Blowing Pico Fuses In Unloaded Channels While Testing

[Matt Salazar]

We are currently working on a complete cleaning and re-cap of our console and have run up against a problem when attempting to install the cleaned and re-capped channels back into the console.

We started with a bucket of eight. Four out of the eight channels didn't work when we applied power to the bucket.

Upon further inspection, we found a blown pico fuse on each of the four dead channels. It was always the same fuse (the one closest to the top of the channel).

Our initial thought was that there was a card or cards that had been recapped and had errors. We went back and after a visual inspection could not find any errors. So we decided to start with an unloaded module and add a card at a time until we blew the fuse to discover the bad card.

Since we had taken 32 two channels out of the console to clean we had plenty of spare channels that had been cleaned but not yet loaded with any daughter cards. We checked the pico fuses on these "raw" channels with a multimeter before applying power (via an extender) and they tested good. Upon applying power, half of the unloaded channels experienced a blown pico fuse in the same position as the fully loaded channels.

Thus far we have a 50% fail rate on channels regardless of whether they have been loaded with daughter cards or raw channels.

Not sure where to start looking for the problem.

[Matt Sartori]

I cannot remember if the top fuse is for the positive or negative rail but I had this before.
check that the large routing matrix cards and all the EQ in DYN in cards are sitting properly and not shorting any of the pins.
it sounds like you might have one (or more) of those cards misplaced.

Matt Sartori.

[Matt Salazar]

We ordered some pico fuses and replaced the blown ones on the non-function 611's. We also checked the routing cards, EQ, and Dynamics switches. After changing the pico fuses 7 of the 8 blown channels are now powering without blowing fuses. One of the channels is still blowing fuses.

On the non-functional channel, we have double checked the seating of the routing cards and other switches and they all look visually fine. We have also double checked the new caps we soldered to the film pan card with a multimeter and they also seem fine.

The channel is still completely unloaded at this point.

What would be the next thing to check?

[Chris Simon]

The next thing to check is measuring resistance between the rail that keeps blowing the fuse, presumably +18 volts, and 0 volts, and +18 volts and -18 volts.. Assuming that the strip is still blowing fuses unloaded, you should read a very low resistance between these points. If that's the case, then methodically start removing the routing and EQ and Dyn switch cards, measuring resistance after each step. Don't discount the flying leads coming off the soldered-in switches; occasionally they can short to ground if not properly insulated. In extreme cases, I've even seen leads coming off the Total Recall taps on the pots short, where the strip will blow fuses only when a particular pot is rotated.

[radardoug]

Have you checked the diodes on those cards? I've had them go.

[Matt Salazar]

After measuring the resistance between the rails and found the Dyn switch card to be causing the problem. We switched it out with one from a working channel and after replacing the picos, we were able to get the channel to power on without failure.

Next, when we installed the daughter cards we got smoke from two different 22uf caps (C35 & C36) on the Input amp card. These were new caps that were just installed during the recap. The polarity was correct and the caps were 50V (original value on schematic was 25V).

Is it possible that there is a malfunction that could be causing the Dynamics switch issue and the input card issue?

[Chris Simon]

Usually there's not too much to go wrong on the Dynamics switch card apart from proper alignment of the Varelco pins on the motherboard. The caps on the Input card (presumably a 291 card) are across the rails as they enter the card; I'm not sure what would cause them to blow apart from reverse polarity. The two issues seem unrelated.

[Loopzilla5]

"We ordered some pico fuses and replaced the blown ones on the non-function 611's"

Matt,

I need to order some pico fuses as well. What did you get? Are they 2 Amp? The schematic only says 2A...

Thanks,

Chris

[Matt Salazar]

Hey Chris,

Here’s a link to the ones we purchased. They’re working great so far.

https://www.westfloridacomponents.com/mm5/merchant.mvc?Screen=PROD&Product_Code=K310APM12

[Loopzilla5]

Thanks! Those will be perfect.