Yup, that's why I suggested measuring at the plug with the power switch on. This avoids any confusion for wiring.
I checked the fuse inside the plug as well, which is fine, and both individual plug pins are continuous into the unit, so i guess the only explanation here is that the primary is blown.
So, I guess it brings us back to square 1, where I had stumbled into the right problem but with wrong reasoning (secondary not primary) that the TX is blown.
What options do I have now, and what would the replacement transformer's specs be?
thank you
So, I guess it brings us back to square 1, where I had stumbled into the right problem but with wrong reasoning (secondary not primary) that the TX is blown.
What options do I have now, and what would the replacement transformer's specs be?
thank you
Looking at this from the perspective of what is important and what will be available ...
1.) Lights, convert to LED and use the main supply to run them, or you can get cheap inverter PCBs from China to down convert to the voltage you need. So ignore the lamp winding. Our lives just got simpler. I would run them in series with a single series resistance. You can make that up with a couple resistors in series to spread the heat out if you don't want to use a switching regulator.
2.) The secondary AC voltage needed will be approx 64 VCT, or 32 V on each leg. 32 - 0.8 * 1.414 = 44 VDC which is close enough. Just pick a common voltage near this. Current rating, you have two fuses rated at 1.5 amps plus the lamps. So 3 and a bit amps secondary current. Since we want to leave some head room, add current rating. However the core size gives a clue also, so measure the dimensions of the iron core (only) and compare to replacements available for that voltage output. This isn't exact by any means and the original may have been undersized as it wasn't designed to deliver that current constantly. Core size at a given current determines the running temperature of the transformer. Another clue would be the primary fuse rating divided by the secondary output voltage, so 230 VAC / 64 VAC = 3.6, so 2 * 3.6 (or 7.2 amperes) on the secondary. That is probably optimistic, so sanity check with the core size. A 2,200 uF main filter cap supports about 2 amperes draw using engineering standards as a further check. Therefore, I believe the original transformer was not sized high enough.
3.) Don't forget it has to physically fit.
1.) Lights, convert to LED and use the main supply to run them, or you can get cheap inverter PCBs from China to down convert to the voltage you need. So ignore the lamp winding. Our lives just got simpler. I would run them in series with a single series resistance. You can make that up with a couple resistors in series to spread the heat out if you don't want to use a switching regulator.
2.) The secondary AC voltage needed will be approx 64 VCT, or 32 V on each leg. 32 - 0.8 * 1.414 = 44 VDC which is close enough. Just pick a common voltage near this. Current rating, you have two fuses rated at 1.5 amps plus the lamps. So 3 and a bit amps secondary current. Since we want to leave some head room, add current rating. However the core size gives a clue also, so measure the dimensions of the iron core (only) and compare to replacements available for that voltage output. This isn't exact by any means and the original may have been undersized as it wasn't designed to deliver that current constantly. Core size at a given current determines the running temperature of the transformer. Another clue would be the primary fuse rating divided by the secondary output voltage, so 230 VAC / 64 VAC = 3.6, so 2 * 3.6 (or 7.2 amperes) on the secondary. That is probably optimistic, so sanity check with the core size. A 2,200 uF main filter cap supports about 2 amperes draw using engineering standards as a further check. Therefore, I believe the original transformer was not sized high enough.
3.) Don't forget it has to physically fit.
The Rotel part number for the transformer is 205001317. You might try contacting them here to see if they could supply one. https://www.rotel.com/spare-parts. I attached the technical manual w/schematics in case you didn't have it.
If I read the posts correctly, the measurements of the transformer secondary where made with one end of the voltmeter to the amp chassis. This will not work, as the transformer AC has to be measured on both it's leads in AC mode. Also the way the primary voltages where measured is questionable.
You need the AC voltage setting of the meter and connect to both wires coming from the transformer, ignore the chassis!
I'm not sure the thread starter has any deeper knowledge about amps and power supplies, AC and DC, while the answering members may have too much practice to notice such beginner faults.
So if the measurements where made as described, any conclusion that was following is questionable or wrong.
I would repeat the measurments of the secondary voltages first.
