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Old 1st Dec 2022, 10:14 am   #41
stevehertz
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Default Re: Newly found VEF Spidola

As this set uses germanium transistors, I'm assuming that the strange, 'S shaped spring legs' detector diode is also germanium. Can I replace it with a common or garden silicon signal diode for checking purposes?
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Old 1st Dec 2022, 9:31 pm   #42
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Default Re: Newly found VEF Spidola

Probably not. A germanium diode will start to conduct at around 0.3 volts, whereas a silicon required 0.6v or maybe more for a power diode. The "small signal diodes" are exactly that - designed to be the detector at the end of an IF amplifier and give maximum sensitivity. A 1N34A will be ideal - you can even see the old-fashioned "cats whisker" inside!
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Old Yesterday, 8:27 am   #43
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Default Re: Newly found VEF Spidola

Thanks Martin. I'll see if I've got a germanium diode hiding somewhere. If not I'll place a wanted ad on here.
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Old Yesterday, 10:47 am   #44
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Default Re: Newly found VEF Spidola

I seem to think that I've used any old diode just for testing purposes before and got away with it. Test the diode with a meter - you may struggle to get a result with it being an old germanium type, but I've had them in old TVs as detectors etc., where they test virtually open circuit and still work perfectly fine in the set. While you may get lucky taking stabs in the dark with such as the detector diode, you may in the end have to crack out the RF signal generator and go down the RF/IF stages and fault find it.

I noted quite early on the amount of views that this thread has had over a relatively short time compared to other concurrently running threads, with these particular sets perhaps proving to be of more interest than I, for one, may have originally thought. Perhaps it's because it's the first time that any have been featured on this forum for over 15 years and it makes a bit of a change from all the run of the mill DAC90 etc. stuff.
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Old Yesterday, 11:18 am   #45
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Default Re: Newly found VEF Spidola

Thanks Techman. And good to hear (I hadn't looked at viewing figures) that these type of sets are creating interest. Until it failed (again) I was quite impressed with the performance, and although others have said otherwise, the build quality and ease of taking it apart and working on. I'll sort it eventually, I'm just not a great RF electronics engineer, I tend to use 'techniques' that have seen me in good stead for many years and I generally get there in the end!

I'm going to check those 500uF caps and the detector diode next.
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Old Yesterday, 11:44 am   #46
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Default Re: Newly found VEF Spidola

The large decoupling capacitors used across the supply lines in battery sets either tend to cause instability/motorboating as they lose capacitance or cause more current drain on the battery if they go excessively leaky. These are virtually always duff in battery valve sets where they're permanently connected across the HT line regardless of whether the set is on or off, which is not the case in a transistor set, so a lot less likely to be faulty, but still worth a check, although unlikely to be causing your fault unless they really are leaking badly and killing the battery.
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Old Yesterday, 12:39 pm   #47
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Default Re: Newly found VEF Spidola

Just an observation, in the schematic I'm looking at the detector diode has some forward bias applied, a change in that if fitting a silicon diode might change the AGC characteristic.

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Old Yesterday, 12:57 pm   #48
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Default Re: Newly found VEF Spidola

I've got shedloads of GEX34 point contact diodes and am happy to send the OP a couple FOC if need be. They are on the large side though.
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Old Yesterday, 1:23 pm   #49
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Default Re: Newly found VEF Spidola

Thanks Graham, Lawrence. Let me see if I can find something first. In the photo the detector diode is in the upper right hand side with a red dot on on it. It has 'S' shaped springy legs.
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Old Yesterday, 1:43 pm   #50
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Default Re: Newly found VEF Spidola

Quote:
Originally Posted by stevehertz View Post
Thanks Graham, Lawrence. Let me see if I can find something first. In the photo the detector diode is in the upper right hand side with a red dot on on it. It has 'S' shaped springy legs.
The signal detector diode is in the central area of the board so far as I can make out.

Lawrence.
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Old Yesterday, 2:09 pm   #51
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Default Re: Newly found VEF Spidola

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Quote:
Originally Posted by stevehertz View Post
Thanks Graham, Lawrence. Let me see if I can find something first. In the photo the detector diode is in the upper right hand side with a red dot on on it. It has 'S' shaped springy legs.
The signal detector diode is in the central area of the board so far as I can make out.

Lawrence.
If that's the case, what is the component that I refer to?
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Old Yesterday, 2:30 pm   #52
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Default Re: Newly found VEF Spidola

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Originally Posted by stevehertz View Post
Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by stevehertz View Post
Thanks Graham, Lawrence. Let me see if I can find something first. In the photo the detector diode is in the upper right hand side with a red dot on on it. It has 'S' shaped springy legs.
The signal detector diode is in the central area of the board so far as I can make out.

