I am trying to get a Carver TFM-25 up and running again but it keeps going into protection mode only when a signal is applied. I plan on doing modifications per order of a service bulletin included in the service manual I attached below but I need to sort out the main problem first.
I am looking for someone to shove me in the right direction of what to look for, I am more familiar with hollow state technology. Plus this amp's topology is different than what I am used to seeing.
Thanks
I am looking for someone to shove me in the right direction of what to look for, I am more familiar with hollow state technology. Plus this amp's topology is different than what I am used to seeing.
Thanks
bump
The right channel is fine it's only the left that goes into protection mode when a signal is applied.
The right channel is fine it's only the left that goes into protection mode when a signal is applied.
With no load a 16mV 1kHz sine wave trips the protection circuit at input, input is rated at 2Vrms.
There is service bulletin with known failures and calls for replacing predriver and driver transistors, attached below is the service bulletin. These look like they could be the culprit, the board is all black around the area like stated by service bulletin.
I checked the output transistors for shorts in circuit and they appear to be fine, plus the protection doesn't kick in until a signal is applied, which if one of the output transistors were shorted I would think it would trip immediately with no signal.
Attachments
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The protection IC, TA7317P, only monitors a couple of things and provides the turn on delay. Download the spec sheet for that IC and start comparing voltages. All of the power supply voltages and offset are good? Since this amp is a rail switcher maybe the signal is causing it to switch one rail prematurely causing DC offset. Try monitoring the output for DC "BEFORE" the relay while a signal is applied. I've noticed on many Carvers I've repaired that the commutator transistors are dead, usually the TO3 type transistors. They check OK on a DMM but are bad on an analyzer, they just don't do anything. PN junctions seem fine, no leakage, just plain dead.
Craig
On a good note you don't have to deal with the "Magnetic Field Power Supply" like other Carvers.
Craig
On a good note you don't have to deal with the "Magnetic Field Power Supply" like other Carvers.
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The DC offset for the right channel is 25mV and for the left 385mV.
Monitoring before the relay when a signal is applied I get a negative voltage that changes when input amplitude changes. I measured from -1 up to -5 volts, -5 volts was with a 1Vrms input signal.
TA7317p pin voltage testing:
Service manual specs: pin1) -.6 pin2) 0 pin3) 0 pin4) 0 pin5) -.7 pin6) -.6 pin7) 0 pin8) 1.3 pin9) 3
my reading w/ no signal: pin1) -.6 pin2) -190mV pin3) -49mV pin4) 1mV pin5) -.7 pin6) 1.17 pin7) 7mV pin8) 1.3 pin9) 3
my readings w/ input signal: pin1) -.6 pin2) 190mV pin3) 20mV pin4) 7.5mV pin5) -.7 pin6) 48v pin7) 2.9mV pin8) 48mV pin9) 3
Pin 6 with no signal isn't what it should be, I will scratch my head on this for a while now. I did download the datasheet for TA7317 and will familiarize myself with it's operation.
Is that a fancy term for a transformer? Or does it actually get it's power from the earth's magnetic field because the latter would be quite revolutionary 😉
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No, it's a power supply that Carver developed for his amplifiers back in the 80s(?), not sure if he invented it or not. The supplies are not real complicated just hard to troubleshoot compared to the stone-age, which I like, power supplies with xfmr, rectifier, and filter capacitors. If you have an amp with the Mag Field style of power supply the protection circuitry will shut the amp down completely until you find the problem but since it won't even start up you can't even measure anything. It takes some time to get used to them. But on top of that most of the magnetic field amps are built in such a way that it makes it hard to get to points you want measure. It's been some time since I worked on one but I think I remember they don't care for Variacs or dim-bulb-testers either. The Carver Cube is a nightmare to troubleshoot until you get the hang of it just because of the mechanics.
Craig
Craig
Pin 6 is high because the amp is protect, normally it would be the gnd for the speaker relays. The temp. switches are in series with Pin 6 and the relays, heat sink(s) get hot switch opens and the relays loose their gnd, and they open. So whatever is causing the problem has to be an input, Pin 6 is an output sort of. Pin 2 looks interesting.
