VEF Spidola - Help?

[davegsm82]

I have a VEF Spidola which is completely deaf on all wavebands.

The amplifier runs and if you poke strategic points then you get audio in the form of mains hum, also if you poke around the first IF you get what sounds almost like radio but you can't hear any stations and the tuner does nothing, it's the same when each band 'turret' is connected on the tuner.

Refering to the schematic on the following page....

http://www.rahutchinson.eclipse.co.uk/VEF_Spidola.htm

When I scope around the collector of T1 (bottom left on the diagram) I can get a 800~odd KHz sine wave about 300mV P/P however the oscillations are killed completely if you try to measure them on the Base or Emitter. 800+ KHz seems incredibly high for a germanium local oscillator?

T3 (Presumably mixer, top left) has zero detectable modulation on it, all voltages around the transistors where shown seem to read quite close to those on the diagram, within about 10%. All resistors seem to read the correct values, can't vouch for the capacitors but have read some of the electrolytics and they seem to be ok (have experienced leaky russian electrolytics in other equipment.

Have swapped T1 and T3 as they are the same device and all transistors are socketed so also very easy to do, made zero detectable change. Is the voltage on C61/C62 likely to be AGC? Appears to connect the output of the final IF stage back to the input of the first.

Would it be worth injecting my own LO so see if that improves matters?

Any common faults on these that members are aware of?

Have been on with this for a few nights now, starting to get frustrated with it!

Dave.

[crackle]

I have worked on a couple of Vega 215 type radios. The connections on the rotary switch are notorious for unreliability. Vibrations of the contacts as they go clunk as the switch is rotated can cause dry joints to fail.
I would concentrate on cleaning all contacts and wipers and re-flow the solder joints.
Then inject an IF signal and trace it through each stage from the last IF to the mixer, but dont adjust anything at this stage.
Mike

[nuvistor]

I
800+ KHz seems incredibly high for a germanium local oscillator?
Dave.

Why do you think that? All early sets used germanium, even in TV tuners.
Mikes idea sounds good for starting out.
If I have misunderstood you apologies.

Frank

[davegsm82]

Hi Mike,

I'll give that a shot when I can get at my sig-gen etc. I've already checked continuity of the 'fingers' on the front side of the radio, they all seem to be fine.

Frank,

800KHz just seems really high to me, aren't LO's usually around the 450KHz area?

Dave.

[nuvistor]

Hi Dave,
In a superhet the IF frequency is the difference between the LO and the wanted signal. In a single conversion superhet for MW the signal frequency is approximativ 500khz to 1500khz, the LO, usually above the signal would be approx 1000khz to 2000khz. Using those numbers the IF would be 500khz, usually the IF in most the broadcast receivers made since the 50's have an IF as you say between 450 and 470khz so the LO would be slightly different to my example.

You will note the the frequency ratio of the LO and signal are different LO 3:1 signal 2:1, this is why the front end needs to be aligned to "gang" them, usually it is only correct at 3 places, the ends and centre, in between the tracking or ganging will be slightly off and therefore slightly less sensitive.

hope that is clear, my writing skills are not great.

Frank

[sideband]

hope that is clear, my writing skills are not great.

Frank

No problem with that description. It describes the basic superhet function perfectly

[nuvistor]

Re reading it again there is a mistake, I cannot edit it, is it possible for the mods to sort it?
"the LO, usually above the signal would be approx 1000khz to 1500khz"

should read "the LO, usually above the signal would be approx 1000khz to 2000khz"

(mods note - done.)

Have written a paragraph about Image or second channel, is it worth posting?
Frank

[davegsm82]

feel free to post it up Frank, I understand about the upper and lower images, i.e. LO+tuned freq & LO-tuned freq, but it's always good to have this information refreshed

If you have any information about how to gang/align the IF's/filter (just after the Mixer) on this set then I would be most happy to hear about it

I have a VIZ WR-50C signal generator so can inject the IF and generate the frequency for the IF alignment, also if necessary create an modulated AM signal.

Dave.

[crackle]

The IF is 465kHz according to the Radio Museum, but at the moment I would only use the signal generator to establish which stage the fault lies, the set may not need aligning.
I suppose you have tested the audio stages by injecting a signal at the volume control?

