Alpine MRV-T707

[pippolonia]

Hi guys,

first of all sorry for my (bad) english ... I'm italian and i studied it at school a long time ago.

Now i'm repairing an Alpine MRV-T707; the damage was very easy to find (one of the right channel FET in short circuit), but now i have some questions about trimmers on board.

1) I set VR701 to have + (and - too) 35 V: is it right ?
2) How do j set the BIAS trimmers (VR401 - VR402)?

I don't have any diagram or technical reference (berissogl@tiscali.it)

Thanks for everything in advance

Giovanni

 

[Perry Babin]

The regulator should be set to 34v.

For VR401 and the probes across test points 3 and 4, set the voltage to 0.001v. Do the same for VR402 and TPs 5 and 6.

The bias can be set as shown in the demo below when you don't have the manufacturer's specs.
http://www.bcae1.com/temp/ausettingbias.swf

 

[pippolonia]

Hi Perry, thanks.

I had just finished the adjustment that you suggested and I made a mess !!!

Now with VR401 fully CCW the input current is 2,5 A (too high) and rises quickly just touch the trimmer. Probably FETs are broken ...

That's enough for tonight !

Thanks again

Giovanni

 

[Perry Babin]

Have you confirmed that the FETs are shorted?

 

[pippolonia]

No Perry, apparently everything is ok (no low resistance between gate and drain/source, only internal diode between drain and source): components are (almost) new !

But I don't understand what mess i do: two days ago the channel worked fine, yesterday no .. Why ?!?

Thinking well, probably I turn on amplifier with PCB on the table (with some residues of copper wire on it) and i make a short. I'm not sure ... if was like this, this will be the first time ... But there is a first time for everything !!!

Today i'm in bed with the flu ... next day i'm going to do some measure in my lab (the garage !!!). Have you any suggestion ?

Thanks again

Giovanni

 

[Perry Babin]

The next time you try to adjust the bias, insert a 10 amp fuse in the B+ line to help protect the amp in case you accidentally set the bias too high.

 

[pippolonia]

Thanks for the answer, Perry.

I normally connect B+ with my multimeter set to DC AMPS, protected with a 10 Amps fuse inside it. Is it a good protection or is much better have another external fuse ?

I probably found the mess I've made. A zener diode signed as ZD401 (connected between -34 V and bias circuit) is open.

With my bionic eyes 🙂 it's signed 24 V ... Could you confirm that, please ?

Another electronic question. I don't have a 24 V zener ... Is it right if I connect two 12 V diodes in series ?

Last question. you talk about "test point 3, 4, 5, 6" ... Where are they? I haven't found any reference write on the board.

Thanks again for your help.

Giovanni

 

[Perry Babin]

I'm guessing that you're not using a Fluke meter since the fuses are about $12 each.

What points were you using to set the bias if you didn't know what the test points were?

 

[pippolonia]

Message understood Perry ! I'm using a FLUKE 75 multimeter, but I never change the internal fuse ... I've inserted another fuse to protect the circuit, 8 amps !!!!

Until now to set bias I simply measured B+ current as you suggest. But if there is a better way to do it I prefer .... I think thath measuring between two test point is more precise, or it's the same ?

Giovanni

 

[Perry Babin]

Either biasing method will work but if you're repairing very high-end equipment it's generally advisable to use the manufacturer's specs if they are available.

email me
babin_perry@yahoo.com

 

[pippolonia]

Hi guys,

last updates.

Zener ZD401 is not marked 24, but 2 A ... The voltage, read on the same diode of the working channel is 9,2 V. I changed the "open" diode with 9,1 V zener.

Found another zener broke. This is identified as ZD403, and this is marked 2 A too. Tester reads 2 V on the same diode on the good channel. I've changed it with a 2,7 zener (the lowest value in the lab) ... Now the bias trimmer (VR401) works, but it's very difficult to set properly.

The amplifier is working, but when temperature of output FETs raise, bias voltage raises too ...

Any suggestion ?

Thanks

Giovanni

 

[Perry Babin]

The 2.7v Zener is allowing too much current to flow through the constant current sources.

 

[pippolonia]

Message understood again Perry.

Yesterday I placed an order for electronic components and I'm waiting for it ...

Thanks

 

[pippolonia]

Last update. 2V zener replaced, but B+ current increase too fast even with small rotation of the bias trimmer.

Bias trimmer was too "long": 3,3 k instead of 1,5 K. Trimmer replaced, bias current set ...

The amplifier works well ... Sound is very "spacious". Really a nice amp.

Thanks Perry.

Tomorrow I'll start to repair another ALPINE: the MRV-1000 ...