Help - Zen4 running too cool

[BeaufortRalph]

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

[Zen Mod]

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

[BeaufortRalph]

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?

[MRupp]

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 ?

[BeaufortRalph]

Vgs around 4 volts on Q5,Q2both channels
Q1 3.0 hot channel
3.5 cool channel

[BeaufortRalph]

Voltage drop across R1 and R0 within 0.007 on both channels
0.636, 0.643

[BeaufortRalph]

Q3 Ve to gnd 27 on hot channel and 32 on cold channel

[Zen Mod]

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 )

[BeaufortRalph]

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

[Zen Mod]

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