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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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3rd Aug 2017, 12:11 pm | #1 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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AR88 (yet another!)
I've just acquired an AR88 for restoration. There's a note inside that says it was completely realigned and repaired in 2007 but more capacitirs have since developed the classic leaking oil (PCP?) symptoms. One suspect is C99/112/114 which if I read the reports correctly puts the output transformer under threat so I haven't even applied power through my variac.
However there's such a mess that I'm wondering if the main smoothing capacitors on the HT are leaking. (I'm away at the moment so can't double check.) I've started to research what to replace them with if the oil filled paper ones are gone and this is where I need some advice. When I've built GV supplies for Linear Amps I've series some electrolytics.) But it would be easy to achieve much more than 4uF per cap. I understand this stresses the rectifier valve. Any suggestions please? Edit: I should have added that I can find any number of 450 v capacitors but with the secondary marked up at 345 volts, then when off load the HT smoothing caps could charge up to approaching 490 volts. I suppose one could add the mods I heard mentioned to ensure the HT line is never off load? Last edited by john_morris_uk; 3rd Aug 2017 at 12:31 pm. |
4th Aug 2017, 7:51 am | #2 |
Heptode
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Re: AR88 (yet another!)
John will you please put some Photos on when you get the time.
Kevin |
4th Aug 2017, 8:54 am | #3 |
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Re: AR88 (yet another!)
The oil may be just mineral transformer oil or it mat be a Poly Chlorinated Bi-phenyls (PCBs). No-one has determined which but those capacitors were made in the era when both could have been used. The best advice seems to be to play safe and use impermeable gloves. Do not burn. Heated PCBs form dioxins which are a much bigger problem.
The existing capacitance values are fine. More C does increase peak currents in rectifiers which shortens their life. Having an AR88 powered with the switch in 'Trans' lets the smoothing capacitors go to a ridiculous voltage, so use 630v capacitors. Plastic film types are fine. RS/Farnell had them last time I needed any. I'n inclined to link the HT so the trans position leaves the set running. David
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4th Aug 2017, 10:17 am | #4 |
Dekatron
Join Date: Nov 2011
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Re: AR88 (yet another!)
A little oil goes a long way, and there's plenty in that smoothing block.... It wouldn't surprise me if it's been gently seeping past those three-score and ten rubber seals, it seems to get mentioned a fair bit and it certainly happened to mine. An AR88 in good condition is very hum-free, even using a loudspeaker with good LF response, so using more than 4uF won't achieve anything useful. (I used 4.7uF 630V plastic film types because I had them anyway, I'm sure 3.9uF would have been just as good). The fact that HV electrolytics are abundant, cheap and compact thanks to the SMPSU world seems to have encouraged some to use bananas amounts of capacitance in e.g. the audiophile world, but it's not really good engineering. (The noise limiter double diode can introduce hum when switched into circuit on some sets, but that's a different thing entirely).
The HT switch link is easy to apply with the set inverted and is a wise addition- switching the whole HT off to mute the set in "TRANS" is a rather crude and brutal way of doing things in an otherwise sophisticated design. |
4th Aug 2017, 10:42 am | #5 |
Dekatron
Join Date: Apr 2012
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Re: AR88 (yet another!)
A possible source of high voltage capacitors to re-stuff the big smoothing-block is 'motor run' capacitors
https://www.rapidonline.com/ducati-4...acitor-10-2938 though these are usually only working-voltage-rated to 450V or so. |
4th Aug 2017, 11:29 am | #6 |
Dekatron
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Re: AR88 (yet another!)
Looking through the .pdf data sheet for those Ducati caps, it states that they can indeed be used with DC voltages of (root 2)x the rated AC voltage- as one would expect but it's nice to see the manufacturer state it in print. They ain't exactly a king's ransom, either, and if they're rated for motor run use then smoothing HT in a radio that draws around 100mA should be an easy life. Small enough to safely and comfortably get three in the original can volume, too.
Last edited by turretslug; 4th Aug 2017 at 11:37 am. Reason: Spurious word! |
4th Aug 2017, 12:38 pm | #7 |
Dekatron
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Re: AR88 (yet another!)