Mike

[nuvistor]

I have only repaired a couple theses and was 40 years ago, dirty switches was all that was wrong. the two I saw worked very well.
As before follow Mikes advice.
If you know about the mixing schemes I will leave the image, like I say not that good a writer and make mistakes.

Frank

[Rebel RafterMembership Retired]

Hi from RR. One word of warning for those poking around in old Russian radios with turret type waveband selectors, DON'T use "servisol" type switch cleaners on the turret, I did that once to my cost way back 30-odd years ago on a "VEF transistor" set, and the "biscuits" that carry the little coils for each band all cracked and fell to bits, so now you know, just in case some folk reading this thread didn't know about this already, which surely some folk will. It was the older servisol back in the early 80's that caused the old Russian plastic to disintegrate and some folk might still have some of it kicking about so I thought I'd better mention this item just in case some folk don't know. I think the switch cleaners of today are better in this respect or at least some are, but I wouldn't like to risk it on something so old and precious and possibly irreplaceable. The same thing also happens with servisol if you spray it on the old British interlocking push button switches that were made from the rather biscuity brown plastic, so watch out. By the way I can read Russian Cyrillic letters but I haven't got a Cyrillic keyboard! RR.

[nuvistor]

I used Electrolube, they did a range of spray cleaners and grease for contacts.

Frank

[davegsm82]

I will abstain on the use of servisol until i can test an 'inconspicuous area'. I have an ancient can of 80's servisol somewhere, but I use the RS equivalent these days -which I suspect from the look of the can and the smell- is the same thing just more modern.

Back to the Radio, I've injected audio and the AF stages are fine. This set has an odd backwards Volume control in which the wiper is connected to the signal from the detector and the track ends are connected to the input to the AF stage. To me this looks like it will affect the AGC line when the volume is down low, perhaps this is intentional.

I don't have time in the workshop at the moment but should get in on monday so will do some further checks then.

Dave.

[Terrykc]

Have written a paragraph about Image or second channel, is it worth posting?

My brother and I used to use the image frequency to our advantage.

If the LO is 1000 to 2000kHz, as in your previous example using 500kHz as the IF for simplicity, the normal MW coverage is the LO minus the IF, so 1000 - 500 = 500kHz and 2000 - 500 = 1500kHz BUT this only works if the aerial circuit is tuned to the difference frequency.

If the aerial circuit is tuned to the sum frequency, the coverage becomes 1000 + 500 = 1500kHz and 2000 + 500 = 2500kHz.

From this it is obvious that the aerial circuit must reject the sum frequency to avoid interference to normal MW listening.

So, what did my brother and I do? We wired a spare MW ferrite rod aerial in parallel with the one in the set. This now tuned to the 'wrong' frequency which, when we tuned the extra aerial coil for maximum, gave up a receiver that now covered Top Band (1.8 - 2.0MHz) instead of MW!

We usually had a set modded like this running in the workshop and listened to the 'old boys' daily net every morning.

[davegsm82]

Interesting anecdote Terrykc

Sadly, I'm still at a loss with this set.

I had it on the bench yesterday, Scope and sig-gen at the ready. I tried simply injecting a signal into the 1st IF and tracing it through, to be honest I couldn't see much evidence of the signal getting from A to B.

I also tried substituting a 450KHz signal into the LO, lo and behold, nothing to report. This thing is driving me crazy.

Could someone point out on the diagram what I should inject and where (freq/amplitude) in order the see results at say, the detector or output of the final IF.

Could it be possible that the old russian transistors have gone low gain? or are they generally quite reliable?

Dave.

[nuvistor]

Why a 450Khz signal into the LO? see earlier posts on mixing from Terry and myself.

Start from the last IF stage and work towards the first, you should get more gain as you work backwards on each stage. Use a divide by 10 probe on the scope to minimise the capacity. Mike stated the IF is 465Khz.

I cannot comment on the transistors going low gain but there are numerous incidents on IF transformers going low Q, but usually on old valve equipment, no history with the ones in this set.

Have you tested the detector diode?

Frank

[merseyswimmer]

I will abstain on the use of servisol until i can test an 'inconspicuous area'. I have an ancient can of 80's servisol somewhere, but I use the RS equivalent these days -which I suspect from the look of the can and the smell- is the same thing just more modern.

You could try the Electrolube 2X switch cleaner/lubricant oil that comes in a pen. Doesn't contain solvent or propellant.

MS