Interesting bit of kit. Since you said you examined the meter movement for damage, I assume this means the "guts" of the meter can be removed, while preserving the scale and LEDs? If so, you might be able to find a similar size and shape meter whose movement could be swapped in to the existing meter case. Even if the sensitivity isn't the same, you may be able to add a shunt to obtain the required sensitivity.
I just acquired one of these after a long search. Unfortunately, it has what seems to be several very odd issues. The Ohms function has a 4-transistor circuit where every semiconductor runs suspiciously warm. The meter deflects and moves around, 'responding' to inputs, but in very odd ways that makes it seem possessed. I can't get the meter pointer to rest over the 0 on the scale (with power off) using the zero screw on the meter face. Because it responds to inputs, I will probably find that most or all of the front-end switching and attenuation circuits are OK. I suspect that the op-amp (which drives the meter movement) is the culprit, but have not obtained a suitable replacement so far. Anyway, a couple observations on this video: While Heathkit calls this an "FET Multimeter", it actually has no discrete FETs in it. The only claim it has to FETs is that the op-amp uses an FET front end inside the chip to get higher input impedance, something that is pretty much taken for granted by modern op-amps, but then this is a 1970s design, and solid state op-amps were fairly new then, and ones with FET front ends were fairly unusual at the time. Calling this an "FET" 'anything' is much like putting "Automatic" on the trunks of cars back when automatic transmissions were a new an novel thing. Jeff mentions that this IM-5225 was Heath's only benchtop solid state multimeter, but that is not quite true. At around the same time, Heath also had the IM-25, a benchtop solid state meter capable of measuring AC & DC volts, AC & DC current, and resistance, i.e. a "multimeter". A close second was the IM-16 which was similar to the IM-25 but without the ability to measure current. Comparing this IM-5225's circuit design to the almost contemporaneous IM-25, it is interesting to see just how profoundly differently various Heath design teams, or designers, could approach design problems in devices that otherwise do exactly the same thing. Somebody could probably write a book on the massive disparity in design philosophy between the two.
I have one of these that I built in the 1970s as well as the the AC voltmeter that uses the same case. I used it to peak and dip alignment coils in FM receivers. It is much easier than using a DVM. I also have a working Mercury VTVM from the late 1960s.
Hi Jeff I am curious, when you replaced the op-amp in this meter, were you able to lay your hands on the same kind (CA3130S), and if not, what kind did you use?
Yes, I purchased a couple of used RCA CA3130S devices from eBay. I turned out that issue was with the meter movement and not the op amp. It actually seemed to work more poorly with the devices I bought, so they may not have been fully functional.
ernestb, the meter movements I have studied are NOT made having internal resistors, rather getting a certain amount of resistance from the moving coil, and usually below 1k. Having said that, I don't know if some meter movements might have been made using additional internal resistors. The meter movement on MY IM-5225 seems to be possessed by evil spirits. The electronics driving it seem to be working correctly, but the meter always points to apparently random points, and seems to read far off of anywhere near where it should be, before jumping to some other point. I suspect that the moving coil, or perhaps its pivot points, have have become damaged, or maybe Heathkit got a bad batch of meters from their supplier. One of these days, I plan to rig up a constant current loop and insert the meter movement into it, so I can isolate the rest of the meter circuit from the movement itself, so I can get a better idea of what's going on. But between my experience with it, and Jeff's, and one other vintage meter collector I have heard from, this model meter seems to often suffer from "PMS" (possessed meter syndrome).
@@youtuubaI have no idea why I have put this comment regarding the resistors. I suppose it made sense at the time, haha. Indeed I would also decouple the meter movement and the electronics, and test those separately. Recently (a few months ago) I have worked on a BBC Goerz Metrawatt MA 4E fet input analog meter. It uses almost the same IC, CA3130. This one also suffers a partial and variable PMS. I tried to evict the bad spirit from it but I suppose I have not really succeed. I suppose we have made an agreement. This meter has problem that on the largest Ohm's range of x10k it (sometimes) shows about 2 to 10MOhm with open circuit leads. Other times, I suppose when there is no full-moon, it just shows (as supposed) the infinite resistance. I could not find what is causing this irregular anomaly. Other thing is that the meter is not completely linear across the scale, so calibrating it is just impossible. If you calibrate to full-scale then you have bad reading on the lower side. If you calibrate in the middle of the scale you come short at full-scale. I couldn't solve that either. One thing about this IC. It has a small compensation capacitor of 10pF in your case C121. The op-amp tends to oscillate if this is not properly compensated. You can see that with an oscilloscope. I ended up using CA3140 DIL that does not need this negative feedback compensation, instead of canned CA3130S. And regarding the linearity, the coil meter movement are somewhat complicated after all. The basic principle seems rather simple, but there is a lot going on in the movement itself (the strength and field uniformity of the permanent magnet, the Hooke's law regarding the springs, friction in the bearings, current-->magnetic field of the coil linearity and such things). And yeah, as long we don't understand it we will call it PMS 🙂 I hope you evict the bad spirit from your meter.
erin19030, that is an odd assertion. The entire circuitry of this meter, not counting the primary winding of the power transformer, is galvanically isolated from the AC mains voltage. That makes it isolated by any normal definition of the word.
Interesting bit of kit. Since you said you examined the meter movement for damage, I assume this means the "guts" of the meter can be removed, while preserving the scale and LEDs? If so, you might be able to find a similar size and shape meter whose movement could be swapped in to the existing meter case. Even if the sensitivity isn't the same, you may be able to add a shunt to obtain the required sensitivity.
