Hifi amp signal flow question

Divad123

Member
2019-09-08 12:57 pm
Hi all,


Trying to do some fault finding on a Rotel RA930 AX.



Would it be true to say that in terms of the input signal flow - the eg CD phono 'in' signal would go to the front panel control button/switches/knobs circuitry first, from there into the pre-amp area, then on to the power amp section and from there on to the speaker outs ?
Or would it be CD phono 'in' > to pre-amp > then to front controls section > then to power amp > then to speaker out ?


I have a good quality copy circuit diagram but it doesn't help with the above



Any help gratefully appreciated
 

Divad123

Member
2019-09-08 12:57 pm
Hi chaps,
Thank you for the prompt replys.

So, I switch the amp on via variac which I left at 22v as I forgot to plug in speakers. I plugged in both left spkr wires, no hum - plugged in right black spkr wire no hum, but when plugged in right red spkr wire I got hum which increased/decreased with variac level (no music at spkr outs)

I had CD playing music through the phono connections so I tried the headphones - the left channel was playing (but distrorted when I increased the volume) - no sound from the right


Volume control has no effect on hum. Comparing with online tone generator, hum from unit is at 100hz



If it helps, here are the DC readings at the spkr outs:
Left black : 0
Left red : .0083mv
Right black : 0
Right red : 1.59v


I am getting voltages into board from transformer but no idea if they're correct as I don't want to push too much power into the unit from variac and burn something out. The hum at 22v is disconcerting enough lol.


So, I hope the above may give you experienced guys a few clues. I have variac, isolation transformer, little audio probe thing, DMM, an oscilloscope which I'm just reading the instructions of, 100w dummy load, so I've got some useful stuff - so it's just everything else I need to learn lol


Thanks
 

rayma

Member
2011-04-29 8:37 pm
First check the output fuses, which are inside the feedback loop (probably on the board).
If blown, there will be poor or no sound. Once these are good, the left channel may work ok.

Now check all the semis in the bad channel(s) of the power amplifier section, Q601-Q622,
with your meter's diode function. Also check the emitter resistors. 0.22 ohm, two per channel.
 

adason

Member
Paid Member
2004-11-10 8:31 pm
Maryland
Hi chaps,
Thank you for the prompt replys.



I had CD playing music through the phono connections so I tried the headphones - the left channel was playing (but distrorted when I increased the volume) - no sound from the right




Thanks

When you put cd player signal into phono input, you are overloading the rria phono section, it will always sound distorted. CD signal can be up to 2V, phono is typically expected ~5mV. It will distort even on fully functioning gear. Use tape input.
 

Divad123

Member
2019-09-08 12:57 pm
Hi Rayma


I put the fuses back in after checking them and left it powered off for 2 hours (I didn't have it powered up during the first set of checks either). I did a repeat check of the transistors and they all came up virtually the same as before.


I checked the last two transistors in the circuit q901 and q902 and here are the results :
q901 (C2362) B to C .677 and B to E .712
q902 (A1016) B to C .688 and B to E .714


Cheers
 

Divad123

Member
2019-09-08 12:57 pm
Sorry for being a bit unsure - do you mean the 2 op amp IC's 401 and 501 ?

I'm really not sure how to test them without power.

On another amp I have I can power it up to the required voltage level and that lets me measure/compare voltages of the IC on the PCB with the amp's circuit diagram using my DVM, but to be honest, I'm a bit lost trying to test an 8 leg IC with no power going through it - could you give me a clue please ?
 

rayma

Member
2011-04-29 8:37 pm
I'm really not sure how to test them without power.

The preamp IC won't be a problem. The problem will be in the power amp (after the tone controls).
You check each transistor in the power amp with the meter's diode test function.
Check B-E and B-C as diodes (in both polarities of the test leads) for each transistor.
Like a diode, one way will conduct and beep, the other will be open circuit.
Also check the C-E for each, which should be open either way. Some readings can be affected by
being connected in circuit, so just write down what those are and compare with the other amp.
 
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Divad123

Member
2019-09-08 12:57 pm
Sorry - I checked all 24 transistors earlier, thought I'd posted results but pressed the wrong button - I didn't check to see if it had posted, it didn't - what an idiot. luckily I wrote them all down, so here they are :

q601 to q606 transistors, the results B to C ranged from .688 to .695 B to E ranged from .715 to .719

q607 was B to C .665 and B to E .178 ***
q608 was B to C .667 and B to E .177 ***

q609 to q612 ranged from B to C .644 to .687 and B to E .648 to .693

q613 to q615 ranged from B to C .735 to .741 and B to E .735 to .742

q616 B to C was .003 and B to E was .633 ***

q617 to q618 ranged from B to C .641 to .643 and B to E .647 to .648

Lge transistors on heatsink :
q619 B to C .562 and B to E .564
q620 B to C .502 and B to E .364
q621 B to C .505 and B to E .362
q622 B to C .505 and B to E .362

The very low results I double checked, they were correct. Checked in both polarities but my DVM doesn't bleep in diode check mode - only get beep in continuity check mode.

Cant really check/compare with the other amp I mentioned I don't think, as it's a Pioneer not a Rotel and 10 yrs younger

Hope the above info gives you a better idea, now I have actually been successful in posting them this time
 

Divad123

Member
2019-09-08 12:57 pm
Hi Boydk - yeah, the results with asterisks beside them have been triple checked and are correct



The emitter resistor results are :
R629 = .28ohm R630 2.8ohm*** R631 .28 ohm R632 .25ohm


Rayma - checked all transistors C to E in continuity mode. Got 'OL' readout on DVM and no bleeps
(wasn't sure if I should have checked C to E in diode mode...or what results to expect if I did)
 

rayma

Member
2011-04-29 8:37 pm
checked all transistors C to E in continuity mode. Got 'OL' readout on DVM and no bleeps

Ok, for now we can assume the semis are ok. There are two diodes in each power amp channel,
check those in the same way. If they are normal, start checking all the resistors in the power amp.
Some may read off because of being in-circuit, but compare the readings with the other channel.
 
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