RCA AR88D Comms Receiver

[Station X]

I can recommend this site for valve data:-

http://www.r-type.org/

When looking from under the chassis the valve pins are numbered clockwise from the gap or spigot.

[Radio Wrangler]

Changing those capacitors will make your AR88 pretty safe from the point of view of risks to ITS existence. What we also need to worry about are risks to your existence.

Swapping components can be done with everything off, cooled-down and discharged. But once you've swapped several things, errors can creep in and you're in the business of trying to fault-find in a machine with exposed connections at 300v DC as well as mains. You need to learn some potentially life-saving survival skills.

David

[Silicon]

Unless your AR-88 has been messed about and is in a terrible state, I would not recommend the wholesale replacement of every resistor.

Measure the value of each resistor and compare it to the schematic or parts list.
If it deviates more than 20 per cent from the correct value it may require replacement.

The paper capacitors and switches should be your priority at this stage.
A few critical capacitors should be replaced before turning the receiver on.

A radio can work satisfactorily even if the non critical capacitors are a little bit leaky.
My AR-88 is still working with the bathtub capacitors.

Faulty valves can often be identified by the voltages on their socket pins.
It's nice to have a valve tester but many enthusiasts make do without one.
They use electronic knowledge and logic to work out what is happening.

After you have replaced the critical capacitors I would recommend that you study and understand the negative bias and AGC circuit because a fault there could 'hobble' your receiver.

Do you have a lamp limiter?

[Radio Wrangler]

The series of valves used in the AR88 are very long-lived, and tend to gentle loss of emission rather than abrupt failure. There are resistors in handy places where you can check currents and spot anything thats going low in emission, but generally, the valves are very little trouble.

Valve testers are OK if you've got one, but they are now ludicrously overpriced due to demand from the hifi and guitar fraternities. For some reason a lot of people seem to behave as if being rested on the right make of tester will improve the sound. A photo of a meter pointer nicely 'in the green' certainly increases auction prices for valves.

Resist any urge to buy a tester, checking the anode and grid currents in the actual circuit a valve is used in is a much more meaningful test anyway.

David

[ms660]

Originally Posted by frank rabbets

Thanks for all the advice. I feel confident in what I am attempting but what I want to do is get rid of theses two potential death trap caps first. I think I have worked out which ones they are but just to double check does anyone have a link to where I can find the valve base pin numbers and their internal valve connections for this receiver. E.G. what numbers are the heaters, anodes etc. I've searched the web but can't find a thing. There must be a diagram of a valve socket somewhere with the pins numbered and what they connect to inside the valves.

Both the original manual and the EMER manual show the valve pin numbers and the electrodes they connect to in the schematics, as has been suggested use The Valve Museum's website if unsure.

Lawrence.

[dave walsh]

As I said Frank I would be behind you in actual practical work but I've been interested in these sets since as I was fourteen and couldn't possibly afford the pristine one that a Radio Amateur showed me. It may be a result of War Time production demands that this "Rolls Royce" set had a power supply circuit that was very prone to destroying Output Transformers via the three routes described ie the two caps or the valve HT being switched off via the RX/TX function. Any of these can create a heavy rise in current that takes out the OT. I think users were always advised to keep a speaker in circuit as well.

Replacement OT's were never common [except during WW2 perhaps] and this always seemed to be a knotty problem regularly complained about until it was demonstrated that relatively commonplace transformers can be substituted. This has probaly happened with your set so you are very wise to concentrate on this potentially disastrous aspect. There should be many postings re the burnt out OT problem on here or on the web you can read.

It's a complex and compact receiver [ie not really a "starter"] but that's your choice of course. It is very common to imagine that replacing all the caps wholesale etc will somehow get things going but its a false premise and will probaly lead to wholesale confusion as already mentioned. That's why the advice is always don't do it. As far as I can see it only works for very experienced people [ie not me] who have been there before many times and even then, it's usually only applied to component types that are very well known to fail. "Step by Step" is by far the best approach then you know were you are [hopefully]:

Dave W

.

[frank rabbets]

These are the "2" caps in question I think ?
What value and voltage do I replace then with please?
Presumably I can use 1 cap for the 2 shown in parallel?

Once I've replaced these and turned the receiver on I'll revise the thread and make a summary.

[Radio Wrangler]

Those are indeed they.

The single one is the tone correction capacitor. It needs to be an especially high voltage rating as has been said before in order to survive some of trhe inductive transients from the output transformer. For some odd reason they wired it from anode to ground, which is what allows it to destroy the transformer if it goes short-circuit. Most radios wire it ACROSS the input winding of the output transformer (Anode to HT connection in other words) This works just as well for the intended purpose, it reduces the stress on the capacitor a bit and it stops a failed capacitor killing the transformer. Earlier AR88 threads have recommend wiring the replacement this way, and I will do so to mine when I get started.