Lawrence.
If that's the case, what is the component that I refer to?
It's a diode, the only Spidola info I have with a schematic and board layout etc is this one:

https://archives.doctsf.com/document...7677&ref=17362

In that, the diode you mentioned is connected between the base of T3 and ground.

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Old Yesterday, 2:48 pm   #53
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Default Re: Newly found VEF Spidola

It's a different model to your link Lawrence, but I think it's just the outward appearance as the the cct - without scrutinising it - seems to be the same. Anyway, there are two diodes on the board as I now see. The detector is indeed in the middle of the board, and the second one (the one I was referring to with the red spot on it) is in the upper right quadrant. I'm not sure what its function is. I'll check both.

If you look on eBay and search 'Spidola' you'll see two types come up.
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Old Yesterday, 3:27 pm   #54
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Default Re: Newly found VEF Spidola

Quote:
Originally Posted by stevehertz View Post
the second one (the one I was referring to with the red spot on it) is in the upper right quadrant. I'm not sure what its function is. I'll check both.
So far as I can make out that diode along with T3 etc form a stabilizer circuit for the Oscillator and Mixer.

According to radiomuseum it's a silicon diode:

https://www.radiomuseum.org/tubes/tube_d101.html

Lawrence.

Last edited by ms660; Yesterday at 3:56 pm. Reason: extra info
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Old Yesterday, 7:15 pm   #55
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Default Re: Newly found VEF Spidola

I couldn't resist having a hunt through the filing drawers to look for the original instruction book for mine and after about the fifth drawer I found it. I hadn't looked at it in decades and was surprised to find the original receipt and guarantee card inside. I was also surprised to find that I'd also kept within the book the paper wrap from one of the original batteries that I'd mentioned earlier. I've taken a quick photo of the circuit diagram from the back of the book - not the best as it was on glossy paper and has a bit of reflection from the light on it - remembering also that my set is a bit later than the one belonging to the OP. Also shown is the other papers mentioned, plus the original supplied plug for the speaker socket at the rear. Shown next to the plug is one of those typical interconnect ones that you find on old electronic gear, particularly on some 405 line TVs, so if you no longer have your original supplied plug, those old Paxolin and metal types are virtually the same and will fit, the top pin being very slightly higher than on the original Russian plug, but I've tried it and it fits, with it being just very slightly tighter.
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Old Today, 12:07 pm   #56
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Default Re: Newly found VEF Spidola

Here's a better schematic than the one I linked to earlier, minus the error that was in the former schematic:

http://www.abetterpage.com/wt/photos...la1600x827.jpg

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Old Today, 12:16 pm   #57
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Default Re: Newly found VEF Spidola

Thanks Lawrence. The one I already had was like your previous one but higher resolution. This new one is somehow clearer.
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Old Today, 4:49 pm   #58
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Default Re: Newly found VEF Spidola

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Originally Posted by stevehertz View Post
As this set uses germanium transistors, I'm assuming that the strange, 'S shaped spring legs' detector diode is also germanium. Can I replace it with a common or garden silicon signal diode for checking purposes?
Have you checked that diode with a meter yet, just to rule it out?

It will measure with both a DVM on diode test and an AVO. You'll get vastly different results between meters, but no need to worry about that. I've just tried it with both from the print side of the board, working blind, as the board and chassis are still in place, but looks like I've identified the correct solder pads. I doubt it'll be the detector diode causing your fault, but you never know...stranger things and all that!
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Old Today, 7:05 pm   #59
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Default Re: Newly found VEF Spidola

Just to clarify the above - I'm talking about the detector diode in the middle of the board and not the one that you originally thought it was. looking from the back, the connections are basically sort of either side (above and below) of the right hand side connection of that orange tubular capacitor on the print side of the board, if your set has it. You'll probably be able to get to it easily from the component side, as I'm guessing that you've still got the board disconnected and removed from the set. I said earlier that these sets were all much of a muchness and from what I've now seen of the different model styles and the various copies of the circuit, I think we can safely now say that they all use the same circuit. The only oddity I've seen is a version with FM in place of long wave - goodness knows how that's supposed to work!
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Old Today, 7:29 pm   #60
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Default Re: Newly found VEF Spidola

Thanks Techman. I've presently got a limited amount of time to work on sets but as soon as I can I'll get on to this ands report back.
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