bypassed the 100 ohm dropping resistor and the AM band woke up. Voltage after the rectifier bridge measured 302 volts. At the 6AJ8 mixer plate I now had exactly 285 volts and 125 volts at the oscillator plate. Two local stations came in strongly, but none of the others.

Here is a detail of the mixer/oscillator section:

Attachment:
6AJ8detail.jpg
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Voltages with a half-moon symbol are the AM voltages. Here's a link to the full schematic:

http://elmphotography.com/radios/radiojpgs/RCA9int1.jpg

I connected my external long wire antenna and was able to pick up all the local stations. So I may have a problem with the rotating loop antenna and/or with the piano keys. I plan to do some more cleaning and working the switches to see if that will help.

FM, on the other hand, is still not working properly; I'm only getting the upper half of the FM band, and it shows up on the lower end of the dial scale. I have several 12AT7's around, so I may try substituting and see if that makes any difference before I dig any deeper.

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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 8:11 pm 
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Just out of curiosity, what kind of meter are you using to take your voltage measurements? Something is unusual. An 8 volt difference on the oscillator shouldn't stop it from running. Most radios with this tube run the oscillator at b+ of about 110volts, and they work fine. I can run most German sets at 85 volts AC in and still get AM to play. And it still seems like you have a problem with sensitivity. Did you try running with the resistor both in and out to see if you can duplicate the problem?

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 Post subject: Re: RCA 9-INT-9 Restoration Update, Step 3
PostPosted: Nov Sat 10, 2018 11:02 pm 
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I’m using a Fluke 27/FM. I did some more work on the switches, which improved reception using the ferrite loop. I removed the dropping resistor, so I didn’t test again with it in circuit.

On this radio, the TAPE key is partially depressed in AM mode to engage the loop antenna. After more cleaning, it worked better, and rotating the loop increased or decreased signal strength. However, it is not as sensitive as it should be. More switch cleaning may help.

With the long wire, I was also able to pick up several stations on SW2 this afternoon between 11 and 14 MHz, with good signal strength.

I still need to go through the radio and check for out of tolerance resistors.

On FM, trying a different 12AT7 didn’t help. Voltages at the 12AT7 RF AMP were all good, but low at the converter. I’m only getting 142 volts at the plate (pin 6) of the converter; it should be 185 volts. At the grid, pin 7, voltage is -.62; it should be -2 volts. Voltage at pin 8 was 0 volts, which is correct.

Voltages at the 1st FM IF AMP (6AJ8) were all close to spec. That’s as far as I got today.

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 Post subject: Re: RCA 9-INT-9 Restoration, AM working
PostPosted: Nov Mon 12, 2018 6:28 pm 
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Ed:

Keep up the good work. This is one of my favorite RCA radios. In fact, please keep me in mind should you ever decide to sell it!

BR,

Terry


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 Post subject: Re: RCA 9-INT-9 Restoration, AM working
PostPosted: Nov Mon 12, 2018 10:59 pm 
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It's great to see all the pictures detailing this project. They give anyone new to restoring a German-made radio insight and a comprehensive overview for what to expect before diving in. In my experience, with some minor differences, almost all of the brands use similar circuits and chassis construction techniques.

Dave


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 Post subject: Re: RCA 9-INT-9 Restoration, AM working
PostPosted: Nov Mon 12, 2018 11:04 pm 
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Terry: I will.

OK, I really don't want to start replacing components beyond the recap I've already done until I have a better understanding of what's going on. This will probably be a long post as I update those of you following this with what's going on.

First, I don't think there is a sensitivity issue. With the external long wire, I've got wall-to-wall reception on the broadcast band at night, and SW2 is picking up in the daytime, and SW1 at night. When I kick in the AM loop antenna, though, I get very little. I think it may be an RF amplification issue in the AM loop coils or circuit. FM that I can pick up come in strong and with great audio quality. There is no distortion on AM or FM.

