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 Post subject: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sat 07, 2017 6:44 pm 
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Bought this radio on eBay the other day and I want to use if for a troubleshooting demo I have planned. AM works fine, but the FM oscillator doesn't seem to be working. I can get a 10.7 Mhz IF signal through the radio, but it will not tune in any stations. Below is the result of voltage checks on Q2 and Q1.

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I also pulled both transistors and tested them, they both test good. Checked all of the resistors in the area and they are all within tolerance. All of the caps are disc ceramic. My feeling is the voltage on the base and emitter of Q2 is the reason it isn't oscillating, but what is the solution?

From all the reading I have done, I know transistor based oscillator circuits can be problematic. Any suggestions?

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sun 08, 2017 4:42 am 
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Update, I got another one of these radios in the mail today. Put it on the bench to test it out, AM works fine, but same exact problem with FM. Plenty of FM hiss, but no stations. I think it's a conspiracy. :(

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sun 08, 2017 5:24 am 
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mosslack wrote:
Update, I got another one of these radios in the mail today. Put it on the bench to test it out, AM works fine, but same exact problem with FM. Plenty of FM hiss, but no stations. I think it's a conspiracy. :(



I guess I missed it, but how did you determine that the oscillator was not operating? I would have expected the discriminator (particularly, the electrolytic cap across the output of the discriminator) was dead in both cases.

Brett


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sun 08, 2017 6:01 am 
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I put a 10.7 Mhz IF signal through the radio (just put it near the antenna terminal) and it goes through just fine, even the AM 400 khz tone comes out the speaker no problem. I always thought if that was the case and it could not tune any stations in that it must be the oscillator isn't working. Plus the voltages are not right on the base and emitter of the converter transistor.

I will check the cap you spoke of and report back. Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Tue 10, 2017 7:30 pm 
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Brett_Buck wrote:
mosslack wrote:
Update, I got another one of these radios in the mail today. Put it on the bench to test it out, AM works fine, but same exact problem with FM. Plenty of FM hiss, but no stations. I think it's a conspiracy. :(



I guess I missed it, but how did you determine that the oscillator was not operating? I would have expected the discriminator (particularly, the electrolytic cap across the output of the discriminator) was dead in both cases.

Brett

Did as you suggested, old one had gone higher, supposed to be 4.7uf and was 8.1uf. Replaced with a NOS 4.7uf, no change.

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Wed 11, 2017 10:00 pm 
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Okay, even though Q2 tested good, I decided to try another transistor to see if it made any difference. Only thing I had on hand was a 2n3904 which I put in. Voltages are now more normal on the base and emitter (less than a volt on each) and still 11 volts on the collector, so that is good, but still no apparent oscillator action. My plan it to buy a proper replacement (NTE 108) and try that. Hopefully that will get it going.

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 5:05 am 
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To check the oscillator, you can tune another radio 10.7 mhz above or below the dial setting of the radio under test. (Don't remember if it's on the high or low side).

I did this one time to my second cousin, who is a brat and playing the stereo too loudly. Asked her to turn it down and got a snarky reply. I just turned on my radio which was on the other side of the wall and the LO from my radio made her stereo go nearly dead. (Dead carrier). She had no clue what was happening and turned off the stereo.

Yup, I am a real jerk at times. hahaha.

Looking at the schematic, I see the collector of Q1 connected to ground via a tapped coil. Can't understand how there is 11 volts present? Wondering if the schematic is correct????

Hope that helps,
Mitch


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 1:51 pm 
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Q2 also has it's collector finding ground through coils but has 11 volts on it. I assume that chassis ground is positive and that the reference when measuring voltages in this set is the negative power rail.

Dave


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 3:01 pm 
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PBPP wrote:
To check the oscillator, you can tune another radio 10.7 mhz above or below the dial setting of the radio under test. (Don't remember if it's on the high or low side).

I did this one time to my second cousin, who is a brat and playing the stereo too loudly. Asked her to turn it down and got a snarky reply. I just turned on my radio which was on the other side of the wall and the LO from my radio made her stereo go nearly dead. (Dead carrier). She had no clue what was happening and turned off the stereo.

