AM/FM radio troubleshooting

"klem kedidelhopper has six toes "

** Might be another dodgy chemical fuse - eh ??

.... Phil

Somehow I feel looking at an oscilloscope might help with determining the amplitude of the signal.

and I would hate to blow up an otherwise good IF amplifier with

Somehow I feel that an excessive signal could be tamed with a resistor divider circuit. Knowing what the input impedance of IF circuits is might help.

One source of IF signal might be another FM radio. Just a wild, crazy thought.

Further, if this is an AM-FM as the subject header states, you forgot to mention if the AM worked.

I would first look at mechanical failure of the connections. ...PCB/ traces/etc.

Especially around the FM Detector area where you heat shocked the PCB when changing that transistor.

I know it's hard to believe battery can heat that much but with age on a PCB, you'd be surprised. Ever try cold spray when radio failed?

Any 4 pin Ge transistors in there? if there is then the usual simple fudge is worth a try

l e

Well the AM is not working either when this set fails. And since the set uses the same transistors but different coils for each stage that doesn't really help me to track this down. I could look at the CW signal from the 249 with my 50MHZ. scope or my spectrum analyzer but I still don't know what a safe signal amplitude would be. Does anyone know what the proper typical amplitude of an IF signal would be so that I would be able to simulate this and inject this signal into the IF amplifiers and detector stage? BTW except for the outputs, they're all 4 pin GE transistors. What do you mean by the "simple fudge"? Thanks, Lenny

One might reasonably assume that the IF signal would not have a peak-to-peak value larger than the net forward bias on the IF transistors. You can be reasonably certain a signal of that level will not damage the transistors.

Fuses, chemical or not, don't become intermittent.

Idiot.

John

Well the AM is not working either when this set fails. And since the set uses the same transistors but different coils for each stage

Does it have the old TO7 transistors like OC170/171/AF116/117 etc?

They're prone to internal whiskering, most commonly the can/screen to one of the 3 electrodes, lifting the screen lead might get it going, some people zap the whiskers with 47uF charged to 50V (twist C,B & E together first).

If the whiskers have reached 2 or more electrodes you're pretty much stuffed, some people use BF450/451 as replacements but you usually have to tweak the base bias resistors to suit silicon 0.7V and you might have to align out the different collector capacitance (do one stage at a time).

If you have extreme manual dexterity and bloody good eyesight you can melt the solder seal between can & header and put the TO92 inside the TO7 can to preserve the appearance of originality.

"John-Del" "Phil Allison"

Fuses, chemical or not, don't become intermittent.

** Then you have not seen everything.

Imbecile.

... Phil

Check the collector voltages on the IF and mixer stage. Use at least a 1 meg resistor between the probe and the transistor to keep from disturbing the circuit. I saw a lot of this in old germanium transistor car radios. Noise induced into the circuit would momentarily restore the transistor enough to work for a while, but of there was zero collector voltage, one of the IF transformers was open. If it was high, the transistor was bad. I troubleshot those radios and repaired most of them in under five minutes with that simple method. :)

--
It's easy to think outside the box, when you have a cutting torch.

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The IF's are AF124,and 126. And to answer a previous question I did use cold spray on everything but the detector, which is inside a can. It didn't seem to have any effect. I plan to open the can and hit the detector too. The "whiskering" as well as the method to remove it are both kind of interesting, sort of like what happens to nicads it seems. Well the transistors seem to have lasted forty some odd years so I can't complain much. The NTE 160 is a germanium and it seems to replace all of these however it does not have a fourth can lead. I don't know if that could make much of a difference.To check one of these for whiskering, would you simply look for leakage between any junction and the can? I never thought of this. On another note, considering the bias issues, I really would not want to use a silicon transistor if I don't have to. Lenny

fudge

I may still have a couple new ECG160 in stock. I'll try to get into the old shop and check, if I can climb through some of the junk that's piled up to be hauled off.

is the datasheet for the NTE version.

I only used freeze mist as a last resort. On early transistor equipment it seemed to cause more failures. Thermal expansion & cooling inside the transistor case caused metal fatigue.

AM or FM? A bad diode shouldn't affect both bands.

A short between the shel & the collector was a common failure.

Using an analog meter would caue open juctions to start working again, for a short time. All this did was make you waste time, since they invariably quit again. You probably have a crack weld inside the can, or a fractured bonding wire. That is why I monitored the collector voltages, with added isolation and used a VTVM, and later, a FETVM.

It's not that difficult to change. If it uses the typical two resisotor bias, you need to raise the E_B voltage to get .6 volts instead of .1 volts. Since the silicon parts have less leakage currents, raise the value of the bottom resistor in the string. Tack a trimpot or use a resistors sub box to find the new value, and solder it in.

--
It's easy to think outside the box, when you have a cutting torch.

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Good idea about the VTVM. I don't get to use my RCA Senior Voltohmyst much anymore. I bought it as a kit from the RCA distributor at the time, Bruno NY in Manhattan and built it when I was 17. It still works great. I use an Energiser for the ohmmeter but I really do worry about the battery leaking. I've been meaning to replace the battery with a little isolated DC supply for a long time but just have never gotten to it. Lenny

The IF's are AF124,and 126. And to answer a previous question I did use cold spray on everything but the detector, which is inside a can. It didn't seem to have any effect. I plan to open the can and hit the detector too. The "whiskering" as well as the method to remove it are both kind of interesting,

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So far I've never heard of whiskering problems with the AF12x series - that's not to say it never happens though.

As the fault is intermittent, continuity checks may not tell you anything useful.

Simply remove the solder from the copper pad around the screen lead and make sure its isolated, if there is whiskering that will cure the symptom.

Phil:

I've seen an intermittant 2AG fuse twice, Drove me nuts the first time. By intermittant, I mean it would open in circuit, but would test OK with an ohmmeter.

I mean 3AG fuse.

"Ron D."

** Yep.

Typically, the fuse wire has come loose at one end - or was never soldered in the first place.

It looks fine and has continuity due to light pressure contact alone.

When the wire heats under load, it opens and closes.

..... Phil

Start with the first RF amp. It's probably a superhet so there will be a

455 osc and mixer. After that a couple rf amps and the the detector.

Google a superhetrodyne receiver. This one uses ICs but the basics are the same

Should give you some insight as to where the signal is being lost in the path.

--
Live Fast Die Young, Leave A Pretty Corpse

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How do you tune the radio in that schematic?