Roland JC77 problems part2

[TarheelTechinTraining]

The resistors all check good including the ceramics. Gotta but the pcb back in to check +/-15 Vdc.

[oc disorder]

"The resistors all check good including the ceramics" oh I've run out of ideas apart from if you altered or "hot wired"
the chorus switch perhaps should remove this and start again.

Just to re cap..from previous forum thread
Roland JC77 has broken chorus/vibrato switch.
Circuit board removed switch removed and some contacts hardwired?
Amp reassembled blows mains fuse.
Appears to be short on power supply.
Aprox 50v AC at secondary of power transformer + & - 25v DC after rectifier.
Amp has approx 6 volts DC at both speaker outputs.
With 25watt lightbulb limiter and both speakers connected bulb shines brightly.
All transistors test ok as do emitter resistors.

Anyone any idea where to go from here ?

[TarheelTechinTraining]

That's about the short and long. In addition +/- 15 Vdc at c6 & c7. The switch repair has been removed.
Now I'm getting 1 Vdc at the speaker and 12+ Vdc on the other, no speakers on the amp.
I'm thinking IC's

[TarheelTechinTraining]

With speaker plugged in one set of leads read 6 Vdc and the other about 5 Vdc also there is static coming from the speaker at this time

[oc disorder]

"I'm thinking IC's" well I wouldn't ! The power amps are coupled to the pre-amp via coupling caps so there is no way DC could come from
a faulty IC and make DC appear at both outputs.
The only thing I can suggest at the moment is to try to isolate one power amp and get that working with the power supply.
It may be possible by lifting links or extreme measure cutting pcb tracks where they can be repaired easily.
Removing two output transistors from one amp should disable it.
C22 and c59 1 uF BP are the inputs to the power amp.

[Enzo]

You need a larger bulb.

If there is any DC on the output, DO NOT connect a load. Period. Once we eliminate the DC, we can think about load. As to the bulb, if you have DC on an output, it usually doesn;t draw current UNTIL a load is connected. SO disconnecting the spaeker load might be the difference between bright bulb and dim.

The point of the bulb is to try to protect the circuits from excess current. The other side of that coin is that the amp will not properly operate with a bulb in series, especially a small bulb. So if we can get the amp to fire up and not light the bulb bright, we can then get rid of the bulb to further troubleshoot.

You have 50VAC at the transformer? Center tapped, that is then 25-0-25vAC. 25vAC rectifies and filters to about 35vDC. You are getting 25v, 10v less. How much ripple is on those 25v rails? Even the schematic says 30-33vDC.

[oc disorder]

Enzo the dc supply was measured without the filter caps. If you care to trawl through the previous thread which became a bit unmanagable for me you will see that we overcame the faulty transformer theory and attempted to check the whole thing!
He has tested all the transistors and checked emitter resistors and I am at a loss to see where the fault could be.
The fact that both the power amps show DC is unfortunate I still wonder what could be common to both that would cause this apart from a solder splash !
O.C.

Greg perhaps with no speakers connected and light bulb limiter in place you could check the supply now the bridge rect. is
attached to the board and the filter caps.

[TarheelTechinTraining]

Its more like 17-0-17vAC, 33vAC rectifies and filters to about 22vDC no speaker connected
Also c22, c59 are removed.

[oc disorder]

Um Greg in Post 34 previous thread you told me

When I test the secondaries ,plugged into the wall ,unhooked from pcb ,the voltages are 25 or less from center tap to Vac and about 50 Vac to Vac. All three wires are off the pcb

I accept the fact that is a little low and planned at the end to check your wall volts 100? 110? 117? 120? (i'm in Australia) and check the selected tapping on that amp.
But even if the incoming is a little low it won't produce DC at the output.

are you saying that C22 and C59 are not there? or you have taken them out of circuit ?

[Enzo]

17-0-17? Is that WITh the bulb? The bulb robs voltage from the circuit, so if the 17v is with bulb, one hopes the voltage will rebound without it. If 25vDC was measured without filter caps, then that answers my how much ripple question. I have not doubted the transformer is OK in my heart.

ANy time you provide a DC reading on unfiltered supplies it should always be mentioned as "unfiltered."

I did read that whole thread, but I can't go back and refresh each day, so I must have forgotten.

The two channels do not have the same symptom. One has a volt offset, the other 12v. That doesn;t look like a common problem offhand. Though they could both spring from a common wound.

Without review, there are various possibilities. Even in an intact circuit we must not focus entirely on parts. Test all the transistors? FIne, what about the connections? An open trace from point A to point B is just as much of a problem as a burnt out resistor along the way.

One pair of wires connected in reverse can upset your entire day. I see 12 wires from the main board to power transistors, if one of them was ever misconnected, there is a problem. Were ANY of those wires EVER disconnected during this repair? A missing ground connection can also play hell with you. Wire 37, just to the right of the bridge is VERY important.

I want to see a pair of nice clean 30-35vDC power supplies. If the whole thing was 10v low, the amp would still work, but not if those low voltages were due to no filters. We need to see if we can get off the bulb. It seriously gets in the way unless it is truly needed.

[TarheelTechinTraining]

I'm sorry for all the miscommunication , my only excuse is I am a newbie.Thanks for your patience. Now I got that off my chest,lol.
The outputs/power transistors were removed for testing and put back correctly. I'm sure because its almost idiot proof, correspoding # on pcb and a smaller pcb at the tranny.
On the trannys viewed from the top its Base-Collector-Emitter, correct? They are 2SD716s. Diode test B to C , B to E and nothing else, right?
#37 is connected and good. I do still have c22 c59 disconnected plus r1,2,3,4,83,90 seperating the power amps from everything else ,hopefully.
When I connect one speaker and turn it on the bulb is bright and there is a hum coming from the speaker. Same for the other speaker connection.
All the above still with 25w bulb. Should I use a 60w bulb?

[52 Bill]

The outputs/power transistors were removed for testing and put back correctly.

And the insulators were carefully checked for cracks and tears?

All the above still with 25w bulb. Should I use a 60w bulb?

Try at least a 60 watt bulb.

[TarheelTechinTraining]

the insulators are intact and are fine.
I will get a 60w bulb
right now I've got the power amps isolated from the rest of the circuitry, except the power supply.
Also I've isolated the 15.5v power supply from the pre-amp,reverb, and chorus section.
In addition I've isolated a 30v via r89,82,83,90.

[Enzo]

If you don;t quit connecting a speaker, I'm gonna smack your head. Use a volt meter. If you see DC voltage on an amplifier output, that is enough to tell us you will hear hum if a speaker gets connected. We do not need to connect one to find that out. If DC is on an output, connecting a speaker potentially damages the speaker, and potentially causes futher damage to the amp.

[TarheelTechinTraining]

ok no prob.thanks
sorry sometimes the ocd and add kick in all at once,lol