The rectifier was blown for sure, this is in most cases a secondary defect, the primary cause being a short of probably some capacitor in the DC part of the amp. So the new diodes may be toast as well.
Usually you can measure such a short with the OHM setting of the meter, but the amp has to be disconnected from the mains! The display on the meter may need some time to stabilize while you measure.
C907 is most suspect.
You need the AC voltage setting of the meter and connect to both wires coming from the transformer, ignore the chassis!
I'm not sure the thread starter has any deeper knowledge about amps and power supplies, AC and DC, while the answering members may have too much practice to notice such beginner faults.
So if the measurements where made as described, any conclusion that was following is questionable or wrong.
I would repeat the measurments of the secondary voltages first.
The rectifier was blown for sure, this is in most cases a secondary defect, the primary cause being a short of probably some capacitor in the DC part of the amp. So the new diodes may be toast as well.
Usually you can measure such a short with the OHM setting of the meter, but the amp has to be disconnected from the mains! The display on the meter may need some time to stabilize while you measure.
C907 is most suspect.
Hi Turbowatch2,
He measured the primary from the mains plug end, power switch on. He read an open. The tested the mains fuse and it was okay.
The thermal fuse inside the transformer is probably open if the measurements were done correctly.
He measured the primary from the mains plug end, power switch on. He read an open. The tested the mains fuse and it was okay.
The thermal fuse inside the transformer is probably open if the measurements were done correctly.
Maybe read this:
"Hello anatech
The fuse is fine and so is the switch. I see 240VAC at the input terminals of the TX (transformer).
I measure almost 15 ohm resistance at the primary.
I also measure c 1.8 ohm at the secondary . I snipped off what look to be the only 4 wires coming out of the TX on the secondary - 2x green and 2x red . The greens mutually, and the reds mutually are continuous, but no continuity between either red to either green. Which makes me think these are two independent secondary coils. The greens must be the pilot lamp ones, the red must be the business ends."
For a replacement a 30V transformer should be suitable, ignoring the different voltages, not 22x50 Volt.
"Hello anatech
The fuse is fine and so is the switch. I see 240VAC at the input terminals of the TX (transformer).
I measure almost 15 ohm resistance at the primary.
I also measure c 1.8 ohm at the secondary . I snipped off what look to be the only 4 wires coming out of the TX on the secondary - 2x green and 2x red . The greens mutually, and the reds mutually are continuous, but no continuity between either red to either green. Which makes me think these are two independent secondary coils. The greens must be the pilot lamp ones, the red must be the business ends."
For a replacement a 30V transformer should be suitable, ignoring the different voltages, not 22x50 Volt.
Also:
"
this TX has multiple input (primary taps) for selecting various input AC voltages, again with very confusing wiring, so i think the 1ohm was an erroneous measurement. I am more confident in saying its 15ohm on primary.
Primary to secondary is (correctly) open.
Secondary red-red and green-green give about 1ohm."
"
this TX has multiple input (primary taps) for selecting various input AC voltages, again with very confusing wiring, so i think the 1ohm was an erroneous measurement. I am more confident in saying its 15ohm on primary.
Primary to secondary is (correctly) open.
Secondary red-red and green-green give about 1ohm."
Here things get confusing:
"I have now isolated the TX from the onward rectification circuits by chopping off the 2 red and 2 green wires I can see (with much effort) coming out of the TX.
I have plugged the unit in directly to power (no DBT)
With the -ve probe on the chassis, and +ve on the input primary side,I get 240vac
I get nothing on either output pair, red-red or green-green, either between the two or with -ve lead on chassis and +ve on either."
I can not see why the transformer should not work. These old Rotel sure had not thermo fuse build in.
If there is a primary and secondary with these values, it should be OK or at least blow a fuse.
I find no logic in the measurements.
PS most common fault is measuring AC in DC mode. Which will only show some inconsistant low voltage numbers depending on the meter.
"I have now isolated the TX from the onward rectification circuits by chopping off the 2 red and 2 green wires I can see (with much effort) coming out of the TX.
