Will have another go
OK guys. Decided not to give up!
I made matters worse yesterday by shorting one of the new transistors while testing it against the negative speaker terminal. Turned amp off before removing the probes but there was residual power left in caps. DC at 50V on speaker terminal now and other channel up to 350mv from 78mv.
I think I tried to rush things as I had a day off yesterday and ended up having a hissy fit.
What I have learned is electronics repairs are an exercise in patience! I have decided to check each component systematically on the entire board before replacing anything.
OK guys. Decided not to give up!
I made matters worse yesterday by shorting one of the new transistors while testing it against the negative speaker terminal. Turned amp off before removing the probes but there was residual power left in caps. DC at 50V on speaker terminal now and other channel up to 350mv from 78mv.
I think I tried to rush things as I had a day off yesterday and ended up having a hissy fit.
What I have learned is electronics repairs are an exercise in patience! I have decided to check each component systematically on the entire board before replacing anything.
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Yes, that's great 
Quick passive checks of semiconductors (looking for shorts) is a good start, as is checking any low value resistors to make sure they have not failed open circuit.
If the amp is in a condition that safely powers up (always use the bulb tester) then the next most useful thing is to print the circuit out and pencil all the main voltages on the transistors.
Concentrate on just one channel, don't try and fix both together.
Quick passive checks of semiconductors (looking for shorts) is a good start, as is checking any low value resistors to make sure they have not failed open circuit.
If the amp is in a condition that safely powers up (always use the bulb tester) then the next most useful thing is to print the circuit out and pencil all the main voltages on the transistors.
Concentrate on just one channel, don't try and fix both together.
Hi.
Tested all the resistors and diodes. I had put in some incorrect values when I replaced the blown resistors so will have to put on parts list! Got my K's mixed up!
Any ideas why I cannot adjust bias on the "good" right hand channel. Trim pot makes no adjustment. It stays at 0MV on the Test Points. Tested trimpot and varies from 0 to 1K ohm. Could it be one of the smaller transistors I have not tested yet?
Tested all the resistors and diodes. I had put in some incorrect values when I replaced the blown resistors so will have to put on parts list! Got my K's mixed up!
- Diodes OK but D609 is not blocking and does not get to OL. Others are (D10 - 12) "slow" so may need to replace.
- Tested Q629/ 630; Q633/634 and Q625 / 626
- Will test all the power transistors and smaller transistors when I have time.
- Unit still powers on OK via Bulb tester.
Any ideas why I cannot adjust bias on the "good" right hand channel. Trim pot makes no adjustment. It stays at 0MV on the Test Points. Tested trimpot and varies from 0 to 1K ohm. Could it be one of the smaller transistors I have not tested yet?
- DC on right now back to 78mv after reinstalling correct resistor> changed in error on Monday at frustration stage.! I have lost the other original so will need to find replacement.
- DC on left is 48V but have not replaced as Q617 as it keeps blowing!
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That "minimalist" stuff from Rotel, and the like, is not trivial to repair. You have to be right in the 1st attempt or something smokes back. There is not a single protection mechanism in the whole circuit, except the fuses. Japanese Kamikaze Electronics. Either you are an expert already, or you have to check the circuit component by component. Diode test for transistors, in case of doubt removing and testing also hFE, a factor of doubt is too dissimilar Vbe or Vbc between transistors of same model (plus this circuit requires matched input transistor pairs, ce and ec shall read open). ESR or capacitance test for medium/small electrolytics (dry C607/608/609/610 can cause the circuit to become unstable, oscillating, and stress VAS transistors).
In this case it is clear that the VAS transistors (the ones with 100ohms from emitter to +48V and -48V) are not OK, and probably one of them was the start of the chain effect of destruction. The bias setting transistor (Vbe multiplier) and potentiometer complain of too much current and voltage, smoking.
Form a quick look at the circuit, the VAS can pass up to 40mA before it runs out of drive from input Long Tailed Pair. 40mA times 48+48V are 3.84W, this is surely enough to damage a TO-92 part. I don't think 40mA are enough to cause the Vbe multiplier and pot to smoke.
Concerning input stage and VAS: Sometimes a damaged small signal transistor does not get shorted into low impedance, it gets semishorted, passing more or less current than commanded through the base-emitter junction (or it can also get noisy or variations in Vbe upon overload).
My recommendation is to use something more sophisticated to power the amplifier, like a combination of a 40W bulb in series and another 100W or 60W in parallel (incandescent bulbs in series unbalance very easily, as filament resistance has high positive temperature coefficient), a variac is the tool, a variable transformer often used for that purpose, but it can also be a spare fixed transformer producing 1/3 or 1/4 or 1/5 mains voltage, or a bench supply injecting +/-15V in bulk capacitors. The bias scheme employed for input stage (R621, R622) allows the amplifier to be tested at very low voltage, like +/-15V instead of +/-48V.
The technique of the single series bulb can prevent output devices to fail but the smaller transistors can still smoke, in Japanese Kamikaze Circuits.
