Dear DIYers.
I recently encouraged my girlfriend into the purchase of an amplifier (annotated schematic attached) so that I can listen to music at her flat, I then took it upon myself to perform a "simple" recap.
Everything was going fine, replaced a few of the caps that I had stock for, ran out, put in an RS order and decided to plug in and test. Plugged in with case off and checked the bias, looking good. Theeeeen I put the case back on and prepared for audio testing. Fired the amp up to be greeted by whiffs of smoke coming from resistors (ringed purple). Turns out I'd forgotten to knock the tails off some of the caps (ringed red) and these were grounding onto the case! 😱
I knocked off the tails tested the smoked resistors out of circuit and they are showing up correct, so went for a switch on test. Now the relays wont engage, I have done no further testing as I really don't know where to start. Can anyone point me in the right direction?! I thought about going through all of the semi's one by one but wondered if a wise one on here could give me a tip off on where to start?!
I recently encouraged my girlfriend into the purchase of an amplifier (annotated schematic attached) so that I can listen to music at her flat, I then took it upon myself to perform a "simple" recap.
Everything was going fine, replaced a few of the caps that I had stock for, ran out, put in an RS order and decided to plug in and test. Plugged in with case off and checked the bias, looking good. Theeeeen I put the case back on and prepared for audio testing. Fired the amp up to be greeted by whiffs of smoke coming from resistors (ringed purple). Turns out I'd forgotten to knock the tails off some of the caps (ringed red) and these were grounding onto the case! 😱
I knocked off the tails tested the smoked resistors out of circuit and they are showing up correct, so went for a switch on test. Now the relays wont engage, I have done no further testing as I really don't know where to start. Can anyone point me in the right direction?! I thought about going through all of the semi's one by one but wondered if a wise one on here could give me a tip off on where to start?!
Attachments
Yep, really caused a few problems there. you could start by spotting the voltages marked on the schematic in fine letters and test to see if they are correct with 5%, say. This will help find the location or extent of damage. The larger caps (eg C001,C002) tend to be associated with the power supply so they need to be checked as a priority. (no power - no relay function, right?) Measure directly across the main smoothing caps. and observe polarity.
Q901 is the regulator for the preamps and protection relays so it must be working to have normal function of any sort. Check Emitter and collector voltages there as a starting point.
A note of caution - the quickest way (even quicker than your novel method 🙂 ) to wreck an amplifier, is dicker about with probes whilst trying to read a meter or something else. Probes that are worn smooth will slip from where you are pushing them and eventually short 2 or more voltages together and cause maybe a little or even an awful lot of damage. Get insulated clip leads or IC clips to attach to at least one test point, such as ground, and test with just 1 probe and be sensible where you wave it about. Good luck and take your time checking the principle supply voltages first, assuming nothing is still in imminent danger of overheating etc.
Q901 is the regulator for the preamps and protection relays so it must be working to have normal function of any sort. Check Emitter and collector voltages there as a starting point.
A note of caution - the quickest way (even quicker than your novel method 🙂 ) to wreck an amplifier, is dicker about with probes whilst trying to read a meter or something else. Probes that are worn smooth will slip from where you are pushing them and eventually short 2 or more voltages together and cause maybe a little or even an awful lot of damage. Get insulated clip leads or IC clips to attach to at least one test point, such as ground, and test with just 1 probe and be sensible where you wave it about. Good luck and take your time checking the principle supply voltages first, assuming nothing is still in imminent danger of overheating etc.
Thanks for all the helpful input Ian (actually thought your appraisal of my work was very moderated!). I put a lightbulb in series with the amp to make sure it wasn't drawing loads of current and plucked up the courage to turn it on again. I have taken a small set of readings and you were right to test Q901! The readings came out:
E 29.4v
C 59.8v
B 30v.
So that's knackered I guess, do I need to change that before I can test anything else or should I hunt around a bit more? Voltage readings on the res caps came out at 34.5v and 34.2v.
E 29.4v
C 59.8v
B 30v.
So that's knackered I guess, do I need to change that before I can test anything else or should I hunt around a bit more? Voltage readings on the res caps came out at 34.5v and 34.2v.
Finally got round to doing some further testing on the amplifier and I have found that all of the voltages in the power amp stage seem to be higher than expected and still that big problem of the collector of Q901 being about twice as high as it should be.
