Musicman RD 112. low voltage resistor desoldered itself

[Richwebe]

Musicman RD 112 with EL34's has had a lot of use before I got it . I replaced the caps last yr as they exploded.. A few weeks ago it began having intermittent static and crackling which was apparently arcing somewhere.. then one day it quit and nothing but static. I opened it up and there are two 390 ohm ceramic resistors in the rectifier circuit. one had over heated and desoldered from the board. I resoldered it and when I turn it on I have a faint signal from the guitar covered by a lot of snow type static. also the ceramic resistors get real hot after being on for about a minute. Since the rectifier resistors are getting real hot there must be a component ahead of them that is shorting? I am not at a level to use an oscilloscope and don't have one. I see on another thread that you have replaced the IC’s. JE 1692's are bad ? or maybe the LM1458's? rweberviolin@gmail.com

[52 Bill]

I opened it up and there are two 390 ohm ceramic resistors in the rectifier circuit. one had over heated and desoldered from the board. I resoldered it and when I turn it on I have a faint signal from the guitar covered by a lot of snow type static. also the ceramic resistors get real hot after being on for about a minute. Since the rectifier resistors are getting real hot there must be a component ahead of them that is shorting?

You didn't post the chassis number or schematic, but will assume from your description, that the 390 ohm resistors are input resistors to the Zener diode regulators. These normally get hot, but not so hot as to desolder themselves. If you read the dc voltage at the end of the resistor that connects to the Zener, you should get the plus and minus 16 volts dc. Are both voltages there?

If not first check the Zener diodes and then the filter caps right next to them. After that then any op amp in the amp could be the problem.

[R.G.]

The use of two power resistors and ~15 - 16V zeners to make a bipolar supply for opamps is a very common one in solid state amps.

However, it's also a common point of failure. Simple zener supplies like this are not durable if there is a short on their output. It runs the current in the dropping resistor (the 390 in this case) up to the point that the resistors overheat, and may char the PCB under them, may melt out the solder in the resistor joints, and can even start fires in extreme cases.

I personally consider it an example of cheapening a circuit design to satisfy a cost target, and thereby lessening relibility.

The resistors' power rating is based on there being no load on the zeners; the voltage across the resistors is then the power supply minus the zener voltage, and the power in the resistors is a max of v-squared divided by R. But if the load on the zener is a short, then the resistors have to withstand the entire incoming power supply NOT minus the zener voltage, and the squared term in the power calculation means that if the zener is 40% of the main power supply (i.e. the power supply is about +/-40V, which is common) then the resistor power dissipated doubles. When a short happens, the resistor overheats.

One way to more reliably fix this is to replace the resistors with big honking resistors of double the power rating. Now a short makes them heat more, but they can take it. Another way is to put in some kind of current limiter. This can be as simple as a one- or two-transistor circuit that won't let more than the design current through the limiting resistors.

The obvious way to fix this, using three terminal regulators, is not all that simple in this case because three terminal regulators don't usually withstand more than 30-35V on their input. So what would otherwise be a premium solution is not available without putting in more parts, and the accountants don't like that.

[lowell]

Richwebe I got your pm. Sorry I didn't see it until now. Where are we at with this repair?