Hello!
Probing around in my vintage amp I noticed the voltage present on the speaker relay coil was about 39 VDC, way higher than its rated 24V. The relay is a Potter & Brumfield RTE24024F, 1440 ohms/400mW coil. Below is the relevant part of the schematic; sorry for the bad scan quality this is the only one I could find online.
The relay is fed from the left channel output stage's negative rail which is -42 VDC unregulated. 680 ohms series resistor R3 drops some volts to bring that down close to 24V. I went ahead and simulated the circuit and in this configuration the relay would see 27 VDC.
In my amp this resistor is only 220 ohms and the relay coil then dissipates about twice its rated power. All the other components are operating correctly.
I only have the schematics for the rev. 3 revision of the circuit whereas my amp is rev. 1. The original relay failed about 20 years ago and I had it replaced by an authorized dealer - perhaps the original circuit really had 220R or the repair technician changed it.
At any rate this amp has been running 24/7 for like 15 years without problems... Do relays typically can handle this kind of abuse or I was just lucky on this one?
In a few weeks/months I'll need to replace some old caps and was planning to preemptively replace the relay with a fresh one, to be sure to have less oxydised contacts. It's already 20 years old after all. I'm thinking of replacing the series resistor at the same time with a 680 ohms like specified in the rev. 3 schematic.
Is that a good idea or a case of "If it ain't broke don't fix it" ?
Thanks in advance.
Probing around in my vintage amp I noticed the voltage present on the speaker relay coil was about 39 VDC, way higher than its rated 24V. The relay is a Potter & Brumfield RTE24024F, 1440 ohms/400mW coil. Below is the relevant part of the schematic; sorry for the bad scan quality this is the only one I could find online.
The relay is fed from the left channel output stage's negative rail which is -42 VDC unregulated. 680 ohms series resistor R3 drops some volts to bring that down close to 24V. I went ahead and simulated the circuit and in this configuration the relay would see 27 VDC.
In my amp this resistor is only 220 ohms and the relay coil then dissipates about twice its rated power. All the other components are operating correctly.
I only have the schematics for the rev. 3 revision of the circuit whereas my amp is rev. 1. The original relay failed about 20 years ago and I had it replaced by an authorized dealer - perhaps the original circuit really had 220R or the repair technician changed it.
At any rate this amp has been running 24/7 for like 15 years without problems... Do relays typically can handle this kind of abuse or I was just lucky on this one?
In a few weeks/months I'll need to replace some old caps and was planning to preemptively replace the relay with a fresh one, to be sure to have less oxydised contacts. It's already 20 years old after all. I'm thinking of replacing the series resistor at the same time with a 680 ohms like specified in the rev. 3 schematic.
Is that a good idea or a case of "If it ain't broke don't fix it" ?
Thanks in advance.
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Probably ok. Relays are surprisingly adaptable to a pretty wide voltage range. I looked at the spec and the power rating of 400mw is the ideal, 24v/1440ohms is 400mw. the coil operating graph is more telling. They are showing recommended coil voltage up to 1.4x or 33.6v and they show the max range up to 60c as 1.8x or 43v. I expect you are under 60c temp. With that said, if you are opening it up anyway, I'd increase the R to 680 to get it closer to the 24V range. They may have chosen 27 since it is unregulated and allow for voltage drooping. And really almost certainly fine to go even a bit more as the "must operate" voltage for the 24V coil is 16.8. As I said initially, relays have an incredible voltage range they work over.
Is it possible that the original was a relay with a 720 ohm coil and the service technician changed it with what he had on hand but neglected to change the 220 ohm resistor?
I guess it's possible, I have a second amp from of the same vintage which is the little brother model with less power but uses the same motherboard. Its relay may be the original part, a Norslo RM-82P, and the manufacturer probably used the same relay for all of their boards. I'm digging on the interwebs but can't find a datasheet for it; I'll measure it when I open up the amp, as it also needs a complete re-capping.