UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 21st Apr 2017, 3:40 pm   #1
Eddy_Stone
Triode
 
Eddy_Stone's Avatar
 
Join Date: Jun 2014
Location: Boston, Massachusetts, USA
Posts: 32
Default Roberts R505 VHF tuning range off

I bought a Roberts R505 transistor radio from Ebay. It works nicely, however the lowest frequency that it tunes to is 89.7 MHz instead of 87.4 MHz. also a 100MHz station comes in at 97.7 MHz on the dial. I thought this will be simple, just tweak the oscillator coil to shift the frequency range to match the dial. Unfortunately tweaking the oscillator coil or trimmer cap does not have any affect. Could it be related to the AFC voltage on pin 8 of the Mullard LP1402/2 VHF tuner module?

Any ideas?
Eddy_Stone is offline  
Old 21st Apr 2017, 5:28 pm   #2
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,328
Default Re: Roberts R505 VHF tuning range off

Does the tuning pointer traverse the full range of the dial?

I repaired a R505 that had the drive cord strung incorrrectly, so that the tuning range of the set was restricted.

If you want to disable the AFC as an experiment, you could just temporarily short out pin 8 to chassis with a jumper.

Ron
ronbryan is offline  
Old 21st Apr 2017, 6:09 pm   #3
Eddy_Stone
Triode
 
Eddy_Stone's Avatar
 
Join Date: Jun 2014
Location: Boston, Massachusetts, USA
Posts: 32
Default Re: Roberts R505 VHF tuning range off

I checked the pointer travel and it covers the entire dial length. I will try shorting the AFC.
Eddy_Stone is offline  
Old 21st Apr 2017, 6:19 pm   #4
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,328
Default Re: Roberts R505 VHF tuning range off

Failure of one of the discriminator diodes could cause an incorrect offset to the AFC voltage, but there would also be considerable distortion on the FM audio. Does the audio sound normal on FM?

Ron
ronbryan is offline  
Old 21st Apr 2017, 7:32 pm   #5
Eddy_Stone
Triode
 
Eddy_Stone's Avatar
 
Join Date: Jun 2014
Location: Boston, Massachusetts, USA
Posts: 32
Default Re: Roberts R505 VHF tuning range off

The audio sounds great. After some research it turns out that the oscillator coils adjustment hole in the tuner module is covered by the part number label. I removed the paper label to access the coil but the coil slug won't turn. I suspect the slug has some sort of Loctite or other material to prevent adjustment. Where do I go from here?
Eddy_Stone is offline  
Old 21st Apr 2017, 7:54 pm   #6
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,328
Default Re: Roberts R505 VHF tuning range off

Core breakage is a serious risk. Get the correct trimming tool that fits the core slot, but don't use excessive force. Don't use a normal screwdriver. You could try a little acetone in the core threads if they have used locking compound, but usually some sort of rubber string is used to secure the cores and a small squirt of switch cleaner will lubricate the core or soften wax, if used.

In your first post you said that adjusting C2 has no effect. Do you want to experiment with adjusting C2 now to see if the high frequency end of the band can be calibrated to nearer the correct scale position?

I take it that disabling the AFC had no benificial effect - is that correct?

Ron
ronbryan is offline  
Closed Thread

Thread Tools



All times are GMT. The time now is 5:52 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Copyright ©2002 - 2017, Paul Stenning.