Rotel RA-1412 strange preamp problem

[ViperZ]

I'll start documenting repair of my Rotel RA-1412 integrated amplifier. Sometimes it would just kick into protection randomly.

Settings that I am measuring at: all tone controls are in 'out' position. AUX input is selected.

I checked voltages on both power supplies, and all voltages are present.

When amp and preamp sections are disengaged from each other, DC offset on both channels does not exceed 20mV, with no input signal.

When I connect preamp to amp sections, DC offset would randomly jump to different values for a very short time. I have seen it as high as 1.2-1.4 VDC, and protection did not engage (too slow?). I monitored each channel for a few minutes. Protection does catch up eventually, maybe to higher or maybe to longer spikes.

The strange part is that it is happening on both channels, so the cause must be something common to both channels...

I assume the problem originates from tone control amp board, not from phono and mic board, since Aux input is selected.

I'm not sure what could cause such problem in both channels. I'll start monitoring supply voltages to that board tomorrow.

If anyone has seen such problem on Rotel and can point me in the right direction, I'd really appreciate it. Thank you.

 

[Oldsansui441]

Hi, sounds like a good possibility that the 2SC1222's and 2SA750's are starting to break down in the tone/pre amp on both channels. KSA992's and KSC1845's will do fine here.

 

[ViperZ]

Still no time to dig into it properly. Looks like the tone board only has two power supplies - 17.4V (b1 and b2 connections) and -12V (-b2 connection point). I'll put a scope on those supplies to monitor them for spikes for several minutes. If I don't see any, then I will start running scope through the channels to see in which section of the board these spikes originate or get injected... Divide and conquer...

 

[ViperZ]

Don't see any spikes on each of four power supply pins, using oscilloscope to monitor with scope channel set on AC.
The receiver has been playing for hours for a couple of days. Spikes appear every several minutes, but way too fast for protection to kick in, and to hear them in the speakers.

I'm honestly not sure what part common to both channels could cause such strange behaviour.

 

[Ylli]

You are seeing them on the scope but do not hear them in the speakers? Dual channel scope? Are these 'spikes' occurring on both channels simultaneously?

 

[slimecity]

if theres a transistor input differential pair at the front end of your power amp section - I'd look at that first.

 

[ViperZ]

You are seeing them on the scope but do not hear them in the speakers? Dual channel scope? Are these 'spikes' occurring on both channels simultaneously?

I know it happens on both channels, but I have not checked if it happens on both at the same time. My Tek is 4-channel, so I can check that.

if theres a transistor input differential pair at the front end of your power amp section - I'd look at that first.

Why? Look at what?

 

[ViperZ]

Still poking in the dark on this one... Took the receiver apart, now I can access preamp tone control board in front of the receiver. All soldering looks good on the board. Replaced all 22 electrolytic capacitors while I was there.

 

[ViperZ]

OK, so only a year later... Replaced two NPN transistors at the output of each preamp channel. 2SC1222 with KSC1845. Also replaced a couple of 220K carbon resistors on one of those transistor (but they were spot on anyway). No more multi-volt spikes, protection not triggering (been playing for hours) but I still see DC offset floating +/-30mV with absolute max that I saw was 80mV (source connected to input, but turned off). Replaced every single transistor in both channels (2SC1222 with KSC1845, 2SA750 with KSA992). The only thing I found is that gain of two transistors in one differential pair was way off (~550 vs 350), but in other they were well matched (444 and 439). Replaced them with matching pairs. Still the same issue.

 

[ViperZ]

Preamp tone board gets +17.4V and -12V power supplies. I can see tiny spikes on the supplies corresponding to the big spikes on the outputs. Replaced electrolytic capacitors (except filters) on both power supply boards - no change. Disconnected +17.4V and -12V rails from the tone board, so now preamp tone board is turned off. Obviously no spikes on the outputs, and no spikes on each power supply. Back to the tone preamp board - all new transistors and all new electrolytics... I am really grasping for straws at this point.

 

[Oldsansui441]

Back to the tone preamp board - all new transistors and all new electrolytics.

Hi ViperZ, as l mentioned earlier l would be going ahead and replacing the transistors on the tone/pre amp as you have also mentioned today. I have had amplifiers with the exact same spikes through the main amp actually being caused by faulty transistors on the tone /pre.

 

[ViperZ]

Hi ViperZ, as l mentioned earlier l would be going ahead and replacing the transistors on the tone/pre amp as you have also mentioned today. I have had amplifiers with the exact same spikes through the main amp actually being caused by faulty transistors on the tone /pre.

