WAD Kit88 rebuild - help needed.

[edhill]

Hi, my partners late husband was an audio geek and was the proud owner of a World Audio Designs Kit88 amp. I repair iphones for a living and because I know what a soldering iron is it has fallen to me to repair it.

I'd very much like to refurb it, the amp has been in storage for the better part of several years and some of the components look a little corroded in places, although it does look in reasonably good order overall.

My plan is to test each component, and remove and replace each thing if required.

Is it possible to get hold of a circuit diagram and other useful paperwork for this amp please? The PCB is stamped 07/01/00 WAD KIT88.

Are there any other pitfalls I should look out for or components that will likely need replacing?

Thanks for your help.

Ed.

[Richard]

Hi Ed and welcome,

This was a very poplular amp and many of us made and run them. If you send me your direct email address I can send the circuit and build instructions.

Corrosion doesn't sound good but just clean gently or replace if on sockets or volume pot. Most likely a squirt of Servisol and fitting/removing valves and plugs will help. Avoid using in volume pots as it will remove the nice damped feel when turning.

Replace all the electrolytic caps as they will be 20 years old and many run at high voltage. Look for over heated resistors. Check and change valves. Turn on and do a full voltage check as per the instructions. Pay particular attention to the mains and HT at the KT88 anodes firstly, then, voltages around the cathode and grids of the KT88's and check which value cathode resistors are fitted. These set the bias which was often a bit high so it was common to adjust bias by changing them.

[John Caswell]

Hi Ed,
Be extremely careful of the PCB tracks as after this time they will be very fragile.
If the previous builder has folded the component leads over hard against the PCB (I hope not) you are in for a difficult time and will need to cut the component out and carefully remaining remove the leads with a minimum of heat and stress. The Octal valve bases were also notorious for their plating of the pins failing causing all sorts of intermittent faults. Richard has covered pretty well everything else but if in doubt ask here before delving in. I did look at producing a modern Plated Through Hole (PTH) version but cost ruled it out.
Greg should be able to supply all required info, and I suspect he will be reading this anyway so expect a call/email/pm
Sorry to be a harbinger of doom, but preparing you.

John

[Greg.]

Yes, either Richard or I can supply you with the build instructions and diagrams including the schematic. Just PM either one of us with your email address and the documents will be sent to you. One thing though, I am currently not well with seasonal (not Covid) flu and have a pre-Christmas rush on, so you might get a quicker response from Richard.

[edhill]

Originally Posted by John Caswell

Hi Ed,
Be extremely careful of the PCB tracks as after this time they will be very fragile.

is the board multi layer or single layer? (form a quick visual it looks single)

A fair amount of the work I do on iphones is long screw damage repair. (apple in their infinite wisdom use different screw sizes in their phones and it's really common for someone to put the long screw in the wrong hole and screw it through the multilayer tiny pcb, causing a heap of trouble)

So I'm confident I can repair/replace pcb damage if needed.

The Octal valve bases were also notorious for their plating of the pins failing causing all sorts of intermittent faults. Richard has covered pretty well everything else but if in doubt ask here before delving in.

This unit did have an intermittent fault that the previous builder wasn't able to find from what I have been told.

Sorry to be a harbinger of doom, but preparing you.

John

no probs always good to be informed in advance, I shall take my time with this and will no doubt be posting more questions in due course.

Thanks all for your help.

[John Caswell]

Hi Ed,
It is a two layer PCB with connections between both layers made by component legs being soldered both sides, a pain in the ****
The best way to remove a component is to cut the leadout on the component side and then grip the lead with some fine nose pliers, heat the component lead with a soldering iron which will melt the solder which will then allow you to remove the wire and then clean up the joint either side. If you think the track is poor do not clean up with a soldering iron but drill through with a 1mm cobalt drill top to bottom.
If you think a valve base is poor first of all break it up with some large side cutters then remove the pins one by one.

John

[edhill]

Originally Posted by Richard

Replace all the electrolytic caps as they will be 20 years old and many run at high voltage. Look for over heated resistors. ....

Thanks for the welcome and the tips, I have sent a PM with my direct email address.

[Richard]

Originally Posted by edhill

Thanks for the welcome and the tips, I have sent a PM with my direct email address.

Thanks Ed, I've just sent the info, good luck and do let us know how it goes!

[edhill]

Thanks for the info.

Keeping in mind how fragile the PCB can be when it's this old I'll keep the work I do to it to the minimum for now then once I have it running, if it does have other issues then I can look to rectify those then.

So far I have tested the valves and they appear to be OK (or at least they haven't shorted out and blown) however for the 4 small valves all of the printing on them has worn away.

For the 6au6 valves - I can't find a source for these (I live in Germany) as several sites I looked at are all showing as unavailable, the description of one site says pentode, 7-pin, so will any 7 pin valve described as a pentode work? as there are lots of those everywhere and they're easy to find.

It looks on closer inspection that a couple of the caps have been changed out already. The parts list has 3 4700uF 16V caps, and on the unit I have 1 of these is a different size and colour to the other two (and looks very bulged with what I assume is dead capacitor juice underneath it)

I am going ahead and changing out all of the electrolytic caps as advised, so no problems there, though are there any brands of caps to go with, or any I should avoid like the plague?

[Richard]

Here's some pairs of the more reasonably priced ones; 6AU6 / EF94,


https://www.ebay.de/itm/225164806459...Bk9SR6TkhuavYQ


https://www.ebay.de/itm/125586267916...Bk9SR6TkhuavYQ


http://www.r-type.org/exhib/aam0079.htm


For caps use good commercial quality from a recognised brand and supplier. you can use a higher voltage ie 25V instead of 16V and higher temp may last longer ie 105c instead of 85c.