Brown Wima caps in AC15

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Malcolm Irving
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Brown Wima caps in AC15

Post by Malcolm Irving »

I'm bringing a 60's AC15 back to life. Most of the non-electrolytic caps are brown Wima. I think they are paper-type with a dark brown lacquer coating.
(Nothing like the silver or mustard coloured caps I've seen in most gut-shots of early Vox amps.)
They are a real pain, every one of them that had any DC voltage to hold back has failed, the lacquer is always cracked and I guess moisture got in.
Anyway, just out of curiosity, does anyone know when Vox used these?
lovetone
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Re: Brown Wima caps in AC15

Post by lovetone »

These are used on the early amps 59 to 61 I have an AC30 chassis with the same problem, these suffer the same problems as the Erie and Hunts capacitors as they dry out they expand and crack.
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Malcolm Irving
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Re: Brown Wima caps in AC15

Post by Malcolm Irving »

Thanks for the information.

So far I've got Channel II working. It has an original Mullard EF86 which sounds fantastic. I swapped in a current production EF86 while testing and I was amazed at the difference in the sound. (Not so good with the current production tube!)

Channel I (vib./trem.) is proving to be a challenge - suffering from a 'crackle' which is difficult to pin down. Chop-sticking almost anything affects the crackle, but so far - no particular sensitive component or connection. Step-by-step elimination using a scope also hasn't been effective as yet. Keep getting into logical paradoxes such as: It must be coming from X,Y or Z, so I prove that it's not X, then that it's not Y, so it must be Z. So I test Z and find that it is not Z - :evil: . I'll persist - I'm sure we've all been there. :)
lovetone
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Re: Brown Wima caps in AC15

Post by lovetone »

LoL the sonic difference between new and old production valves can be quite dramatic.
One problem I find with new production valves is the inconsistency in the gain (mu) if I buy NOS in the tester they are much closer from the same batch.

Crackles I find are generally noisy anode resistors, probing the anodes with a scope can help find the stage the noise is in. Other than that make sure the valve base sockets are clean and tight I have found this to be a problem on old English amplifiers, some times I have found them to have algae.

Hope this helps

I have sent you a pm

Geoff
lovetone
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Re: Brown Wima caps in AC15

Post by lovetone »

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Malcolm Irving
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Re: Brown Wima caps in AC15

Post by Malcolm Irving »

Thanks.
sluckey
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Re: Brown Wima caps in AC15

Post by sluckey »

Here's a more legible schematic for the 1960 AC-15.

http://el34world.com/charts/Schematics/ ... x_AC15.pdf
RockinRocket
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Re: Brown Wima caps in AC15

Post by RockinRocket »

Are we talking the Wima TFM caps?
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Malcolm Irving
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Re: Brown Wima caps in AC15

Post by Malcolm Irving »

Thanks sluckey for the further schematic.

The caps are not the TFM (which have a good reputation – as far as I know). They are a dark brown (like plain chocolate) and just have the WIMA logo, capacitance and voltage values, plus a part number (such as 80Np or 80Cd).

I’ve got both channels working now, but still a crackle to track down. There are still 4 cap.s which I need to replace.

I think the next time I’m offered an old Vox to work on, I’ll check for these ‘dreaded brown Wimas’ and either politely decline or suggest new tag boards with all new resistors and caps!

I found a mention of them on a vintage radio site, and someone said he used to work at a place where they had a look inside any brand new oscilloscope / test equipment they purchased and if they saw the ‘brown wimas’ they routinely replaced the lot, before putting the equipment into service!

Sounds like I am getting a fixation about these caps, and I suppose every electronic component has its place, but in this case it has to be ‘in the bin’. :roll:
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Malcolm Irving
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Re: Brown Wima caps in AC15

Post by Malcolm Irving »

Replaced the last of those caps and it all now works - the crackle has gone. :D
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