Yup, I am a real jerk at times. hahaha.

Looking at the schematic, I see the collector of Q1 connected to ground via a tapped coil. Can't understand how there is 11 volts present? Wondering if the schematic is correct????

Hope that helps,
Mitch

Thanks Mitch. Yeah I knew another radio could be used, but I didn't remember exactly how to go about it. I watched a YouTube video that shango066 made about a similar problem on a GE and he covered using another radio to test the oscillator. That radio also used an NTE 108 for that oscillator and he had tons of problems getting it to oscillate.

I wondered about the schematic also, but if you look, the reference point for the meter is labeled as just plain B. Not B+ or B-, just B. I've had other transistor sets I've worked on that use this backwards setup, almost like they use a positive ground or something like that.

At least now I know the there was a problem with Q2 cause when I swapped it with the 2N3904, the voltages on the base and emitter fell into place. Hoping a new NTE 108 or equivalent will fix the problem, but you never know.

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 3:07 pm 
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Dave Doughty wrote:
Q2 also has it's collector finding ground through coils but has 11 volts on it. I assume that chassis ground is positive and that the reference when measuring voltages in this set is the negative power rail.

Dave

I believe that would be a correct assumption Dave. Odd they just call the reference point "B". I've worked on other transistor radios that have a positive ground also.

Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 5:02 pm 
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Nice catch Dave. I didn't see the path to ground on Q2.

Still, something is going on which we aren't seeing. You tested both transistors and they checked good. Subbing in a modem silicone part brings the voltages back in line but no oscillation. I am assuming the original transistors are germanium. If so, it doesn't tell us much.

In addition, you have a second set with the same symptoms. That would suggest a design flaw. I'd hold off on getting an NTE equivalent since they are famous for selling replacements which may or may not work in your particular circuit. I got a replacement part from them years ago and it cost me a lot for this little replacement, which only worked halfway. Found a similar radio with the exact same part. Bingo! It worked again. Had spent days trying to fix it. If you tell me what the original part is, I may have one from a donor set.

So what are we missing here? Some design flaw/issue common to both sets. I get the feeling it might be mechanical. Did you check for any hairline cracks in that section? Gently applying pressure with your finger in that area and wiggling some associated parts might reveal the culprit. Won't hurt to try.

I'll pull up the schematic and look at this further.

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Mitch


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 8:28 pm 
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PBPP wrote:
Nice catch Dave. I didn't see the path to ground on Q2.

Still, something is going on which we aren't seeing. You tested both transistors and they checked good. Subbing in a modem silicone part brings the voltages back in line but no oscillation. I am assuming the original transistors are germanium. If so, it doesn't tell us much.

In addition, you have a second set with the same symptoms. That would suggest a design flaw. I'd hold off on getting an NTE equivalent since they are famous for selling replacements which may or may not work in your particular circuit. I got a replacement part from them years ago and it cost me a lot for this little replacement, which only worked halfway. Found a similar radio with the exact same part. Bingo! It worked again. Had spent days trying to fix it. If you tell me what the original part is, I may have one from a donor set.

So what are we missing here? Some design flaw/issue common to both sets. I get the feeling it might be mechanical. Did you check for any hairline cracks in that section? Gently applying pressure with your finger in that area and wiggling some associated parts might reveal the culprit. Won't hurt to try.

I'll pull up the schematic and look at this further.

TTYL
Mitch

As far as I know this is a 70's radio so the transistors are silicon. I do plan to look closer at the circuit and re-solder the connections, but the odd thing is the second radio with the same exact symptoms. So either this was a design flaw or manufacturing flaw of some kind I'm sure.

Zenith part number for Q2 is 121-613. Q1 is a 121-612. I will report back once I take a closer look and re-solder. Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 13, 2017 10:36 pm 
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Check the continuity of the "J" line: from the "left" end of R5 to and through FM-AM switch.
That line supply the "negative" power supply line - zero volts. It clearly is missing.
Also, since you have measured a reversed voltage on B-E junction of Q1, that can degrade its
noise characteristics. Be prepared to replace it, if the FM noise level will be elevated.