Brilliant idea to use those caps, certainly won't fail in a hurry.
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4th Aug 2017, 1:43 pm | #8 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
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Re: AR88 (yet another!)
Thanks for the responses. Capacitors ordered.
I expect I'll get sone other questions and I've got to check the sweep function on my Hewlett Packard signal generator when I eventually get round to setting up the IF. Of course if anyone had an S meter I could buy I would be very interested. I appreciate they are like hen's teeth (The original one started full scale and started biased back to zero if I understand the circuit correctly....) |
4th Aug 2017, 1:46 pm | #9 |
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Re: AR88 (yet another!)
That's correct. A right hand zero meter was used. A left hand zero meter mounted upside down will serve the same function.
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4th Aug 2017, 1:59 pm | #10 |
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Re: AR88 (yet another!)
I took a quick photo of an original one in my AR88 (not original cabinet colour, but I was a youngster at the time...)
The legend says "Decibels above 1 microvolt" David
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4th Aug 2017, 2:21 pm | #11 |
Dekatron
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Re: AR88 (yet another!)
Here's the one in the AR88 I had, not an original AR88 S meter but it's right handed and the right sort of colour.
If I remember right my old Hallicrafters SX28 and SX17 also had right handed S meters. Lawrence. |
4th Aug 2017, 6:07 pm | #12 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
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Re: AR88 (yet another!)
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4th Aug 2017, 11:16 pm | #13 |
Dekatron
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Re: AR88 (yet another!)
Chapter and verse!
https://www.vintage-radio.net/forum/...ad.php?t=77650 |
5th Aug 2017, 9:07 am | #14 |
Dekatron
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Re: AR88 (yet another!)
John, hit reply then manage attachments to up load pictures from your desktop folder etc.
Forum will re size them as required.
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7th Aug 2017, 11:45 pm | #15 |
Tetrode
Join Date: Mar 2014
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Re: AR88 (yet another!)
As requested, some photos. A few shots of the underchassis to start with.
Contrast the cover for the RF stages (that's had a few moments work with acid metal cleaner and wire wool and a rinse of fresh water with a few minutes on the Aga) with the rest of the metal work. You can see where a previous person has made several repairs and replaced various cap's. |
8th Aug 2017, 10:59 am | #16 |
Heptode
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Re: AR88 (yet another!)
Thanks John for taking the time to put the photos on the Thread.
Kevin. |
8th Aug 2017, 11:05 am | #17 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
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Re: AR88 (yet another!)
If the oil has left a mess, you should find the spray version of jizer does a good job of removing it.
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8th Aug 2017, 11:48 am | #18 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
Thanks. I hadn't tried that.
Whatever it is it's put very light corrosion on the chassis. I've tried white spirit as a solvent but the best results so far have been with Screwfix metal cleaner and rinsing with water. The problem is that Screwfix metal cleaner is an acid and I don't want to get it near the wiring loom, coils or IF cans etc. Metal work I can remove and clean separately comes up almost like new... |
8th Aug 2017, 11:51 am | #19 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
I've also found that whoever sorted it previously didn't appear to check any (or many?) of the resistors. Some of the sample few that I've checked are way off. E.g. 560R (R13?) is 1.2k
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8th Aug 2017, 2:47 pm | #20 |
Octode
Join Date: Jun 2013
Location: Fleet, Hampshire, UK
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Re: AR88 (yet another!)
Yeah its horrible stuff. It hadn't corroded mine, but the stuff that came out of the smoothing block had a tar like consistency when cool, could be a mix of the oil plus the potting compound.
In the process of re-doing the smoothing block and bathtubs on mine, just too much going on and workshop issues preventing progress at the moment. As my workshop is quite damp, I daren't leave my receivers in there, so each bit of work I do I have to hump the damn thing in there then back out again! Will look forward to seeing your progress. I had some problems removing the innards of the smoothing block myself, tried immersing in boiling water and all sorts, the contents just wouldn't budge. Hope you have better luck ! |