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Old 31st Oct 2017, 3:09 pm   #21
PsychMan
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Default Re: AR88 Low HT

Success! After tracing things out it became apparent R43 was in fact missing completely. I later found it under the table I'm keeping the set on, it must have been loose in the case when I got it and didn't realise when it fell out.

I replaced this with a new 5w resistor as I had done with R44. I also identified the other resistors nearby and replaced R53, R36, R37 and R45. They were either visibly heat damaged, or were out of spec. When R44 had burned it made a hole in the paxolin(?) board underneath, so you can imagine parts near it were not in the best shape.

The receiver now powers up, I'm seeing 156v on the stabilised rail, and 304vdc wrt chassis at the anode of the output valve. All bands work to some extent and I can pull in at least something on all with just a 4 metre ength of wire in the back in a concrete building. The receivers ground tag is shorted to chassis and the set is mains earthed. Bands 5 and 6 seem weaker than others, the wave change switch could use a clean, along with the pots. No apparent faults, but this is my first proper comms receiver so I can't say for sure. It does wipe the floor with any portable shortwave device I've used!! I managed to catch a bit of CW so the BFO is at least doing something.

I think at this point I'll continue with the usual tasks of neons on the output TX, bathtub rebuilds and go from there
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Old 31st Oct 2017, 3:46 pm   #22
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Default Re: AR88 Low HT

Just added the neons, they strike each wave change or mode change on high volumes, and actually flicker continually at high volume levels, almost like flicker flame bulbs, so they're clearly sparing the transformer some pain, this was near max volume.
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Old 31st Oct 2017, 5:15 pm   #23
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Default Re: AR88 Low HT

This all sounds good now and yes they in my opinion perform well when serviced.

PS I knew something was missing
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Old 1st Nov 2017, 2:03 pm   #24
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Excellent news- I wonder if the resistor lead-outs and connections had become brittle and granular with prolonged heat. Those ceramic body wirewounds are capable of pretending to be bar-fire elements, but the things surrounding them will suffer in the end!

An AR88 in good working order is a lively and rewarding set to use (without the fake liveliness of images and other spurios), there must have been many people over many years who enjoyed the smooth tuning, good tuning rate (at least for SWL AM-type use) and good stability as they dug across a busy band. Just a shame that HF broadcasting isn't the eclectic world-wide kaleidoscope that it was. The front-end alignment on the two top HF bands could well be worth going over- not that difficult but there are quite a few individual tweaks to get right. With band 6 in particular (22-32MHz) it may well be that it is genuinely a bit quiet out there currently. At least a signal generator won't deceive.

If anything, the 304V HT sounds a bit (not desperately) high- substitute or later type rectifier valves (often 5Z4s in metal or glass end up here) have a lower forward voltage than the original somewhat lossy 5Y3GT and may cause this. Also check the voltage selector setting- ISTR that some of the numbers on the voltage selector differ from the tag-plate on the transformer base! I just removed the selector and hard-wired to the 240V tapping, I don't like voltage selectors from the contact liability or disaster-in-waiting point of view and the AR88s isn't the best of the breed.
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Old 1st Nov 2017, 3:14 pm   #25
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Default Re: AR88 Low HT

I think it will need aligning, I can already see the dial doesn't reflect the signal being received on band 1(from an sstran), performance wise I noticed it's a bit of a whistle box on some bands. I'll have a go at that after redoing the bathtubs and possibly all the micamolds. I try and avoid the shotgun approach now, but I might be chasing too many faults caused by rubbish capacitors otherwise.

After playing with it for a short time it's also gone almost dead on any non noise limited mode. man nl, avc nl behave fine, others low volume and audible hum.

So, no more fun for now! Still chuffed to get some life from it, I expect it to need a lot more TLC until I can properly use it. The rectifier is original, and the mains lead has been replaced in the 60s I believe and hard wired to the correct tap - far as I can see, will double check that. The lead is in good nick (industrious pvc I think), as well as the plug so I'll be keeping those.

In case anyone is interested it came with an old photo copy (or Banda) of the original manual with "SSB products" and the name Norman Birkett stamped on it, with an address in Truro. I'd imagine this was the chap the previous owner bought the receiver from, perhaps in the early 80s. There's a call sign also but can't recall just now and am away for a few days
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Old 1st Nov 2017, 3:17 pm   #26
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Default Re: AR88 Low HT

That would be "S.S.B Products" 7 Little Castle Street. Truro - owned by Norman Birkett G3EKX.

