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Old 26th May 2018, 7:16 pm   #21
crackle
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Default Re: Dynatron Elan Repair Guidance

I believe the AF115 transistor in the FM module is TR2.
20v should be enough, but remember the tin whiskers may grow again in years to come. So when you have done the test, and proved everything else is OK, if the transistor is in an inaccessible place it is best to replace with a AF125 before you finally put the radio back together..
Some FM modules and front ends can be very particular to what transistor they use, I like to prove everything else is working before swapping with a different type of transistor.
Mike

Last edited by crackle; 26th May 2018 at 7:27 pm.
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Old 26th May 2018, 7:33 pm   #22
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Default Re: Dynatron Elan Repair Guidance

Quote:
Originally Posted by OldTechFan96 View Post
I have tested two AF115 transistors and they both have E to C shorts. Will 'zapping' clear this short?
If that really is the case then possibly not, unless both the E and C are shorted to the case, in which case both electrodes will have their shorting whiskers fried and hopefully the short will go.
But remember to solder the 3 EBC legs together.
Mike
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Old 27th May 2018, 9:26 am   #23
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Default Re: Dynatron Elan Repair Guidance

Zapping it should clear the shorts. If not then let me know and I will zap an AF115 then pop it in the post to you or I can let you have a GT322b. Sorry for not following up on voltage checks for you but it totally slipped my mind. Let me know if you still want me to do that for you.

Kind regards

Graham
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Old 27th May 2018, 10:57 pm   #24
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Default Re: Dynatron Elan Repair Guidance

I've tried zapping the AF115 and it is still shorted E to C. I tested a few AF11X transistors and they were all shorted C to E. One AF117 had shorts to the screen but 20V and some vigorous tapping did not clear the shorts.

I also tested TR2 (AF178, RF amp) and it was also bad. I plan to replace both of these transistors.

Thanks for the offer Radiocruncher. I will send you a PM.

I've just googled the VHF front end, AE 02119, and can't find anything about it.

Last edited by OldTechFan96; 27th May 2018 at 11:02 pm.
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Old 28th May 2018, 8:23 pm   #25
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Default Re: Dynatron Elan Repair Guidance

All these transistors are actually S/C are they? Zero ohms? I always used to check these out on a Selectest (the small one, but same sensitivity as an AVO 8) and expect to see a hundred or so ohms, but if genuinely S/C, full deflection to zero mark. Germanium transistors totally confuse most modern Chinese testers, and technicians only used to silicon devices.
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Old 29th May 2018, 11:37 pm   #26
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Default Re: Dynatron Elan Repair Guidance

Hi!

Not normally – the "tin–whisker" syndrome is normally screen–to–collector!

I think you'll find if you use an AF179 (an RF amplifier device in VHF tuners, if you can get one cheap) in the FM tuner module RF stage,and an AF178 for the remainder, you will be very gratified with the results!

I would think that it's possible there could have been lightning damage at some point in the past – it doesn't have to strike anywhere near the set to cause damage, and in bad storms, sometimes very severe "atmospherics" can destroy germanium RF devices, which are only rated at 15–20V in some cases!

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Old 30th May 2018, 12:17 am   #27
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Default Re: Dynatron Elan Repair Guidance

Quote:
Originally Posted by OldTechFan96 View Post
I will have a look into sorting TR1.

I have tested two AF115 transistors and they both have E to C shorts. Will 'zapping' clear this short?

My bench PSU only goes to 20V. I will try zapping the transistor.
Yes zapping the transistor as Mike ("Crackle") suggested would normally clear the shorts if tin whiskers are at play. As Chris has mentioned the whiskers usually appear between screen and collector. In this case I don't think tin whiskers are the problem as, you mention you've already had a go at zapping the transistors to no effect. it's likely some sort of surge or static has damaged the transistors, causing them to short out between collector and emitter. There are many germanium transistors that would work in place of a AF115. I have in the past got away with using a 2N1307 in place of a AF115 and it has worked quite well. If you decide to go with a silicon transistor instead then you will no doubt have to change the biasing resistor to suit.

Thanks,
Peter
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Old 30th May 2018, 9:02 am   #28
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Default Re: Dynatron Elan Repair Guidance

A freshly zapped and tested AF115 will be off in the post today. It’s got short legs as it’s lived in a module for many years. Some pictures of the results on my component tester attached.

Regards

Graham
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Old 30th May 2018, 10:28 pm   #29
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Default Re: Dynatron Elan Repair Guidance

Quote:
Originally Posted by MotorBikeLes View Post
All these transistors are actually S/C are they? Zero ohms? I always used to check these out on a Selectest (the small one, but same sensitivity as an AVO 8) and expect to see a hundred or so ohms, but if genuinely S/C, full deflection to zero mark. Germanium transistors totally confuse most modern Chinese testers, and technicians only used to silicon devices.
When testing transistors with my DMM I always use the diode check function and probe the following:

B to E
B to C
C to E

Testing the AF115 with the diode check:

B to E: 0.3V
B to C: 0.2V
C to E: 1.9V

Your post made me check the resistance between the electrodes of the AF115:

B to E: 1.5KΩ
B to C: 770Ω
C to E: 12.5MΩ, 5.6KΩ with leads reversed.

