Laney RB9 +HT rail dropping. Last ditch attempt!

[allante666]

Hi. last attempt before it goes in the skip!
Story so far:- RB9 head was given to me by another engineer who ran out of patience with it. After seemingly following him around the circuit board I can see why.

Ive attached the schematic, hopefully, so you can see if Im making sense.

Switched it on initially and my limiter lit up like a xmas tree. short outputs thinks I. Nope, all four outputs are new and ok. The low voltage rail (19v) works fine, the fan and all the front panel led's etc work as they should.
The high voltage (60v) is the problem. If you look at the circuit diagram you can see that the + and - parts of the output board are basically mirrored and I found that if I connect the -ve side I have no problem. Th eproblem arises when I connect the +ve side. So, as I couldnt measure a short to earth resistance wise I figured maybe TS12 and/or TS17 are being turned hard on and its dumping the + side supply to earth.
To test that theory I disconnected R33 and R47, one at a time to see if it switched off the transistors to agree with my theory. It did, with those disconnected the rail came back up. so, working backwards I reconnected those and disconnected R46, This also allowed the rail to run up ok.

Now Im stumped. Looking at the pcb it looks like the previous guy has been working on the same theory and also got nowhere. TS15 and TS16 both check out ok as does D16 and TS11.

Ive checked various other bits as well but this is the main story.

Maybe I'm missing something obvious or maybe my theory is totally wrong. Hopefully someone on here can shine a light?

In the meantime I have three or four Marshalls to do, why arent all amps so simple?? (yes, I know, there's Marshall's that defy engineer logic as well but thankfully there seems to be far fewer!)


TIA
Andy

[Mick Bailey]

Are you running the amp with no speaker load?

[allante666]

Yes, all knobs turned to zero. shouldn't be a problem for a solid state?

[MarkusBass]

To test that theory I disconnected R33 and R47, one at a time to see if it switched off the transistors to agree with my theory.

Disconnecting R33 and R47 is a very bad way to test any theory since they form a load for TS16 and by doing this you make TS16 floating (and you may get unspecified results). I think that Laney provided links LN20, LN19, LN22 and LN24 to be used in such cases (they disconnect collectors of TS12 and TS17). I would rather diconnect those links and verify whether the input stage of the amp is working correctly. BTW, you haven't mentioned checking any of the input transistors, or making any voltage measurements of the input transistors. Have you measured them? If I had such a problem (output transistors OK but big current through them), I would imediatelly check input transistors (since they are DC coupled). My suggestion is that you remove the links to collectors of TS12 and 17 and verify whether the input stage of the amp works correctly (of course without a load) - starting from DC voltages on transistors and if they are OK checking the input with a generator and oscilloscope.
Another reason for such a strange behaviour is that in some amps the ground connection for the input stage of the power amp is provided with a separate wire. Technicians often disconnect the input of the power amp and then the amp does not work. Do you have the input of the power amp connected to the preamp (including ground connection)?
I also could find the R46 on the schematic - it's to small for me.

EDIT: speaking of measurements: if you say that the current through output transistors is high and the transistors are OK, the only reason for this is that the base-emitter current is high. And the current is high since the voltage just before R33 and R47 resistors is much higher than expected. And this is the voltage on collector of TS16. So your first check would be to measure voltage on collector of TS16. If this is the case (the voltage is to high), I would check TS16 and TS15 for shorts.

Mark

[allante666]

Hi Markus, you have basically covered what Ive pretty much done, As i said i made a few other checks as well. The first thing i did do was to disconnect those links you mention basically to rule out short output transistors. The edit you added basically is the theory i was going on. TS 16 and TS15 woudl cause the symptom if they were short but neither one of them is, I had them out to double check, looks like the guy before me also did!

What woudl you class as the voltage on TS16's collector as too high?

Preamp is connected by a ribbon and looks pretty much undisturbed, I admit I havent checked any signal paths as the way this goes it looks like something much more obvious, which of course it isnt!

R46 in the diagram is just to the left of TS16 in the base supply.

[Enzo]

Unless you have a load connected, there is no path to ground for the plus side transistors to shunt 60v to ground, or the negative side either for that matter. What happens - without a load - is both sides turn on at once and current flows through them essentially shorting the 60v rails together.

You connected the V- side and no burn, but then added the V+ side and saw excess current. You then assumed the problem was on the V+ side. But did you ever try it with the V+ side connected and the V- side not? And then adding the V-? You might find the same scenario. You need to answer that.

The collector of TS16 is the base drive for the outputs, so it ought to be near zero. The outputs follow that collector voltage. Actually more like a volt or two above zero, but not 60v, OK?

[allante666]

HI enzo, apologies if this repeats but i just answered you and it disappeared! Anyway, despite thinking I had tried the supplies individually it appears i hadnt. I just tried disconnecting the negative side and leaving the +ve connected and as you say, no burn.
Ah, it also appears I have 60Volts on R46, (TS16 base)

[MarkusBass]

Now I see R46. But this was also a very bad test . TS16 in order to work has to have at least 0.6V less (on the base) than the V+ rail. So disconnecting it is useless. Enzo may be right that the problem may be in the "negative" part of the amp.
The expected voltage on the collector of TS16 I think should be in a range between 1.8V and 3.5V. You provided voltage for the base of this transistor (and I asked you for the collector's voltage). When you specify voltage on the base of TS16, you should measure it in reference to the V+ rail. Otherwise, we don't know whether the transistor is conducting, or not. Is it less by 0.6V than the V+ rail?
My advice is to disconnect output transistors but in a smart way: don't disconnect the base and emitter of TS12, TS17 - disconnect the collector only. Otherwise, you break the DC path and there is no chance for the amp to work. The same applies to R46 - don't disconnect it. Also do the same with TS6, TS7 - disconnect collectors only. The input stage should work correctly in this case (both regarding DC voltages and AC signal - of course without a load, or with 1k resistor as a load).
Print out the schematic in a larger scale and take notes what voltages you have on each transistor. Then think whether the voltages are correct. I'm sure some of them will be incorrect. And this where you should start.

Mark

[allante666]

Sorry, you did say collector I misread it. Ill do that tomorrow, thanks

[allante666]

Hi again. Managed t get a few minutes on this again. I followed your advice and disconnected the collectors of the four output transistors. This does as you say bring up the rest of the board working ok. Theh speaker relay cuts in and out as it should etc.
So, voltage on TS16 base is 58.4 against 59.9 on the +ve rail. Collector has 3.7 on it. Its oppo TS4 has -61.1 on its base against -61.8 on the -ve rail. Collector has -3.7 on it. TS11 collector has 6.4 and collector of TS10 has -6
This all seems in order yes? Do you think I should be looking after this stage or before it?