Second, I'm still only getting the upper half of the FM band on the lower end of the dial scale. I think someone may have messed with the oscillator coil. In the photo below, L4, the FM RF coil, still appears to have the adjustment slot plugged with a sealant, but the sealant has been dug out of L6, the FM Oscillator coil, as if someone has attempted to adjust the coil.

Attachment:
AlignFM6.jpg
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Third, before doing anything else, I decided to do complete voltage and resistance check against the values in the SAM's Photofact schematic.

The chart below shows the voltages I observed. The values in RED are values that appear to vary significantly from the expected value (shown below in BLUE).

Attachment:
Volts.jpg
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I don't know enough to know which values are of concern, but it seems to me that the 145 volts on the 12AT7 converter plate (pin 6) is a concern, and some of the 6DA6 values (both tubes) are way off.

As for resistances, while there are some anomalies, I didn't see anything to far out. But again, I'm may not recognize an issue that I should.

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Ohms.jpg
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Here are the expected resistance values:

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ohms2.jpg
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I apologize for the lengthy post here, but I am grateful for any guidance at this point. I do plan to go through and check resistor values as the next step.

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 Post subject: Re: RCA 9-INT-9 Restoration, New Update Nov 12
PostPosted: Nov Sat 17, 2018 4:18 pm 
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No real progress to report. I checked all the resistors, and I only found one that was out of tolerance, a 20 Meg Ohm resistor, R36 on the Sams schematic, which measured almost 25 Meg Ohms. All the other resistors were well within 10%, in fact, most were within two to three percent of value (see photo below, which is typical of most of the resistors in this radio).

I had hoped to find more that had drifted high that would account for some of the low voltages. They only thing left are the small value caps.

I still have two issues, the weak signal on the AM loop antenna, and the FM tracking.

Attachment:
RCAg24.jpg
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 Post subject: Re: RCA 9-INT-9 Restoration, New Update Nov 12
PostPosted: Nov Mon 19, 2018 3:16 am 
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Which capacitor in the FM oscillator circuit is most likely to be open?

Today I tried adjusting the FM Oscillator Trimmer to see if I could shift the FM band up towards the top of the dial scale. At present, I have 107.3 coming in at about 98 MHz.

Turning the oscillator trimmer adjustment screw about 1/4 turn in either direction moved the station about 1/16" of an inch up or down on the dial scale. Any more either way, and the station just disappeared.

So could I have an open oscillator trimmer or is some other capacitor in the path likely to be open?

Working on the FM sub-chassis is going to be very difficult, and before I start lifting caps, if anyone has some suggestions on which are most likely to cause the problem I have, I would be very grateful.

Below is a photo of the sub-chassis and detail from the SAM's Photofact. I'm really in over my head here, and don't want to make the problem any worse.

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FMsc4.jpg
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Attachment:
FMsc2.jpg
FMsc2.jpg [ 124.59 KiB | Viewed 2980 times ]

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Mon 19, 2018 6:37 pm 
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The obvious place to start would be C15 and C16.

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Mon 19, 2018 6:59 pm 
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John, thanks for the advice. I have ordered some trimmers and fixed capacitors, including the 4pF caps I would need for C15 and C16, from Mouser. I won't get them until after the holidays.

I have been going through old threads on FM oscillator issues and found a thread from 2007 with a comment from ARF member Jkaetzjr:

"Bad osc. trimmer normally drives the entire band toward the low frequency end so many low end stations aren't received...Failure of the osc. trimmer open shifts the band down. If that is the case, you can tell by tuning a station and adjusting the trimmer, in which case it will have little or no effect. The ceramic kind often fail open. If it shorted, FM wouldn't work at all. "

This seems to describe my symptoms closely. What do you think? Hopefully when I get the trimmers and capacitors in, I can find the culprit, and get the full FM band working.

Thanks again, John, for your help. I will update the thread when I have some news.

