RCA AR88D Comms Receiver

[frank rabbets]

I am about to embark on total replacement of all the caps and resistors of tired AR88D.

Could someone recommend a very good brand for both caps and resistors please.

Thanks

[frank rabbets]

P.S.

I've been told by David RW to replace the 6sj7-6v6 cap (done). Also the tone correction cap. I can't find it. Should it be on the back of the tone pot? If so it's gone. Could someone also give me the cap number please so I can identify it from the manual.

Thanks

[Radio Wrangler]

You won't get them all of one brand.

I'm planning rebuilding my AR88 (no suffix, very early model) and I've been buying capacitors. I found all I needed at Farnell. 630v yellow film capacitors to restuff the bathtubs, and for film capacitors to redo the main smoothing block, I had to go for some audiophile jobs at several quid a go. Don't replace the real mica capacitors, they're prob OK. DO replce the 'Micamold' fake ones, they're known trouble.

David

[ms660]

Originally Posted by frank rabbets

I've been told by David RW to replace the 6sj7-6v6 cap (done). Also the tone correction cap. I can't find it. Should it be on the back of the tone pot? If so it's gone. Could someone also give me the cap number please so I can identify it from the manual.

The capacitor that RW was referring to is probably the tone correction capacitor that is connected between the anode of the audio output valve and chassis, if it breaks down it can take out the sound output transformer or maybe worse.

On the schematic I'm looking at its component number is C119 with a value of 0.003uf, that value seems to vary in different schematics/parts list but not by a lot, I would go for 0.0033uf with a rating of least 1,000 volts DC.

Lawrence.

[frank rabbets]

Thanks for both replies. The smoothing block does not get hot, it does not leak nor is there any hum on the speaker. Does that mean it's OK at the moment?

Thanks for identifying C119. It's just in a blurred area in my picture so I'll replace everything within a 3 inch radius before turning it on again. LOL.

I am a real amateur at this and only understand half of what is said so far.

Resistors? Do they have a voltage rating? What make is best ?

Thanks

[ms660]

Yes, resistors do have a voltage rating, in the comms receivers I've worked on I used ones rated at least 350 volts, all mine were from Farnell.

The original resistors in the bias network have a habit of changing value.

If you are changing the lot then do no more than a few at a time and check your work and the receiver's performance as you go.

EDIT: Note that when the switch is in the Transmit position the HT will increase to high level....Think safety.

Lawrence.

[Radio Wrangler]

The mode switch, bottom left, breaks the HT feed to the receiver. Off load, the power supply charges itself up to a very high voltage, and the valves are left stewing with their heaters on but no other supplies.

This puts a lot of stress on the reservoir and smoothing capacitors as well as general insulation. You do need 600v capacitors in this area of the set. When the switch is moved out of "Trans" the HT is re-connected with a nasty surge into the rest of the set, and everything else gets stressed. Oh, and running valves heated, but no HT, slowly degrades them by a process called 'cathode poisoning'.

The transmit position also closes a contact linking two terminals on the rear of the chassis, used to enable a transmitter. On the whoole, it's not a good way to switch a station from receive to transmit. I'd disable the HT-Off condition by soldering in a link to bridge the HT contacts.

David

[frank rabbets]

Thanks Lawrence. I'll do all resistors as you say. I'll take close up photos of every part before I do anything.

Thanks again David. I'm not experienced enough to understand your post but would be grateful if you could tell me where to put the knobs on the front to safe working positions? I just want to browse through short wave reception.

Someone has replaced the audio output transformer. There are 2 spare wires. Do you think these were from the 600 ohm output of the original transformer. Audio woks on the 2.5 ohm screws.

There's something called a Crystal Filter. I can't find it, probably because I don't know what it looks like and don't know where it is. Is it linked to sensitivity switch as this does not work in the 3 most clockwise positions.

Your help is most appreciated.

[dave walsh]

If you go on the VMARS [Vintage Military Amateur Radio Society] site Frank you can down load an AR88 [D] Manual [with schematics] from that section for free. It's No 317 on the list. They very generously make a point of open access. There are also a couple of repair/mod articles you can view. I'm limited in tech experience myself but it's good info.

