Harris MS-15R Stereo Generator (RF Broadcast Device)

[jackstraw]

Hello everyone. I was searching around the net for some help on this device and found this website. I thought maybe someone here might have some advice on this unit. I got it used and have no idea what the PO did to this poor board but I posted some pics of it at the bottom. It seems as if the one filter on it was bypassed for some reason. When the unit is powered up, the carbon comp resistor R66 (bottom left of first pic) gets very hot, so there seems to be a problem with the power regulation or something of that nature. Anyone with any ideas on what to replace or test would be great...

 

[Resqueline]

Hi Jason, welcome to the forum.
First of all, you'll have to be able to replicate the existing filter to replace the one that is missing.
R66 is 1-2 W, 22 Ohms. Measure the voltage across it and calculate the power. You can compare all measurements with the mirror-image circuits (R63=R66). Follow the tracks from R66 and see where they go to.
Check all zeners (like CR19) and tantalum-capacitors (like C47) for shorts.
At least one of the power "transistors" (U13) is an integrated voltage regulator. The writing on it will reveal what it is, for instance 7812 means it's a 12V positive regulator. Measure input & output voltages. Center pin & tab is ground, except if it's a 79XX - which is a negative regulator.

 

[jackstraw]

Huh, that's one lucky guess! C47 is a dead short. So I guess I'll start by replacing that guy. I measured all the other caps on the board and the rest of them seem to be okay. I'm also going to replace R66 due to all the heating it received... Any other thoughts??? The filter that was removed seems to be different then the one that is still on the board. I'm hoping that it isn't needed for operation...

 

[Resqueline]

Hehe, lucky maybe, but it was a natural first choice, being close to the resistor & regulator. Zeners & tantalums are both prone to shorting out if they are exposed to too high ripple currents and are the first suspects in cases like this.
Carbon comp. resistors can drift off spec. when they are run hard, but just measure it first. If it's within 21-23 ohms you don't have to replace it. These circuit boards are easily damaged during such replacements, and the exact value of it won't matter to the circuit anyway.
I don't think much else is wrong with the board, except the missing filter for one of the channels; that will have to be put back in. Admittedly it looks somewhat battered - but still functional.

 

[jackstraw]

Ok, this sounds likes what's going on. The power coming into the MC7915 (U13) is going through R66 then being shorted to ground by C47 before getting to the MC7915 thus creating the heat in R66. Thanks so much for the help! The resistor is still in spec so it's a go after I replace the cap. Guess we'll see if the other filter has to be there or not...