What is a C100k pot?

[JohnH]

I have a question about pot references

I need to replace the treble tone pot in my Crate Powerblock amp, which has become intermittent, resulting in a dull tone until it is fiddled with, then the treble comes back, then it goes again etc. It’s the pot, not connecting properly between wiper and track. But it is a small, sealed 9mm pot, (a green square thing), pcb mounted, single gang. The usual cleaners that I find very effective on normal pots can’t get inside.

So I think I must replace it, and it is labelled C100k. What do we think that is? Linear, log. reverse log? I’m not convinced by internet references to it that I have found.

The amp was probably made about 4 years ago, and other similar looking pots on the same board include A100k and A10k.

Cheers
John

[sumgai]

John,

It should be a reverse-taper, thanks to the "C" designation. However, the only way to tell for sure is to dismount it and have at it with your DMM.

I had a pair of Crate Power Blocks awhile back, damned nifty little units. However, they had a slight hum, both of them, and I was afraid that things could only get worse from there. Rather than play "techie" on them, I found them a new home for $40 more than I paid for 'em!! (Apparently my buyer had never heard that GC cleared them out at $60 apiece. eBay was full of them for ~$80-100 at the time, so I took advantage of the opportunity, so to speak. )

They were supposed to be superceded with a newer model that had DSP and a bunch of other features, but as you can see, that never happened. I don't particularly miss them, except for the power-to-weight ratio.

How's Dick Smith on tiny PCB-mounted pots of that nature?

HTH



sumgai

[JohnH]

Thanks sumgai. I saw evidence that they were either linear or reverse audio. I went out at lunchtime and got myslef a B type linear one, since there was no C available. My thinking is that this will work OK, but the neutral point may be off centre which iI can live with.

Dick Smith is giving up on components, theres more money in Ipods for the masses. Luckily we still have Jaycar hanging onto the business of supplying the requirements gentleman amateur electro-dabblers such as myself.

I find the PB to be a great little unit, and I admire the cool (ie cool running) D type amp topology. There are many ways to use it and its natural sounds are pretty good, once one finds out how to get past the inherent 'cab simulation' circuit.

John

[cynical1]

I was looking at one of these a while ago and I vaguely remember reading somewhere that Crate discontinued the Powerblock as it was prone to catastrophic failure...as in flash, spark, poof, smoke...

Hopefully your results may vary.

[ashcatlt]

I know you've already replaced the thing. You could run wires from the pot (a standard log pot) to the PCB (rather than soldering the pot right on there), reverse the outside lug connections, and then it would just work backwards. 10 would be 0 and vice versa.

Also, as the late great Mr. K was fond of pointing out, a pan pot usually has both log and reverse log elements. Not sure if a 100K pan pot would be easier to find than a reverse log one, though...

[sumgai]

ash,

The results wouldn't be reversed (10 for 1, 1 for 10), the reaction would be reversed, due to the opposing taper.

Your trick is one I've used often, figuring that it's cheaper, easier in most cases, far quicker than waiting for the correct part to arrive from a parts house, and doesn't harm the external appearance of the unit. But in the CPB, the close quarters might make it a bit difficult. When there's room aplenty, there's no doubt about what I'd do, but if it's really, really tight.....

And yes, if there's room, using the correct half of a pan-pot would be the ideal solution. They do exist, but more than likely, not in your spare parts/junk bin. That means a bit of waiting time for the Postman to ring your doorbell.


sumgai

[JohnH]

Just to finish this small story:

I went back into the amp and measured the pot resistance in-situ, assuming that the reading would not be too confused by surrounding components. It read 93k total, and 15k from centre to outer lug when set at 12 o'clock, in a direction consistent with reverse audio, as advised above.

I haven't replaced it - it is soldered to a double sided board, with extra anchoring lugs off the body, and getting the board out and back in with enough access to do the change looks like a days work for me. It would also require disassembling the case, which although very sturdy, is not particularly accurately made hence the small screws that hold it together have to be forced to find the slightly mismatched holes, and so there's a great chance to shear them off.

I found a work-around though. As designed, the offending treble pot works just as a variable resistor, with two legs shorted on the PCB. Full treble is when set to zero resistance, fully clockwise. This position works fine, as does minimum treble, fully anticlockwise. I find I either use full treble or about 70% treble, since this is a dark sounding amp otherwise. So I bridged the pot with a 10k resistor, and now I get the two settings that I use either at fully clockwise or anticlockwise. Job done, and thanks for the input above.

cheers

John