I have plugged the unit in directly to power (no DBT)
With the -ve probe on the chassis, and +ve on the input primary side,I get 240vac
I get nothing on either output pair, red-red or green-green, either between the two or with -ve lead on chassis and +ve on either."
I can not see why the transformer should not work. These old Rotel sure had not thermo fuse build in.
If there is a primary and secondary with these values, it should be OK or at least blow a fuse.
I find no logic in the measurements.
PS most common fault is measuring AC in DC mode. Which will only show some inconsistant low voltage numbers depending on the meter.
Hi Turbowatch2,
A 64 VCT secondary will generate +44 VDC approximately, the original supply is about +42 VDC (from the service manual). Remember, it is not a bridge rectifier, just two diodes, or full wave.
Please let me know if my math is wrong and maybe show yours as I have mine.
Now, 30 VCT will generate 15 - 0.8 V, or 14.2 V, peak rectified to +20.1 VDC approx. If you were using a 60 VCT transformer, that gives you 30 VAC per leg, so the calculation becomes 29.2 V (correcting for approx diode drop) * 1.414 = 41.3 VDC approx. The secondary voltage has a lot of latitude, so whatever transformer available close to 60 VCT, or 64 VCT would be just fine.
Pars suggested getting the original part, if available. If it is, it would be the easiest solution.
* Edit: I had him measure primary continuity.
A 64 VCT secondary will generate +44 VDC approximately, the original supply is about +42 VDC (from the service manual). Remember, it is not a bridge rectifier, just two diodes, or full wave.
Please let me know if my math is wrong and maybe show yours as I have mine.
Now, 30 VCT will generate 15 - 0.8 V, or 14.2 V, peak rectified to +20.1 VDC approx. If you were using a 60 VCT transformer, that gives you 30 VAC per leg, so the calculation becomes 29.2 V (correcting for approx diode drop) * 1.414 = 41.3 VDC approx. The secondary voltage has a lot of latitude, so whatever transformer available close to 60 VCT, or 64 VCT would be just fine.
Pars suggested getting the original part, if available. If it is, it would be the easiest solution.
* Edit: I had him measure primary continuity.
Wouldn't it it be simpler to use a common 30V AC transformer and a simple bridge rectifier to replace the origional part? I see no problem with such a modification.
Anyway, before throwing money at this thing, shouldn't he test for the cause of the blown rectifier and blown internal fuse (which usually should be visible in the schematic). If any transistor is gone these can be quite rare and problematic to replace.
Anyway, before throwing money at this thing, shouldn't he test for the cause of the blown rectifier and blown internal fuse (which usually should be visible in the schematic). If any transistor is gone these can be quite rare and problematic to replace.
I generally don't deviate much from the original design, you would need 2x the current rating so I guess what is available may determine which path is followed. I can't see where going to a full wave bridge would be less complicated to be honest. Changing things is a complication.
If he can't power this thing up, he needs the transformer. I have seen rectifiers like this one self destruct without a failure elsewhere. My guess, marginal current rating on the rectifier. There are no expensive parts used in the amplifier, so if there is also a failed output he didn't find yet, not really a big deal. I would use 6 ampere rectifiers to replace what was there, and a bridge may screw onto the chassis if that is easier. He would ignore the negative terminal on the bridge (common anode).
If he can't power this thing up, he needs the transformer. I have seen rectifiers like this one self destruct without a failure elsewhere. My guess, marginal current rating on the rectifier. There are no expensive parts used in the amplifier, so if there is also a failed output he didn't find yet, not really a big deal. I would use 6 ampere rectifiers to replace what was there, and a bridge may screw onto the chassis if that is easier. He would ignore the negative terminal on the bridge (common anode).
Hello all
Sorry for the radio silence, I was ready to throw in the towel, but something struck me last evening.
Given the plug showed an open, I decided to try replacing the plug as it looked really old and seemed to have a patina on the legs.
Lo and behold, I now have 30ohm between the two. When I power on, the TX hums very softly. A small victory!
(the 240v seen in prior posts i guess was just the line vs ground. the 15ohm reading was possibly part of the primary as its a multivoltage setup)
Now, recall there are 2x green and 2x red wires out of the TX on the secondary.