Craig
Craig
Pin 2 is for the left channel, C307 "should" take all AC signal to gnd unless it's bad. Try replacing C307 with a new cap, something close doesn't have to be exact for now. The signal on Pin 2 also goes all the way back to the input IC101, maybe something there. Check for DC on the outputs of IC101, left is Pin 1.
Craig
Craig
I figured it was normal that in protect mode pin 6 was 48v, but not in protect mode it should be -.6 but I am reading 1.17
I will look around pin 2 which I believe to be the DC detection input, thanks again for helping me out with this.
And it sounds like I don't want to come across a magnetic field power supply, I like the stone age myself. But it is interesting and I will do some reading up on the subject.
I will look around pin 2 which I believe to be the DC detection input, thanks again for helping me out with this.
And it sounds like I don't want to come across a magnetic field power supply, I like the stone age myself. But it is interesting and I will do some reading up on the subject.
Remember Pin 6 in common to both channels so whatever you read there is probably OK since the right channel works.
Craig
Craig
Yes that would make sense.
So I got to some more probing and prodding tonight and removed C307, it tested fine but I replaced it anyway with no improvement.
I did find 1.5v at output pin 1 of IC101😱 Comparing to the right channel that works pin 7 only has 13mV on it, so I think that IC101 is bad. I plan to remove IC101 and see if I get 1.5 volts at the trace which goes to the base of Q103 and Q105 through R117 and R119.
I found this:
NE5532DR Texas Instruments | Mouser
It says SOIC package but the picture is SMD😕 Will this work as a replacement?
I'm assuming given the vintage of the amplifier you need a PDIP (plastic dip)
SOIC is smd "small outline IC"
SOIC is smd "small outline IC"
Yes that's what I need an in-line package, thanks.
I think I may have learned something the hard way, I turned the amp on without the IC101 and after a couple of seconds I thought I saw smoke so I instantly shut it off. I put the IC back in and turned it on and no smoke but hopefully I didn't do any damage.
Could I damage anything turning the amp on without IC101 in? I feel like an idiot
I just replaced IC101 and I still get 1.5vdc at the broken channel's output specifically pin 1😕
So back to square one, I have 350mVdc at the broken channel speaker output, no load and no signal. The DC voltage increases when input signal's amplitude increases until the point of the protection circuit kicking.
Pin 2 of IC301 is getting increasing DC with increasing input signal and tripping the relay. This pin is also connected to the base Q229 through a DS446 diode (D233) and the base of Q230 through another DS446 diode (D236). Also connected to pin 2 is IC151.
I think I will replace IC151 next and then D233 and D236.
Attached are a schematic and a scope shot of the output of the amp, input is a 1kHz sine wave.
So back to square one, I have 350mVdc at the broken channel speaker output, no load and no signal. The DC voltage increases when input signal's amplitude increases until the point of the protection circuit kicking.
Pin 2 of IC301 is getting increasing DC with increasing input signal and tripping the relay. This pin is also connected to the base Q229 through a DS446 diode (D233) and the base of Q230 through another DS446 diode (D236). Also connected to pin 2 is IC151.
I think I will replace IC151 next and then D233 and D236.
Attached are a schematic and a scope shot of the output of the amp, input is a 1kHz sine wave.
Attachments
What do the voltages and signals look like on the collectors of Q103/Q105? Should be the same but 180 out of phase. The offset on the output of the IC could be caused by Q103/105 and supporting circuitry.
Service update done?
Craig
Service update done?
Craig
I was testing around Q103/Q105 and all of a sudden the problem disappeared😕 I didn't replace anything all I did was probe the solder joints. Maybe just a bad connection? I am going to reflow all solder connections around that area or even the whole board, hopefully it stays fixed. I did replace IC151 but I don't think that is what fixed the problem.
Nope, I have all the parts and plan to tackle that tomorrow.
The board they are on show no signs of heat damage. I have seen guys touch transistors to see if they are getting hot but I am not keen on sticking my hand inside a live amp and touching them, I guess as long as I stick my other hand in my pocket and wear some sneakers I should be ok. My body always tests on the low side for resistance (high sodium?) so I assume I am a good candidate for electrocution.