Hello, I have a Fisher 440T that has almost no output to the speakers. It also keeps blowing the speaker protection fuse. The dc offset is 26mv on the left channel and 1.6mv on the right. The only sound I get is a very quiet crackling sound when playing a song, there is no hum. The amplifier was partially recapped about 5 years ago. The remaining caps have tested good. I tested all of the output transistors and they are good, but while in the amp the Vbe is zero volts on the right channel, and it seems good at 0.7 on the left. All of the power supply voltages are correct according to the service manual.
I have trying to figure it out for hours, I'm just stuck now.
Any help would be appreciated.
I have trying to figure it out for hours, I'm just stuck now.
Any help would be appreciated.
Fisher 440-T - Manual - Transistor FM MPX Receiver - HiFi Engine
Hi, the 440T comes from an era when capacitors were used to pass audio to the speakers but block the DC (half the single rail voltage) normally present at the output of the amplifier. That means any DC offset you can measure at the output terminals is just an inevitable slight leakage through the caps or a meter error. The actual DC voltage at the amplifier output stage should be more like 23VDC but isn't critical as long as the output stage is functional with correct bias and all semis checked properly. Look at Q1-Q8 for Collector-Emitter shorts and verify Vbe (voltage between base and emitter) is around +/- 0.65V for each when powered. Remove any signal, turn down volume and remove speakers for these checks. Good hunting!
Your problem may be that you are misinterpreting the operating conditions. Take a look at the voltages marked on the schematic linked above (free download with registration). The bottom RH side of the last drawing, ref. P-1340 refers. Check all voltages at the output stage (the RH side of T2, 3) carefully to avoid shorts, slipping about etc.
Hi, the 440T comes from an era when capacitors were used to pass audio to the speakers but block the DC (half the single rail voltage) normally present at the output of the amplifier. That means any DC offset you can measure at the output terminals is just an inevitable slight leakage through the caps or a meter error. The actual DC voltage at the amplifier output stage should be more like 23VDC but isn't critical as long as the output stage is functional with correct bias and all semis checked properly. Look at Q1-Q8 for Collector-Emitter shorts and verify Vbe (voltage between base and emitter) is around +/- 0.65V for each when powered. Remove any signal, turn down volume and remove speakers for these checks. Good hunting!
Your problem may be that you are misinterpreting the operating conditions. Take a look at the voltages marked on the schematic linked above (free download with registration). The bottom RH side of the last drawing, ref. P-1340 refers. Check all voltages at the output stage (the RH side of T2, 3) carefully to avoid shorts, slipping about etc.
440T
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I will replace all of the caps in the signal path, but i still have another problem. It keeps blowing the speaker protection fuse and one of the .5 ohm resistors on the left main output transistor is dissipating much more heat than all of the others.
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I will replace all of the caps in the signal path, but i still have another problem. It keeps blowing the speaker protection fuse and one of the .5 ohm resistors on the left main output transistor is dissipating much more heat than all of the others.
Hi klh2000
Like Ian said, look at the schematics and compare voltages in the circuits. Do not start replacing anything, get it fixed first. You have a fault in the left side I'd say from your description.
If you aren't handy with electronics and audio equipment, for goodness sake, get a good technician in there. In other words, if you aren't knowledgeable, stop and get someone who is involved!
-Chris
Like Ian said, look at the schematics and compare voltages in the circuits. Do not start replacing anything, get it fixed first. You have a fault in the left side I'd say from your description.
If you aren't handy with electronics and audio equipment, for goodness sake, get a good technician in there. In other words, if you aren't knowledgeable, stop and get someone who is involved!
-Chris
Well, I tested every voltage rail of the power supply and they are all correct.
The four output transistors are good, but the driver transistors were bad on the right channel only. The driver transistors of the drivers were both bad, so i replaced them. I still don't get any sound. I checked the pre-amp stage, that works fine, The tone control board works, and is connected to the amp stage properly. It still blows the 4a speaker fuse. I have checked every transistor twice, and they are all good, all voltages are good, There is just no output
My oscilloscope won't really work because it only goes up to 20v, it is just a cheap DSO138, I've been looking for a nice old tek or hp scope. But anyway, I Fixed the fuse blowing by replacing a shorted emitter resistor on the left channel. The only problem now is some of the voltages. All of the supply voltages are correct, but the voltages at the collectors/emitters of the transistors are wrong according to the schematic.
Hi klh2000,
You had a shorted emitter resistor?? They tend to fail open, not shorted. Output transistors fail short normally. I've only seen output transistors go open if there was enough energy in the circuit to literally blow the case apart, or to burn through a TO-3 case (Carver PM1.5). That one also burned out the bottom cover. We're talking about holes here - large ones, not a pin hole.
Can you ramp the AC voltage up slowly so you can make measurements? A DC supply might work for this as well.
-Chris
Is that with a 10:1 probe? You can buy 100:1 probes that would extend your range for worst case. Your scope should have an AC coupling feature (AC vs DC). That should allow you to see oscillations. To blow fuses, the oscillation would be a very large amplitude.
You had a shorted emitter resistor?? They tend to fail open, not shorted. Output transistors fail short normally. I've only seen output transistors go open if there was enough energy in the circuit to literally blow the case apart, or to burn through a TO-3 case (Carver PM1.5). That one also burned out the bottom cover. We're talking about holes here - large ones, not a pin hole.
Can you ramp the AC voltage up slowly so you can make measurements? A DC supply might work for this as well.
-Chris
My scope doesn't really use a probe, just alligator clips. But it is set to X5 internally.
