1981 Roland Jazz Chorus 120 CH1 Problems - Solved

[gbono]

It is possible the output transistors have been or were originally damaged but look at the DC voltages at the base, emitter and collector of Q1-4 (wouldn't hurt to look at the same junctions of Q14-15. See this link if you need some help in understanding how to trouble shoot transistor circuits: http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/

I sometimes use the LBL to help establish what is getting hot in the circuit: a cheap IR thermometer is a good tool to use and scanning the board before and after power is applied sometime gives a clue as to what is not happy on your board.

[substatica]

It is possible the output transistors have been or were originally damaged but look at the DC voltages at the base, emitter and collector of Q1-4 (wouldn't hurt to look at the same junctions of Q14-15. See this link if you need some help in understanding how to trouble shoot transistor circuits: http://www.allaboutcircuits.com/worksheets/bjt-amplifier-troubleshooting/

I sometimes use the LBL to help establish what is getting hot in the circuit: a cheap IR thermometer is a good tool to use and scanning the board before and after power is applied sometime gives a clue as to what is not happy on your board.

I have a cheap IR thermometer -- I'll give that a go

[substatica]

Replaced Q1, Q2.

Q14 is bad, but it's 2S1628-Y, any chance I could replace that with one of the 2sC1624's I picked up? At least temporarily?

http://alltransistors.com/transistor.php?transistor=12170
http://alltransistors.com/transistor.php?transistor=12177

Otherwise I have to order some parts. The local shop didn't have any 2S1628-Y.

[substatica]

I swapped in a 2SC1624 for the 2SC1628-Y temporarily. Looks like Q1, 2SD845, an output transistor? is completely shorted. Any suggestions for a replacement? Does that mean I have use a new matching pair? Once I pulled Q1 the amp powers up with the LBL and CH2 plays, I heard another snap, I'm guessing from Q2 again, but it still tests fine.

Update: Seems like a Sanken 2SC2921-ND will work for the 2SD845, though a little pricey at $9 per. Still trying to find a replacement for the C1628-Y, NTE190 is supposedly equivalent but the only source I've found wants $21, yeesh.

[phatt]

Because you have fly leads to the power transistors you are not limited to using the same case. 8|
You can simply remount any flat pack power device that has the *same or higher ratings*.

You may need to drill a couple of holes to accommodate the new devices but not a problem as you have plenty of space. TO264 or similar flat devices should not be hard to find.
Your 2SD845 is an NPN, 120Watt, 150Volt, 12Amp device.

Might be overkill but 2SC5200 is an NPN flat pack 150Watt, 250Volt, 17Amp device but I have no doubt it would work. Mouser stock them for $3~$5 depending on brand.
an Yes Wise to replace *Both* Output transistors.
You may have to cut those molex connectors in the process but they are obviously failing anyway. Revert to the old block connectors if you can't find anything suitable.

re the other Tr's;
If you can't get 2SC1628 then use 2SC1624, *AFAIK* it will make little difference.

Q13 is the *Voltage Amplifier section* (VAS) it swings the largest signal then the rest just do the heavy lifting and convert that into current so it can drive the speaker.

Yes *gbono* is onto the thermal issues,, a good manual feedback with ye old finger burn test tells you if something is running too hot,,   :lmao:
Of course making sure you are not touching anything that has dangerous voltage lurking. 8|
If something runs very hot at idle then it's bound to blow up at full power.

If it all goes south you can always throw in a LM3886 power amp chip,, just another option for you. :tu:
Phil.

[substatica]

Replaced both output transistors, Q1, Q2 with Sanken 2SC2921-ND's. Things were still being wonky. Pulled all the transistors off the board and tested them. They all tested good so I soldered them back in place. Now CH2 plays fine, the LBL bulb lights up then dims, voltage at CH1 output is 0.01 VDC or so -- in other words everything seems fine but I'm getting no sound out of CH1. I have to test that speaker, will report back when that's done. Feels like I'm getting close to cracking this nut.

[substatica]

Speaker seems alright. When I measure resistance across the good CH2 speaker outputs where they connect to the board I get an expected 7 ohms and change, when I measure resistance across the problematic CH1 speaker outputs where they connect to the board I get a slowly climbing and falling resistance that climbs and falls, 100 ohm, 200, 70, 50. Does that mean anything to anyone?

[phatt]

You have a dead speaker.
Ideally you need to disconnect the speaker from the amp and Then measure the resistance.
With your meter connected by clips on the speaker terminals wiggle those braided wires that go from the speaker terminals across to the Cone. They sometimes fray and give intermittent signal.

The braids tend to break right next to the solder join and I've saved a few otherwise dead speakers by simply resoldering.
If the coil is burnt out which is probably what has happened (DC on the output) then I'd say the coil has burnt out in which case you need a recone or a new speaker.

Meantime you can use the working speaker to test if the repair passes signal,, as long as there is no DC on the output.
Phil.

[substatica]

The speaker works fine when used on the working channel and measures 7.5 ohms, so I don't think it's that.

Though I just tried it again and I get DC hum on the good channel when a speaker is plugged in on the bad channel.

It's changed again, now the bad channel is measuring 20 VDC at the speaker outputs with no speaker.

[phatt]

Oh dear
I think we need better minds to look at this one.
Enzo, JMF, Teemu, anyone care to comment? xP
As I'm sure you know I don't wish to turn substatica's Amp into landfill with wrong advice.
Phil.

[substatica]

Oh dear
I think we need better minds to look at this one.
Enzo, JMF, Teemu, anyone care to comment? xP
As I'm sure you know I don't wish to turn substatica's Amp into landfill with wrong advice.
Phil.

Thanks for all your help. I'm pretty confident that the transistors are all good now. I guess I'll go over all the resistors and caps on that board again.

[gbono]

Is the problem in the preamp or the power amp section? It appears that the problem was ch2 and now it's ch1??

If you don't have a scope you can make a "signal chaser" https://www.youtube.com/watch?v=IEcyDzYAsuU

[substatica]

Is the problem in the preamp or the power amp section? It appears that the problem was ch2 and now it's ch1??

If you don't have a scope you can make a "signal chaser" https://www.youtube.com/watch?v=IEcyDzYAsuU

Pretty sure the problem is with CH1, it only affects CH2 when the CH1 speaker is plugged in and the short continues over to CH2 -- if that makes sense. If there's no speaker plugged into CH1 then CH2 works perfectly fine. I have a used a signal probe/chaser on effect pedals but I was wary of using it with such high voltages around, I don't want to fry another amp trying to test this one. I do have a scope, an old Tektronix, but I'm not fully proficient at it.

[substatica]

Using a signal generator I ran a square wave through and it seems to lose the signal (compared to CH2) at R63, in other words, past Q11 and Q12, which I guess I'm going to test again.

Replaced Q11 and Q12 and I still lose the signal at R63, it may be lost before that, but I definitely have it at R62. See attached portion of the circuit. Any thoughts?

Update: Seems I lose the signal at the emitter of Q11, R64 tests alright, C34 was OL so I replaced it with a 12pf I had on hand, no change, D3 tests alright.

[gbono]

What signal do you have at the collector of Q13?