I blew out AM section of radio - Page 1
Forum section: electronics / repair

author: tom_servo
author_rank: Newbie
timestamp: March 17, 2020, 09:17:37 pm
content: Greetings, everyone! Hope you are staying safe.So I finished a repair on an Eico tube signal generator and wanted to try aligning a radio with it. I don't really know what I'm doing so I used the Elenco kit radio to keep from destroying a good one. No surprise, ended up damaging something in the radio.I've spent several hours troubleshooting the radio an am getting nowhere. My confusion is coming from the final IF transistor before the detector diode. I can trace the signal on my oscilloscope from antenna to this transistor's base and it appears good. However, the signal coming out of the collector is so small it's almost nonexistent on the scope, and there is a huge signal going from emitter to ground. I tried replacing the transistor with a new one and it behaved the same way. To my uneducated brain, the voltages at all the transistors and the AGC look normal. I changed the detector diode and it made no difference, but I believe there is a tiny signal getting to this diode anyway. The IF coil between the transistor and the detector also checks out good.What am I missing?

author: duak
author_rank: Super Contributor
timestamp: March 18, 2020, 12:40:01 am
content: Tom, what are the DC voltages on the collector and emitter of the suspect transistor? Does the emitter connect to a resistor and is the resistor OK? Is there a capacitor across that resistor and is that OK?The following site shows schematics for typical transistor AM radios:http://www.angelfire.com/planet/funwithtransistors/AJ6-1.html

author: tom_servo
author_rank: Newbie
timestamp: March 18, 2020, 03:06:56 pm
content: I'll check again when I get home from work, but if I recall, I have battery voltage at the collector and something like half a volt on the emitter. There is a resistor with a capacitor across it between the emitter and ground. I have not checked these particular components yet.

author: duak
author_rank: Super Contributor
timestamp: March 18, 2020, 11:09:20 pm
content: The DC voltages sound OK. If there is a large AC signal on the emitter, the cap from the emitter to ground might be open. If there is less AC voltage on the collector than on the base or emitter, then either the IF transformer or the diode on the secondary of the IF transformer cound be shorted,

author: Circlotron
author_rank: Super Contributor
timestamp: March 19, 2020, 01:37:48 am
content: Quote from: duak on March 18, 2020, 11:09:20 pm+1

author: tom_servo
author_rank: Newbie
timestamp: March 19, 2020, 02:51:56 pm
content: Ahh, of course, that makes perfect sense! That is a ceramic disc cap, so I hadn't expected there to be a problem with it, but I will look at it closely after work tonight.By the way, is it better to use a digital or analog meter to measure these small AC voltages? I get almost zero voltage on my digital meter, but a very large voltage on my analog. I forget exactly, since I thought it must be faulty, but it was something like 2v on the emitter and 10 on the collector or vice versa.

author: duak
author_rank: Super Contributor
timestamp: March 20, 2020, 12:04:06 am
content: Tom,It's a rare DMM that can measure AC much above a few kilohertz. Many analog meters can measure higher but they're often wildly inaccurate. Good for detecting the presence of but not accurately measuring such frequencies. When you get to RF, if you don't have a scope that's capable, or an RF AC voltmeter then you can whip up an RF detector probe. Here's an interesting one:http://www.techlib.com/electronics/detect.htmYou can use it as a front end to the meter you prefer to use.

author: tom_servo
author_rank: Newbie
timestamp: March 20, 2020, 04:35:29 am
content: Thanks, that's a really interesting page. I had a feeling my digital meter wasn't capable of measuring that kind of AC voltage.Anyhow, I tried substituting another cap between emitter and ground and it made no difference.The IF coil has the same resistance as the other coils, something like 1.5 ohms if I remember right. Is there a better way to check for shorts?I think the diode is ok, it behaved the same way after swapping in another one.