Dave W

[ms660]

Originally Posted by frank rabbets

There's something called a Crystal Filter. I can't find it, probably because I don't know what it looks like and don't know where it is. Is it linked to sensitivity switch as this does not work in the 3 most clockwise positions.

Yes, a bit of a complicated city block to find your way around.

Crystal (Xtal) Filter:

The actual Xtal should be underneath the chassis, go to this link:

http://www.antiqueradios.com/forums/...p?f=5&t=238621

When you get there scroll down to the set of AR88 photo's, look at the 6th photo down, you can see the underside of the mains transformer, above that are the undersides of the two HT chokes, to the right of that you will see a preset variable capacitor (C75) to the right of that capacitor is the Xtal, to the right of the Xtal is T3.

Here's one of the better manual copies on the web for the under chassis view:

http://www.qsl.net/g3giq/AR88images/Underchassis.JPG

You can see the word "crystal" at the bottom of the page towards the right hand side, following the arrow will take you to the same point as I described above.

Xtals don't often fail, sometimes the xtal itself can be a bit lack lustre when they are old, cleaning the actual Xtal itself and internal coupling plates can sometimes bring about an improvement in performance, some Xtals aren't designed to be opened up, some are, such as in the early HRO, Hallicrafters receivers for example.

If I were you I wouldn't interfere with the Xtal or any of the Xtal's transformer/alignment adjustments unless you know what's involved and know what you're doing for sure.

All switch contacts in the receiver should be given a damn good clean.

Good luck.

Lawrence.

[frank rabbets]

Thank you for your reply Lawrence.

I am still struggling to find the caps referred to by RW.

1. 6SJ7-6v6
2. Tone correction cap.

I have indeed found a cap between the two valve's bases quoted but you describe this as RW's tone correction cap so I am still missing one cap that I should change. I wonder if you could perhaps describe both caps again so I can proceed with confidence.

Thanks for the links. Really interesting.

[frank rabbets]

6th photo down shows the cap I've identified to the RHS of the main transformer. It is diagonal between the 2 valve bases. Where is the other one please ? I shall get some sleep when I know. Been awake for days now !

[trh01uk]

Originally Posted by dave walsh

If you go on the VMARS [Vintage Military Amateur Radio Society] site Frank you can down load an AR88 [D] Manual [with schematics] from that section for free. It's No 317 on the list. They very generously make a point of open access. There are also a couple of repair/mod articles you can view. I'm limited in tech experience myself but it's good info

Dave,

yes, I did that manual many years ago. I also instituted the free access. Glad to see the resource is being used.

Richard

[ms660]

Originally Posted by frank rabbets

I have indeed found a cap between the two valve's bases quoted but you describe this as RW's tone correction cap so I am still missing one cap that I should change. I wonder if you could perhaps describe both caps again so I can proceed with confidence.

The tone correction capacitor C119 is connected between pin 3 (anode) of the audio output valve and chassis according to the schematic I'm looking at, it's shown as a 0.0027uf capacitor.

The audio output valve grid coupling capacitor is connected between pin 8 (anode) of the audio amplifier valve and pin 5 (control grid) of the audio output valve, the same schematic shows two of them in parallel, C118 & C122, both those capacitors are shown as 0.0047uf each.

Looking at that copy of the under chassis layout it's difficult to see exactly where they are but they will be there somewhere, trace the connections from the valve pin numbers on the valve holders, that should lead you to them with a bit of luck.

Those capacitors must be replaced.

Lawrence.

[trh01uk]

Originally Posted by frank rabbets

Could someone recommend a very good brand for both caps and resistors please.

I will make some recommendations bearing in mind you say you are out of your depth in some of the other posts in this thread. Don't let that put you off - as long as you check in here with what you are doing, chances are some of us with more experience will put you right if you are going to make mistakes.