He is/was a relative of the Lincoln Birketts better known as component-suppliers.
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Old 1st Nov 2017, 3:51 pm   #27
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Default Re: AR88 Low HT

It's worth improving the almost non-existant fusing arrangements. I put the spare fuse holder on the rear apron to use in the centre tap of the HT winding (I think I used a 150mA part). This was a "stable door bolting" measure having just had the mains transformer burn out due to capacitor failure!
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Old 1st Nov 2017, 3:52 pm   #28
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Default Re: AR88 Low HT

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Originally Posted by G6Tanuki View Post
That would be "S.S.B Products" 7 Little Castle Street. Truro - owned by Norman Birkett G3EKX.

He is/was a relative of the Lincoln Birketts better known as component-suppliers.
I remember Norman... used to service his TV, he lived out at Playing Place when I knew him, bought some radio stuff off him too.

Lawrence.

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Old 1st Nov 2017, 8:52 pm   #29
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It's worth improving the almost non-existant fusing arrangements. I put the spare fuse holder on the rear apron to use in the centre tap of the HT winding (I think I used a 150mA part). This was a "stable door bolting" measure having just had the mains transformer burn out due to capacitor failure!
There's quite a bit of chassis space under the mains transformer- as a safety weenie, I made a bracket secured by the 3x 0.25uF bath-tub fasteners featuring 4x 1.25" fuse-holders for mains, 2x FW rectifier anode and 6.3V protection, as the rectifier heater registered 5.4VAC with a a calibrated Fluke, it got a 0.1 ohm fusible film in series. Some countries (UK, US) seemed quite blasť about the amount of power involved around HT transformers, others really went to town on protection (Scandinavia).
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Old 6th Nov 2017, 8:05 pm   #30
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Default Re: AR88 Low HT

The fusing sounds a good idea, I'll have to look at doing that.

I spent some time on the receiver the last few days, rebuilt one of the bathtubs that was visibly leaking oil (C79,C84, C92). Not too bad a job with the rebuild itself, soaking the empty case in Jizer removed all the gunk nicely. It was a sod to remove and refit with all the wiring so close by. Made somewhat harder by the fact I decided to snip the connected wires and resistor. There was so much gunk on the terminals I dared not heat it with an iron for fear of heating up the delicious PCB oil. I replaced R22 as it had become brittle and snapped.

No discernable improvement from that tub rebuild, but would have needed doing sooner or later.

I checked the mains transformer wiring and the mains lead has been hard wired to the 0 and 240v terminals, so that seems fine.

A few more micamolds gone, one of the black ones registered high leakage when tested (43ma), and some of the light pink RCA capacitors as some were split. Those RCA ones all tested fine however in terms of capacitance and leakage.

At some point some work has been done to this and some of the micamolds in the RF units replaced with Dubilier micas, I will leave these as the odd ones I tested behaved very well and I suppose if "proper" micas, they should be fine.

A few more "solar" branded capacitors remain, the square ones of a red/brown colour. All pots have been cleaned, as has the wave change switch and selectivity switch.

I still have the fault where volume is generally low on all modes now (it wasnt when I got it running). On noise limited modes its much louder, and non-NL modes its so low its almost inaudible now, with more hum than audio. Will have to take a close look at the diagram to see why that might be
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Old 6th Nov 2017, 10:24 pm   #31
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Default Re: AR88 Low HT

At some point it's probably worth replacing the three 33K screen resistors in the coil box (shown as 33M on the circuit). They go high in value , dropping the front end screen voltages and reducing the sensitivity. Snag is, they're a pig to get at - a good training excercise if you are considering taking up brain surgery!
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Old 8th Nov 2017, 8:45 pm   #32
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Default Re: AR88 Low HT

After tracing out the connections for S21 (I think) and much time checking resistors and capacitors around V9, turns out the 6H6 was bad. I first swapped it with V8 and had more or less the opposite effect (manual and plain AVC very low volume, not the NL modes). Luckily I have an LF model and borrowed one from that and its now working as it was.

Will continue with the bathtub rebuilds and micamold replacements. Picked up some heavy duty degreaser from screwfix to help the clean up task, it looks like jizer but was nearly half the price for 5 litres

@Keith thanks for the tip about the screen resistors, a lot of components in there are a sod to get to - I can relate to the brain surgery comment!!
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Old 10th Nov 2017, 9:31 am   #33
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Default Re: AR88 Low HT

Had another look last night, though no time to delve into the bathtubs just yet.

I worked out how to disable the "Trans" feature by shorting two links on the switch, which means I can comfortably use the switch now without the neons getting too excited!

