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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Feb 2019, 8:39 am   #21
Boater Sam
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Default Re: Ge to silicon osc replacement AF117

I've used AFZ12 as I had a load of them, I posted here
https://www.vintage-radio.net/forum/...d.php?t=148257
If it helps.
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Old 27th Feb 2019, 9:06 pm   #22
poppydog
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Default Re: Ge to silicon osc replacement AF117

I have had numerous problems with this radio even before trying the silicon as the replacement for the AF117 osc but have now tried it and it does work, in a fashion. Both AF117s had the whiskers so I ended up trying a 29306 for the IF amp and the BC557b for the osc , the 29306 was not good as I had very little vol with it in place so I ended up biting the bullet and fitting one of the few GT322b that I have left in its place that made a massive improvement.

The BC557b is working quite well with stations on lw/mw but I cannot get lw/mw trimming right. I have to screw the aerial trimmer c3 almost all the way in to receive stations on the low end of mw but then have to screw c3 almost all the way out to receive stations on the high end of mw, there is no inbetween ,if you adjust so that you can pick up stations at both ends of the band they are very quiet, adjusting the coils on ferrite doesn’t help much. This applies to LW as well, also go anywhere near the ferrite or touch tags on the tuning cap and the volume of the stations increases massively. I have done all the obvious checks such as bad/missing connections on ferrite, waveband switch working correctly poor joints etc and everything checks out ok. It seems to be picking up all the stations on both wavebands and they are in the right places on the tuner dial so that side of things is ok.

The other thing is that it wont work on mw with a voltage below 8.20v (LW is ok) which is a problem as it takes a pp7 size battery which I haven’t got so I end up using rechargeable AAs which when charged arent that much over 7.2v. Are all these problems due to the silicon osc?
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Old 27th Feb 2019, 9:11 pm   #23
ms660
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Default Re: Ge to silicon osc replacement AF117

I don't know what the model number is but normally.....

….LF end = coil.

HF end = trimmer.

Repeat until no further improvement

Lawrence.
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Old 27th Feb 2019, 9:27 pm   #24
G6Tanuki
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Default Re: Ge to silicon osc replacement AF117

Last time I had to replace an AF117-type transistor [in a Pye Bantam] I used a Texas Instruments GM0278 I happened to have lying about.
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Old 27th Feb 2019, 11:20 pm   #25
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Default Re: Ge to silicon osc replacement AF117

Quote:
Originally Posted by ms660 View Post
I don't know what the model number is but normally.....

….LF end = coil.

HF end = trimmer.

Repeat until no further improvement

Lawrence.
Image from a Stella service sheet attached. L3/L4 is the MW coil on ferrite and C3 is the trimmer.
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Old 28th Feb 2019, 8:44 am   #26
poppydog
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Default Re: Ge to silicon osc replacement AF117

Quote:
Originally Posted by ms660 View Post
I don't know what the model number is but normally.....

….LF end = coil.

HF end = trimmer.

Repeat until no further improvement

Lawrence.
I have already stated "adjusting the coils on ferrite doesn’t help much"

poppydog
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Old 28th Feb 2019, 9:18 am   #27
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Default Re: Ge to silicon osc replacement AF117

Forgot to mention its a stella st415t
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Old 28th Feb 2019, 10:21 am   #28
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Default Re: Ge to silicon osc replacement AF117

Quote:
Originally Posted by poppydog View Post
I have already stated "adjusting the coils on ferrite doesn’t help much"
Then you have a problem unrelated to simple alignment. There should be a very obvious peak as you slide the coil along the rod.
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Old 28th Feb 2019, 2:26 pm   #29
poppydog
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Default Re: Ge to silicon osc replacement AF117

Yes it looks like I have issues elsewhere, hence my checking of ferrite wires and switching etc. I have had far too many radios with these exact tuning issues ie quiet stations touching ferrite coils improves etc that have ended up going to the grave. I can never seem to solve these types of tuning problems and trawling through the forum it seems I am not the only one, lots of abandoned threads that contain no problem solved.

As my question has been answered and the bc557b works in a fashion as a replacement for the af117 in the osc position on a stella st415t, the thread can be closed. Thanks to those that suggested the replacements.
Regards Poppydog
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Old 28th Feb 2019, 2:29 pm   #30
ms660
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Default Re: Ge to silicon osc replacement AF117

Quote:
Originally Posted by poppydog View Post
I have to screw the aerial trimmer c3 almost all the way in to receive stations on the low end of mw but then have to screw c3 almost all the way out to receive stations on the high end of mw, there is no inbetween
There won't be.

