WAD Kit88 rebuild - help needed.

[edhill]

Originally Posted by Richard

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.

thanks for the links.

I've removed all of the electrolytic caps from the pcb now and have taken measurements for the lead spacing from the board.

I'm looking to get caps at the correct capacitance as per the spec sheet, but buying ones rated for higher voltage, am I correct in thinking that a higher ripple current is better (slightly cooler running and longer lasting potentially?)

Looking at the circuit diagrams in the destructions, the board I have has jumper wires all over it, and I damaged 1 pcb trace that I need to repair. It looks like the previous builder thought the board was cracked somewhere, but I've spent a long time looking at it under my microscope and the pcb generally looks in good nick. I can't see any cracks or corroded traces, it had a lot of dust and dirt on it, but it cleaned up well.

I'm not sure about the valve bases, I looked at them carefully for signs of corrosion/damage, but there's nothing obvious, I'm not really sure what I'm looking for and replacing them looks like a real PITA so I'll leave them be for now. If it starts throwing intermittent faults at me later I'll look to change them out then.

As this was built by my partners late husband I am going to try to leave it as is as much as I can. If I can get it working well but keep it as 'original' as possible then so much the better.

Will post more progress updates (and no doubt more questions) once the new bits arrive.

[Richard]

Yes, in general low ESR, high ripple, and long life go hand in hand as resistance causes heat and buying all electro's using these factors will be good.

With the large 4 it will be important to get them of a size to physically fit and they're 10mm snap in iirc. I fitted a loop of tinned copper wire to make a more accessible centre earth point - easier to do before the caps go in.

[edhill]

Yeah I looked at cap diameter C1 and C2 the 2 biggest I went with the same as was on it as they don't interfere with any connections, the other 2 large ones were blocking some pins in the centre so I went with a slightly smaller diameter but higher rated cap. My thinking being that smaller will allow more access and more airflow to help a little with temps.

I'll definitely look at adding a loop of wire to the earth pin in the centre.

The 4 larger caps were indeed all 10mm snap ins.

Going through the resistors today as some of them look a little toasted and will order up any of those that need replacing at the same time, then I shall wait for my goodies to arrive.

[edhill]

and more questions:

resistors, I've tested all the resistors, and they do all resist, which is nice.

but, there's a few variations from the specs in the part list that I'd like to ask about.

The notes file suggests switching r3 from 13k to 18k to increase negative bias at g1 or switch out the cathode resistors to 220R wirewound. The board I have has both of these mods done, is both OK at the same time?

the doc also suggests changing anode Rs to 2W varieties, the ones installed look like 1w versions and a couple are reading higher than advertised 19 and 24 are both hooked to anodes of the B9A valve bases (ECC82?). spec says 47k and these are reading 50-53k.

R21 & 22 spec says 15k 3W and these are both reading 12kish those are both hooked to the cathodes of the same valves.

lastly Rs 28/29 spec says should be 1M and the ones installed here are 100k.

The circuit shows a selector and a bunch of inputs, this amp has a single input only. Not sure if that makes a difference for 28/29.

Am I right that resistors reading high are on the way out and need changing? more resistance = more heat = soon to be released magic blue smoke?

Apologies for all the questions, I know a lot about iphones, a little about general electronics and very little about amplifiers.

[Richard]

and more questions:

resistors, I've tested all the resistors, and they do all resist, which is nice.

but, there's a few variations from the specs in the part list that I'd like to ask about.

The notes file suggests switching r3 from 13k to 18k to increase negative bias at g1 or switch out the cathode resistors to 220R wirewound. The board I have has both of these mods done, is both OK at the same time?

Some folk did both and no-one reported problems that I recall. It fixed the red patches on the KT88 plates and sound was just as good. I kept R3 at 13k and changed the cathodes to 220R. You’ll see the mains TX is wound for old UK 240V mains so new UK 230V and EU mains should be fine.

the doc also suggests changing anode Rs to 2W varieties, the ones installed look like 1w versions and a couple are reading higher than advertised 19 and 24 are both hooked to anodes of the B9A valve bases (ECC82?). spec says 47k and these are reading 50-53k.

They are too high and may well readeven higher when hot, mine did when failing. (They are rated too low, ratings are for free air which they don’t run in and amp insides can be around 50C.) Change R19/24 to 47k 2W and R7/20 to 36k 2W

R21 & 22 spec says 15k 3W and these are both reading 12kish those are both hooked to the cathodes of the same valves.

Change them too, modern film resistors should read very close to spec.

lastly Rs 28/29 spec says should be 1M and the ones installed here are 100k.

The circuit shows a selector and a bunch of inputs, this amp has a single input only. Not sure if that makes a difference for 28/29.

They should be 1M with a 100k vol pot before them or 100k if driven from a pre-amp (single input without vol pot).

Am I right that resistors reading high are on the way out and need changing? more resistance = more heat = soon to be released magic blue smoke?

Yes they can go high or open circuit and read higher when hot when they are failing. Modern film resistors are very accurate (2% metal film and 5% carbon film) so change any which are not very close to spec as they will be faulty.

Apologies for all the questions, I know a lot about iphones, a little about general electronics and very little about amplifiers.

[edhill]

Originally Posted by Richard

lastly Rs 28/29 spec says should be 1M and the ones installed here are 100k.[/left]

The circuit shows a selector and a bunch of inputs, this amp has a single input only. Not sure if that makes a difference for 28/29.


They should be 1M with a 100k vol pot before them or 100k if driven from a pre-amp (single input without vol pot).

This makes sense then as there will be a preamp in front of this when it's all running.

Thanks for the help, I'll order up the bits I need and post a progress update once everything is installed.