Hacker Mayflower tuning problem

[Nuvistor]

Just a thought, confirm the IF alignment is OK at 10.7 MHz before digging into the tuner, if the the IF alignment is off by a considerable amount it can affect the tuning range.

[Pilot Mariner]

Originally Posted by Nuvistor

Just a thought, confirm the IF alignment is OK at 10.7 MHz before digging into the tuner, if the the IF alignment is off by a considerable amount it can affect the tuning range.

I can tune into the IF @ 10.6Mhz.

[Pilot Mariner]

I have now gained access to the tuner unit.

Again suspicions aroused because the bottom plate is held in position with clips and aluminium tape.
The aluminium tape has been previously cut so proves someone has definitely been here before trying to fix this fault and probably got put off by what he saw - don't blame him.

The core looks like it is sliding in and out of the perspex former smoothly.
I will post some pictures tomorrow to see what members think.

[Pilot Mariner]

The pics show the whole tuner, and extremes of travel of the core, the first being the core fully wound out.

[Mr 1936]

Hi

The error in the IF won't help, but it is only 100 kHz so won't account on its own for your problem.

It's not impossible that your radio has always been like this. Do the sliding cores look OK, i.e. no cracks or bits missing ? Are the cores spaced apart by the same amount as the coil windings ?

[Pilot Mariner]

Its difficult to tell, but I'm just wondering if the core on the right exits the 3 turn coil before the other one so there is less change in inductance in that coil at that end of the band, perhaps?

[Nuvistor]

It looks like a 220k resistor, is it in the grid of the oscillator triode? It appears to be a cracked carbon type, I.e. spiral that can have inductance and upset the is oscillator.
It’s a long shot but something is causing the problem. The difficulty with VHF radio tuners is slight movement of components can have effects on its operation.

[Pilot Mariner]

Originally Posted by Nuvistor

It looks like a 220k resistor, is it in the grid of the oscillator triode? It appears to be a cracked carbon type, ie. spiral that can have inductance and upset the is oscillator.
It’s a long shot but something is causing the problem. The difficulty with VHF radio tuners is slight movement of components can have effects on its operation.

Yes the 220K connects the oscillator grid to ground.
There's also the problem of changing something then faffing about putting it all back together only to find it's still not working.
I wonder if I could somehow run it in a standalone mode on the bench?

[frsimen]

Hello Mark,

Having seen the internal arrangement, I suspect that something has got out of line in the mechanical adjustment of the tuning drive. Mr 1936 has suggested it already in post 19.

The maximum inductance of the inductors seems to be when the pointer gets to about 90MHz, so you need to adjust things so that the pointer is at the low frequency end of the dial when you reach maximum inductance, rather than at 90MHz.

You will then need to adjust the trimmer capacitor to bring the calibration back as it should be. It will need a little additional capacitance. Hopefully that will correct the problem while still allowing stations at the 104MHz end to be tuned in.

Paula

[Pilot Mariner]

Hi Paula - It looks the other way round, the point of minimum inductance is at the lower frequency end of the the band, ie the cores pull out of the coil when tuning towards 87MHz. Hence I was thinking maybe cores were out of alignment or exiting the coils influence to early at 90MHz in post 26.
I need to try and set it up standalone to prove this.

Mark

[Nuvistor]

What are the cores made of? Brass will lower the inductance when inserted in the coil.

See https://www.vintage-radio.net/forum/...d.php?t=162194

[Pilot Mariner]

Originally Posted by Nuvistor

What are the cores made of? Brass will lower the inductance when inserted in the coil.

See https://www.vintage-radio.net/forum/...d.php?t=162194

Aha OK I stand corrected.
Its difficult to see what the cores made of, some sort of silvery metal with an orangy/brown band.

[ms660]

That's a Gorler tuner, so far as I know the cores are aluminium.

Lawrence.

[Mr 1936]

Hi

Interesting, I hadn't considered the use of Aluminium cores for this application.

I have encountered them for fixed VHF inductors having a small amount of adjustment. All I know about them is that they are more stable than ferrite, the presence of the aluminium core reduces the inductance and the Q is worse than ferrite so they are more popular for LO use than for RF tuning/filtering.

My gut feeling is that you would struggle to get the 40 % change in inductance needed to cover 88 to 104 MHz, but I can't find any quantitative data on performance at the moment, so maybe it is possible.

It does occur to me that with the RF and LO coils mounted on the same tube, there might come a point where the "proper" core is pulling out of the LO coil but at the same time the "wrong" (RF) core is approaching it and starting to negate its effect.

[Mr 1936]

Hi

Something else that might shed some light. See a previous post on this forum about a Hacker RV20 by Steve Browne on 19th May 2014.

https://www.vintage-radio.net/forum/...d.php?t=106306

He seems to have had a similar problem. His photo shows the 220K as being a traditional carbon composition type, rather than the newer one which you have which is possibly spiral cut.

[Pilot Mariner]

Hi - thankyou for that, very interesting, unfortunately the posts seem to finish abruptly and he does not state if the fault was fixed
The 220K is reading 230K and the 1M is reading 1.16M, the diode tests OK.
I think the next step is to put the unit on flying leads like the other post and have a poke around.

[Pilot Mariner]

Just a quick update - I have the tuner now operating on flying leads, so I can see what is happening. My hypothesis is that CCR comes in as the slugs are exiting the coils. So I don't think that there is much further change in inductance to bring in lower frequencies.
Going full circle on the oscillator trimmer will only move CCR slightly, not enough to bring in R2.
I'm not sure what the next step is at the moment.

[Mr 1936]

Hi

Your next step is to add a small extra fixed capacitance across the LO trimmer. Try 1 pF at a time. Miniature ceramic suggested. Voltage rating not important as there is no DC across it.

Calculation time:

Present minimum RF freq is 89.6 MHz, LO is 100.3 MHz
Wanted minimum RF freq is 87.0 MHz, LO is 97.7 MHz
Desired change in LO freq is -2.7 %
Required change in LO capacitance is +5.4 %

Guessing LO fixed capacitance to be 20 pF, you'd need to add 1 pF

[majoconz]

Where is the 'end stop' that prevents you pulling the core out of the coil former and how much does the core actually move from end to end. That 'safety pin' spring doesn't appear to have much free movement in it before it either hits the coil former or the metalwork - what is that orange blob at the cord joint to the spring - just a blob of glue?

A 1pF capacitor can be easily made with a turn or two of two insulated wires - a 'gimmick'.

[Pilot Mariner]

Originally Posted by majoconz

Where is the 'end stop' that prevents you pulling the core out of the coil former and how much does the core actually move from end to end. That 'safety pin' spring doesn't appear to have much free movement in it before it either hits the coil former or the metalwork - what is that orange blob at the cord joint to the spring - just a blob of glue?

A 1pF capacitor can be easily made with a turn or two of two insulated wires - a 'gimmick'.

Hi - The endstop is as shown in post 32 - ie the slug is virtually up against the metalwork.
Not sure what orange blob you are referring to, but the cord does not pass through the coils. The cord is attached to a piece of metal onto which the cores are placed, The blob is probably a bit of solder joining the metal to the spring.
Anyway 4-5 pf is enough to pull in R2 on 88.3 close to the 'event horizon' of the coils, and tune up past CFM on 100.1 and a couple of other stations.

Next step is to make this permanent and re-assemble in the hope that everything can be brought into cal.