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Tue 20, 2018 3:30 am 
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Here's a You Tube video of the radio on the workbench:

https://youtu.be/JcSaGI8Lns0

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Tue 20, 2018 2:05 pm 
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Ed Morris wrote:
John, thanks for the advice. I have ordered some trimmers and fixed capacitors, including the 4pF caps I would need for C15 and C16, from Mouser. I won't get them until after the holidays.

I have been going through old threads on FM oscillator issues and found a thread from 2007 with a comment from ARF member Jkaetzjr:

"Bad osc. trimmer normally drives the entire band toward the low frequency end so many low end stations aren't received...Failure of the osc. trimmer open shifts the band down. If that is the case, you can tell by tuning a station and adjusting the trimmer, in which case it will have little or no effect. The ceramic kind often fail open. If it shorted, FM wouldn't work at all. "

This seems to describe my symptoms closely. What do you think? Hopefully when I get the trimmers and capacitors in, I can find the culprit, and get the full FM band working.

Thanks again, John, for your help. I will update the thread when I have some news.


Ed, I was under the impression that the trimmer was indeed moving the oscillator frequency somewhat, but not enough to get it to properly track. I suppose there might be a problem with it, but you'll just have to experiment and see. I have a good quality capacitance meter which really comes in handy for problems like this. It's unfortunate that you have problems in this part of the set. It's never easy to do a repair in an FM front end.

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Tue 20, 2018 2:44 pm 
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It only moved a fraction of an inch. I've got a BK 815, a L/C IIB, and one of those cheap Chinese meters. Hopefully one will be good enough. I'm going out of town for a week, but my Mouser order should be in when I get home and I can track down the problem.

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 Post subject: Re: RCA 9-INT-9 Restoration, Need Help with FM
PostPosted: Nov Wed 28, 2018 10:38 pm 
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It might be time to move this thread to the Electrical/Mechanical forum. I got back home last night and my Mouser order was here. So here's what I've done so far:

1. This morning I proceeded to test C15. It tested good at 4pF, but since I had to remove it to test it, I replaced it with a new 4pF cap. I tested the FM, and no change: good audio--half a band. I then tested and replaced C16 with a new 4pF cap, with the same results.

2. Next I disconnected one lead from C14, the oscillator trimmer. It tested good, with a pF range from 3pF to 10pF. It was set at about 6.5pF when I first tested it.

3. I then connected a jumper to reconnect the clipped lead to the oscillator trimmer, and had no FM at all. I tried adjusted C14 throughout its range with no effect.

4. I then tried bypassing the trimmer with fixed capacitors, using a 4pF, a 6pF, and a 8pF. I could very faintly hear one station at maximum volume, not well enough to identify the station.

5. I re-soldered the clipped lead to the oscillator trimmer and I once again had good audio on FM, but just the top half of the band coming in on the lower end of the dial scale as before.

6. I tested and replaced C17 with a 15pF capacitor with no improvement.

So that's where I am now. I was not able to find exact values for the other capacitors in the FM section, so I have not done any further testing or replacing. All the caps I have removed so far have actually tested very close to their stated values.

The questions I have now are

1. Whether I should try adjusting any other coils or trimmers? The only coil that appears to have been tampered with is L6, the oscillator coil. Before I mess with it, what would adjusting L6 likely do?

2. All the other coils appear to have some kind of wax seal in the top of the coil. The other two trimmers are the FM antenna trimmer and the FM radiation trimmer. Should I try adjusting those?

3. Should I continue looking for an open capacitor and replace it with the closest value I have if I find one?

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 Post subject: Re: RCA 9-INT-9 Restoration, Update 11/28
PostPosted: Nov Thu 29, 2018 6:11 pm 
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Ed, At this point I don't think you have anything to lose by tweaking the oscillator coil. I would leave the rest alone. But you may end up finding that nothing you do will cure the problem. FM front ends can be real mysteries when not working right. I would imagine there are other front ends you could substitute for that one, as long as you can get something that has the same turn ratio as the one that's in there. Ideally, you'd find a junker RCA or a Graetz. That would be your last resort. Clearly this has been an ongoing problem on this radio since someone else has been dickering with the oscillator section before you.

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