The trimmer capacitor is for RF HF end adjustment, the same applies to an oscillator trimmer, HF end adjustment.

If the transistor and the coil circuits check out ok it might be worth checking the oscillator alignment.

Sometimes with receivers the oscillator alignment might seem ok because the stations are being picked up at what might appear to be the correct place on the tuning dial/scale, but that doesn't meant to say that the alignment is definitely ok, for instance if the oscillator is out, say by 15kHz as an example, you would be hard pressed to see a resolution to 15kHz on many tuning scales yet 15kHz out can seriously affect the IF output from the mixer in relation to the IF transformers pass band.

If for instance say the IF transformers in a receiver are tuned to 470kHz and have a bandwidth of 15kHz and the receiver is tuned to a 600kHz transmission, the oscillator should be oscillating at 1.07 MHz in order to produce an IF from the mixer that is in the center of the IF transformers pass band, eg: an IF of 470kHz, now consider what happens if the oscillator was oscillating 15kHz higher than it should be, the IF is now 485kHz, that means, depending on the slope of the pass band response, that the IF could now be very much towards the limits of the IF transformers pass band or even surpassed it and the signal to the detector will be much reduced or possibly not there at all.

The biggest problem that some beginners have is understanding the principle of frequency conversion in a superhet and why the tracking of the tuned circuits is important.

Lawrence.

Last edited by ms660; 28th Feb 2019 at 2:39 pm. Reason: addition
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Old 28th Feb 2019, 3:53 pm   #31
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Default Re: Ge to silicon osc replacement AF117

I recently had "fun" with a KB FB10 "toaster" with similar tracking problems.
Turned out that someone had moved the I.F. frequency by about 35 Khz !
Realigning the I.F. to the correct frequency cured the tracking problem.
Perhaps worth a check. Tony.
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Old 28th Feb 2019, 7:19 pm   #32
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Default Re: Ge to silicon osc replacement AF117

Concur with Boater Sam #21 and on his previous recommendation have used AFZ12 with success, bought as a lot of 5 for less than £4 rapidly delivered from an eBayer in Bulgaria. Thanks Boater Sam!
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Old 1st Mar 2019, 10:03 am   #33
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Default Re: Ge to silicon osc replacement AF117

The schematics and component diagram I have for this radio show on the component layout sheet 4 the trimmers in the right place on the main tuner body, but if the tags at the side are for the trimmer next to it the numbering appears to be wrong. C3 is labelled C4 and visa versa, or am I mistaken in this? The schematic is correct showing C1,C3 going to S2a then via switch L3, L1 aerial coils, but the component layout has C3 side tag going to L6 the oscillator coil and S2d.
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Old 1st Mar 2019, 11:39 am   #34
ms660
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Default Re: Ge to silicon osc replacement AF117

So far as I can make out the schematic numbering/board layout numbering is correct, I think the trimmer connections to their respective sections of the tuning gang are at an angle, so that C4 is connected to the smaller V/C (C2...Osc section, lower number of plates, which is nearest the board) and that C3 is connected to the larger V/C (C1...RF section, higher number of plates, which is furthest from the board)

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Old 1st Mar 2019, 12:12 pm   #35
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Default Re: Ge to silicon osc replacement AF117

Out of interest I see there's an RF coil damper diode fitted, would it's operating point be the same if silicon transistors are used for replacements for the germanium ones?

Lawrence.
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Old 1st Mar 2019, 12:17 pm   #36
GeoffK
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Default Re: Ge to silicon osc replacement AF117

I thought the diode might have been a delayed agc the way it is biased by R1 R2. It may not have suited the b/e volt drop of a silicon transistor.
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Old 1st Mar 2019, 12:19 pm   #37
ms660
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Default Re: Ge to silicon osc replacement AF117

6:28 in...:

https://www.youtube.com/watch?v=uNUUaj6M-cs

Lawrence.
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Old 1st Mar 2019, 1:07 pm   #38
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Default Re: Ge to silicon osc replacement AF117

Quote:
Originally Posted by ms660 View Post
Thanks Lawrence, yes the top trimmer is going to the oscillator section nearest the board. The diagram could have been laid out better to show this.
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