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 > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 21st Nov 2019, 10:52 pm   #1
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Desoldering large IF box screens

Having decided to replace the electrolytic in the ratio detector in a Grundig Elite-Boy 500 to deal with a sibilance issue, I was faced with removal of the steel screening box that contains all the IF transformers and other parts.

The box has tags soldered to ground tracks on the main circuit board. Desoldering these for removal is proving very, very difficult. Does anyone have experience of this and tips for doing it? I've tried the solder-sucker and the braid. Using a 450 degrees C iron.

Mike
Attached Thumbnails
Click image for larger version

Name:	G pic.jpg
Views:	233
Size:	77.3 KB
ID:	194112  
Boulevardier is online now  
Old 21st Nov 2019, 11:17 pm   #2
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,361
Default Re: desoldering large IF box screens

Am i right in thinking these are soldered "at right angles" with printed circuit pad on IF amp touching the similar pad on the mother board? This is how Grundig did it on the IF preamp and main IF amp on the 717 and 3010 hybrid colour TVs.
If so, get some GOOD QUALITY solder mop. Use a hot iron (I used to use a Weller gun) and with care, plus a bit of strategic "bending" of the module will clear the solder joint. When all clear, it will pull out cleanly.
Once it is off and you can see the PCB, you may well find that a strategic touch with a hot iron where the thin wires meet the printed coils will cure most faults. -- I am assuming it is very similar to the TV board cited above.
I fixed dozens of them, usually just dries due to the PCB design.
Les.
MotorBikeLes is offline  
Old 22nd Nov 2019, 1:04 am   #3
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

Yes, Les. Inside the box is a small pcb mounted at right angles to main circuit board. Problem is that the box acts as a large heatsink for the tags that go through the main circuit board - heat just vanishes (I'm using a temp. controlled iron that claims it's max 65W - this struggles to even melt the solder on those tags). The tags are soldered to a fairly large ground pcb trace which adds further heatsinking. I have tried all the usual tricks - adding more solder and flux to aid the wicking, etc.

These things just weren't designed with repairs in mind. I will struggle on, but not really getting anywhere.

Mike
Boulevardier is online now  
Old 22nd Nov 2019, 10:01 am   #4
poppydog
Banned
 
Join Date: Oct 2015
Location: Cornwall, UK.
Posts: 849
Default Re: Desoldering large IF box screens

If your expecting a aluminium elec under that screen you may find that there is a "tant" in there.

poppydog
poppydog is offline  
Old 22nd Nov 2019, 12:30 pm   #5
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,504
Default Re: Desoldering large IF box screens

I've found that blowing away the molten solder with a blast of compressed air works better than any solder sucker. Just make sure to remove all traces of the solder from the rest of the PCB afterwards.

Or if you have several irons and a helper you can heat all the tags at once and push them out of their holes.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is offline  
Old 22nd Nov 2019, 12:58 pm   #6
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Desoldering large IF box screens

Just because your iron goes to 450 deg C when the tip is not in contact with anything doesn't mean it will get anywhere near that when attached/coupled to a large thermal mass.

What you need is a larger sized iron and iron tip and preferably going to 480 deg C, so it has much more thermal inertia and can keep maintaining the heat in the areas you want to solder/unsolder.

When they soldered it up in the first place, they simply had a bigger geometry & hotter iron or bath system than you are trying with. So you are simply sending a boy to do a man's job with your current soldering iron.
Argus25 is offline  
Old 22nd Nov 2019, 1:06 pm   #7
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Desoldering large IF box screens

Big hot chisel bit soldering iron and some wick is what I always used back in the day, always worked for me.

Further back in the day before solder wicks were around it was ditto for the soldering iron and use an old toothbrush to sweep away the melted solder.

Lawrence.
ms660 is offline  
Old 22nd Nov 2019, 1:36 pm   #8
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,162
Default Re: Desoldering large IF box screens

Yes, this is one of the very few 'radio' applications where a big old-fashioned high-power iron [Henley Solon?] has its uses.

Don't necessarily expect the metal cover to come free easily though - it's quite common for the cover's lugs to have been deliberately twisted after the cover's fitted but before it's been soldered. Some even have a little slit in the sides of the lugs to make this easier.
G6Tanuki is offline  
Old 22nd Nov 2019, 1:44 pm   #9
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

Agreed - my soldering irons are probably working at the edge, and I don't have any bits larger than about 3/16 inch. I now seem to have all the tags loosened in their slots, but it's still catching on one or two of them. Difficult to get in there to apply leverage (see picture in first post). Is it safe to assume that the small PCB inside the box is not fastened to the inside of the box (ie, that I'm not levering both box and board away from the main pcb)?