Here are my findings:
Rotel RA-712 Voltage readings
AC3 5.4v AC
AC5 24.5v AC
AC6 24.5v AC
C001 34.5v
C002 34.2v
C906 0.87
Left power
Q601 E 0.560v C -32.1v B -0.0049v
Q603 E 0.558v C -33.1v B -0.0238v
Q605 E -32.9v C -0.655v B -32.2v
Q607 E 0.517v C 33.7v B 1.118v
Q609 E 0.0607v C -33.4v B -0.654v
Q611 E 33.7v C 0.0013v B 33.6v
Q613 E 0.0343v C 33.9v B 0.515v
Q615 E -33.5 C -0.0421v B -33.9v
Right power
Q602 E 0.568v C -31.5v B 0.0052v
Q604 E 0.567v C -32.1v B -0.00219
Q606 E -32.5v C -0.653v B -31.9v
Q608 E 0.541v C 33.4v B 1.143v
Q610 E -0.0591v C 33.5v B -0.655v
Q612 E 33.2v C 0.0015v B 33.4v
Q614 E -0.0335v C 33.8v B 0.539
Q616 E -33.4v C -0.00410v B -33.4v
Q617 E 0.0008v C -6.42v B 0.0024v
Q618 E 0.0098v C 29v B 0.0031
Q620 E 0.0009v C 0.0193v B 0.682v
Q621 E 0.001v C 8.94 B 0.0234
Q901 E 29.4v C 59.8v B 30v
Having got to the bottom of this bout of testing I have noticed that the caps parallel to the rectifier diodes have started to swell a little. I assume that this must be a symptom of the problem but where does this take me? Do they need replacing or will any replacement go the same way? I have tested the diodes and they still seem to be a one way street.
Wise people please help!
Here are my findings:
Rotel RA-712 Voltage readings
AC3 5.4v AC
AC5 24.5v AC
AC6 24.5v AC
C001 34.5v
C002 34.2v
C906 0.87
Left power
Q601 E 0.560v C -32.1v B -0.0049v
Q603 E 0.558v C -33.1v B -0.0238v
Q605 E -32.9v C -0.655v B -32.2v
Q607 E 0.517v C 33.7v B 1.118v
Q609 E 0.0607v C -33.4v B -0.654v
Q611 E 33.7v C 0.0013v B 33.6v
Q613 E 0.0343v C 33.9v B 0.515v
Q615 E -33.5 C -0.0421v B -33.9v
Right power
Q602 E 0.568v C -31.5v B 0.0052v
Q604 E 0.567v C -32.1v B -0.00219
Q606 E -32.5v C -0.653v B -31.9v
Q608 E 0.541v C 33.4v B 1.143v
Q610 E -0.0591v C 33.5v B -0.655v
Q612 E 33.2v C 0.0015v B 33.4v
Q614 E -0.0335v C 33.8v B 0.539
Q616 E -33.4v C -0.00410v B -33.4v
Q617 E 0.0008v C -6.42v B 0.0024v
Q618 E 0.0098v C 29v B 0.0031
Q620 E 0.0009v C 0.0193v B 0.682v
Q621 E 0.001v C 8.94 B 0.0234
Q901 E 29.4v C 59.8v B 30v
Having got to the bottom of this bout of testing I have noticed that the caps parallel to the rectifier diodes have started to swell a little. I assume that this must be a symptom of the problem but where does this take me? Do they need replacing or will any replacement go the same way? I have tested the diodes and they still seem to be a one way street.
Wise people please help!
Correction, the rectifier caps are not swelling but the fine coating has bubbled at the top and is peeling off. I would put this down to heat but the amp was on it's side during testing. The tops of the caps are closest to the rectifier diodes so could it be heat coming off them that has caused this? I suppose higher current draw would cause them to get hotter...
First, sorry I hadn't noticed your posts 'til now and though I must get some shut-eye, this part is OK. Note the regulated output is correct at 30V and emitter-base differential is 0.6V. That's about right for all normally functional BJT transistors. The supply voltage near 60V is different to the schematic 38.9V though. Is that measurement correct?