The two green produce c7VAC. Although I disconnected one of the green wires as it led to the bulbs (which i wasn't sure the state of) - when connected the bulbs do light up, and the function select dial does cycle through the correct lights on the panel !
On to the more important parts now. The 2x red wires produce c 37VAC between them. I cannot for the life of me find center tap. I first thought one of the greens is the center tap but either green to either red is Open, so it cannot be so.
No DC is being generated as things stand though. Then i decided to disconnect the board that has the rectification and other voltage divider networks to isolate and simplify, and see the bottom side. I disconnected all the supply voltage wires from the rest of the amp. I also removed the connection to the +ve of c907 the 50V capacitor.
The schematic I have (same as the one also attached by another member above) shows a D902 = IS1850 which looks like a common cathode half recitifier. Whereas the circuit clearly had two halfwave components, one common anode (working) and other common cathode (burnt).
Looking at the physical configuration as-is, the common cathode point is ultimately electrically connected to the chassis, ie ground, which is also the -ve of C907.
Am I nuts or is there a chance the schematic is different to what I have before me, and that there is no center tap and no need for one here?
Thanks again for every one's time and help.
Sorry for the radio silence, I was ready to throw in the towel, but something struck me last evening.
Given the plug showed an open, I decided to try replacing the plug as it looked really old and seemed to have a patina on the legs.
Lo and behold, I now have 30ohm between the two. When I power on, the TX hums very softly. A small victory!
(the 240v seen in prior posts i guess was just the line vs ground. the 15ohm reading was possibly part of the primary as its a multivoltage setup)
Now, recall there are 2x green and 2x red wires out of the TX on the secondary.
The two green produce c7VAC. Although I disconnected one of the green wires as it led to the bulbs (which i wasn't sure the state of) - when connected the bulbs do light up, and the function select dial does cycle through the correct lights on the panel !
On to the more important parts now. The 2x red wires produce c 37VAC between them. I cannot for the life of me find center tap. I first thought one of the greens is the center tap but either green to either red is Open, so it cannot be so.
No DC is being generated as things stand though. Then i decided to disconnect the board that has the rectification and other voltage divider networks to isolate and simplify, and see the bottom side. I disconnected all the supply voltage wires from the rest of the amp. I also removed the connection to the +ve of c907 the 50V capacitor.
The schematic I have (same as the one also attached by another member above) shows a D902 = IS1850 which looks like a common cathode half recitifier. Whereas the circuit clearly had two halfwave components, one common anode (working) and other common cathode (burnt).
Looking at the physical configuration as-is, the common cathode point is ultimately electrically connected to the chassis, ie ground, which is also the -ve of C907.
Am I nuts or is there a chance the schematic is different to what I have before me, and that there is no center tap and no need for one here?
Thanks again for every one's time and help.
Hi woodyp,
Excellent news, and a great example of why to keep an open mind.
With a winding that has no centre tap, you either will use a single diode (very inefficient, lots of ripple), or a full wave bridge rectifier. What you showed was a full wave rectifier (not a bridge). That configuration requires a centre tap on that secondary to work, period.
The wire may have been broken and pulled out of the transformer possibly, but at some point it was there. It may possibly come out at a different point, but that would be unusual. The centre tap should connect pretty close to directly to the filter capacitor negative terminal. Possibly the chassis, but close to the filter cap.
Excellent news, and a great example of why to keep an open mind.
With a winding that has no centre tap, you either will use a single diode (very inefficient, lots of ripple), or a full wave bridge rectifier. What you showed was a full wave rectifier (not a bridge). That configuration requires a centre tap on that secondary to work, period.
The wire may have been broken and pulled out of the transformer possibly, but at some point it was there. It may possibly come out at a different point, but that would be unusual. The centre tap should connect pretty close to directly to the filter capacitor negative terminal. Possibly the chassis, but close to the filter cap.
If the center tap has disappeared from access, a diode bridge could be a fix—- full secondary across AC terminals, negative terminal grounded.
