Roland SH-101 analog synth repair

I am trying to repair a Roland SH-101 analog synth

I started opening the unit and checking voltages.

When the unit is powered on the 9V rail drops from +9v to +2v DC.

I guess I have some problem in the power supply section.

Can you suggest me which component to check and eventually replace? My guess would be replacing TR1 and D8 for starters

Here you can find original manual with schematics etc

http://manuals.fdiskc.com/flat/Roland%20SH101%20Service%20Manual.pdf

Does the voltage drop occur when run on batteries, or only when using an AC adapter?  If it does not happen with batteries, then the problem is in the PSU.  If it does happen with batteries then you have something dead in the synth such as a power decoupling cap, opamp, etc.

I fixed one of these once. It was the regulator..

mjrippe said:

Does the voltage drop occur when run on batteries, or only when using an AC adapter?  If it does not happen with batteries, then the problem is in the PSU.  If it does happen with batteries then you have something dead in the synth such as a power decoupling cap, opamp, etc.

Click to expand...

but don't the batteries run into the same portion of power supply circuit as the dc jack does? where there is the voltage conversion and regulator.

that's where i think the problem might be. what you reckon?

buildafriend said:

I fixed one of these once. It was the regulator..

Click to expand...

do you mean TR1?

On second look, you appear to be correct.  It does seem that both the batteries and DC adapter go to the same point. 

Some updates as I got back at fixing this.

I replaced TR1 and D8 but the problem is still the same.

Once I activate the power switch, the voltage on 9V rail drops to 1.8V

Can you suggest how to proceed? What should I check?

Thanks

Did you try it with batteries or with another power transformer, just to eliminate the possibility of a failure in the AC adapter?

yep. adapter is working with other devices

Check any power supply decoupling caps, particularly if they're tantalum (booger-looking guys).  I fixed an old dbx500 that was having "power supply problems."  Turned out to be one bad tantalum cap shorting the + rail to ground.

hi guys, had some time these days and went back trying to fix this.

first thing i did was to replace all the diodes and electrolytic capacitors in the power supply. transistors T1, T2, T3 as well

still the same behavior: at power on the voltage dropped to about 1.7V

so i completely took it apart and desoldered all the components from the +9V rail, one by one.

now i only have the small transformer (marked T1 on the schematic), and the polarity protection diode.

the voltage still drops, now to about 4.6V.

i am thinking the transformer T1 is dead, but this power supply circuit is too complex for me.

i think i should desolder it and check it with multimeter, but I don't know what to measure since it's impossible to find any datasheet, and Roland customer service is not willing to help

what do you think?

i did some more experiments

replaced also IC1 and IC2 in the power supply and replaced again transistor TR1 and TR2, just to be sure they were not defective

then i isolated the small transformer T1 by cutting traces and pulling one side of components connected to it

basically is connected only on the right side of the schematic. one pin to TR2 and one to TR1

still have the same problem

can you please have a look and confirm T1 is dead, or suggest a way to test it? I can't really understand that circuit

thanks to everyone who is so generous to have a look and give me a hand

> connected only on the right side of the schematic. one pin to TR2 and one to TR1
> still have the same problem

That's probably the side with the problem, no?

Lift one end of R8. That kills ALL power to the T1 subcircuit. (You should have done this sooner.) Does your 9V now stay 9V?

The T1 circuit converts +9V into +/-12.2V which is regulated by IC1b to +10.5V and to some negative voltage by IC2b. There is much which is unclear here.

actually R8 is on the schematic but not on the PCB, don't know why

but wait, if R9 is lifted 9V stays and doesn't drop. forgot to mention that

so do you think the problem is on the right side of the transformer (primary?)

Once i had a similar problem with a Roland JX3P. After a lot of debugging i found the problem was not in the PSU but a failed IC in the audio path. Try to isolate the PSU and see if the voltage still drop.
my 2 cents.

yeah at the moment i isolated all the power supply trying to understand if T1 is faulty

Hi beatnik,

have you managed to sort out your issue ? I have exactly the same : I swapped all IC's except the VCO, VCF and SIL opamps with no result. I now consider cutting the traces after the power circuity to insulate it and see if TR1 stops getting warm.

thanks for your feedback
Philippe

hi Philippe

my sh-101 project is left on standby

my case 99.9% that the small transformer T1 in the power supply is damaged, and it's impossible to find a replacement

i definitely agree you should try isolating the power supply circuit.

this involves lifting some resistor and cutting some traces.

this way you can understand if the fault is in the power supply itself or somewhere else in the audio circuit

good luck and let me know. hope you will find out it's something possible to repair, not as mine !

Hi beatnik,
thanks for your quick reply !
I just cut the traces from D1/C1 to TR3/IC1 and from D2/D5/C8 to IC2 and TR1/TR2 were oscillating fine - a regular square - with no heating. I re-established the connection to TR3 and unsoldered IC1, still no heating... Re-soldered IC1 and back to the problem. 

Seems like this M5218L may be a issue. I just found two chips for sale on eBay and ordered them... we'll see, I will keep you posted

kind regards,
Philippe

i have done some more test on my dead sh-101 and finally i'm sure that the small transformer is broken

i'm so unlucky.... is the only component that is impossible to find a replacement !

do you think is possible for some expert to rewind the transformer, even without datasheet ?

what you think of rebuilding the power supply circuit with standard linear regulators ?

would 78-series regulators provide enough power for the synth circuit ?

Have you fixed the SH-101? Was the transformer damaged?
I received an SH-101 with the same symptom: voltage drops when power is switched on, and could not yet determine where the problem is.
Thanks