Anyway, that's how to refine your listed voltage tests of the transistors - note any that don't have a B-E voltage around 0.67V and then look at those areas more closely. Then compare those parts in R and L channels. Any real problems will show as significant differences, >10%
Your meter seems capable of high precision but sometimes you show only 2 significant figures where it counts (ok, it's a pun) You may have to recheck some B, E measurements for a close look but it probably won't matter too much. .6 to .7V will be near enough, allowing for the reading errors in the meter's spec. Press on......🙂
Ian, thanks again for your support in this matter. I have double checked the readings for Q901 and today they came out at E 29.1 C 50.6 B 29.7. So a bit of a change since my last set of readings (although still over 25% higher than what it should be). I find it unlikely that I misread the meter in the first instance (though not out of the question), is it possible that there are some wandering values going on in a faulty power supply? The decimal places I am giving are as many as my meter will allow according to the different ranges of use.
I just tested the voltage at the foot of R904, C906 and that came out at +0.87v (as opposed to -4.7v). Does this suggest a problem with D904 or C906 affecting the protection relay?
Andreas, I have just tested the diodes out of circuit and the caps (to the best of my ability). Both diodes read a voltage drop of around .660 forward and nil backwards. I ran the caps through the multimeter on 200kr and they seemed to be charging and discharging normally and both read as nil on a diode check. So does this not mean that they are operating normally? OR is my testing not good enough to confirm that the components are ok? I'm following online instructions for testing these components and I suppose I could be misunderstanding some of them.
I just tested the voltage at the foot of R904, C906 and that came out at +0.87v (as opposed to -4.7v). Does this suggest a problem with D904 or C906 affecting the protection relay?
Andreas, I have just tested the diodes out of circuit and the caps (to the best of my ability). Both diodes read a voltage drop of around .660 forward and nil backwards. I ran the caps through the multimeter on 200kr and they seemed to be charging and discharging normally and both read as nil on a diode check. So does this not mean that they are operating normally? OR is my testing not good enough to confirm that the components are ok? I'm following online instructions for testing these components and I suppose I could be misunderstanding some of them.
Dear thefamilybassman
This can be a problem. This sensor is the presence of the AC network.
Must be immediately after the -4.7V, and disappear after a shutdown. If there is no device that will measure the capacity and ESR, then be sure to replace the capacitors are completely new.
Finally got time to look at this again!
I have replaced C905 C906 C907 D902 D903 and D904, all to no avail Andreas!
Here are some readings I took today. I had been taking readings with a bulb tester in place up until now, and I read today that this affects values! duh
Mains recorded at 244v (we run hot in Cardiff! I've measured over 250 before now)
Secondary coil 0.6v
Main coils 25.4v
Amp outputs (pre relay) -0.04v both channels
C906, D904 0.85
Q617 E 0.001 C -6.43 B 0.0005
Q618 E 0.05 C 29.3 B 0.001
Q619 E 0.001 C 0.03 B 0.001
Q620 E 0.001 C 0.02 B 0.66
Q921 E 0.001 C 9.14 B 0.024
Q901 E 29.2 C 48.6 B 30
This leads me to more questions (sorry they aren't answers)
The secondary coil voltage, does this mean that that coil is wasted (both pilot bulbs are blown)?
Q619 (E 0.001, C 0.03, B 0.001) has no voltages on the schematic but Ian does this (and Q621) not suggest a problem as there is not .6v between EB? can you explain this further as they match the values on the schematic?
The 9.14v reading at the Q621 is very far off the 0.1 given and the voltage on the other side of the relay solenoid is 24v. does this suggest a problem with the relay itself?
The -0.04v reading pre relay for the power amp output is normal isn't it? Does this point me in the direction of the problem being in the protection circuit itself?
I've had to buy the amp off my girlfriend now until I can fix it! hah
I have replaced C905 C906 C907 D902 D903 and D904, all to no avail Andreas!
Here are some readings I took today. I had been taking readings with a bulb tester in place up until now, and I read today that this affects values! duh
Mains recorded at 244v (we run hot in Cardiff! I've measured over 250 before now)
Secondary coil 0.6v
Main coils 25.4v
Amp outputs (pre relay) -0.04v both channels
C906, D904 0.85
Q617 E 0.001 C -6.43 B 0.0005
Q618 E 0.05 C 29.3 B 0.001
Q619 E 0.001 C 0.03 B 0.001
Q620 E 0.001 C 0.02 B 0.66
Q921 E 0.001 C 9.14 B 0.024
Q901 E 29.2 C 48.6 B 30
This leads me to more questions (sorry they aren't answers)
The secondary coil voltage, does this mean that that coil is wasted (both pilot bulbs are blown)?
Q619 (E 0.001, C 0.03, B 0.001) has no voltages on the schematic but Ian does this (and Q621) not suggest a problem as there is not .6v between EB? can you explain this further as they match the values on the schematic?
