Egnator rebel 30 channel switching problem

[pontiacpete]

Hi there,
I'm working on this egnator rebel 30, (lucky me) and it has a channel switching problem.
When depressing the channel switch nothing happens. But if you wait a minute or two a hum starts to develop
and then the light appears for the drive channel and the hum gets louder but no drive channel c
Not sure where I'm going to start but with the visual inspection I see this 6 leg IC labeled U1 on the board and it has
a chip off of one corner and a big red dot. Take a look at the picture. Can't seem to find it in the schematic(also attached).
Any thoughts?
Thanks.

[Enzo]

On the schematic, look up top, left of center, find R1. Just below R1 is an optocoupler. it isn't labeled with a part number, but the diode side of it wires to CN1. I am pretty sure that is what your six-legged IC is. The red dot is where the pin 1 indicator would be. I see the corner chip, but frankly it looks OK to me.

[Jazz P Bass]

The optocoupler, H11F1, is marked as a mute circuit.
I don't see how that could make the channel switching circuit misbehave.

I would test each relay to see if they are turning on & off.
D1, D2 & D3 are the test points for the relay power.

Here is the opto datasheet:H11F1.pdf

[mozwell]

A quick check of the power supplies may be in order. Measure DC volts & AC volts (so we can see if there is a lot of ripple on the DC supply)
B+1, B+2, B+3, B+4
+12V, U4 output, U2 output, U3 output
6.3VAC heater

The +12V line (from the simple transistor regulator ZD6 & Q1) powers the preamp heaters, and the channel switching
If the 12V is low, the channel switching relays wont operate & the preamp heaters will have low voltage.

U1 may be at fault. I don't like how the input to U1 hasn't any "spike" protection, adding a 470ohm series resistor on the two connections from the foot switch socket to U1 pin 1 & pin 11 and U1 pin 13 may help (if U1 is faulty). Adding a small 10n cap from U1 pin 1 & U1 pin 13 to 0V may help, as would adding 1n4148 clamp diodes to 0V & +12V.
Generally i never run a CMOS input direct to the "outside world" without a series resistor & diode clamps.

I think the +12V line from Q1 may be your problem, but lets first measure all the power supplies & see how we go from there.

[pontiacpete]

Thanks for the responses
The diodes D1,2 and 3 all have about 10v on their anodes.
At Q1 I am not sure which leg of the transistor is what, but I took reads at the points
where each lead is connected to the board and find that there is 10v, 10v and 12v.
what is that transistor? It says 'MJ?3055T" ??

EDIT: looks like MJE3055
B - 11.6v
C - 13.4v
E - 11v
voltages seem to go up as it warms up.

[pontiacpete]

Finally have these B+1 370v,
B+2 358v,
B+3, 318v
U4 13.4in, 4.9out,
U3 -18in ,-12 out
U2 13.6in, 11out

Are voltages OK here? looks like they are close.

[Jazz P Bass]

The diodes D1,2 and 3 all have about 10v on their anodes..

Yes,but what are the voltages when the relays are engaged?

[pontiacpete]

when the relays are not engaged D1,2, and 3 have 11.5v on there anodes
and 11.5v on the cathodes.

when relays engaged the voltages stay the same on the anodes at 11.5v
but the cathodes fall down to 7.4v each and continues to slowly go down and the hum almost rattling sound starts to get loud.

[Jazz P Bass]

So you have roughly 4 volts across the relays when engaged.

That is not good enough.

I can't make heads or tails out of this schematic.

[pontiacpete]

Well, it's not just me, then.

So it seems to me that it's unlikely that it's the relays themselves, could they all be hung up because they are all bad?
Maybe Mozwell knows something about U1, the big U1 with 16pins not the little one that I took a picture of in the 1st post.
I can't see yet how to work on the board that it's mounted to, it's buried but I think I see it near the power tubes, yikes.

[Jazz P Bass]

Well U1 on the Ft Sw input is an inverter IC.

Look up the datasheet.

A high level on an input pin will output a low level.

[pontiacpete]

so I decided I would pull one of the boards out. Not the one where the power tubes are mounted, but the one where the pre amp tube sockets are mounted. All of these sockets had been a bit damaged by the previous person or gorilla trying to pull tubes out and putting them back in.
ONe particular socket was intermittent so I thought I'd change it and check the board for bad solder joints.
Well in doing this I notice C53 is bulging. I guess my previous 'visual inspection' wasn't so good. Can't believe I didn't notice it before, even after all those test on the regulators..sitting right next to C53.
Anyway that did the trick. Amp is now changing channels.

Q1 Now has even higher voltages on each leg. Something like 15v 13v and 15v. I still don't quite understand
why there is voltage on all of them.

Thanks for the help!!

[Jazz P Bass]

Q1 is a simple 'emitter follower series pass regulator'.
Series Voltage Regulator | Series Pass Regulator | Tutorial

The voltage at the base of Q1 should equal the zener voltage rating.
This should result in a voltage drop at Q1's emitter, which you do not have.

So something is still amiss.
Check Q1 & ZD6 & R68.

[pontiacpete]

Do you think it's still a problem? It's switching normally

[Jazz P Bass]

From an engineering standpoint, it is most definitely a 'problem'.

The circuit is supposed to regulate Q1's output voltage to 12Volts.

If it is not doing that, then, yes, there is a problem.