Good morning - it is snowing again here in northern New Mexico - a perfect day for working on projects. I have a problem with my Zen version 4 which I built about six years ago in 2005. It worked well until last year when I fried one channel (all my fault).
I have just had the chance to repair it - we spent a year touring the country looking for a new place to live and the zen travelled in pieces under the settee in the travel trailer.
The problem it that the repaired channel runs cold - the heat sink barely gets warm, while the other channel functions as a heater as well as an amp. When replacing parts, I found that some parts were not available so I substituted as follows:
Digikey part numbers:
IRFP044N-ND to IRFP044NPBF-ND
The recommended IRFP240 to IRFP240PBF
Voltage readings as follows:
Cool (repaired) Warm
Vin 51.8 51.4
Across R0 .65 .661
Q1 D to gnd 27.76 27.8
Q2 D to S 12.23 12.08
Both channels are showing about 1.5 volt peaks at the speaker terminals at a low listening level
The heatsinks are 12” x 15” with 2” deep fins
Any thoughts?
Thanks, Ralph
I have just had the chance to repair it - we spent a year touring the country looking for a new place to live and the zen travelled in pieces under the settee in the travel trailer.
The problem it that the repaired channel runs cold - the heat sink barely gets warm, while the other channel functions as a heater as well as an amp. When replacing parts, I found that some parts were not available so I substituted as follows:
Digikey part numbers:
IRFP044N-ND to IRFP044NPBF-ND
The recommended IRFP240 to IRFP240PBF
Voltage readings as follows:
Cool (repaired) Warm
Vin 51.8 51.4
Across R0 .65 .661
Q1 D to gnd 27.76 27.8
Q2 D to S 12.23 12.08
Both channels are showing about 1.5 volt peaks at the speaker terminals at a low listening level
The heatsinks are 12” x 15” with 2” deep fins
Any thoughts?
Thanks, Ralph
taking in account that most important voltage - across R0 ( then also current) is similar in both channels ......... I resume that you have poor thermal contact between new mosfets and heatsink
they're (mosfets) nearly scorched , while heatsink is cold
clean well mosfets , micas and heatsink , reapply thermal goo between mosfet-mica and mica-heatsink , squeeze da screw tight and well .
use biggie washer between screw and mosfet
they're (mosfets) nearly scorched , while heatsink is cold
clean well mosfets , micas and heatsink , reapply thermal goo between mosfet-mica and mica-heatsink , squeeze da screw tight and well .
use biggie washer between screw and mosfet
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Thanks, but still running cold!!
Thanks for your answer. I thought that sounded like it must be the problem. Got busy with some work stuff and did not get a chance to check it out till this morning. I checked the tightness of the mosfets (mounted on silpads) and did not find any problems. Decided to measure the temp of each mosfet before dismounting the board and cleaning...
Here are the results after about 20 minutes of operation at around 65˚ F:
repaired:
Q1 - 83˚
Q2 - 85˚
Q5 - 83.5˚
R1 - 86.6˚
Center of heatsink - 80.5˚
not repaired:
repaired:
Q1 - 133˚
Q2 - 109.5˚
Q5 - 104˚
R1 - 104˚
Center of heatsink - 105˚
Temperature measurements taken adjacent to mounting screws and in center of resistors
Anyone have any thoughts?
Thanks for your answer. I thought that sounded like it must be the problem. Got busy with some work stuff and did not get a chance to check it out till this morning. I checked the tightness of the mosfets (mounted on silpads) and did not find any problems. Decided to measure the temp of each mosfet before dismounting the board and cleaning...
Here are the results after about 20 minutes of operation at around 65˚ F:
repaired:
Q1 - 83˚
Q2 - 85˚
Q5 - 83.5˚
R1 - 86.6˚
Center of heatsink - 80.5˚
not repaired:
repaired:
Q1 - 133˚
Q2 - 109.5˚
Q5 - 104˚
R1 - 104˚
Center of heatsink - 105˚
Temperature measurements taken adjacent to mounting screws and in center of resistors
Anyone have any thoughts?
here are mine: The current sensing resistor could be somewhat damaged and has too high resistance (R0, R1, not sure if you have several in parallel, then one of them might be kaputt), or the VBE transistor Q3 could be damaged. Also, did you measure VGS for all MOSFETS ?
look at attached pic ; all important voltages are here
open this pic in any editor ( MS Paint) and write down your voltages in , say, red
most important now - connect your DMM as Ampermeter ,in series with + voltage and read what current is going to channel
(naughty Papa - he deleted post
)
open this pic in any editor ( MS Paint) and write down your voltages in , say, red
most important now - connect your DMM as Ampermeter ,in series with + voltage and read what current is going to channel
(naughty Papa - he deleted post
)Attachments
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Still can not figure it out
I took some more measurements this evening. I guess the one that tells the tale is that the "cool" channel is drawing .5 amps at the fuse while the "normal" channel is drawing 1.37 amps.
While some of the voltages vary from those on the schematic, both channels are very close.
Does anyone have any more ideas?
All help is appreciated
Q1 Vsg 3.5
Q2 Vsg 4
C5+ R11 36.48 but both channels close to this value
Many thanks, Ralph
I took some more measurements this evening. I guess the one that tells the tale is that the "cool" channel is drawing .5 amps at the fuse while the "normal" channel is drawing 1.37 amps.
While some of the voltages vary from those on the schematic, both channels are very close.
Does anyone have any more ideas?
All help is appreciated
Q1 Vsg 3.5
Q2 Vsg 4
C5+ R11 36.48 but both channels close to this value
Many thanks, Ralph
it's already written - you need tp desolder those big resistors and check them with ohm-meter
best - against new ones , to exclude DMM low value treshold from process.
or even better - replace them
best - against new ones , to exclude DMM low value treshold from process.
or even better - replace them
Hi,
What about replacing the other channel with the new transistor? If the channel get hot then you have a problem in the first channel. Any way both channel suppose to have the same transistors.
What about replacing the other channel with the new transistor? If the channel get hot then you have a problem in the first channel. Any way both channel suppose to have the same transistors.
Thanks for the advice. The culprit was R0 which I replaced in the repair. It was way off.
Again thanks for all the advice!!
Ralph
Again thanks for all the advice!!
Ralph
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