The two capacitors wired together must have been to use available parts to get the wanted value. They are 0.0047 microfarads each (which is the same as 4.7 nanofarads, or 4700 picofarads) Fitting one 0.01uF ( or 10nF or 10,000pF) capacitor will be just fine.

David

[frank rabbets]

David thanks. When you say high voltage could you quote please? Also where can I get as good as possible quality capacitors from? There is a "Maplins" close by. Will they do ?

None of these caps we are talking about are polarity specific are they ?

[HamishBoxer]

1000volt at least ,1250 Volt DC is even better.

Maplins may or not have them.

[frank rabbets]

Like I say I need quality. where do I go for best.

Thanks.

[Radio Wrangler]

I normally use Farnell.

They are a professional/industrial supplier with an amazing range and reasonable but not cheap prices. They've always been prepared to deal with individuals on the phone, subject to a minimum order of £20. I just wait until I need enough bits.

Their competitor is RS components and they would not deal with individuals at all. I think they've changed, but I still remember not being able to get something only they kept. It was very frustrating and I avoid them to this day. They were the main supplier to the TV/radio repair trade. They too have a huge catalogue.

These two are broad-range suppliers of good components of known make.

You can get lost on either website and not surface for days!

There are several other component retailers, aimed more at the hobbyist market, like Maplin. However, their components tend to be of generic make. Most are fine, but there's less assurance. I design things for planes, so I guess you want me to be a bit picky

David

[John Caswell]

May I also chuck in the following.
As several other people have stated do not change all components Rs Cs etc willy nilly, that way lies problems.
The current trend of "changing every capacitor/resistor 'cos Joe Bloggs says so" is foolish in the extreme.
I know it is long winded but change one item at a time, that way if it doesn't work properly after the change you know where to look.
You wouldn't credit the number of jobs I get because the customer has gone at it mob handed, and finished up with some thing that used to work, albeit poorly, to something that doesn't work at all, and until you have it working reasonably well leave any coils adjustments/trimmer capacitors well alone.
If you are not familiar with radio alignment find somebody that is, as these things are complicated to align.
Component purchase - try CPC in Preston, part of the Farnell Group but cheaper although not such a wide range, and also Rapid and Cricklewood Electronics.

John

[G8HQP Dave]

Don't worry about "quality" components. Ordinary good quality components are fine, as obtained from any reputable dealer or shop. Specifications are what matter, not the brand printed on the outside. Just avoid fakes and cheap rubbish off ebay.

As others have said, it is vital that you only change a few components at a time and then re-test. I would only change faulty components, and then those expected to fail within a short time.

Personally, I fear you are biting off more than you can chew - the questions you have asked indicate this - but we are here to help.

[M0FYA Andy]

I recommend 'JustRadios' in Canada for resistors and capacitors for repairing valve equipment - http://www.justradios.com/
Orders arrive very quickly!
Andy

[frank rabbets]

I did ask under "repair and restoration services wanted" if anyone here wanted a paying job to restore this receiver but I had no offers. I'm willing to hand the set over to someone capable...

[dave walsh]

You might well find someone capable here or elsewhere Frank but it's likely to be expensive and you've indicated that you want "the best" so a potential repairer may be a little unsure about taking it on-especially if you want it all stripped out and replaced

Dave W

[frank rabbets]

I thought it would be easier for someone to give a fixed price on that basis other than to to ask for "just get it working please".

I am happy however if someone could do an overhaul and service...

[G3VFB_Anton]

Couple of points to add to this interesting discussion

1. Years ago I recall some sort of relay mod to switch the HT off to go to Tx but don't ask for the circuit - long forgotten ! Cathode poisoning point taken but for the shortish time one is 'yakking' live on air it's really not going to be a problem in my humble view.

2. As a repair technician at ARE in London we had a saying which got used for almost everything that came in, but especially for AR88s because it was true. "If it's deaf change R1" - (the RF amp screen-grid resistor).

[frank rabbets]

Andy thanks for the tip on Just Radios.

David. R.W. Just Radios do 0.01 MFD 1600 volts and 0.0033 MFD 1600 volts. Both are called orange dip. I'm sure these will be OK but can you confirm please.

Once I get the receiver fired up at least I will be able to assess it's performance and may well take advice on doing a lot less work on it.

As there is the original twin core cable I've wired an earth to the chassis. I've fitted the plug with a 5 amp fuse as I note it only runs at 100 watts.

Thanks again for the help and hopefully will be sorted soon.