Hi, I replaced all transistors on that board a couple of days ago with KSA992 and KSC1845, but still have the same issue. The damn spiking (obviously with much lower amplitude than on amp outputs) seems to be all over the tone/pre board. I can even see the spikes on the +/- power supply feeds. Disconnecting the power supplies from the board does not help - power supplies feeds lose the spikes, DC offset goes back to solid 0 (since tone/pre board is off). So all transistors and all electrolytics on tone/pre are brand new now. I did think that replacing those transistors lowered the spikes, but it did not. The amp played for a couple of days with DC offset floating +/- 80mV, and then this morning I saw a couple of major spikes that triggered protection.

Maybe the phono/mic board is contributing somehow, since there are feeds from mic amp all the way to the outputs of tone/pre board. I do realize that the outputs of mic circuit should be disconnected when the mic selector is pushed in, but I don't know what else to do. Pulled +/- to mic circuit (+17.5 / -10.9), did not help. I am thinking about actually disconnecting 2 wires that go from mic circuit to the tone/pre board's outputs for mixing there.

 

[Champco]

Wire wrap connectors are suspect to oxidation and will cause noise.
I would loosen them and pull them off. Deoxit all the surfaces with gusto on the pins with q-tips wet with Deoxit. Maybe a tooth pic on the wire wraps. That is what I would do as I have done many many wire wrap connections. There is always a solder option if you could pick the offending pin.

 

[Oldsansui441]

Hi, I replaced all transistors on that board a couple of days ago with KSA992 and KSC1845, but still have the same issue

Fair enough, sorry l didn't realise you had already done this.

I can even see the spikes on the +/- power supply feeds. Disconnecting the power supplies from the board does not help - power supplies feeds lose the spikes, DC offset goes back to solid 0 (since tone/pre board is off)

It's very strange that you can see the spikes on the power supplies to the pre amp and yet after disconnecting them the problem persists or am l misreading this? I guess you have already tried, but does this same spike through pre amp still happen when the input selector is in a position other than Aux?

 

[ViperZ]

It's very strange that you can see the spikes on the power supplies to the pre amp and yet after disconnecting them the problem persists or am l misreading this? I guess you have already tried, but does this same spike through pre amp still happen when the input selector is in a position other than Aux?

No, no spikes on the power supply or on the amplifier output when power supply is disconnected from tone/preamp board.

Interesting that Champco mentioned wirewraps - it looks like all wirewraps are factory finished with solder... at least on the power supply board. They all look solid and well made on preamp too. I will check the ones on tone/preamp board today while disconnecting mic/phono board from tone/preamp.

 

[Oldsansui441]

No, no spikes on the power supply or on the amplifier output when power supply is disconnected from tone/preamp board.

Aah l see, sounds like you have pinpointed the instability to the tone/pre amp then. Is it possible the grounds for the tone/pre are questionable?

with absolute max that I saw was 80mV (source connected to input, but turned off)

I see the source was turned off so one would assume that it was having no influence on the Rotel, I am also guessing you have done other monitoring with no source connected at all just to be sure?

 

[hamrules]

Hope you get it sorted out, sounds like your close.

 

[ViperZ]

Aah l see, sounds like you have pinpointed the instability to the tone/pre amp then. Is it possible the grounds for the tone/pre are questionable?

I see the source was turned off so one would assume that it was having no influence on the Rotel, I am also guessing you have done other monitoring with no source connected at all just to be sure?

I will look at quality of ground connections.. Yes, no difference if line source is connected or disconnected. I keep playing with aux1/aux2 inputs just so that I don't introduce extra circuitry into the equation (i.e. phono)

 

[Oldsansui441]

Yes, no difference if line source is connected or disconnected.

Fair enough, l suspected as much.

I keep playing with aux1/aux2 inputs just so that I don't introduce extra circuitry into the equation (i.e. phono)

Good call, surely it can only be in that darn tone/pre area.

 

[ViperZ]

Every time I get back to this integrated, it just gets more depressing. I have the scope on DC sitting on outputs of tone/pre board, and I can see these several mV random spikes on each channel.
All inputs are off. Mic input is off, which is supposed to disconnect mic board from tone/pre board. Amp is separated from tone/pre.
It has to be this board. Every single transistor and every single electrolytic capacitor is new on the board by now. Several resistors swapped too.
I can see these spikes on power supply pins on this board too. Disconnecting power from this board cleans up spikes on the power supply side.

I truly have no clue what to do next. What can be common to both channels on this board outside of power supply and cause these damn DC spikes.