Mike
Boulevardier is online now  
Old 22nd Nov 2019, 3:17 pm   #10
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

"Don't necessarily expect the metal cover to come free easily though..."

That's my worry - that in applying a bit of force to lift the box I am also lifting the internal PCB - and I haven't desoldered the PCB's connections to the main board. Is this at all likely?

Mike
Attached Thumbnails
Click image for larger version

Name:	Circuit.jpg
Views:	122
Size:	136.3 KB
ID:	194146  
Boulevardier is online now  
Old 22nd Nov 2019, 3:33 pm   #11
poppydog
Banned
 
Join Date: Oct 2015
Location: Cornwall, UK.
Posts: 849
Default Re: Desoldering large IF box screens

If C526 is the one your thinking of replacing there is a t next to it on the sheet you just posted. That means its a tant, and will almost certainly be OK.
poppydog is offline  
Old 22nd Nov 2019, 11:48 pm   #12
MotorBikeLes
Nonode
 
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,361
Default Re: Desoldering large IF box screens

Looking at photo in first post, it seems clear the can should come off on its own. It may be tight, perhaps it has a slit separating one PCB that looks like two, but if all the legs are free, it should come off.
Les.
MotorBikeLes is offline  
Old 23rd Nov 2019, 12:40 pm   #13
dominicbeesley
Octode
 
dominicbeesley's Avatar
 
Join Date: Nov 2004
Location: Hebden Bridge, West Yorkshire, UK.
Posts: 1,886
Default Re: Desoldering large IF box screens

Would loading it with low melting point solder work?
dominicbeesley is offline  
Old 24th Nov 2019, 5:12 pm   #14
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

Done it! Corblimey what a fiasco!
Many thanks to all with suggestions – especially MotorBikeLes and Poppydog. Persisting with the sucking and braiding, I eventually saw with a magnifier that the box’s lugs were moving very slightly with respect to the pcb traces, and were thus clearing the solder. I also managed to find a small gap in the metal of the box with the vertical sub-pcb accessible below it. Levering the box upwards with a screwdriver using the top of the sub-pcb as fulcrum, the box very reluctantly and gradually was lifted away from the main pcb. Having gone through this nightmare, I was determined to replace C526, if only to eliminate it as a possible fault (it was tantalum, and of course read fine on test once removed!). Some minor damage to sub-pcb connectors from all the mechanical warfare, but easily put right with a wire bridge, and the set fired up again with no change in problem. I’ve left the screening box off for now, so that I can do a bit more investigation.
Getting to be a long post, but a few closing questions –
1. Is the sibillance (which isn’t unbearable) likely to be caused by unbalanced discriminator diodes? If so how could I check this, and would schottky diodes be suitable replacements. Is the setting of the core of IF labelled “a” critical for the sibillance issue? Any other component suggestions?
2. Wondering if this could all be a problem in de-emphasis components causing overload of audio higher frequencies. If so , can anyone point me to these components? Circuit attached.

Mike

PS - meant to say, I've not seen this circuit arrangement for discriminator before - no third winding on final IF, but an additional transformer with its primary in series with primary of final FM IF.
Attached Files
File Type: docx IF Amplifier stages.docx (459.1 KB, 67 views)

Last edited by Boulevardier; 24th Nov 2019 at 5:29 pm.
Boulevardier is online now  
Old 24th Nov 2019, 8:18 pm   #15
ronbryan
Octode
 
Join Date: Jan 2003
Location: Guildford, Surrey, UK.
Posts: 1,969
Default Re: Desoldering large IF box screens

What is the voltage you get across C526 when tuned in to an FM station with sibilance?

Ron
ronbryan is offline  
Old 24th Nov 2019, 8:27 pm   #16
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

Ron
I'll have to dismantle and measure that, so will come back to you. I assume it would be a voltage that fluctuates with audio content, so I'm not sure what measurement you want.
I must say again that the sibillance is not very intrusive, just seems more than it should be in a good quality set.

