Roland SPA-120 Amplifier Tech Notes / SM / Schematic Source?

[KeyResults]

Repairing a Roland SPA-120 amplifier, a solid-state early-mid 80's part of the Roland Rack. The PCB is Labled AP-124B. Was thinking lytics would resolve the nasty output on scope but all tested 100% for values, ESR, and leakage (very surprised!). Symptom is nasty wave form on both channels in Stereo mode, but clean in single channel mono (BTL) mode. The Output transistors test ok at room temp, but still could be faulty once heated up. Anyway, a schematic and some test point voltages would sure save some time. Thx!

[petehall347]

what does it look like on the scope ?
p.s take care of those toshiba output transistors .

[CapLeaker]

I see definitely caps that I would change. One of them the sleeve shrunk considerably. Not sure what kind of ESR tester you got, but I wonder if it does work correctly
What about the bias on the output transistors?

[KeyResults]

CAPS: I focused on the same "ugly" Lytics. As stated, I pulled and tested w Kelvin Clips on Calibrated DE-5000 LCR Meter and Sencore LC53 (Value and Leakage). They may look ugly, but they're NOT the problem here. Haven't probed powered on yet. Only passive testing on obvious components, and found no abonormalities. Decided I'd like to find a schematic or Tech Notes for this SPA 120 unit if I can. So far, no dice. Not even for sale.

WAVEFORM / SCOPE: I usually snap pics of scope but I guess I forgot before teardown. When I power it up again I'll try to remember to get a pic. I can describe the clean sine wave on mono/BTL becomes Trapezoidal in Stereo mode. I also noted a high pitch "squeal" that sounded ominous. Evena simple block diagram would help me at this point. There are three preamp ICs which makes me wonder if two are on the stereo mode circuit, and the third is on the mono side.

[check12]

This might be pretty similar just with added protect circuit...

[KeyResults]

This might be pretty similar just with added protect circuit...

Thx. I appreciate that. I found that SPA-240 Tech Notes too with the same thought. The 240 is a much beefier design and apparently a much more popular unit as it seems to be well documented unlike the SPA-100.. There are multiple board revisions on the 240 too, but none match up with the SPA-120. Still, one of the revs may give me some strong hints that prove useful. Thanks again.

[CapLeaker]

CAPS: I focused on the same "ugly" Lytics. As stated, I pulled and tested w Kelvin Clips on Calibrated DE-5000 LCR Meter and Sencore LC53 (Value and Leakage). They may look ugly, but they're NOT the problem here. Haven't probed powered on yet. Only passive testing on obvious components, and found no abonormalities. Decided I'd like to find a schematic or Tech Notes for this SPA 120 unit if I can. So far, no dice. Not even for sale.

WAVEFORM / SCOPE: I usually snap pics of scope but I guess I forgot before teardown. When I power it up again I'll try to remember to get a pic. I can describe the clean sine wave on mono/BTL becomes Trapezoidal in Stereo mode. I also noted a high pitch "squeal" that sounded ominous. Evena simple block diagram would help me at this point. There are three preamp ICs which makes me wonder if two are on the stereo mode circuit, and the third is on the mono side.

Maybe look and see if this PA is made under a different brand(s)? Sometimes you find at least one to 3 sub brands with the identical PA. I guess it's not a capacitor problem if you got that kind of equipment available to your disposal. I wont argue there. Do the transistors get hot while idling?

[KeyResults]

Maybe look and see if this PA is made under a different brand(s)? Sometimes you find at least one to 3 sub brands with the identical PA. I guess it's not a capacitor problem if you got that kind of equipment available to your disposal. I wont argue there. Do the transistors get hot while idling?

I wondered about that too. There are a few amps that I thought might be candidates under the CRATE, PEAVY, or a couple commercial PA system amplifiers. But all were dead ends. I have about decided to just attack this one the old fashioned way and work my way through, tracing and checking for sigs and voltages til I find the culprit(s). I will take a look with the Thermal Camera in various states just for grins and note anything unusual as a matter of routine. I have noted that I am not the first person inside this equip, but the current owner has no idea about that. Whoever was inside changed one of the power transistors

In any case, thanks to all. I will post the resolution as a follow-up along with the symptoms if anyone is interested.

K

[CapLeaker]

Doesn't make sense to me yet why the amp is clean in mono, but then dirty in stereo mode. Is the switch o.k.? Must be something stupid.

[petehall347]

does it have the same problem without a speaker load ?

[KeyResults]

Doesn't make sense to me yet why the amp is clean in mono, but then dirty in stereo mode. Is the switch o.k.? Must be something stupid.

I'm thinking the same. Something dumb.

[KeyResults]

does it have the same problem without a speaker load ?

I've had to dive into several other projects since cracking this one open. I cannot recall if I removed the 8ohm load or not in Stereo mode. At this point I won't be able to work on this guy again until after the holidays. But, I'll get it singing again and follow-up. Hopefully not a nut loose at the controls!