I just acquired one of these after a long search. Unfortunately, it has what seems to be several very odd issues. The Ohms function has a 4-transistor circuit where every semiconductor runs suspiciously warm. The meter deflects and moves around, 'responding' to inputs, but in very odd ways that makes it seem possessed. I can't get the meter pointer to rest over the 0 on the scale (with power off) using the zero screw on the meter face.
Because it responds to inputs, I will probably find that most or all of the front-end switching and attenuation circuits are OK. I suspect that the op-amp (which drives the meter movement) is the culprit, but have not obtained a suitable replacement so far.
Anyway, a couple observations on this video:
While Heathkit calls this an "FET Multimeter", it actually has no discrete FETs in it. The only claim it has to FETs is that the op-amp uses an FET front end inside the chip to get higher input impedance, something that is pretty much taken for granted by modern op-amps, but then this is a 1970s design, and solid state op-amps were fairly new then, and ones with FET front ends were fairly unusual at the time. Calling this an "FET" 'anything' is much like putting "Automatic" on the trunks of cars back when automatic transmissions were a new an novel thing.
Jeff mentions that this IM-5225 was Heath's only benchtop solid state multimeter, but that is not quite true. At around the same time, Heath also had the IM-25, a benchtop solid state meter capable of measuring AC & DC volts, AC & DC current, and resistance, i.e. a "multimeter". A close second was the IM-16 which was similar to the IM-25 but without the ability to measure current.
Comparing this IM-5225's circuit design to the almost contemporaneous IM-25, it is interesting to see just how profoundly differently various Heath design teams, or designers, could approach design problems in devices that otherwise do exactly the same thing. Somebody could probably write a book on the massive disparity in design philosophy between the two.
very nice i had the IM-104
I have one of these that I built in the 1970s as well as the the AC voltmeter that uses the same case.
I used it to peak and dip alignment coils in FM receivers. It is much easier than using a DVM.
I also have a working Mercury VTVM from the late 1960s.
Hi Jeff
I am curious, when you replaced the op-amp in this meter, were you able to lay your hands on the same kind (CA3130S), and if not, what kind did you use?
Yes, I purchased a couple of used RCA CA3130S devices from eBay. I turned out that issue was with the meter movement and not the op amp. It actually seemed to work more poorly with the devices I bought, so they may not have been fully functional.
That part must be 407-729 - Meter - M1 in the manual. Good luck finding a matching part ... seems to be the soul of the device.
Could it be that the meter was initially assembled with a incorrect resistor (value), like 68k instead of 6k8?
ernestb, the meter movements I have studied are NOT made having internal resistors, rather getting a certain amount of resistance from the moving coil, and usually below 1k. Having said that, I don't know if some meter movements might have been made using additional internal resistors.
The meter movement on MY IM-5225 seems to be possessed by evil spirits. The electronics driving it seem to be working correctly, but the meter always points to apparently random points, and seems to read far off of anywhere near where it should be, before jumping to some other point. I suspect that the moving coil, or perhaps its pivot points, have have become damaged, or maybe Heathkit got a bad batch of meters from their supplier. One of these days, I plan to rig up a constant current loop and insert the meter movement into it, so I can isolate the rest of the meter circuit from the movement itself, so I can get a better idea of what's going on. But between my experience with it, and Jeff's, and one other vintage meter collector I have heard from, this model meter seems to often suffer from "PMS" (possessed meter syndrome).
@@youtuubaI have no idea why I have put this comment regarding the resistors. I suppose it made sense at the time, haha. Indeed I would also decouple the meter movement and the electronics, and test those separately. Recently (a few months ago) I have worked on a BBC Goerz Metrawatt MA 4E fet input analog meter. It uses almost the same IC, CA3130. This one also suffers a partial and variable PMS. I tried to evict the bad spirit from it but I suppose I have not really succeed. I suppose we have made an agreement. This meter has problem that on the largest Ohm's range of x10k it (sometimes) shows about 2 to 10MOhm with open circuit leads. Other times, I suppose when there is no full-moon, it just shows (as supposed) the infinite resistance. I could not find what is causing this irregular anomaly. Other thing is that the meter is not completely linear across the scale, so calibrating it is just impossible. If you calibrate to full-scale then you have bad reading on the lower side. If you calibrate in the middle of the scale you come short at full-scale. I couldn't solve that either. One thing about this IC. It has a small compensation capacitor of 10pF in your case C121. The op-amp tends to oscillate if this is not properly compensated. You can see that with an oscilloscope. I ended up using CA3140 DIL that does not need this negative feedback compensation, instead of canned CA3130S. And regarding the linearity, the coil meter movement are somewhat complicated after all. The basic principle seems rather simple, but there is a lot going on in the movement itself (the strength and field uniformity of the permanent magnet, the Hooke's law regarding the springs, friction in the bearings, current-->magnetic field of the coil linearity and such things). And yeah, as long we don't understand it we will call it PMS 🙂 I hope you evict the bad spirit from your meter.
As long as it is connected to the mains its not isolated!
erin19030, that is an odd assertion. The entire circuitry of this meter, not counting the primary winding of the power transformer, is galvanically isolated from the AC mains voltage. That makes it isolated by any normal definition of the word.
great intelligent review, too bad their not still in business
Jeff Tranter, You didn’t recap the unit, regardless of what the caps read? Not sure you did yourself a disservice or not.
I may recap it later, if I can find a replacement for the meter movement.