But I think found what was causing the fuse to blow, the two main driver transistors were shorted and without them the fuse won't blow, so ill have to track them down ad see if new ones help the situation.
Well, I replaced them and now the fuse doesn't blow, but there is still no sound. The voltage are much closer to what they should be now. My problem is that the Vbe of the output transistors is only 0.550 Volts, and the bias adjustment pots will only let me bring them up to 0.611 Volts. So, I still have no sound but everything else is normal again.
Okay, I have all of the emitter resistors dropping 55mV, Which is what the service manual says (40-60mV). So, when I play music I can only hear it faintly through the tweeters while it is at full volume. Do you think it could be the original .22uF caps in series with the signal path?
Hi klh2000,
Okay, so your bias current should be about 110 mA per output transistor. Have you checked Q5 and Q6?
Which 0.22uF capacitors are you asking about? C17 and C18? Those are your audio connection to the amplifier section. They could do this if they were open. Try touching your finger to the metallic connection on these capacitors to see if you pick up a hum. Touching R45 and R46 would create the same hum. C19 and C20 (500uF) would certainly cause this problem as well if they go open. They would cause the gain to fall to unity or close to that. Replace with the same voltage rating and 470uF (standard size today) if they are open. C23 and C24 would cause a partial loss of gain should they be open. If the 0.22uF capacitors are electrolytic types, replace all the capacitors I mentioned. If they are ceramic or film types, change the others. At this point we are just trying to solve your problem before we go and do any improving.
If you look at the control PCB, there are six 1uF capacitors in the audio path in both channels (three in each). Just change them as I'll bet they are electrolytic types.
In a repair like this one, it is wise to eliminate known problem capacitors instead of tracing each fault out and possibly missing a couple. The smaller electrolytic capacitors, and the very low voltage ones tend to lose their electrolyte faster than the larger ones (higher capacitance and voltage ones). It makes sense to simply replace those right out of the gate because the chances are they are at least loosing capacitance if they aren't already open. We're talking about these really old pieces of equipment.
One old timers trick was to take a capacitor and touch the leads across a suspected open capacitor to see if that fixed the problem. Two things to watch out for are, a large thump noise as that other capacitor charges or discharges (can be extremely loud and damaging to speakers), and the fact that you probably have multiple bad capacitors, given their age. The values to not have to be exactly the same, anything in the ballpark will give you the information you need.
The safe way to do this is to tack the jump capacitors across the leads of the suspected capacitors. If you're going to do that, may as well replace them.
Last comment. Never place a new capacitor across an old one (failed or not) and leave it there. Either replace the parts, or leave them in there. This is especially important with those multi-section can capacitors in the power supply. If you have one bad section, the rest will go, so replace the entire can. You can jump an open section for troubleshooting purposes, but do not leave it (them) there.
-Chris
Okay, so your bias current should be about 110 mA per output transistor. Have you checked Q5 and Q6?
Which 0.22uF capacitors are you asking about? C17 and C18? Those are your audio connection to the amplifier section. They could do this if they were open. Try touching your finger to the metallic connection on these capacitors to see if you pick up a hum. Touching R45 and R46 would create the same hum. C19 and C20 (500uF) would certainly cause this problem as well if they go open. They would cause the gain to fall to unity or close to that. Replace with the same voltage rating and 470uF (standard size today) if they are open. C23 and C24 would cause a partial loss of gain should they be open. If the 0.22uF capacitors are electrolytic types, replace all the capacitors I mentioned. If they are ceramic or film types, change the others. At this point we are just trying to solve your problem before we go and do any improving.
If you look at the control PCB, there are six 1uF capacitors in the audio path in both channels (three in each). Just change them as I'll bet they are electrolytic types.
In a repair like this one, it is wise to eliminate known problem capacitors instead of tracing each fault out and possibly missing a couple. The smaller electrolytic capacitors, and the very low voltage ones tend to lose their electrolyte faster than the larger ones (higher capacitance and voltage ones). It makes sense to simply replace those right out of the gate because the chances are they are at least loosing capacitance if they aren't already open. We're talking about these really old pieces of equipment.
One old timers trick was to take a capacitor and touch the leads across a suspected open capacitor to see if that fixed the problem. Two things to watch out for are, a large thump noise as that other capacitor charges or discharges (can be extremely loud and damaging to speakers), and the fact that you probably have multiple bad capacitors, given their age. The values to not have to be exactly the same, anything in the ballpark will give you the information you need.
The safe way to do this is to tack the jump capacitors across the leads of the suspected capacitors. If you're going to do that, may as well replace them.
Last comment. Never place a new capacitor across an old one (failed or not) and leave it there. Either replace the parts, or leave them in there. This is especially important with those multi-section can capacitors in the power supply. If you have one bad section, the rest will go, so replace the entire can. You can jump an open section for troubleshooting purposes, but do not leave it (them) there.
-Chris
C23 and C24 are 4uF in mine, not 50uF. They are both the factory sprague caps too
. But, I will replace the 500uF ones right now.
C23 and C24 are 4uF in mine, not 50uF. They are both the factory sprague caps too
Okay, I replaced all of the 1uF caps, and the 500uF ones. There is no change. I can only hear high frequencies; such as the hiss from an fm tuner. None of the lows come through on either channel?
I figured it out! the two 2N2924's at the begining of the amp circuitrymust have one bad sometime during testing. They fine in the beginning, So I replaced them and it is up and running!
Thank you so much for your help, couldn't have done it without you!
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