ALBA model 70 Radiogram problems.

[trabant]

Knackered volume control pot, as the radio was working to a reasonable volume I never thought to meter it.

[trabant]

The Alba 70 Radiogram I've been working on had a dead volume pot hence the lack of anything at the Gram socket the pot was marked 10,000 ohm so I purchased a new switched 10k linear pot I've installed this I only have should literally at the top end of the volume control it's listenable but quiet the radio also cuts completely on parts of the tuning scale I get a pop then I have to change waveband then change it back again to regain the use of the that waveband any idea. The set is early 1930s output valve is a Pen24m .

[Herald1360]

Hi,


any chance you can post the circuit you obtained for this set. There's been loads of confusion about what we are actually looking at- is it a superhet? TRF? or what? You say there are only three signal valves which suggests TRF. What are these valves?


If it's a TRF, the symptom you describe sounds like it's going into oscillation (the "pop"). As a TRF, the volume control may involve RF +ve feedback (AKA reaction) and changing it may have upset something.

Volume control pots are also normally log types, which may explain your cramped up one end issue.

[trabant]

The circuit appears to be incorrect I'll post the valve numbers when I get back.

[trabant]

Ok valve lineup is VMS4/B , AC/SG, Pen24m.

Pics of the chassis

[ms660]

I think the schematic for the 501 will be the nearest to what's in your receiver.

Lawrence.

[Herald1360]

The 501 circuit shows a reaction control as the bottom left control looking at the front of the set, with volume in the middle and the wavechange/gram selector on the right. Adjusting this may get rid of the "pop" (usually called a "plop"). If the detector goes into oscillation with a "plop" it tends to suggest that the design is a bit marginal for the reaction circuit- something that does seem to be more likely with circuits as here where the reaction control is a variable capacitor in series with the feedback winding. Sets that used something like variable screen voltage to change the gain of the valve tended to me more controllable. "Smooth" reaction was a bit of a holy grail!


Since the loop gain of the detector is affected by the tuning setting as well as the reaction setting, going into oscillation as tuning is varied is quite common. Two handed tuning (one to tune, one to tweak reaction) is a skill worth practising especially on a "twitchy" set.

[trabant]

That would work if it had a reaction control which it doesn't this has volume via the aforementioned pot a single tuning knob and a wavechange switch

[ms660]

There will be some built in regen (reaction) coming into play as the volume is advanced.

Is that a 2 or 3 gang tuning capacitor that's fitted?

Lawrence.

[trabant]

It's a three gang see pic , ok so having had another look at it I realise the wiper contact of the original pot went to the pots casing and subsequently the chassis the new pot a NOS RS job didn't it was isolated so I have rectified that however I still have basically full volume or nothing I've included a couple of pics of the inside of the original pot is this a linear pot ?? I've ordered some conductive paint and I will attempt to repair the original pot I can actually see the break in it which is handy. Connecting the current pot to chassis has eliminated nearly all the tuning drop out.

[G6Tanuki]

That pot looks to have a non-linear curve (whether it's log, antilog or something else it's hard to tell) - the resistance-windings are much closer-together at one end of the track than the other.

[If it was linear you'd expect the winding to be uniform throughout the whole length of the track].

[trabant]

That's what I thought

[ms660]

Ditto, as a first guess I would say antilog.

The 3 gang tuning capacitor would normally make it a front end bandpass job if all three sections are used.

Lawrence.

[trabant]

Ok I have tried a 10k log in it that didn't work so I'm thinking it is anti log problem I can't find anywhere that supplies a 10k anti log in the old style like the good old RS ones they all seem to be these tiny modern things