Ashly SC-50 repair a success

[doorunrun]

I was given two Ashly SC-50's a long time ago by a friend and they have been sitting in storage in a spare bedroom. I decided I needed one of them to tame some vocal tracking I started doing. Of course they weren't working. Symptoms were that they passed audio but neither gain control did anything. One of them has what looks like oscillation in the gain reduction stage.

Big surprise opening up their enclosures, all that rotting foam! First order of business was getting as much as I could off the covers and PC board.
I picked one to work on mainly because the power switch lamp was still working.

To cut to the chase, the problem with it was with the zero-ohm jumpers. The lead connections to the ceramic body had become brittle and would lose contact. I think it might have been from interaction with the decomposing foam rubber. I went through and replaced them all. There were a few that were still in pretty good shape and didn't easily come apart but I replaced them anyway.

I helps that the lead spacing is common to them all at 10mm. That made it easier to form them out of the insulated 24ga. solid wire I used.

Got the unit back together and it acts just like advertised. I've still got to give it a real shake down listening test. Once things settle down I'll work on neutralizing the rust on the cover plates and see if I can come up with a good method to support the PC board other than stuffing more foam in there.

I'll try the same approach with the other unit.

Cheers!

 

[Ike Zimbel]

An SC-50 was the first compressor I ever owned, I'm kind of itching to get my hands on one and try some upgrades...The back story here

 

[doorunrun]

I've just started on the second unit and have the main PCB out of the chassis.

A note about these zero-ohm jumper components. In the first unit they all had a single green band, not the usual black one that is considered normal.

With the second unit there a mix of both green and black banded jumpers. The green ones do pop apart where the lead enters the ceramic body just like the first one I worked on.

But, the black banded ones in this second chassis seem to have aged better and hold together under a little pressure.

We'll see how it goes...

 

[doorunrun]

Finished up on the second SC-50 and it's working again! Replaced all the green banded zero-ohm jumpers. Here's a look at some I pulled. There was one (1) that held together and was probably usable, but it came out, too.

I noticed there's also one the rear panel I/O board. I overlooked it on the first unit.

I took apart this unit's rocker switch to see how practical it is to repair or upgrade to LED. I found what appears to be a good replacement illuminated (neon) rocker switch on Amazon. If it doesn't work I'll get the LED option a try.
Edit: the switch is a Carling LTA201-TR-B/125N and the one from Amazon is a perfect fit.

Cheers!

 

[doorunrun]

After replacing the last suspect zero-ohm jumper on the I/O board. Out of curiosity, I pulled a couple of the 47uF electrolytics to test and saw this on my component tester on the first one:

The second one wasn't as bad, but bad enough.
Yes, it's time to recap...

 

[doorunrun]

The saga continues. After recapping one unit, at first power up it was passing a signal, but there was no gain reductions showing on the front-panel LED. It had developed a problem in the detector section.

It turned out IC5, a LM301a, had 16V on pin 6, the IC's output. I noticed pin 1 was at +14V. The complete opposite of all the other '301's with -14V on them (similar circuitry).

I ordered replacements from Jameco. I decided to pull IC5 from the other unit as a test and as it came out of the socket a few leads broke off. I think it was the long term contact with the rotting foam made the leads brittle similar to what occurred with the zero-ohm jumpers. Ugh!


I substituted an LM307 IC I had on hand. It's of the same family as the '301a, the difference is it's internally compensated. It's a temporary fix 'till the Jameco order arrives. I'm not sure how much operational difference it will make for the time being.

 

[scott2000]

Even the sockets themselves can become suspect. I chased my tail for a while before finding a tiny crack inside one of the sockets that was just in the right/wrong spot ...

 

[doorunrun]

I encountered a problem with one of the Ashly units. In the detector section IC6, LM301A, would latch up and no activity on the front panel LEDs an no apparent compression or limiting.
The detector section of the schematic:


The voltage on IC6 pins 6 and 2 was reading around -14V. I'm not very experienced with bipolar op amp troubleshooting. I tinkered around for a while but couldn't come up with any obvious problem.

One clue was after doing some resoldering of the IC socket the unit worked as designed, but 20 minutes later failed and stayed that way. So heat maybe was the cause and/or a bad solder connection.

I replaced the components in yellow highlighted areas of the above schematic (2 resistors, 2 diodes) and things started working correctly. Leaving the unit powered up for several hours the problem did not come back. The PCB's traces are gunked up from being in contact with foam rubber, that may have caused the problem.

I did a voltage comparison of IC5-6-7 op-amp pins with the other working unit and they are reasonably close. For better or worse here's what I'm reading with my DMM. IC 5 & 6 are LM301A, IC7 is RCA4136 (stock).
IC5: ======== IC6: ====== IC7 (Edit: Trying to fix display formatting)
1. -14.43V === -14.43V === +2.6mV
2 +0.9mV === -1.20mV === +0.8mV
3 +0.3mV === -1.6mV === -6.75V
4 -15.77V === -15.77V === -124.5mV
5 -14.51V === -14.51V === (pin 12: +6.77V)
6 -253.5mV == -505mV
7 +15.66V === +15.65V
8 -0.721V === -1.117V

I think the function of D7 and maybe D6 are there to clamp the op-amp's output preventing a latch up. A similar arrangement is used on IC5.

BTW, the diodes used in this model are type 1N914. The schematic does not indicate their type, they are marked on the component. The replacements I used were 1N4148's.

Fingers Crossed!

