Toastie Rotel RA-840BX4

LektroiD

Member
2014-09-29 3:55 pm
Just got this amp, noticed one of the power resistors on the output stage was toast. In the schematic it suggests 2W, so to make sure it didn't happen again, I ordered some 5W replacements, installed them, recalibrated as per service manual, but after about an hour of use, I noticed the amp could fry an egg on itself, I have nothing above the amp, so it has free airflow all around it, yet it is ridiculously hot.

Service manual suggests I should wait 5 minutes, then calibrate for 2.5mV between TP3&1 / TP4&2 and 0V between 1&E1 / 2&E2. I followed this, but the voltage continued to increase between test points, so I left it warming for about 30 minutes until it stabilised, then calibrated. Now after an hour of use it is working more like a heat generator than an amplifier. Sound doesn't appear to be affected in any way.

Possibly not related, but the mono switch only seems to work on initial power up when the amplifier is cool
 
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LektroiD

Member
2014-09-29 3:55 pm
My guess is DC on the speaker terminal exceeds the limit of 0volts.
I'll check as soon as I'm free from the weekly constraints of work and get back to you...

I wonder which resistor failed, you don't say.
Apologies, R642 was total toast, although all four 1K 2W resistors (R641-644) showed signs of heat exhaustion, I sustituted them with it with Bourns 1K 5W, although they are also heating up way beyond expectation.
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
Always remove the speakers and input signal before testing voltages at the output stage.

Surely, the readings are more like 0.008V (=8mV) and 0.001V (=1mV). I doubt any handheld 3₁⁄₂ digit meter could display anything less than 0.1mV anyway.

1 0r 8mV is vanishingly small and fine for DC offset. When it rises above 0.1V, we become concerned a little, 1V quite a lot and at 10V we panic if speakers were connected when in that condition.
 
If memory serves the specific amp when comes to service it has the exact picture you describe
A lot of heat around those resistors
reverse engineer the schematic to see how much voltage drop is supposed to be on those resistors and then you will know ...
I think that for the specific amp Rotel ppl coocked the driver a bit too much which is fine with me as long those play well
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW

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LektroiD

Member
2014-09-29 3:55 pm
I think I have found the issue, this model was bought secondhand, I spotted the European voltage on the badge at the back (220V), I live in the UK (240V), so I'm getting a higher secondary output from the transformer, thus over powering the components.


According to the schematic, this is the type C power circuit - without the voltage selector switch! It appears the transformer has the ability to run at 3 primary settings, but the schematic gives no indication which coloured wire referrs to which voltage. Inside the unit, Blue is wired to the power switch; this must be the 220V tap (as indicated on the badge), leaving the Orange and Brown floating.

Part number of the toroidal is T-1018GF, although Google couldn't seem to find any info on it. Anyone have any info on these toroidals?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
That ~10% difference in mains voltage isn't the root cause of the problem, it just exacerbates it. The problem of the toasted resistors also exists in amplifiers wired correctly for the local mains, as Sakis comments. Replace the resistors as sgrossklass suggests as well as correct the transformer primary winding connection. (and maintain a decent air space between the PCB and resistors ;))
 

LektroiD

Member
2014-09-29 3:55 pm
You measure the primary side resistance, the higher resistance than current 220v terminal will be the 240v

I'd prefer not to strip back all the live cables in the unit to test, so I checked the schematic for my RA-870BX, it's a different toroidal, but it is labelled brown = 240V, blue = 220V, orange = 120V. Usually companies incorporate a standard wiring colour code, so to make double sure, I downloaded the schematic for the RA-840BX3 (same line, previous model), same toroidal, and brown = 240V, blue = 220V, orange = 120V.



I desoldered the blue wire and wired the brown to the switch PCB, I also desoldered and stripped down the broken mono/tone defeat switch, which was full of carbon, and gave that a thorough clean.


I've powered the amp up and gave it some time to warm up, it does still exhibit heat, although I need to re-calibrate now that it's at the correct voltage...
 
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LektroiD

Member
2014-09-29 3:55 pm
Replace the resistors as sgrossklass suggests as well as correct the transformer primary winding connection. (and maintain a decent air space between the PCB and resistors ;))


Replace the resistors again? With what? I've already put 5W in (as I mentioned in my initial post). Naturally, I kept the full length of the legs to maintain the air gap. Most of the heat appears to come from the small transistor heat sinks surrounding the resistors, although the resistors are also hot.
 
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It's simply a poor design choice for the output stage. Instead of those resistors, remove them and connect the emitters of the driver transistors (Q623, Q625 in the shown diagram) together with a 330 ohm 2W resistor. Re-set the bias accordingly. Bypass the resistor with a 470nF film capacitor if you're feeling fancy.
 

LektroiD

Member
2014-09-29 3:55 pm
I believe a similar issue and possible solution (heat reduction) is described here (Electronics knowledge will be required).

Good Luck!
Thanks for the link, I'll check it out as soon as I've repaired another issue...

While I was recalibrating, my probe managed to push and short one of the pins of R647 against the exposed brass fin next to it carrying -41V (another poor design choice). A pop, a spark and a small amount of magic smoke escaped. There is no visible damage, but Q627 and Q631 directly in the path are likely to have had a load of negative voltage thrown at them. I've sourced replacements on ebay, hopefully they popped and opened circuit before anything else was damaged.

In other news, the right channel has calibrated perfectly :)
 
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Karl vd Berg

Member
2012-07-16 12:49 am
It's simply a poor design choice for the output stage. Instead of those resistors, remove them and connect the emitters of the driver transistors (Q623, Q625 in the shown diagram) together with a 330 ohm 2W resistor. Re-set the bias accordingly. Bypass the resistor with a 470nF film capacitor if you're feeling fancy.
Hi Jaycee,

Would it be something like this (green circled)? And maybe the stock 2SB631/2SD600 (Ic 1A) drivers could be replaced with Toshiba's TTA/TTC004B (Ic 1.5A)?

output-stg.jpg