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sun 15, 2017 5:56 pm 
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swam wrote:
Check the continuity of the "J" line: from the "left" end of R5 to and through FM-AM switch.
That line supply the "negative" power supply line - zero volts. It clearly is missing.
Also, since you have measured a reversed voltage on B-E junction of Q1, that can degrade its
noise characteristics. Be prepared to replace it, if the FM noise level will be elevated.

I've been making YouTube videos of this as well and it's hard to remember what I've said here and there, but continuity was checked from the junction of R4 and R5 all the way back through the AM/FM switch to the power switch. Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Wed 18, 2017 9:34 pm 
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You've got an open trace somewhere on the PC board.

Where is "D" coming from on that schematic?


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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Thu 19, 2017 8:15 pm 
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Primitiveradiogod wrote:
You've got an open trace somewhere on the PC board.

Where is "D" coming from on that schematic?

If you check the legend on the schematic you will see that "D" is a test point for the 1st FM IF Input.

I had other things come up and have not got to check the board over yet, but it will be done soon.

Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 27, 2017 3:08 am 
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Working on this radio again today. Took a lot of time and my magnification lamp and went all over the front end of the circuit board. I touched up any questionable spots. I also removed the 2N3904 and cleaned the solder pads for that transistor.

I found a good deal on 2N3563 transistors (50 for $6.61 shipped) and those arrived today. Soldered one in at Q2, but nothing changed. I made a video of this for YouTube and watching that I think it's possible I had not put Q1 back in from the time before. So I will have to check that tomorrow and replace it if I left it out.

I also used a coil of wire hooked to my frequency counter and put it around Q2 to see if I could detect any change in the frequency when I tuned it, but no change. I don't know what it was picking up, but it was way too low to be the FM local oscillator.

Also the voltages on Q2 are messed up again. Only had 8.72 volts on the collector, 3.01 on the base and 2.33 on the emitter. So I'm not sure what that is all about.

I may do some continuity checks on the circuit tomorrow depending on what I find out about Q1. If anyone has any better ideas, please don't hesitate to let me know. Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 27, 2017 4:41 am 
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I couldn't wait for tomorrow, I found that I must of stuck a 2N3904 in for Q1 also, so I removed that. I also think I put Q2 in backwards (reversed the emitter and collector). Gonna double check that and install it correctly tomorrow. I will also determine which transistor Q1 was and put the original back in.

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Fri 27, 2017 4:32 pm 
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Okay, I put Q2 back correctly this time and then I went to put Q1 back in and ran into a problem. I know the original part numbers for each transistor and thought that was what I would be dealing with, but the ones I took out are not Zenith parts. They are Motorola and are marked SPS 3367 and SPS 3368. A Google search for these numbers turn up nothing.

Anyone know where to find info on these transistors?

Thanks!

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 Post subject: Re: Zenith A 410 L (Chassis 8AT 16X) FM not working
PostPosted: Jan Sat 28, 2017 1:04 am 
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I stuck the one in I thought it might be, not a whole lot of difference in the two, and fired the radio up. Still nothing. I messed around trying to find the oscillator signal, but none was found.

Next I rammed a signal into the antenna and did hear the modulation tone. But something else happened as well. A radio station was coming in at that same frequency, so I turned of the modulation and there it was, clear as a bell. I experimented tuning the RF signal generator around until I found a station which gave their call sign and calculated the sum to see if it added up. It did, so the signal generator was acting like the local oscillator and I was able to tune in various station by adjusting the frequency input.

If that's not a clue that the local oscillator is not working I don't know what is, but it also proves that the rest of the radio is working just fine (including the RF amp) as I didn't have any antenna connected to the radio. So as far as I'm concerned, it now has to be some passive component in or around Q2. I plan to first check all the resistors and then start pulling caps to check their values.

My biggest question is, how much does the primary of T201 have to do with it? I can see the oscillator signal has to pass through it to work, so I assume it should be tuned to 10.7 Mhz as well in order for the oscillator to work? The reason I asked is because in looking at it, the tuning slug appears to be very near the top so I'm wondering if some previous person tried to tune it to make it oscillate when the problem was the transistor all along.

I will report back with my findings. I'm NOT going to let this radio kick my butt!

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