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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. |
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21st Nov 2019, 10:52 pm | #1 |
Octode
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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 |
21st Nov 2019, 11:17 pm | #2 |
Nonode
Join Date: Dec 2012
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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. |
22nd Nov 2019, 1:04 am | #3 |
Octode
Join Date: Sep 2017
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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 |
22nd Nov 2019, 10:01 am | #4 |
Banned
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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 |
22nd Nov 2019, 12:30 pm | #5 |
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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.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
22nd Nov 2019, 12:58 pm | #6 |
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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. |
22nd Nov 2019, 1:06 pm | #7 |
Dekatron
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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. |
22nd Nov 2019, 1:36 pm | #8 |
Dekatron
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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. |
22nd Nov 2019, 1:44 pm | #9 |
Octode
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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 |
22nd Nov 2019, 3:17 pm | #10 |
Octode
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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 |
22nd Nov 2019, 3:33 pm | #11 |
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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.
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22nd Nov 2019, 11:48 pm | #12 |
Nonode
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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. |
23rd Nov 2019, 12:40 pm | #13 |
Octode
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Re: Desoldering large IF box screens
Would loading it with low melting point solder work?
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24th Nov 2019, 5:12 pm | #14 |
Octode
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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. Last edited by Boulevardier; 24th Nov 2019 at 5:29 pm. |
24th Nov 2019, 8:18 pm | #15 |
Octode
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Re: Desoldering large IF box screens
What is the voltage you get across C526 when tuned in to an FM station with sibilance?
Ron |
24th Nov 2019, 8:27 pm | #16 |
Octode
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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 |
24th Nov 2019, 8:40 pm | #17 |
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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 |
24th Nov 2019, 8:55 pm | #18 |
Octode
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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 |
24th Nov 2019, 10:11 pm | #19 | |
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Re: Desoldering large IF box screens
Quote:
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 |
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24th Nov 2019, 10:59 pm | #20 |
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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. |