UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV 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 17th Jan 2022, 6:11 pm   #1
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default RAP 626 Tuning Issue

Hi.
I'm looking at the above RAP for a friend. It has the 6K7 RF amp stage. It works OK on the three short wave bands and the tracking is good. Medium and Long wave are badly mistracking.
There are two caps in the feedback circuit. One is a 50pf in the grid circuit of the 6K8 which was 72pf on test, now replaced, the other cap on the oscillator anode is a 1nf but totally inaccessible without removing the full wave change switch and coil assembly which I'm very loathed to do. Its not too bad at the very HF end of the band but when you get to BBC Scotland on 810 it's coming in at 1050 kHz long wave shows a similar shift.
Before I decide to remove the wave change switch etc has anyone else come across this issue?
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 17th Jan 2022, 10:51 pm   #2
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default Re: RAP 626 Tuning Issue

Addendum the model is a 646.
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 18th Jan 2022, 12:04 am   #3
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
Default Re: RAP 626 Tuning Issue

Seems unlikely it would be the anode coupling capacitor. Is the IF aligned correctly as it is an odd one at 365KHz? There are some alignment instructions in this thread https://vintage-radio.net/forum/show...=178432&page=2.
PJL is offline  
Old 18th Jan 2022, 10:02 am   #4
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default Re: RAP 626 Tuning Issue

Hi.
Indeed I checked the IF alignment and was found to be just about spot on. SW and TV sound are very close indeed. I'll check the padders later but I'm leaning toward dampness in the litz windings on long and medium wave.
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 18th Jan 2022, 3:46 pm   #5
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default Re: RAP 626 Tuning Issue

Hi.
I have stumbled over the issue. Both the mixer and oscillator wafers on the band switch have been coated in a heavy grease. I thought this was effecting the paxolin and carefully removed it and cleaned off the residue with alcohol. After drying over suitable period on power up we now have a sparking switch. Looks as if this heavy grease has been used to insulate the paxolin wafers.
A good coil pack with switches looks like the only answer. I do have grease that would do but the issue with tracking would remain unforunately.
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 19th Jan 2022, 11:04 pm   #6
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default Re: RAP 626 Tuning Issue

Hi.
The set has turned into a bit of a nightmare. There are a few issues and some really rubbish designs. There was a bit of mains hum and this turned out to be a previous replacement 4uf 450 reservoir (fairly modern) being OC. Prior to that the HT off the 5Z4 cathode was 330v now its nearly 380v! After the mains energised the HT is well over 310v which is present on the 6V6, 6K7 & the mixer of the 6K8.
The oscillator and RF amp is fed from the full HT via a 40K resistor which again results in over 300v. So either the mains TX is faulty or these sets over run the valves.

After re greasing the errant wafer we still have issue with it so in the meantime I've disconnected the RF amp and fed the grid of the 6K8 from the aerial coils, in fact the set performs much better than with the RF amp no doubt as the tracking issues are multiplied with two sets of RF tuning.
So we definitely need a full wave change switch with the coils from a scrap set, and possibly a Mains TX rewind although I'd be interested if someone can measure the HT at the rectifier, after the energising coil and the feed after the 40k, This set is a class A with one 6V6.
Help would be appreciated.
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 1st Feb 2022, 9:20 am   #7
murphyv310
Dekatron
 
murphyv310's Avatar
 
Join Date: Sep 2006
Location: Kilmarnock, Ayrshire, UK.
Posts: 5,413
Default Re: RAP 626 Tuning Issue

Hi.
This radio is still exhibiting issues. In well over 55 years of repairing radios (plus many other items) both professionally and as a hobby this set must be one of the worst I've done. The main issue is a rotten layout, especially the wave change switch and access to components behind the assembly. The way the tuning drive is done is whacky, the absence of adjustment at the low frequency end of the tuning coils is corrected mechanically. The circuit is full of errors. It also reminds me that this is most likely a US design rather than UK. Nice chrome chassis and glass viewing panel but the engineering is Diabolical, even the audio quality is nothing out of this world.
The issue now is still the wave change switch with it leaking badly.
Also the diagram has countless errors and really needs to be revised.
In all I've spent 26 hours on this set. Imagine being charged £10 an hour on a repair, and I'm still not happy with the result.
__________________
Cheers,
Trevor.
MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member
murphyv310 is offline  
Old 1st Feb 2022, 1:58 pm   #8
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,354
Default Re: RAP 626 Tuning Issue

If you wanted to you could knock back the high HT somewhat by substituting a U50 or 5Y3GT for the 5Z4. The directly heated rectifiers, as I'm sure you know, seem to have a higher series resistance. Jerry
cathoderay57 is offline  
Old 1st Feb 2022, 3:31 pm   #9
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,354
Default Re: RAP 626 Tuning Issue

What's the anode current on the 6V6 looking like? Obviously a low emission OP valve could explain the high HT. These sets were also designed for a push-pull 6V6 stage option so maybe the PSU is intended to give headroom for that. Jerry
cathoderay57 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:59 am.


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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.