I played with the set a little while and its still working as it was before the 6H6 failure with something on all bands. Band 4 is intermittent but its the range switch that's at fault - will revisit that with some servisol.

3 more Micamolds left in the RF section , plus 1 pink RCA cap, all others I've either done myself or were done a long time ago and replaced with proper MICAs that so far seem good. As advised will check screen resistors, and while I'm at it any other resistors in those boxes. Its a pig of a job so I want to do it once and do it right!

At some point I will need to sort myself out with a proper loop antenna instead of the random length of wire strewn across the floor.

The gearbox works very well, but it sounds like the flywheel sometimes scrapes a little on the chassis. Looking at it there is lots of dust and muck on the gears, so I will have to remove this, soak in degreaser and relubricate it.

Chassis is quite greasy and mucky in parts and will need some washing down, but not till everything else is sorted. I wonder if these machines saw more abuse in amateur radio shacks than they ever saw in the second world war!

Interestingly a lot of it has an almost light green tinted coating on it, tropicalisation I assume. Its different to my LF model which is labelled as having had an anti fungal treatment.

Given the slightly high voltages I checked my mains voltage to find its 250v, a little on the high side and given the highest voltage tapping on the primary is 240v, might explain the higher readings I see.

Oh and one dead dial lamp still to replace!

Thanks everyone for the pointers so far, and for reading my ramblings!
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Old 28th Nov 2017, 3:14 pm   #34
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Default Re: AR88 Low HT

Haven't had much time as I've been ripping out the old worm eaten work bench in my workshop and replacing with a nice slab of spare kitchen work top. Also trying to get things more organised before I delve back in, and install an extractor to keep the solder (and other!) fumes down.

I have managed to rebuild 2 more bathtubs, leaving 3 more. Will probably hit these in one go, this weekend if I can, its a messy and repetitive job.

Once the suspect caps are replaced, I will be able to check remaining resistors, clean the reduction drive, replace the failed dial lamp, and have a think about alignment. I've never properly aligned a set before so will have to do some comprehensive reading on the subject.
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Old 28th Nov 2017, 3:39 pm   #35
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Default Re: AR88 Low HT

I strongly advise you to leave the IF alignment well alone. It can be very difficult to set up even with the correct test equipment.
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Old 28th Nov 2017, 5:57 pm   #36
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Default Re: AR88 Low HT

Unless it has been got at, very rarely is realignment needed with most sets in my experience. These sets perform very well and if not mentioned don't forget the neons mod across the sound output transformer.
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Old 28th Nov 2017, 8:36 pm   #37
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Default Re: AR88 Low HT

Yep neons mod has been done, it was surprising how often they strike at high volumes or when the range switch used.

If I can avoid alignment I certainly will. I suppose I will finish this as best I can, and sort out a proper antenna and give it a proper appraisal. It's only been tested with an 8 foot bit of wire and in a building with a thick concrete roof that typically provides lousy reception on any wireless device.
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Old 28th Nov 2017, 9:00 pm   #38
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Default Re: AR88 Low HT

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Yep neons mod has been done, it was surprising how often they strike at high volumes or when the range switch used..
I don't claim to have invented the neons mod but it's good to know that something I've been promoting for three decades is still doing its bit to save AR88 output-transformers from an early insulation-failed grave.
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Old 29th Nov 2017, 1:43 am   #39
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Default Re: AR88 Low HT

Greetings all.
I am so enjoying this thread along with the others on the AR88.
I have 3 AR88s waiting in the wings for restoration. All work ok but still need to be worked over to make sure they keep on working as desired.
G6Tanuki I am hearing you. I like the idea of the neon bulbs to prevent damage to the output transformer. I will go back through previous threads to identify the correct bulbs to use.
I currently have an R390A RX on my bench followed by a Hammarlund SP600 JX6 to restore then I will hoist an AR88 onto the bench.
Thank you so much for the valuable information I have learned.

Cheers. Robert.
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Old 29th Nov 2017, 11:21 am   #40
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I strongly advise you to leave the IF alignment well alone. It can be very difficult to set up even with the correct test equipment.
Very much agree- whilst the front-end alignment is essentially a much-expanded version of the procedure used for superhets in general, the IF alignment is nothing like the casual tweaking of the four cores in the typical 4 + R with fixed bandwidth. The whole shebang needs to be precisely centred on the narrow-bandwidth crystal, then the varying degrees of over-coupling need to be progressively and painstakingly monitored while adjusting for centering, symmetry and level, and it's all inter-dependent. Not for the faint-hearted, casual or time-pressed approach.
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