Roland DB-500 no sound

[pontiacpete]

Continuing with the signal trace and I find it's goes up until the JFET then there nothing.
So I'm looking at Q15 and Q20 are these the muting circuit? The JFET is good, I believe.
There is a good strong signal in the headphone.
Q15 is showing that it's shorted in circuit.
Am I on the right track?

EDIT: typo: Q15 is not showing that it's shorted in circuit

[Jazz P Bass]

It sure would help if you told which schematic the jfets are on.

And the coordinates.

[pontiacpete]

I'm on the 'power board' drawing, I think this is pdf 1. The coordinates would around E - 14.
thanks for looking!

[Jazz P Bass]

If Q15 shows as a short (which pins?) remove it & test it off board.

Q25 is a mute circuit.
What voltage do you read at the gate pin?

[pontiacpete]

I'm really sorry sorry about the misinformation. Q15 shows no short in circuit.

Q25 has these voltages

S 0v
G -12v
D 0v

[MarkusBass]

Pete,

The mute circuit consist of many components - starting from R39, D6 (just after the transformer), Q16, Q24 transistors (and all components in their neighbourhood), Q25, Q15, Q20 transistors, Q1, Q2, Q3,Q4 transistors, and the most important PHONES jack and Q33 transistor. From your description it seems that the amp is muted and the reason for this may be that you have headphones plugged in to he PHONES jack, or the PHONES jack has failed. Another reason for the failure may be that the PHONES DETECT signal (on the PHONES jack) is not grounded. Can you check whether pin 8 of the jack is properly grounded?

Mark

[pontiacpete]

This morning I decided to change the headphone jack because the threaded barrel had broken off and needed to be replaced anyway and I was started to suspect that it might not be working. But that wasn't it. Still no sound but I also checked pin 8 for ground and it is grounded. So I moved directly to Q33
and it is shorted between the emitter and base.
Replaced that with the correct transistor and still no sound.
I will continue to check all the components you list and report back, hopefully with good news

[MarkusBass]

Please note that the headphone jack is of very special type - with two isolated switches. I hope you replaced it with a correct type.
Instead of checking everything (this may take a long time), I suggest measuring voltages on bases of Q15 and Q20. This will directly tell you whether the amp is muted or not.
Another simple test is to remove temporarily these two transistors.

Mark

[pontiacpete]

I had a jack that had the full 9 pins, had to cut pin 9 and 6 to fit on the board. Would this do?

Voltages are equal on Q15 and 20, the bases have -15v and 0v on the collectors and emitters

[MarkusBass]

I had a jack that had the full 9 pins, had to cut pin 9 and 6 to fit on the board. Would this do?

Haven't you checked that the jack makes the same connections as the previous one? It depends on what connections were made by pins 9 and 6.

Voltages are equal on Q15 and 20, the bases have -15v and 0v on the collectors and emitters

I think that -15V on base means that the power amp is unmuted. And if you touch the emitter of Q20 with a screwdriver (or a finger), do you hear any noise in the speaker?
There is another simple test: plug in headphones and check the voltage on base of Q20. It should change to something like +0.6V.

Mark

[pontiacpete]

The base voltage did change with headphones plugged in to .68v

As for the jack, there are no connections to pin 9 and 6 on the board so I thought I'd be OK with that.

[MarkusBass]

And if you touch the emitter of Q20 with a screwdriver (or a finger), do you hear any noise in the speaker?

Have you done this simple test?

[pontiacpete]

Tried it with a screwdriver, no sound

[MarkusBass]

Continuing with the signal trace and I find it's goes up until the JFET then there nothing.
So I'm looking at Q15 and Q20 are these the muting circuit? The JFET is good, I believe.

I suggest to continue with the signal tracing. You can temporarily remove Q25 and short its drain with source. On what basis do you believe that the JFET is good? Faith, or something stronger?

PS: I assume you checked that the speaker is OK, right?

Mark

[pontiacpete]

Probably faith... but I shorted the source and drain and not a sound. Changed the speaker 3 times and the cable going to the speaker.
The signal is really small on the JFET and master volume and now the signal is very very weak in the headphone.
Something happened. Taking a small break and will have to sort out that issue