 

[Script]

Good work. Would you mind posting a shot of the entire PCB?

 

[doorunrun]

Thanks! Here's one showing most of the board, just missing the 2 LEDs on the far right.

ICs are numbered left to right. Pin 1 on IC's 1-8 is located at upper right. IC9's is upper left.
Black socket at lower left is for ribbon cable connection to I/O panel (the black dot ID's pin 1).

Cheers!

 

[Script]

How are the leads of the ceramic discs doing ? This is really old gear.

 

[doorunrun]

Hello Script,

They're doing OK.
While I was replacing the diodes, resistors and reflowing pads in the area of IC6, I had a couple of the caps out and cleaned their leads up by "burnishing" the leads with needle-nosed plier jaws and they held up well.
I checked their values with an off-shore LC component tester that I've come to trust, they're a bit off their marked value, but not by too much.

 

[doorunrun]

I discovered on one unit IC1, a RC4558 dual op-amp, had +15V on pin 1, OUTput1. There was about 30mV difference between input pins with the non-inverting input at around 40mV. Time for another partial schematic, the VCA section.


I tried isolating (opening up) connections at IC1 pin3 to see where that extra voltage was coming from, but there was no magic bullet.

It wasn't until I popped a fresh LM301a in place of IC3 did the voltage on IC1 pin1 not jump up as it had before.

Since I don't understand how exactly this circuit works I'll have to leave it there.

I started out with 9 (nine) LM301a spares. Each unit uses 5 (five) of them and now I only have 3 (three) left. Now I'm thinking about chip upgrades for at least one of them.

And so it goes...

 

[Script]

I'd replace all IC sockets first. Those are really not good at all -- as was hinted to above. They are cheap and unreliable. I'd only use those sockets for test circuits, if at all. Then test the unit with the original ICs plugged in.

 

[doorunrun]

I guess so. It's relatively easy getting stuff removed from the PCB. I've been using Solder-Wick. The only trouble is that crappy foam rubber residue on the board's traces. It takes bit of heat and extra flux to get it to flow. Slow work, but it will pay off in the end.

 

[doorunrun]

I decided to go with a NE5532 upgrade for the RC4558 and RCA4136 op amps. For now I'm keeping the LM301a's. I also replaced the IC sockets as part of the project.

I used BrownDog '4136 adapter boards and soldered on the SMD 5532's. I used two of them for IC7 and IC8. I opted to keep the RCA4136 for IC9 since it just drives the red gain reduction LEDs.
Both SC-50's were modified.

The most notable change was how the yellow Threshold LED acted. When I first powered up either unit, the Threshold LED lit and would stayed lit for at least 10 seconds, slowly going dim to off. Testing using an active source showed the same kind of slow-to-extinguish behavior.

Before the mod, the LED would just briefly light up and quickly go out. In processing an active source the LED would respond quickly to levels above and below threshold.

I did some probing around and found with the BrownDog/5532 combo replacing IC7, just touching the DMM probe to pin 8 would cause the yellow LED to come on and stay on as long as the probe was attached. IC7 functions as the Threshold LED driver, +/- 7V source and release control for the VCA.

I reinstalled the RCA4136 chip into the IC7 socket and now the Threshold LED functions as I believe it should. I think without trying to redesign the Threshold detection circuitry I'll just leave the '4136 in for IC7.

EDIT: I gather the LED indicator circuits are voltage comparators. The '5532 is not a good candidate as a comparator in this case because it has diode protection on its inputs limiting input swing.

Audio from the detector circuitry highlighted in yellow.
Since the RC4558 IC is still in production I'm going to try mounting two SOIC-8s on a BrownDog '4136 adapter board to see it it's a long term availability solution.

As far as I can tell replacing the RC4558s with NE5532 DIPs has not caused problems.

Replacing the IC sockets was a great idea!

 

[Peter Stoffregen]

Hi Folks! First post for me on this forum. So glad I joined! I have been meticulously cleaning an SC-50. Opening it to clean noisy pots revealed the rotted foam. It was working before I started but I can't seem to leave well enough alone! It will be part of a large electronically crossovered sound system, so avoid ground loops and observe star topology grounding, I lifted the ground lead from the inside of the Ashley case. This resulted in about 50 VAC leakage on the chassis. The SC-50 then started to misbehave. I have restored the ground connection but the problem remained.
You have pointed me to the green banded zero ohm jumpers. Touching one of them restored operation! Thanks again!

 

[Peter Stoffregen]

I will of course be replacing all of these jumpers. I will use solid copper wire. Question; why use these manufactured jumpers in the first place? would wire not be cheaper---and evidently more reliable? BTW, attached is my solution for supporting the PCB's once the foam was removed. The wood was adhered to the pcb edge with hot melt glue, not too hard to knife through if board removal is required.

 

[scott2000]

why use these manufactured jumpers in the first place? would wire not be cheaper---and evidently more reliable?

Maybe easier to stuff boards..... or some sort of fusible link thinking?

 

[doorunrun]

Question; why use these manufactured jumpers in the first place? would wire not be cheaper---and evidently more reliable?

Cheaper, but probably more time consuming putting it together. The IC socket/ribbon combination works well. It would be nice to think the signal wires are separated by a ground wire in the ribbon.

To hold the boards in place I'm using a nice flexible grade of styrofoam with a slit cut using a utility knife. I think the compression of the top and bottom plates are enough to keep it in place.

I've been very pleased with the upgrade. Good luck with yours!!