Basic Elec Question

[hinkasaurus]

Hi all,

I'm replacing piggybacked electrolytic caps with values of 330u/35v and 63u/47v. Can I replace these with a single cap? If so, what value would be appropriate?

Is there a formula I could use to figure this myself?

Tia!

[Nick Sevilla]

Hi all,

I'm replacing piggybacked electrolytic caps with values of 330u/35v and 63u/47v. Can I replace these with a single cap? If so, what value would be appropriate?

Is there a formula I could use to figure this myself?

Tia!

Try to match what you're replacing. Otherwise, mayhem might ensue.

If it's piggybacked, it piggybacked for a reason...

Cheers

[hinkasaurus]

Fair enough, I can match it exactly. Was trying to simplify things.

The reason it's piggybacked:
It is in the erase circuit on an MCI JH-110. I was told the extra cap was put there to bolster the erase signal for 2 channel machines running with 1/2" heads/tape rather than the standard 1/4" setup.
My machine is an JH-110 8 (1" tape) which means trackwidth is same as 1/4". Perhaps I don't need the piggyback anyway and should just go with the single cap.

Thanks for the reply, noeqplease!

[Nick Sevilla]

Fair enough, I can match it exactly. Was trying to simplify things.

The reason it's piggybacked:
It is in the erase circuit on an MCI JH-110. I was told the extra cap was put there to bolster the erase signal for 2 channel machines running with 1/2" heads/tape rather than the standard 1/4" setup.
My machine is an JH-110 8 (1" tape) which means trackwidth is same as 1/4". Perhaps I don't need the piggyback anyway and should just go with the single cap.

Thanks for the reply, noeqplease!

Well, I would test this, and see if you like the results...

[The Scum]

What value do the schematics call for? Are these additional service notes or updates that spec the new parts? Any idea about who added them or why?

My spider sense tells me that the factory spec was a 470 uf cap.

Without know how it's used in the circuit it's being used, it's hard to say what the effect of changing it will be. If it's power supply decoupling, it might not make much difference to swap values. If it's in the audio path, then you'd probably be looking at LF rolloff or phase shift.

[hinkasaurus]

What value do the schematics call for? Are these additional service notes or updates that spec the new parts? Any idea about who added them or why?.

There is a note on the schematic which says:

C5 is 220u/40 for high speed (7.5, 15, 30ips) and 330/35 for lo speed (3.75, 7.5,15ips)

This is from the MCI manual and says nothing about a piggyback nor mentions the need for special needs regarding the erase circuit for 1/2" heads. However, I've got the 8 channels in one JH-110, 2 channels in a 2 track 110 and 4 spare channels and everyone of them has the piggyback setup with the exact same caps, so I'm inclined to think they came from the factory that way.

Without know how it's used in the circuit it's being used, it's hard to say what the effect of changing it will be. If it's power supply decoupling, it might not make much difference to swap values. If it's in the audio path, then you'd probably be looking at LF rolloff or phase shift.

As I said above, this is the part of the erase circuit and, as I understand it, the need for different values is due to the need for increased voltage(?) for erasing the wider track width on 1/2" tape vs. 1/4".

The reason I'm replacing them in two of the channels is because they were put in with the polarity reversed. On another channel, they were simply missing.

My spider sense tells me that the factory spec was a 470 uf cap.

Is this value the net result of the two piggybacked caps?

Thanks again for the input.

[ashcat_lt]

Parallel caps add their capacitance. Series is the reciprocal of the sum of the reciprocals (1/C=1/C1 + 1/C2...). Opposite of resistance.

I'm not completely clear on the max V ratings. I'd think you'd want to err toward the larger value. Heck, couldn't hurt (except a $ or two) to just add the two together. Caps aren't usually used to protect against over-voltage, so you just want to make sure they will handle whatever they can reasonably be expected to take, and maybe a little more. They tend to fail closed (shorted), so we try to avoid causing them to fail if we can.

There are some instances where you'll see two caps in parallel on something like a power filter. One catches the little high frequency ripples while the other catches the bigger low-freq ripples. I think there's usually a resistance between the two in these cases, though...

HTH