author: duak
author_rank: Super Contributor
timestamp: March 21, 2020, 04:19:58 am
content: Tom, Unless you want to build up an RF detector, let's try to test the detector that's already in the radio by injecting a bit of signal into the IF strip. if you put a voltmeter set to low DC volts on the AGC line from the detector back to the converter stage (see the schematics in by link) you should see a varying DC voltage corresponding to the signal level. The AGC line is usually connected the cathode of the detector but if the radio uses PNP transistors, the AGC will come from the anode. Note, you will want to connect to the filtered version, not the signal on the diode itself.I think I had one of those EICO oscillators at one time. Set the frequency to 455 KHz and the output level to a mid setting. Using a small capacitor of 22 to 100p in series with the output, touch the other end of the cap to the detector diode of the last IF stage. Sweeping the frequency up and down a few KHz should cause the AGC voltage to vary. You may need to increase the output to make it easier to see the variation. Move the cap to the collector of the last IF to confirm the transformer is working. Move the cap to the base of the last IF to confirm the transistor is working. You should need far less output to get the same variation in the AGC.Hope this makes sense,

author: tom_servo
author_rank: Newbie
timestamp: March 22, 2020, 11:32:46 pm
content: Hmm...Your explanation was great and I understood what you're saying, but I'm not able to get any varying voltage in my AGC. Not by turning the tuning gang or by touching the signal tracer to any point on the radio. It always shows the same voltage.I don't get any variation on my scope when I adjust the IF coil before the detector diode, either. Would this coil cause the AGC not to react to signal strength?

author: Circlotron
author_rank: Super Contributor
timestamp: March 23, 2020, 12:43:29 am
content: Quote from: tom_servo on March 22, 2020, 11:32:46 pmWhat happens if you sweep the signal a bit either side of 455kHz? You should read a peak in the AGC voltage at 455kHz if things are working. You may need to re-tweak the last IF coil now.

author: duak
author_rank: Super Contributor
timestamp: March 23, 2020, 01:08:43 am
content: What signal level are you applying to the final IF transformer? I think you'll need at least 100 mV to see something on the AGC line. Can you see any RF on the non-filtered end of the diode as you sweep the RF through 455 KHz? If not, then it's good evidence that the IF primary is shorted. I think you said that you were aligning the IF strip when it stopped working. If your power supply had enough current, accidently shorting the collector connection to ground could damage the winding. The resistance seems a little low but it's been years since I measured one. If the primary wasn't open, there's a good chance it was overheated and developed a shorted turn or two - the wire is really thin. There's also a chance that the secondary is open.I looked up Elenco and downloaded a manual for the model 550. The good thing is that the IF transformers are very similar and you might be able pull one from another radio, even if it's a different brand. For the most part, the impedances are very similar and as long as it fits if should work. Note that the first and 2nd IF transformers are different from the 3rd.

author: tom_servo
author_rank: Newbie
timestamp: March 23, 2020, 03:07:36 am
content: Destroying stuff, must be break time...Since you said IF coils tend to be similar...I borrowed one from the FM side. This had the effect of burning out the op amp! I think I might have shorted the pins on the back of the board by pressing down on the board. There is a cap back there across 2 pins. I must have pressed the leads onto the pins. Otherwise, I have no explanation for the chip suddenly overheating.The kit comes with a little transistor audio amp board that swaps into the IC socket, so I'm not totally screwed yet. I don't think I have another LM386.Here's what I learned before the mishap:I was remembering incorrectly the resistance of the IF coil in question. It has resistance of 2.5 ohms on one side and 1.5 on the other, whereas the others have .7 on both.I definitely don't get any voltage variation in the AGC from the signal generator, including sweeping back and forth past 455kc. I'm using an Eico 324 and the voltage I'm measuring on the scope looks like 175mv peak to peak on the medium setting.I managed to find a jpg of the radio schematic:http://denysborysiuk.com/Images/Projects/Radio/schematic.JPGThe radio is called "superhet 108". The instruction book says that this particular coil is the beginning (or end) of the AGC.I strongly suspect the coil, but I'm going to knock off for the night before I destroy anything else.

author: Circlotron
author_rank: Super Contributor
timestamp: March 23, 2020, 03:51:47 am
content: An IF tranny from the FM side will be totally unsuitable for the AM side. Designed to run at 10.7MHz so at 455KHz it would appear almost as a short circuit.

author: tom_servo
author_rank: Newbie
timestamp: March 23, 2020, 04:22:40 am
content: I see...damn.That must be what burned up my op amp.If I may ask... what makes the coils different? Number of turns?

author: Circlotron
author_rank: Super Contributor
timestamp: March 23, 2020, 11:27:01 am
content: Less turns and possibly the core material. Much lower inductance.