HMV 1420 Not receiving anything

[nigelr2000]

I was given this simple little HMV 6 transistor radio, I want to get it working as it's the same model my Gran had in her kitchen for years. On first inspection I found the ferrite rod snapped in half and its a really long one, luckily I obtained another 1420 in rough nick but with an intact aerial. After transplanting and replacing the older caps and red/black plessey electrolytics I was rewarded with......nothing, a quick check of volts showed the rf stages were only getting 3 volts, this turned out to be the on off switch had a high resistance contact so a quick bridge and all voltages were about right however still nothing received, I scoped the LO VT1 and had a lovely strong sine wave which varied with tuning but strangely it climbed and then started going down again as you tuned it, I have done a short video with the probe on collector of VT1 to show what I mean.

https://youtu.be/FXvsEoFdCWs

Now I really haven't fully understood how a superhet works but I don't think this is right and also think it may be the root cause of no signals.

I scoped a dansette gem I have and egt normal behaviour ie frequency increases normally when tuned.

Any pointers gratefully received.

[ms660]

The amplitude looked constant enough to me.

Lawrence.

[nigelr2000]

constant but as you tune up frequency goes up initially then down again as you continue to tune up in the same direction surely it should increase in one direction until the variable cap is fully open not increase then decrease as you turn towards the end

[ms660]

My apologies, I should have washed my ears out.

Lawrence.

[vinrads]

I would start at the audio stage ie is the output stage working , if so inject a signal at the if freq to the base of the mix osc transistor if no signals work towards the detector until you get a signal, injecting at the base first then the collector , oops do you have a signal generator? Mick.

[nigelr2000]

I do have a sig generator, picked it up a while back from a nice man in Braintree with a Taylor valve tester, Audio stages work fine and if I stick my hand on the coil on the ferrite rod it introduces interference pick up from the many Switch mode psu's near the bench but no stations.

[ms660]

Check the osc frequency when tuned right at the top end of the MW band then when tuned right at the bottom end, ditto for the LW band.

Lawrence.

[nigelr2000]

Well hooked up the sig genny, adjusted it to 455 khz injected at the aerial end base of VT1 and scoped at the detector diode, signals coming through and when I adjust the sig genny slightly I can get it to beat against the LO ( https://youtu.be/fsPnNKyt14g )

[nigelr2000]

Aha I just looked at the variable capacitor and guess what one set of vanes are out of alignment when compared to the one from the other radio, no wonder the frequency went up then down again, I have swapped it over and can now receive radio 4 on LW but really low volume and nothing on MW but at least it's getting further

[Freya]

Not seen that happen before !

[ms660]

The good one looks a bit out as well.

Lawrence.

[nigelr2000]

Just a follow up on this, at last I have had time to spend on it. I pushed the out of line tuning cap vanes back into line, Connected it all up and was rewarded with radio 5 loud and clear. Now the difficult bit, neither of the sets had there dial cords attached so now I have the horrible task of trying to decide the route and number of turns etc. I spent an hour messing about with different combinations but got nowhere. I don't suppose anyone has one they could take pictures of to give me the info I need to complete mine.

[Sideband]

Is this any good?

I had to repair one a few years ago for a fellow collector. I've just scanned this from the manufacturers manual.

The full circuit is available from Paul by clicking the top right panel of this page.

[nigelr2000]

Sideband you are a diamond that's just the info I need, was nearly there but didn't have them in the right direction on the tuning drive. Thank you so much