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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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8th Aug 2017, 5:01 pm | #21 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
The front panel removed. It's had a wash and scrub in hot soapy water. Some before and after pictures of bits of chassis and cover for the ganged main variable cap'. Sadly the trimming tools are missing...
Last edited by john_morris_uk; 8th Aug 2017 at 5:11 pm. |
15th Aug 2017, 12:03 am | #22 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
Some progress being made, even though the set hasn't had power applied to it yet...
The main smoothing caps proved challenging to dismantle. Oil filled in a case that was immersed in wax in another case. The ends were soft soldered on and even with minimum heat I was worried about noxious fumes. After very careful use of a blow torch it was all apart and I've decided to build new caps on a bit of veroboard fixed undecthe shell of the old smoothing caps can. It will look original from above... In trying to sort out which wire went where I found a problem that's almost certainly been with the set since it was built. In trying to sort out which wire went where I found R43 open circuit with a dry joint. All now fixed and hopefully Power tomorrow. |
15th Aug 2017, 7:17 am | #23 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
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Re: AR88 (yet another!)
There are series resistors in the power supply negative end, used to create a bit of negative bias for the AGC/RF gain control etc. These seem to take a bit of a hammering, so give them a quick check before applying power. Need I mention 'those capacitors' being a parallel pair of dodgy types to the audio output grid? and there's a tone correction capacitor from anode to ground just ideal to take out the slightly fragile output transformer.
David
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15th Aug 2017, 10:18 pm | #24 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
It was one of those series resistors in the negative that had a dry joint. I only discovered the fault when I found that the resistance to ground was in the multi k's instead of 165 ohms or so. I traced it through and found one of the resistors apparently reading 'open' so I assumed that the wire wound resistor had gone open circuit and unsoldered it. I double checked it out of interest and the meter said it was 105 ohms (against 100 ohms declared) which is when I noticed that the 'unsoldered' end wire at one end looked dirty and tarnished and obviously had been a dry joint. A scrape with a knife to produce shiny wire and soldered back into position and all resistances in this bit of the circuit are now within reasonable tolerance.
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15th Aug 2017, 11:07 pm | #25 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,799
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Re: AR88 (yet another!)
It was probably once a fine joint, but the heat from the resistors bakes the living daylights out of the solder, close to its plastic point and it goes granular.
Pretty common. David
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Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done |
16th Aug 2017, 4:49 pm | #26 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,385
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Re: AR88 (yet another!)
The bias chain resistors are fairly large tubular types (at least the 100 and 150 ohm ones are, the 15 ohm end-of-chain providing the -1.5V for the 1st AF amp is only a little carbon comp type as it doesn't develop much power) and look capable enough- but they are close together, underneath a sparsely-ventilated chassis and wired close to the Paxolin (or whatever trade-name Americans generically use) sub-panel. In other words, a classic example of when generous de-rating needs to be applied. If replacing, something like 6W rating or higher would be wise, the green vitreous Welwyn-type with stiff steel leads that allow for resilient spacing would make a good choice here.
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16th Aug 2017, 5:21 pm | #27 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,951
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Re: AR88 (yet another!)
I did the ones on my AR88 using 7-watt ceramic coffin-style resistors. I can see the possible attractiveness of using the aluminium-clad bolt-to-the-chassis-for-heatsinking type of resistors too (I like to use these when replacing the more-traditional carbon or wirewound resistors used between reservoir- abd smoothing-capacitors in HT supplies, in order to reduce component surface-temperature).
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17th Aug 2017, 9:21 am | #28 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
Apologies if this is a dull question but what threads are the nuts and bolts and fixings in the AR88? I assumed that they were BA but they're not. They look too coarse to be AF.
The reason I found out is that a few of the nuts securing the RF stages bottom plate were missing so I ordered some thin 6BA nuts. I've now got some thin BA nuts in my stock... |
17th Aug 2017, 11:43 am | #29 |
Moderator
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
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Re: AR88 (yet another!)
They're small size UNC. UNC threads below 1/4" diameter are specified by a number and thread pitch such as 10 X 24 UNC.
https://www.britishfasteners.com/threads/unc.html
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9th Sep 2017, 11:48 am | #30 |
Tetrode
Join Date: Mar 2014
Location: Farnham, Surrey, UK.
Posts: 61
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Re: AR88 (yet another!)
A quick update: I'll try and post some pictures.
All the bathtub caps seemed slightly leaking round the edges so out they came. The purists might weep but I removed some of the connecting wires and fitted the replacement decoupling capacitors by shorter routes on many of them. A couple of small tag boards had to be added to accommodate some. All tubes removed, (I want to call them valves, but is a Canadian built radio...) and some basic insulation checks with the multimeter, I applied power through a variac and checked voltages and signs of insulation breakdown. The panel lights came on and the transformer showed 365-0-365 etc so in with the rectifier tube and check the HT. All good and my plan had been to start with just the audio output and gradually check the stages backwards but in a fit of enthusiasm I put all the valves in and turned on. Triumph! Local radio and various broadcast stations on the medium wave frequencies when I put a bit of wire on the antenna connection. All controls seem to be functioning after a fashion, although the bandwidth selection needs a little attention as predicted. Nothing on the other short wave ranges so something's wrong, but I'll get the signal generator out to have a look when I get a minute. It doesn't help that we're moving house in three weeks time! |