The 9.14v reading at the Q621 is very far off the 0.1 given and the voltage on the other side of the relay solenoid is 24v. does this suggest a problem with the relay itself?
The -0.04v reading pre relay for the power amp output is normal isn't it? Does this point me in the direction of the problem being in the protection circuit itself?
I've had to buy the amp off my girlfriend now until I can fix it! hah
Just a comment:
Bulb testers are load-variable resistances. When the amplifier is under load, the bulb resistance shoots up from around 200R cold, to whatever resistance the glowing filament needs to be, up to its full brilliance and power rating, as when the device under test is virtually a dead short. If the bulb doesn't glow because the load is very small, measurement errors will be minor. After all, you are only trying to get a circuit just to operate when idling, rather than at high power.
However, I suggest you don't test mains voltages unless you have at least a genuine CAT2 meter and safety, shrouded or fully insulated plugs and clips. Think about this when you use a cheap DMM with dinky connectors and questionable PVC leads. 240 or even 115VAC is fatal, even if we here seem to be still alive.
I can't follow your transformer voltage measurements. Are you using the DC range to measure AC? You have 60VDC power in there somewhere yet you say you only measure the transformer primary at 25.4V and the secondary at 0.6V - was the bulb glowing when you took this more recent AC measurement?
What counts, is the voltages of the power supplies and these will all be down by some percentage if not regulated. There certainly will be problems if the input voltage of a linear regulator is closer than a few volts to the output voltage. It will "drop out" of regulation and the output becomes noisier until it quits altogether as the supply continues to fall. With the latge input-output differential of the the regulator here though, the should be no problem. Obviously, a little forethought when testing and a basic knowledge of common solid state circuits will be necessary to get a grip on this.
Seriously, if you are not quite understanding the technical basis of power supplies and regulators, do some text book reading. Forums are not the right place to learn theory. Posters really only have the time and space to drop a few hints and suggestions in a post and this will likely only be helpful for those who already have the basic knowledge.
If you are a strictly on-line type of guy, here's our go-to guy:
Linear Power Supply Design
Bulb testers are load-variable resistances. When the amplifier is under load, the bulb resistance shoots up from around 200R cold, to whatever resistance the glowing filament needs to be, up to its full brilliance and power rating, as when the device under test is virtually a dead short. If the bulb doesn't glow because the load is very small, measurement errors will be minor. After all, you are only trying to get a circuit just to operate when idling, rather than at high power.
However, I suggest you don't test mains voltages unless you have at least a genuine CAT2 meter and safety, shrouded or fully insulated plugs and clips. Think about this when you use a cheap DMM with dinky connectors and questionable PVC leads. 240 or even 115VAC is fatal, even if we here seem to be still alive.
I can't follow your transformer voltage measurements. Are you using the DC range to measure AC? You have 60VDC power in there somewhere yet you say you only measure the transformer primary at 25.4V and the secondary at 0.6V - was the bulb glowing when you took this more recent AC measurement?
What counts, is the voltages of the power supplies and these will all be down by some percentage if not regulated. There certainly will be problems if the input voltage of a linear regulator is closer than a few volts to the output voltage. It will "drop out" of regulation and the output becomes noisier until it quits altogether as the supply continues to fall. With the latge input-output differential of the the regulator here though, the should be no problem. Obviously, a little forethought when testing and a basic knowledge of common solid state circuits will be necessary to get a grip on this.
Seriously, if you are not quite understanding the technical basis of power supplies and regulators, do some text book reading. Forums are not the right place to learn theory. Posters really only have the time and space to drop a few hints and suggestions in a post and this will likely only be helpful for those who already have the basic knowledge.
If you are a strictly on-line type of guy, here's our go-to guy:
Linear Power Supply Design
😱thefamilybassman said:
If all the voltages are substantially higher than expected, is the mains voltage selector set for 240V?
N.B. The Collector of Q901 appears to be supplied from a classic voltage doubler (C907, D903, D902 & C905). If D901 is a 30V Zener (your measurements suggest that it is...), the Collector voltage is probably 'normal' given that the Base & Emitter voltages also appear to be 'normal'.
You've already shown that the transformer windings are OK - AC3=5V4!thefamilybassman said:
Check the voltage @ AC4? It should be the same as AC3 unless F904 is open circuit.
Good Luck!
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