Hello everybody!🙂
I am working on a Carver CT-Seven preamplifier / tuner for someone & could use a little help on a power supply problem.
A few of the lights came on when plugged in & turned on, but did not fully come on, found the fuse blown for the PT801 transformer & noticed someone replaced it with a 2-amp fuse which is never a good sign. Anyway I replaced the fuse & it fully lit up & all looked to be working & then the fuse blew. I found the positive 15 volt regulator IC804 78M15 shorted from input pin to ground which obviously overloaded it.
I removed the transformer & hooked it up out of circuit on the bench & it pulls over 1- amp which is not normal & gets really hot with no load, but does put out voltage that seems correct or close as it does power the regulators fine until it over heats or blows a fuse as I did retested it with the fuse temporarily bypassed. I think some of the winding are shorted from over heating, I suspect with the regulator shorted & the 2-amp fuse installed where a 315ma one should have been let it overheat. (I am also going over a Carver M-0.5T amplifier & they completely bypassed the fuse inside!!!😱) I don’t have high hopes for repairing this stuff, but thought I would inquire.
Anyway, does anyone know a suitable replacement transformer for the CT-Seven?
The primary is 120vAC & for 220vAC.
It has two secondaries; here is what I measured with it on the bench with no load before it got too hot: I will include a picture I put together when measuring the voltage, might make it more clear, I don’t know, but may help.
47.5v CT (23.75v – 0 – 23.75v)
3.40v CT (1.7v – 0 – 1.7v)
I imagine finding a relatively inexpensive transformer may be difficult.
Here is the service manual with the schematic: http://thecarversite.com/manuals/files-manuals/Carver CT-7 service manual.pdf
Thanks for any info on this.
I am working on a Carver CT-Seven preamplifier / tuner for someone & could use a little help on a power supply problem.
A few of the lights came on when plugged in & turned on, but did not fully come on, found the fuse blown for the PT801 transformer & noticed someone replaced it with a 2-amp fuse which is never a good sign. Anyway I replaced the fuse & it fully lit up & all looked to be working & then the fuse blew. I found the positive 15 volt regulator IC804 78M15 shorted from input pin to ground which obviously overloaded it.
I removed the transformer & hooked it up out of circuit on the bench & it pulls over 1- amp which is not normal & gets really hot with no load, but does put out voltage that seems correct or close as it does power the regulators fine until it over heats or blows a fuse as I did retested it with the fuse temporarily bypassed. I think some of the winding are shorted from over heating, I suspect with the regulator shorted & the 2-amp fuse installed where a 315ma one should have been let it overheat. (I am also going over a Carver M-0.5T amplifier & they completely bypassed the fuse inside!!!😱) I don’t have high hopes for repairing this stuff, but thought I would inquire.
Anyway, does anyone know a suitable replacement transformer for the CT-Seven?
The primary is 120vAC & for 220vAC.
It has two secondaries; here is what I measured with it on the bench with no load before it got too hot: I will include a picture I put together when measuring the voltage, might make it more clear, I don’t know, but may help.
47.5v CT (23.75v – 0 – 23.75v)
3.40v CT (1.7v – 0 – 1.7v)
I imagine finding a relatively inexpensive transformer may be difficult.
Here is the service manual with the schematic: http://thecarversite.com/manuals/files-manuals/Carver CT-7 service manual.pdf
Thanks for any info on this.
An externally hosted image should be here but it was not working when we last tested it.
Looking at the circuit diagram, I think, that 47.5Vac a bit on the high side. It's used to feed the 78M15/79M15 and 78M12/79M12 regulators. A 23.75Vac rectified and decoupled gives you a 33Vdc at the input of the regulators, meaning that more than half of the power consumption is directly dissipated as heat.
I would expect a secondary of 2x15Vac or 2x18Vac. As for the 2x1.7Vac. When you use a torrodial transformer, it's quite easy to add a few separate windings to get the desired output voltage.
I would expect a secondary of 2x15Vac or 2x18Vac. As for the 2x1.7Vac. When you use a torrodial transformer, it's quite easy to add a few separate windings to get the desired output voltage.
Thank you for the reply!
The voltages I posted may not be correct since the transformer may have problems. I just figured I would measure it while I could & that is what it showed with no load.
I am not real familiar with toroidal transformers, I have never worked with them anyway, but it sounds like a reasonable solution & should work fine, adding the low voltage winding should not be too difficult.
I am not certain if the 3.4v I measured is correct or not. Looks like it might be for the tuners display lighting, which lit up fine & looked normal, so hopefully that is about right. I guess I will browse around online & see what toroidal transformers I can find around the voltage you suggested. Thank you again for the information I greatly appreciate it!
The voltages I posted may not be correct since the transformer may have problems. I just figured I would measure it while I could & that is what it showed with no load.
I am not real familiar with toroidal transformers, I have never worked with them anyway, but it sounds like a reasonable solution & should work fine, adding the low voltage winding should not be too difficult.
