Help needed on testing a Rotel RX-400

[woostersoz]

I purchased a Rotel RX-400 receiver - when I plug it in, nothing happens, no lights, no output. So, I lifted the covers and started poking around with my DMM. What I have found so far :

(a) The power cord is fine because the 120 VAC is coming through into the power supply

(b) The transformer looks OK because I measured the output and found it in the range of 12 VAC. I am assuming this is OK.

Now, I am stuck - I have no idea what to look at next. The AC fuses are fine (measured the resistance across each of them and they are not open).

I have the schematic that I am happy to email to anyone who volunteers to help me understand what items to test next

Thanks

 

[Fred Longworth]

The primary of the transformer, as measured from the line cord pins -- looking through the power switch and any fuses -- should measure somewhere around 1 ohm.

* * * * *

The AC voltage in the secondary circuit should be a good deal higher than 12vac. That's looks like a low voltage line to power lamps. Look for a high-voltage secondary with 50-70vac -- or more -- to run the power amps.

Fred

 

[woostersoz]

Reviving old thread on Rotel RX-400 receiver

I finally managed to revive my desire to fix a Rotel RX-400 receiver that I had bought.

The bias on the left speaker is about 65mV DC (as measured through my DMM) which I believe indicates that something is wrong with this channel.

Also when I plug in headphones, I can hear the right channel but the left is dead.

The schematic PDF I have is too big in filesize to be uploaded here. Happy to email it anyone who wants - can anyone please provide pointers on where I should start the troubleshooting?

thanks

 

[cademan]

I would start with the output transistors for the dead channel.

 

[petehall347]

there should be according to the manual a speaker fuse for each channel . check these first .
also did you remove the fuses you tested ? you need to remove fuses for testing them properly .
if yours is the csa model it will have 2 speaker fuses for each channel . 2 on the outside at 2.5 amps .and 2 on the inside at 3 amps
if all speaker fuses are good check ac volts at the fuses f901 f902 f903 ..all these ac volts reference chassis ground

 

[woostersoz]

Thanks.. all the fuses tested OK. The AC volts at F901 and F902 are 18.5V whereas it is 11.8V at F903.

Does this give any indication of what may be wrong?

 

[petehall347]

those voltages look about right to me .
the power light should be on with voltage at f903 .
f901 f902 are power for the amplifier section .
being as there is one channel working there could be a break in the power going to the dead side .
could you get voltages for q613 q614 q615 q616 base collector emitter . ..be very careful not to short anything out .tape your meter lead so only a tiny bit is showing at the tip .

 

[woostersoz]

Sorry, despite my best efforts, I managed to short something.. Had a mini fireworks display in my garage

So I am afraid I am going to look for someone in the SF Bay Area who can help me repair this and a NAD 7220PE that I have

 

[petehall347]

oh no sorry to hear that . hopefully just blew a fuse .

 

[petehall347]

can you remember exactly what you shorted ?

 

[woostersoz]

can you remember exactly what you shorted ?

I don't remember that. When I opened it up, I found that the 3 internal fuses (F901-903 rated 4A) were blown.. So I replaced them and as soon as I switch on the power F901 and F902 are blowing up. This has happened twice so need to figure out why they are blowing up as soon as the power is switched on.

thanks

 

[petehall347]

looking like possibly output transistors shorted plus maybe more things like driver transistors and resistors .

 

[woostersoz]

looking like possibly output transistors shorted plus maybe more things like driver transistors and resistors .

Thanks.. i need to go back to basics - how do I go about testing the output transistors? Pls let me know if these are questions I shouldn't be asking

 

[petehall347]

always ask if not sure .
you need to remove the power first . like make sure its not plugged in .
the output transistors will be on a large heat sink .
set meter on low ohms like 200 .
test for shorts between the pins . its pretty likely it will be just one channel at fault .
there are 2 outputs per channel . all 2sc1826 ..

 

[woostersoz]

always ask if not sure .
you need to remove the power first . like make sure its not plugged in .
the output transistors will be on a large heat sink .
set meter on low ohms like 200 .
test for shorts between the pins . its pretty likely it will be just one channel at fault .
there are 2 outputs per channel . all 2sc1826 ..

Thanks. looks like Q614 and Q616 are shorted (both of them don't show an open circuit when +ve multi-meter on Base and -ve on either C or E). If my understanding is correct, do I need to replace these? the space next to the heat sink looks very tight so not sure how I can go about doing this

 

[petehall347]

c to e is the way they generally go short . maybe you have to desolder all transistors on heat sink then undo and remove heat sink complete with transistors . or maybe first remove whatever is in the way of the screws that hold the transistors .

 

[woostersoz]

I managed to remove the 2 transistors and have ordered replacements. Is there any other testing I can do while waiting for them to show up?
thanks

 

[mbz]

need to go back to basics - how do I go about testing the output transistors?

Nice graphic from markthefizer, different colours refer to probe orientation. Use diode function available on most multimeters, nominally expect 650mV, otherwise
use resistance as per above.

looks like Q614 and Q616 are shorted (both of them don't show an open circuit when +ve multi-meter on Base and -ve on either C or E)

Q614,6 are NPN. Norminal 650mV (not OL) is expected with +ve probe on base and -ve
probe on either collector or emitter.

Is there any other testing I can do while waiting for them to show up?

Maybe build yourself a dim bulb tester, search AK.

Suggest retesting Q614,6 using above graphic. However, please note a transistor can pass diode testing and still be faulty, it's not an absolute test method.
Sometime a transistor needs a "few" volts to fail. Certainly, if a transistor fails diode test then it is faulty.

 

[woostersoz]

thanks much. I removed all 4 transistors and checked them (found that my DMM had a diode test function) and Q614 is the one that's faulty according to the above graphic. The other 3 seem to be OK.

Will check out the dim bulb tester

 

[mbz]

Bit unusual for one output to fail solo. Usual fault scenario is people fiddling with spkr cables while amp powered up, short, both outputs toast, maybe drivers.
Q614 was probably working hard before it failed, take a look at Q610, incircuit diode test. Maybe also check R624, again in circuit measurement, make sure not open.