Watkins Copicat MkIV : No Echo

[kraussss]

Someone gave me a Copicat echo that they found in a charity shop.

Here is the schematic:
http://britamps.mysite.wanadoo-members.co.uk/watkins/schematics/copicatmk4.html

The unit had all the leads cut off.
So I hooked up a mains lead, and an output lead, and a tape loop, to see if it would go.
It powered up fine, and turned the tape-loop.
Initially it made a good echo, but then, over a couple of minutes, the echo effect got quieter and more distorted, with less repeats.
Now it only makes an echo if you really crank up the input gain, and slam down on the strings, hard. 
The echo is just a distorted "spppt" noise.

I changed the tape loop: same sound.

I know I should be getting in there and measuring voltages.
However I'm not sure where to start.
I thought I would make an initial post to see if someone could suggest a possible cause for this behaviour, and which transistors are the most likely culprits.

Any suggestions welcome.
Thanks,
Kraus

[Nasse]

The easy thing you can do is to clean the tape heads, if you have not done it yet. That cotton the girls use to clean their face and some denatured near 100% alcohol is best. Do not put that solvent in other places than tape heads.

If that does not work maybe you need audio probe and some thinkin "how it works", luckily you have the schematic, and maybe that big "capacitor job" is not a bad idea (change the electrolytic caps).

Dunno where you live but those "instro" players have much knowledge about these, and there exist few repairmans that know these things and can make it work almost "as new". It might cost some money but such is life. And Watkind company is still in business, some spare parts and info is available I believe.

[bajaman]

Try changing all the caps in the bias oscillator circuit - youn should hear good echo again
cheers
Steve

[kraussss]

OK, Nasse and bajaman, thanks very much for those suggestions.
I gave the heads a good clean before I tested, so I don't think it can be that.
I had a feeling it might be the electrolytic caps, because of the way the echo seemed to mysteriously die over a period of time.
I guess they are very old, maybe 30 years?  Anyway past their usable lifetime.

I have been pondering the schematic, I am starting to understand what each stage does.
However, I do not know which is the bias oscillator circuit.
Would you be able to help me identify it?
I would really appreciate it.

Thanks again,
Kraus

[Nasse]

I believe that thing that says "65 kHz" (drawn from right to left) is bias oscillator, only one electrolytic cap if I can see near it but some further up the road. Maybe they can get old too. Remember the high voltage, dangerous stuff

[kraussss]

Right, I was wondering what that stuff did.
It looks like an astable connected to a transformer, yes?

Can you check for bad electrolytic caps with a capacitance meter?

Remember the high voltage, dangerous stuff

Thanks for the warning.
I certainly need it, as I haven't much experience with mains electricity.
Yes, I am taking GREAT care.

Thanks,
Kraus

[Nasse]

bajaman suggested changing ALL caps in the bias circuit, quess there are some types other than electrolytics that go old sometimes, perhaps because of heat or aging

Just wondering what is the rectified DC supply voltage for this thing...

[bajaman]

Change the polystyrene capacitors to the left of the bias oscillator - they will almost certainly not be the correct value anymore - 200pf and 1800pf.
Also it wouldn't hurt to change the 4700pf rolloff and the 0.01uf across the erase head. Hope this helps - let me know if it doesn't. I have a Mk II valve watkins unit, and I found these caps to be the main culprits - from memory the 200pf measured around 20pf on test!!!!!
good luck
Steve

[kraussss]

Oh, I see, ALL the caps in the bias oscillator circuit.
I didn't realise that polystyrene caps could go bad.

Thanks for identifying the specific parts to replace.
Hopefully that will take care of it.
It will be a couple of weeks by the time I get the caps and put them in.
I will measure the DC voltage as well.

Thanks again for all the helpful advice.
Kraus.