I am not certain if the 3.4v I measured is correct or not. Looks like it might be for the tuners display lighting, which lit up fine & looked normal, so hopefully that is about right. I guess I will browse around online & see what toroidal transformers I can find around the voltage you suggested. Thank you again for the information I greatly appreciate it!
If the display is a vacuum flouresent display, than it makes sense that the the voltage is around 1.7 CT volts. Usually that is the filament supply for the VF display. Measure the DC voltage across C812 and C811. Both should be about the same and will tell you if the D810-D813 are working. My guess is that the voltage will be pretty high if you are seeing 23 VAC from each transformer winding. Around 33-34 volts DC on each cap?? Yes, that is a lot for the 78M12/15 or 79M12/15 but if the current is low, power is no big deal. VF displays do need rather high voltage for the grids and plates, so 33-34 volts (usually negative) makes some sense to me. VF displays work well with old PMOS 4-bit micros.
BTW, IC301 is a CMOS 4-bit micro from NEC that can drive a VF display. I used to be a Apps. Engr. for NEC many moons ago........
Just some thoughts....
BTW, IC301 is a CMOS 4-bit micro from NEC that can drive a VF display. I used to be a Apps. Engr. for NEC many moons ago........
Just some thoughts....
I appreciate the info.
Alright, I put the transformer back in but I can’t power it up for too long since the transformer gets pretty hot, but long enough to measure voltage. Here is the DC voltage I measured at C811 & C812:
C811: +24.2vDC
C812: -25.3vDC
I also measured the AC voltage at the rectifiers D810 – D813 with the transformer in circuit is as follows:
Measured about 20.9vAC from the center tap to each of the outer windings, so slightly less in circuit with load, unless something else is wrong.
The lower voltage winding measured 3.03vAC with it in circuit & about 1.5vAC from center tap to outer windings.
I looked over the component list in the service manual & it shows it as a “Fluorescent Lamp” & IC251 is a TC9158P fluorescent display driver so I think you are correct on the display. I do want to make sure I get the voltage correct if that is what it is powering. I checked the voltage at pin-1 & pin-19 of what appeared to be the display pins & it also measured the 3vAC there too, actually is a direct connection that does indeed go to the display by the looks of it. Do you think the 3vAC is correct for this type of display? It lights up fine & looks good.
Again I appreciate all the help as I am no pro at this stuff.
Alright, I put the transformer back in but I can’t power it up for too long since the transformer gets pretty hot, but long enough to measure voltage. Here is the DC voltage I measured at C811 & C812:
C811: +24.2vDC
C812: -25.3vDC
I also measured the AC voltage at the rectifiers D810 – D813 with the transformer in circuit is as follows:
Measured about 20.9vAC from the center tap to each of the outer windings, so slightly less in circuit with load, unless something else is wrong.
The lower voltage winding measured 3.03vAC with it in circuit & about 1.5vAC from center tap to outer windings.
I looked over the component list in the service manual & it shows it as a “Fluorescent Lamp” & IC251 is a TC9158P fluorescent display driver so I think you are correct on the display. I do want to make sure I get the voltage correct if that is what it is powering. I checked the voltage at pin-1 & pin-19 of what appeared to be the display pins & it also measured the 3vAC there too, actually is a direct connection that does indeed go to the display by the looks of it. Do you think the 3vAC is correct for this type of display? It lights up fine & looks good.
Again I appreciate all the help as I am no pro at this stuff.
Hi ananto09,
Sorry for not responding sooner, I rarely check my email & did not see your reply until now. I was just browsing the forum & seen your reply!
I do still need a transformer, but I have not done anything to the CT-7 preamp yet as I wanted to get the Carver M-0.5T amplifier working first, I still need to order some parts for that before I can troubleshoot much further. The CT-7 looks like it should be a relatively simple repair mainly needing the transformer which looks like worst case I can use a toroidal for a replacement as mentioned above, but I figured if I Can’t fix the M-0.5T amp there is no point for me to repair the CT-7.
Thank you for the post about having a transformer, it may be a while until I actually get to repairing this unit (if I do), but I will keep it in mind if / when I get a transformer, if you still have it by then! Thanks again!
Sorry for not responding sooner, I rarely check my email & did not see your reply until now. I was just browsing the forum & seen your reply!
I do still need a transformer, but I have not done anything to the CT-7 preamp yet as I wanted to get the Carver M-0.5T amplifier working first, I still need to order some parts for that before I can troubleshoot much further. The CT-7 looks like it should be a relatively simple repair mainly needing the transformer which looks like worst case I can use a toroidal for a replacement as mentioned above, but I figured if I Can’t fix the M-0.5T amp there is no point for me to repair the CT-7.
Thank you for the post about having a transformer, it may be a while until I actually get to repairing this unit (if I do), but I will keep it in mind if / when I get a transformer, if you still have it by then! Thanks again!
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