Roberts RT22 Unusual FM problem

[martinhog]

Good afternoon. I have been reviving a dead RT22 and have rewarded with surprisingly good reception on both Medium and Long wave. However, if I select FM, the radio seems "stuck" on Radio 3 with the reception improving and deteriorating, but never totally disappearing as I tune from one end of the scale to the other.

All suggestions gratefully received, thank you.

[Nuvistor]

Presume you have checked the tuning gang on VHF is moving?

[martinhog]

Must admit, I can only assume so as it is hidden in a screening can. As i tune towards the high frequency end of the scale, the background hiss gets louder. Perfect tuning on AM
though ! I did consider a " local" transmitter was bleeding over the signal, but another radio works correctly in the same location.

[Nuvistor]

Use another radio to listen to the LO on VHF, if it does not change frequency as you move the tuning control you probably have a mechanical fault.

[martinhog]

Thanks I'll give that a go. Appreciate your help.

[martinhog]

Struggled a little with this.The Roberts is stuck on Classic FM (not Radio 3,sorry ) so I should hear the LO at 10.7 MHz below that frequency ? . However set up a frequency counter and sniffer coil and could detect the 10.7 MHz , this frequency seemed to alter as I adjusted the tune controller ? .

All suggested gratefully received, thanks.

[ukcol]

The FM tuning is done inside the RR1412/4 module but there is no clue in the manual as to whether it is done with vari-cap diodes or a tuning capacitor.

There is an RR1412/4 module for sale on ebay here :

http://www.ebay.co.uk/itm/Roberts-R2...-/121864313493

I have no connection with the seller.

[DangerMan]

The tuning on both am and fm is by a mechanical multi-section variable capacitor.
This also contains the respective aerial and oscillator trimmers (the four screws visible in a diamond formation in the linked photo).

[Nuvistor]

Without a circuit is a guess, if there is a capacitor in series with LO section of the tuning gang and it's gone low or O/C the tuning would be high, (Classic FM) end of the dial and the frequency would only change very slightly.

Frank

[ukcol]

This circuit snip from the manufacturers manual is not very helpful. It just shows the FM tuner module in block form, it even suggests (probably wrongly) that the tuning only changes the local oscillator frequency and doesn't change the tuning of the RF stage.

[Nuvistor]

That diagram shows the "AGC" from the FM detector going to the LO, either its AFC or it does not feed the LO.
I suppose if they didn't want these modules repaired in the field it would not matter but it's still a bit poor Roberts not supplying complete service data.

Just in case there is a peculiar AFC fault it may be worth removing the AFC from the module to see if it changes the fault.

Frank

[DangerMan]

The module does provide tuning on the output of the RF amp despite that block diagram.
There is a diagram on the forum showing the basic circuit, and quite a bit about the variable capacitor causing crackles etc.
If you enter LP1412 in the forum search box, they should show up... it's a thinly disguised Mullard module.

[ukcol]

Originally Posted by Nuvistor

That diagram shows the "AGC" from the FM detector going to the LO, either its AFC or it does not feed the LO.

That's my typo. As that diagram is a snip I had added that "AGC from FM detector", it should as you say Frank be "AFC from FM detector".

It is, as you suggest, a good idea to check there is no AFC fault.

Originally Posted by DangerMan

If you enter LP1412 in the forum search box, they should show up... it's a thinly disguised Mullard module.

Thanks for that, it does look the RR1412/4 is probably very much like the Mullard LP1412, the connections are certainly consistant with the Mullard module. See below.

[Nuvistor]

Looking at the circuit it could be as simple as a dry joint on the tuning cap LO section or a bad run of print. The RF section of the gang would tune giving the variation in output but the LO would be fixed frequency.

After checking the AFC the option is open up the module or replace it. Worth a look inside, if it's a dry joint or cracked print it may be an easy fix.

Frank

[martinhog]

Thanks for all your replies, I really appreciate it.
I William be back on the radio in the week, so I will report my findings .
Those Mullard Modules !
Thanks

Martin

[ukcol]

Originally Posted by Nuvistor

Presume you have checked the tuning gang on VHF is moving?

Originally Posted by martinhog

Must admit, I can only assume so as it is hidden in a screening can.

Can you, perhaps using a marker pen, at least establish if the shaft is turning?

[martinhog]

Good evening. Managed a quick half hour on the Roberts tonight. Removed and opened the dreaded Mullard module. Can confirm the tuning vanes are moving. Looks like it had got a little damp at some point, the rest of the radio has not suffered this though. Cold checks revealed 22K resistor was O/C. More when I get time . Thanks again for your help .

[martinhog]

Hi, I think i must admit defeat with the infamous Mullard module !. I changed a few "tired looking" ceramic caps that were about 50% out, but no joy.I can confirm the tuning vanes are moving. Still surprised to see the obvious water damage although this is not obvious in the rest of the radio, maybe its Mullard Water ?.

Was the Mullard module used extensively by Roberts ? My 505 has one but that's the workshop radio and my first Roberts. I would like to sort the RT22 as it belongs to a colleague.

Thanks so much for all your help , time for a trip to eBay..............

[Mike Noble]

This fault reminds me off a similar problem on an R23 which is virtually the same radio. See

http://www.vintage-radio.net/forum/s...ad.php?t=89322

or search for Roberts R23. Dirty slip rings on the tuning gang!

[martinhog]

Thanks for that Mike, there are certainly similarities in the two cases. The back ground noise gets louder as I tune "up" and quietens in the other direction. Since replacing some of the questionable components , you can now tune Classic FM in and out rather than picking it up across the whole scale. Time to look further, thanks again Mike .