Very good you you found the tansformer to be in working order. Please do not make experiments in connecting it now, as the voltage of 37V between red and red you measured, may not be correct and too high. As long as the transformer is running at idle this is no problem. Did you change/ try anything in the 110-120-220-240 Volt AC selection? It has to be set to 240V! Please check first.
Next, ROTEL is lazy. Once they have a working receiver the don't change anything and build exactly as the schematic shows you. It may not be easy to match both on this old stuff, I know...
If you have a look at the schematic, page 10, on the left you can see a wire conected to both amplifier channels "To chassis ground".
Find it and follow it. This should go to the center tap of the transformer.
Can you please check the large capacitor C 907? The rectifier may have blown by it self or because of a short in the capacitor.
Things would be simpler for us if you provided more and better pictures. On the one with the transformer I can see a black wire running throug a rubber grommet in the sheet metal of the chassis. This may be the chenter tap.
BSST is right, but that fix might not be neccesary at this point.
Next, ROTEL is lazy. Once they have a working receiver the don't change anything and build exactly as the schematic shows you. It may not be easy to match both on this old stuff, I know...
If you have a look at the schematic, page 10, on the left you can see a wire conected to both amplifier channels "To chassis ground".
Find it and follow it. This should go to the center tap of the transformer.
Can you please check the large capacitor C 907? The rectifier may have blown by it self or because of a short in the capacitor.
Things would be simpler for us if you provided more and better pictures. On the one with the transformer I can see a black wire running throug a rubber grommet in the sheet metal of the chassis. This may be the chenter tap.
BSST is right, but that fix might not be neccesary at this point.
Hello all
Thanks for your responses.
I have made some more progress.
I isolated the board which recitifies the AC and creates all the other DC voltages.
I decided to remove the other ESA device and brought in a GBPC2502W bridge rectifier.
Now get the main DC rail voltage of 43v (B1 in the schematic). This is below 47V as per the schematic, but I am on the DBT so that might be limiting it? I am also getting all the other lower voltages including the 12v on the zener (B6).
So re the transformer - my conclusion is that it is (obviously) fine. There is no center tap. The circuit design probably changed vs the schematic I and another user has found on the internet (posted above). Rotel decided to use a bridge rectifier configuration using two ESA devices listed in a prior post, and use a TX without center tap. To answer some one above.. the black wire going thru the grommet is the -ve DC point of the bridge that's grounded to the chassis.
Since C907 wasnt very noticably shorted, I cautiously connected a 15kOhm resistor leading in to the C907, and measured voltage rise up to c 5v. Then used a 6kohm and it rises to 9V and now I have a 10ohm fusible in series for the moment. So far the Dim Bulb never lights up at all.
I have left the other DC voltages unconnected for the moment till I figure the amp section is OK.
I get no DC on the speaker outputs but I havent tested beyond this.
Thought I would post this as a little milestone. But more work to do.
Thanks for all your help and input.
Thanks for your responses.
I have made some more progress.
I isolated the board which recitifies the AC and creates all the other DC voltages.
I decided to remove the other ESA device and brought in a GBPC2502W bridge rectifier.
Now get the main DC rail voltage of 43v (B1 in the schematic). This is below 47V as per the schematic, but I am on the DBT so that might be limiting it? I am also getting all the other lower voltages including the 12v on the zener (B6).
So re the transformer - my conclusion is that it is (obviously) fine. There is no center tap. The circuit design probably changed vs the schematic I and another user has found on the internet (posted above). Rotel decided to use a bridge rectifier configuration using two ESA devices listed in a prior post, and use a TX without center tap. To answer some one above.. the black wire going thru the grommet is the -ve DC point of the bridge that's grounded to the chassis.
Since C907 wasnt very noticably shorted, I cautiously connected a 15kOhm resistor leading in to the C907, and measured voltage rise up to c 5v. Then used a 6kohm and it rises to 9V and now I have a 10ohm fusible in series for the moment. So far the Dim Bulb never lights up at all.
I have left the other DC voltages unconnected for the moment till I figure the amp section is OK.
I get no DC on the speaker outputs but I havent tested beyond this.
Thought I would post this as a little milestone. But more work to do.
