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Old 9th July 2014, 09:38 PM   #1
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Default Carver TFM-25 help

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
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Old 10th July 2014, 02:50 AM   #2
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Looks like the PDF was too big, here is the schematic.
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File Type: jpg Carver TFM-25 service manual.jpg (1,021.9 KB, 150 views)
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Old 10th July 2014, 06:35 PM   #3
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bump

The right channel is fine it's only the left that goes into protection mode when a signal is applied.
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Old 10th July 2014, 08:33 PM   #4
llwhtt is offline llwhtt  United States
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With or without a load?

Craig
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Old 10th July 2014, 09:37 PM   #5
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Quote:
Originally Posted by llwhtt View Post
With or without a load?

Craig

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.
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Last edited by famousmockingbird; 10th July 2014 at 09:41 PM.
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Old 10th July 2014, 10:29 PM   #6
llwhtt is offline llwhtt  United States
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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.

Last edited by llwhtt; 10th July 2014 at 10:33 PM.
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Old 11th July 2014, 01:14 AM   #7
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Quote:
Originally Posted by llwhtt View Post
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

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.

Quote:
Originally Posted by llwhtt View Post
On a good note you don't have to deal with the "Magnetic Field Power Supply" like other Carvers.
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

Last edited by famousmockingbird; 11th July 2014 at 01:16 AM.
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Old 11th July 2014, 03:17 AM   #8
llwhtt is offline llwhtt  United States
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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
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Old 11th July 2014, 03:26 AM   #9
llwhtt is offline llwhtt  United States
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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
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Old 11th July 2014, 03:34 AM   #10
llwhtt is offline llwhtt  United States
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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
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