Mike
Boulevardier is online now  
Old 24th Nov 2019, 8:40 pm   #17
poppydog
Banned
 
Join Date: Oct 2015
Location: Cornwall, UK.
Posts: 849
Default Re: Desoldering large IF box screens

I have several grundigs now and some of them the sound is quite distorted on fm and fine on mw and a couple make speech sound like the person has a lisp. Ive never got to the bottom of this, the usual reply (when nobody knows the answer) is put it on a scope..
Though this may be of some help to you

https://www.vintage-radio.net/forum/...ad.php?t=82713

poppydog
poppydog is offline  
Old 24th Nov 2019, 8:55 pm   #18
Boulevardier
Octode
 
Join Date: Sep 2017
Location: Bristol, UK.
Posts: 1,709
Default Re: Desoldering large IF box screens

Poppydog - thanks, yes, I've been wondering if I'm just on a wild-goose chase here, and if it's just a quirk of the particular radio. Also, I've seen a few posts on other threads about the generally increased sibilance on FM broadcasts.

I'd still be interested to investigate the diodes. Also the de-emphasis circuit - just can't work out which components those are. It seems a slightly unusual variant on the standard ratio detector circuit, and I've noticed that Grundig used something similar on other models, such as some of the Satellit models.

I may investigate with scope, but I don't have an FM generator (perhaps I should try and get one), so I can't put a waveform through the whole set.

Mike
Boulevardier is online now  
Old 24th Nov 2019, 10:11 pm   #19
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Desoldering large IF box screens

Quote:
Originally Posted by Boulevardier View Post

I was determined to replace C526, if only to eliminate it as a possible fault (it was tantalum, and of course read fine on test once removed!). Some minor damage to sub-pcb connectors from all the mechanical warfare.

PS - meant to say, I've not seen this circuit arrangement for discriminator before - no third winding on final IF, but an additional transformer with its primary in series with primary of final FM IF.
With these unsoldering jobs as I implied it does pay to have a suitable power soldering iron to get the solder removed quickly with wick and limit the damage to the pcb that happens from struggling for longer with it.

Also, a large part of a successful diagnosis & repair is understanding the circuit you are dealing with, the circuit is not a discriminator, it is a ratio detector. A discriminator works quite differently.

The job of C526 is to stabilize the voltage so that the sum of the two output voltages from each diode is constant and doesn't vary at audio frequencies. Therefore C526 could not likely be a suspect if there was good audio output level, even distorted audio output. If it had shorted, as Tants sometimes can, you would have had near zero audio output.

The ratio detector has the advantage of being insensitive to amplitude variations and therefore the system does not require an amplitude limiter stage leading to this detector (unlike a discriminator), that is why is became ubiquitous in many transistor FM receivers.

It does ideally require balanced and matched diodes, so you could check the fwd drop of each diode on a meter to see if they look the same, in case one diode was faulty.

It is also better to help make an accurate diagnosis of a fault, vs a design issue when it comes to distortion, to sweep the IF to see if the ratio detector is working properly with a sweep generator & scope in case the issue was elsewhere or simply an adjustment issue. Otherwise you can find yourself trying to hunt down & repacing faulty components when there are none.

You could replace these with signal Schottky signal diodes & it will work, but the germanium ones used commonly are good for the task. A common garden schottky signal diode that will work is the 1N5711:


https://au.rs-online.com/web/p/produ...E&gclsrc=aw.ds

Last edited by Argus25; 24th Nov 2019 at 10:18 pm. Reason: add link
Argus25 is offline  
Old 24th Nov 2019, 10:59 pm   #20
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Desoldering large IF box screens

...PS I see you mentioned it was a ratio detector and called it a discriminator later.

One thing about both these types of detectors, there is really no satisfactory way to adjust these without sweeping them with markers. The +/- slope of the output needs to be right over the correct frequency range and be substantially linear and correctly centered. It doesn't take much to get them out of alignment, less than 5 degrees on a tuning slug can do it and you can end up with a distorted output.

There are people around who think they can adjust these by ear or with other methods, but they likely never get it right, unless by accident.Will be interesting to see how many argue with this remark.

This was quite unlike the days of superhet AM radio, where the system bandwidth, in most cases, was set by the bandpass characteristics of the IF transformers, so for the most part the technician only had to worry about signal peaks and meters would work. In the post WW2 era when America had introduced FM sound to its TV receivers, it required all the TV repair shops tooled up with sweep generators that would cover the 4.5MHz range to adjust the FM detectors.

Later when FM radio came in the '60's the same thing happened with the requirement for a sweep generator that would cover an FM radio's IF, so as to be able to properly set the ratio detector or discriminator. And every factory making radios, all swept the IF to make this adjustment. Later on other methods of FM detection, PLL's etc eliminated the need for the sweep in most cases as there were no critical adjustments to set up.

Last edited by Argus25; 24th Nov 2019 at 11:06 pm.
Argus25 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:38 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.