In this case it is clear that the VAS transistors (the ones with 100ohms from emitter to +48V and -48V) are not OK, and probably one of them was the start of the chain effect of destruction. The bias setting transistor (Vbe multiplier) and potentiometer complain of too much current and voltage, smoking.
Form a quick look at the circuit, the VAS can pass up to 40mA before it runs out of drive from input Long Tailed Pair. 40mA times 48+48V are 3.84W, this is surely enough to damage a TO-92 part. I don't think 40mA are enough to cause the Vbe multiplier and pot to smoke.
Concerning input stage and VAS: Sometimes a damaged small signal transistor does not get shorted into low impedance, it gets semishorted, passing more or less current than commanded through the base-emitter junction (or it can also get noisy or variations in Vbe upon overload).
My recommendation is to use something more sophisticated to power the amplifier, like a combination of a 40W bulb in series and another 100W or 60W in parallel (incandescent bulbs in series unbalance very easily, as filament resistance has high positive temperature coefficient), a variac is the tool, a variable transformer often used for that purpose, but it can also be a spare fixed transformer producing 1/3 or 1/4 or 1/5 mains voltage, or a bench supply injecting +/-15V in bulk capacitors. The bias scheme employed for input stage (R621, R622) allows the amplifier to be tested at very low voltage, like +/-15V instead of +/-48V.
The technique of the single series bulb can prevent output devices to fail but the smaller transistors can still smoke, in Japanese Kamikaze Circuits.
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You do need to be very sure you have the correct value parts in all the right locations. Its also important to make sure you haven't mixed any NPN devices for PNP and vice versa.
Non adjustable bias on the right channel... the first thing is to put your meter across Q618 and see what the voltage varies between as you turn the preset.
You need to overcome all the 'base-emitter' volt drops of the predrivers, drivers and outputs before the output stage draws current which means somewhere around 3.5 to 4 volts is needed.
Q617 burning on the left channel can only be because of failed or incorrect/incorrectly fitted pre drivers. There is no other possible low impedance path present that could deliver a damaging current.
It is possible however that the driver and outputs could be faulty/incorrect/incorrectly fitted and that these have caused failure of the pre drivers.
In the last Rotel I repaired I had to repair twice due to a probe slip while adjusting bias, it blew the full output stage due to a few dozen milliseconds of probe slip.
To be honest, and up to date, my latest work gives an error message through RS-232 interface when an output device is shorted or open, it is microsecond fast so the opposite transistor does not blow.
To be honest, and up to date, my latest work gives an error message through RS-232 interface when an output device is shorted or open, it is microsecond fast so the opposite transistor does not blow.
Are there up to date replacements for 2SA1208 + 2SC2910?
Actually I have ordered a cheap transistor tester with a display which I saw on the blog link below for £5 (not recieved yet). I am now testing all the smaller transistors by taking them off the board but doing it slowly! It is a pain as you have to look up each one to check what the pins are.
ariejan de vroom - Repair: Rotel RB-970BX
Are there up to date replacements for 2SA1208 + 2SC2910?
I found an equivalent for 2SA1208 on RS components but not for 2SC2910
Actually I have ordered a cheap transistor tester with a display which I saw on the blog link below for £5 (not recieved yet). I am now testing all the smaller transistors by taking them off the board but doing it slowly! It is a pain as you have to look up each one to check what the pins are.
ariejan de vroom - Repair: Rotel RB-970BX
Are there up to date replacements for 2SA1208 + 2SC2910?
I found an equivalent for 2SA1208 on RS components but not for 2SC2910
The best thing to substitute them with is 2SA1209/2SC2911 which is the same transistor but in a TO-126 package (like the driver transistors are). These will still be difficult to source however as they are out of production.
Unfortunately there are no easy substitutes. Don't be tempted to buy the parts from eBay - it is highly likely you will get fake parts.
The parts I would use for replacement are KSA1220/KSC2690, but this would be getting into the realm of an "expert" repair. The transistors used here are for the VAS stage of the amplifier, and this is critical - substituting different devices isn't something I'd recommend here without experience and test gear such as an oscilloscope to check for instability/oscillation.
Cross your fingers and hope that the 2SA1208/2SC2910 are still ok
Replacements for 2SA1208 + 2SC2910?
Would these work?
KSA992/KSC1845 TO92
Mentioned here: Cyrus 2 Amplifier - 2SC1775A + 2SA872A Equivalent/Replacements?
Companies like Littlediode stock 2SA1208 + 2SC2910 but pricey. I have used them before to get an AN7062 IC for a Technics amp and it has worked.
2SC2910 on both sides has failed - I think. Will double check and still need to check all the other small transistors.
Would these work?
KSA992/KSC1845 TO92
Mentioned here: Cyrus 2 Amplifier - 2SC1775A + 2SA872A Equivalent/Replacements?
Companies like Littlediode stock 2SA1208 + 2SC2910 but pricey. I have used them before to get an AN7062 IC for a Technics amp and it has worked.
2SC2910 on both sides has failed - I think. Will double check and still need to check all the other small transistors.
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