Roland GP-100 Repair

Roland GP-100My dad handed me another item to repair this weekend, a Roland GP-100 Guitar Processor, circa mid-1990’s. He reported that it had been acting strangely, so he replaced the backup battery in the unit, but after that, it wouldn’t boot (screen would light, but then it wouldn’t show anything, or respond to controls). To me, symptoms like this always scream power supply problems, and given the age of the unit, to me that meant capacitors. So I agreed to repair it, and brought it home. After confirming the problem myself (you never know how someone’s AC power can influence issues like this), I started looking quickly for the service manual, which I found quickly at this wonderful site! And quickly found the schematic for the power section of the board. Holy crap, I quite literally explained, they put 25V capacitors on the 21V rails. That’s a pretty small margin, and bound to lead to excessive heating1, and while the ones on the 7V rail are at least better (15V), it’s still a bit closer than one likes to see2. Throw in there’s no ventilation for the enclosure, and these things are bound to be bad… let’s take a look.

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  1. Be warned, I’m actually going to include some pictures in this repair []
  2. Derating capacitors according to the US Navy. TL;DR: at 75C operating temp for a 105°C Aluminum electrolytic capacitor, the max voltage should be 70% of it’s rated value. 21V across a 25V capacitor is 84%, and 7V across a 15V capacitor is ~46%, so while the 15V ones were okay, the 25V ones were WAY under rated. []

Midland 75-785 Handheld CB Repair

Midland 75-785I recently picked up a Midland 75-785 handheld CB at Goodwill for $6, and thought I would see how it worked. I’ve thought several times about taking the HAM exam, and figured I might want to start out using a CB, repairing a CB, etc. I got the unit home, and fired it up, and it seemed to work okay with a simple 50R rubber duck antenna I got off my scanner. One thing that didn’t seem to work, however, was the battery charging circuit. I had some older Ni-Cad batteries to try in the unit, but they would quickly drain, and worse, the charging circuit didn’t seem to work (plugging it into my bench power supply, it would draw about 150mA at 13.8V, then quickly ramp down to 10mA, and sit there. Never really charging.

So, being one to never throw something out without first trying to fix it, I took the thing apart! Which consisted of 4 screws on the back piece, then another screw hidden under a battery spring on the remaining battery holder. Taking the board out is a bit challenging, but nothing is holding it in, you just have to work it past the case.

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HP 5316a Repair

5316AOnce again, knowing no end to repairing things, I saw an HP 5316a come up on eBay for cheap ($20), that I bid on first thing, and ended up winning. Not sure I WANTED to win it, but hey, it’s a repair, I’m cool with doing those. Maybe I’ll be able to resell it, maybe not. *shrug*

3 days later, it showed up on my doorstep via FedEx, and looked to be in good condition, except the missing handle and feet, as described by the seller. Initial power up showed it power up okay, but never register any count. The gate LED would trigger, as would the sensitivity LED, and both could be adjusted. I did get a alternating flashing of the HP-IB LEDs on the front panel though (at this time, I didn’t know what this meant).

After opening the unit, I noticed two empty sockets, one on the HP-IB (GPIB) card, and one on the main board. It appeared they were used for a ribbon cable between the two, but the cable was missing. Also, another power up showed an E2 error (which translates into a failed HP-IB card)… which isn’t surprising given the connection to the HP-IB card was missing. I’m just not sure why it booted okay previous to that.

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