Alpine MRV-1000 repair

[Perry Babin]

How did you get 90v from 42 and 20?

 

[Kei]

That is what I cannot understand. Collector to collector on Q226 to Q224 definitely gives 90V. GND to collector of Q226 gives -20V and GND to Q224 gives 42V. I've quadruple checked this at many different points from the rectifiers all the way to the outputs and the readings are definitely correct. (using both two different meters)

 

[Perry Babin]

With no RCAs plugged into the amp, what's the resistance between the primary and secondary grounds?

 

[Kei]

Started off at roughly 17ohms and rose until showing open circuit suggesting a capacitor between the two.

 

[Perry Babin]

Have you been using the secondary ground for the voltage readings? If not, use the secondary ground. Does that make a difference?

 

[Kei]

I've been using primary ground for all measurements. Using secondary ground I get +45V on the collectors of Q220 & Q224 and -45V on Q222 & Q226 which sounds bang on target.

 

[Kei]

OK, since I've now found out why my voltage readings for anything post transformer was wrong, I need to ascertain whether the channel with correct voltage rails is actually working correctly and why the other channel seems to shut down.

These are the measurements from the channel with "working" rails. (left)

IC801
1. 0.96v
2. 0.96v
3. 3.71v
4. 0.328v
5. 1.58v
6. 5.03v
7. 0.0048v
8. 14.25v
9. 0.16v
10. 0.16v
11. 14.25v
12. 14.25v
13. 5.03v
14. 5.03v
15. 5.03v
16. 0.00v

IC801 - Pin 9

An externally hosted image should be here but it was not working when we last tested it.

IC801 - Pin 10

An externally hosted image should be here but it was not working when we last tested it.

Q808 - Drain - all 4 inverters seem to match.

An externally hosted image should be here but it was not working when we last tested it.

These are the measurements for the right channel which seems to shut down

IC802
1. 0.00v
2. 0.89v
3. 0.08v
4. 0.33v
5. 1.59v
6. 3.81v
7. 0.0047v
8. 14.25v
9. 6.95v
10. 5.40v
11. 14.26v
12. 14.26v
13. 5.03v
14. 5.03v
15. 5.03v
16. 0.00v

IC802 - (Right channel) Pin 9

An externally hosted image should be here but it was not working when we last tested it.

IC802 - Pin 10

An externally hosted image should be here but it was not working when we last tested it.

Q809 - Drain - I will check the levels on the others again to be sure but believe they matched. There really is that big an offset.

An externally hosted image should be here but it was not working when we last tested it.

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[Perry Babin]

The drive signals from pins 9 and 10 are not going to ground as they should.

Check R842 and R844. They should read 110 ohms.

If they're within tolerance, I think the FETs have failed.

 

[Kei]

R842 & R844 are definitely good. Just tested Q815 and gate to drain is essentially short. Drain to source with the bridging wire (G to S) is also short in both directions. I'll assume that the rest of the FET's in that half are also duff too.

I've got some new IRFZ44NPBF's coming in a day or two anyway as I need a full set for my MRV-F400 which is also in the "to be fixed" pile.

 

[Kei]

All tested and all definitely failed. No signs of hot running or anything to suggest a reason as to why they failed. Will have to check the outputs for this channel too.

 

[Kei]

How close do the gate resistors need to be to each other? The ones in the damaged side measure as follows:

R846 - 101.3
R848 - 98.9
R850 - 102.4
R852 - 101.2

 

[Perry Babin]

Whatever they're marked, likely ±5%.

 

[Kei]

Look like gold markings so 5%. The two PNP 2SA1265 tested good as did the two NPN 2SC3182.

Should I check the 494 without the 4 FETs in place or do you think it's Ok?

 

[Perry Babin]

It's so easy to check it, you may as well do it. Make sure that the output swings from ground to almost the voltage on pins 12 and 11 (about 1v below whichever is lowest).

 

[Kei]

DC voltages on IC802

Pin 9 = 5.75V
Pin 10 = 3.80V
Pin 11 = 14.45V
Pin 12 = 14.45V

Peak voltage of waveform (clean square wave)
Pin 9 = 13.3V
Pin 10 = 9.5V

IC801 shows 12.7V peak across both pins 9 & 10 with 14.4 on 11 & 12.

 

[Perry Babin]

If the two 110 ohm resistors are very nearly identical in resistance and the voltage on pin 8 is the same as pin 11, I'd replace the 494.

 

[Kei]

Yes, pins 8 & 11 measure identically (14.4V) and the 110 ohm resistors measure 109.6 and 109.9. I've got a TL494IN which is the higher temperature range part, will I need to replace the other channel to match?

 

[Perry Babin]

no

 

[Kei]

Much better now. I've got an even 13.2V across 9 & 10.

I'll put some new FETs into the left channel now and see how it goes.

 

[Kei]

New FETs fitted and channel still shuts down. I'm seeing the square wave at the gates of all four FETs and nothing at all on drain or source.

Pin 1 on IC802 is at 0V too.