First thing I would advise if you are a newbie to restorations is to make sure you know what a good quality soldered joint looks like, and how to achieve one. You may well know that already so I won't go into it, but its an absolutely essential first step if you are not to totally wreck your set.

Then regarding resistors and the replacement of all them. Download the manual from the vmars website as already advised. Look for the parts list, and the resistors in particular. You will note that you are given a value of resistance in ohms, and a power rating in watts. I see that nearly all the resistors are 1/2 watt.

If you go to the Farnell website, you can search on "resistor through-hole" (i.e. ones with leads on), and you will turn up thousands. I then selected using the side panels 1/2W (or 500mW) and 350V (as someone else suggested), and got a good selection of values, made by TE Connectivity supplied in packs of 5, and costing 4.4p+VAT each. That's not cheap, but I dare say cost is not your primary concern here.

You will find these resistors pretty small compared to the ones you are taking out. Some people like to put in much larger ones (though I find it pretty arbitrary myself) - if you do that you can find biggere ones with a much higher power rating (e.g. 2 watts), but they will cost quite a bit more.

And finally, looking down the parts list there are few resistors rated at 4 watts. That's R30, R43, R44, R56. In the user manual's part list its unclear what type of resistors these are. However referring to EMER L722 (vmars ref no. 639 also downloadable for free), the parts list in that confirms these are all wirewound types. Again Farnell has a range of these at 4 watt or 5 watt rating.

Richard

[trh01uk]

Originally Posted by pwdrive

The tone correction capacitor C119 is connected between pin 3 (anode) of the audio output valve and chassis according to the schematic I'm looking at, it's shown as a 0.0027uf capacitor.

The audio output valve grid coupling capacitor is connected between pin 8 (anode) of the audio amplifier valve and pin 5 (control grid) of the audio output valve, the same schematic shows two of them in parallel, C118 & C122, both those capacitors are shown as 0.0047uf each.

Looking at that copy of the under chassis layout it's difficult to see exactly where they are but they will be there somewhere, trace the connections from the valve pin numbers on the valve holders, that should lead you to them with a bit of luck.

And just to help you spot the capacitors that Lawrence mentions, here is the underchassis diagram with the capacitors ringed. They appear clear enough on the diagram - but maybe your set has been wired differently to that?

Richard

[ms660]

Following the arrow for C119 seems to point clear enough, following the arrow for C118/C122 is not so clear to me, I don't have my AR88D anymore otherwise I would take a photo of the said.

Maybe someone has already snipped the capacitor(s) out with a view to replacing them and never did.....Or maybe not.

I'm sure the OP will figure it out.

Lawrence.

[Radio Wrangler]

"EMER" was mentioned a few posts age.

It's a british military manual for servicing and repairing the AR88. It's very well written with much better explanations than the American original. It also breaks the big circuit diagram into a number of sections. You'll find it a lot clearer.

It's freely available on the web.

http://www.vmarsmanuals.co.uk/archive/files_index.htm

as number 639, and 664

David

[Hartley118]

Originally Posted by frank rabbets

I am a real amateur at this and only understand half of what is said so far.

All the advice offered so far is sound, but I do worry that we're encouraging you to try and swim the Channel before you've done a few lengths in the local pool.

In what way is your AR88 'tired'? Lack of sensitivity? My own example is original and unmodified, but still works pretty well. The original components were good quality and typically don't leak or drift beyond what the circuit can tolerate. The really critical capacitors have already been identified, but I'd be surprised if you need to replace more.

I'd aim to find the fault first before going for wholesale component replacement. For example, is there a low emission valve? A valve tester would be ideal, but you can learn a lot from measuring anode, screen and cathode voltages in circuit and comparing with the service data to check whether a valve is too weak to draw its specified current.

You may feel that you don't want to get into this level of circuit detail, but I can assure you that wholesale component replacement will involve you in some detailed fault diagnosis anyway in rectifying the errors that would inevitably creep in!

Martin

[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.

Once I have beaten these 2 caps I'll assess and ask advice on what to do next. just don't want to cause damage.

Thanks again.