Thanks for all your help and input.
PLEASE DISREGARD THE ENTIRE PREVIOUS POST. I CANNOT EDIT IT OR DELETE IT - SOME TYPOS IN THERE, SO HERE IS THE CORRECT VERSION:
More updates:
Please refer to the schematic in post #13
Checked output transistors and seeing no major issues, plugged in some sacrificial speakers and .... the unit plays beautifully! The sound is lovely and warm.
Some pilot lamps behind the frequency band are missing - will have to hunt down online.
Radio (FM)seems to work in that it catches some signals and I can hear hissy but identifiable sounds but probably needs an antenna that I dont have at the moment.
I checked some voltages when on, and one issue I spotted was on the RIGHT channel transistors.
Left channel:
Collector voltage of TR509 = 45V
Collector voltage of TR510 = 23v (approx half of above)
RIGHT channel:
Collector voltage of TR609 = 45V
Collector voltage of TR610 = 17.5 volt (!)
Amp sounds pretty good despite it and no significant volume or distortion on left channel vs right to the ear.
I can see the dim bulb glow faintly at high volume and bassy passages but otherwise not glowing.
TR509 and 510 are Toshiba 2SC789 in the amp, but as per the schematic they need to be
TR509, 510 : ''2SD317 or 2SC1107''
TR609 is TIP41A (very hard to read, has a line above it probably 80456) and TR610 has all lettering erased. As per schematic they should also be ''2SD317 or 2SC1107''
Just judging that the 610n and 609 look a bit older than the 510 and 509 some one repaired the left channel at some point.
The voltage difference is of course bothering me, so I need to investigate further down the chain. But pretty chuffed with the progress so far.
As I listen to the music, no component seems to be heating up to the touch even after a good 15-30 min of playing at 75% volume.
Any further ideas and tips welcome on the outstanding issue.
More updates:
Please refer to the schematic in post #13
Checked output transistors and seeing no major issues, plugged in some sacrificial speakers and .... the unit plays beautifully! The sound is lovely and warm.
Some pilot lamps behind the frequency band are missing - will have to hunt down online.
Radio (FM)seems to work in that it catches some signals and I can hear hissy but identifiable sounds but probably needs an antenna that I dont have at the moment.
I checked some voltages when on, and one issue I spotted was on the RIGHT channel transistors.
Left channel:
Collector voltage of TR509 = 45V
Collector voltage of TR510 = 23v (approx half of above)
RIGHT channel:
Collector voltage of TR609 = 45V
Collector voltage of TR610 = 17.5 volt (!)
Amp sounds pretty good despite it and no significant volume or distortion on left channel vs right to the ear.
I can see the dim bulb glow faintly at high volume and bassy passages but otherwise not glowing.
TR509 and 510 are Toshiba 2SC789 in the amp, but as per the schematic they need to be
TR509, 510 : ''2SD317 or 2SC1107''
TR609 is TIP41A (very hard to read, has a line above it probably 80456) and TR610 has all lettering erased. As per schematic they should also be ''2SD317 or 2SC1107''
Just judging that the 610n and 609 look a bit older than the 510 and 509 some one repaired the left channel at some point.
The voltage difference is of course bothering me, so I need to investigate further down the chain. But pretty chuffed with the progress so far.
As I listen to the music, no component seems to be heating up to the touch even after a good 15-30 min of playing at 75% volume.
Any further ideas and tips welcome on the outstanding issue.
I tried to get the mods to delete the post with the typos, but instead had the earlier one deleted ... not really my day.
In case that post cannot be resurrected, I will add a few quick summary points:
1. Transformer was established good. Conclusion was that plug was faulty. Also the schematic was out of date and the TX has no center tap.
2. put in a bridge rectifier GBPC2502W to replace the ESAB devices
3. All voltages are working fine - power board reconnected
In case that post cannot be resurrected, I will add a few quick summary points:
1. Transformer was established good. Conclusion was that plug was faulty. Also the schematic was out of date and the TX has no center tap.
2. put in a bridge rectifier GBPC2502W to replace the ESAB devices
3. All voltages are working fine - power board reconnected