With my modern Chinese tester the AF115 tests as if it has a diode between the E and C.

Quote:
Originally Posted by Chris55000 View Post
Not normally – the "tin–whisker" syndrome is normally screen–to–collector!

I think you'll find if you use an AF179 (an RF amplifier device in VHF tuners, if you can get one cheap) in the FM tuner module RF stage,and an AF178 for the remainder, you will be very gratified with the results!

I would think that it's possible there could have been lightning damage at some point in the past – it doesn't have to strike anywhere near the set to cause damage, and in bad storms, sometimes very severe "atmospherics" can destroy germanium RF devices, which are only rated at 15–20V in some cases!
Thanks for the suggestion of alternative RF transistors. I think that you could be right about the cause of the transistor trouble. Of the transistors in the RF/IF section the RF ones are the only ones to test bad. I think that the faults the radio had designated to the scrap pile as the speaker and FM aerial was missing along with looking a bit rough.

Quote:
Originally Posted by marconi_pete View Post
Yes zapping the transistor as Mike ("Crackle") suggested would normally clear the shorts if tin whiskers are at play. As Chris has mentioned the whiskers usually appear between screen and collector. In this case I don't think tin whiskers are the problem as, you mention you've already had a go at zapping the transistors to no effect. it's likely some sort of surge or static has damaged the transistors, causing them to short out between collector and emitter. There are many germanium transistors that would work in place of a AF115. I have in the past gotten away with using a 2N1307 in place of a AF115 and it has worked quite well. If you decide to go with a silicon transistor instead then you will no doubt have to change the biasing resistor to suit.
I found an AF117 that had shorts to the screen and tried zapping it. It did not seem to work but I will have another go. In the future I'll try to use Ge transistors in the FM RF section. The BC557 I used just may not have been suitable replacing the AF115. In a Roberts R700 I used BC557s to replace an AF114 and an AF178 and the radio worked fine. I'll do some experimenting
with Si transistors in the future with a different radio. I'll fit some sockets for convenience.

Radiocruncher,

Thanks for the transistor! I can't wait to get in on the radio!. I also bought some AF178s.
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Old 31st May 2018, 8:00 pm   #30
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Default Re: Dynatron Elan Repair Guidance

OTF96, those readings do not suggest to me that the AF115 is faulty!
Les.
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Old 1st Jun 2018, 12:22 pm   #31
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Default Re: Dynatron Elan Repair Guidance

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Originally Posted by MotorBikeLes View Post
OTF96, those readings do not suggest to me that the AF115 is faulty!
Les.
When testing transistors I always thought that there should be a reading of O/L between E and C. The testing method I use can be found on this website.

I wonder if the above only applies to Si transistors? I'll assume that with Ge transistors there should be a reading between C and E?
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Old 1st Jun 2018, 10:32 pm   #32
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Default Re: Dynatron Elan Repair Guidance

Germanium transistors are somewhat leaky (it's in the physics of the things) and whereas the voltage "gap" for a silicon transistor is 0.6v, it is around 0.2 to 0.3 for germanium types. If you are checking with an AVO or similar where a 1.5v battery is used on the resistance range, 0.5/1.5 looks much more leaky than 0.6/1.5 when reading across the junction.
That is why most chinese testers, designed for more modern semicons, get confused and often say faulty or why?.
Les.
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Old 2nd Jun 2018, 9:46 am   #33
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Default Re: Dynatron Elan Repair Guidance

if you short the base and emitter together then measure the collector to emitter leakage it should be high. the leakage current can forward bias the base if it is left floating and the transistor turns on and appears leaky during the test.

If it is truly faulty it will still be leaky even with the base and emitter shorted
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Old 2nd Jun 2018, 5:29 pm   #34
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Default Re: Dynatron Elan Repair Guidance

I replaced the AF115 and AF178. FM now works! Well, sort of.

The only station to come in well is Classic FM. Everything else is either poor, quiet or non existent. I can't find Radio 2 on the low Mhz side of the band and I can poorly make out Smooth Radio on the high end.

The tuning dial has not been reinstalled so I'm turning the tuning capacitor as carefully as possible.

I'm wondering if this could be an alignment issue?

The radio was hooked up to a 10M long wire aerial.
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Old 2nd Jun 2018, 5:51 pm   #35
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Default Re: Dynatron Elan Repair Guidance

Each transistor has its own capacitive characteristics even with the same type number, this can be very critical at these VHF frequencies. This is why I always find it best to try and use the original transistors.
It does sound like you need to realign the FM front end.

Mike
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Old 3rd Jun 2018, 9:35 pm   #36
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Default Re: Dynatron Elan Repair Guidance

I would agree that an RF alignment is needed. I don't have a wobbulator but it does not look like it would be needed for the RF alignment.

I've had a look at the trader sheet and the alignment should not be a problem.
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Old 4th Jun 2018, 5:01 pm   #37
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Default Re: Dynatron Elan Repair Guidance

I performed steps 1 to 4 of the RF alignment and it did not yield much improvement. Radio 2 can now be found at the bottom end of the band.

A full alignment is probably needed. I don't have a wobbulator to do this.
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