Super-sonic 22 preamp acting funny..

[Telekaaster]

Hi, again.
Been going quite a bit scientific on it, and I have checked and double checked ground connections for all three pots with amp in locked state. Resoldered them all. Been all over to check for broken tracks. Found zilch.

I've got it on my bench, and I can induce the failure by putting some cloth on top to accumulate heat. Also when I push gently down on the pcb in the area where V1 connects it occurs and then goes away when I let go. The whole preamp circuit is on one separate pcb strip, and it's kinda flexible. Found no broken tracks with the meter, nor with bypassing.

Holy smokes, what to do?

[Mick Bailey]

So we're looking at a mechanical failure that can be thermally triggered. The problem with probing with your meter is that while you're doing this the board isn't in its flexed 'failure' mode.

There are a number of possibilities;

An undetected microscopic crack in a solder joint.
A track that's broken through right on the edge of a solder pad.
A mechanical failure in a component
Bad plug/socket or IDC

Sometimes the fault can be some distance away from the point where you apply pressure. I still think your problem may lie between V1a and V1b. There aren't many components in the clean signal path and if you can inject a signal into the input you can ground one meter probe to the chassis. Then you've got a free hand to flex the board whilst probing the signal path. The first thing to do is establish that you're getting readings at the test points on the schematic, and to see what happens to these when the amp fails.

[Telekaaster]

Hi, Mick!
Thank you for a precise analysis.
I was right in the middle of a looong an dreary reply, listing my actions and thoughts on the matter, while fumbling around in the curcuitery.. I had the amp on, pcb sticking up vertically to get to the components underneath.
So I was poking around like one do, and it was then that I found IT!

The wee little bugger called K1 took only one little sqeeze on the side to put the channel in permanent bad mode. I can only bring it back by white knuckle pressure on it lenghtwise..

What do you make of this, is it the explanation?

The thingy is a Hasco CS212DC24, and I bet it is a hard biscuit to come by... Or does those have equivalents?

Cheers

[Mick Bailey]

K1 is a relay. Check the coil voltage marked, but I think its 24v DPDT 8.3 mA, part # 0036613000.

Maybe someone who has a parts list can verify this.

[Telekaaster]

It is a relay 24v. Got 8 legs, approx 10x12x20 mm. They got them at two big US distributors..
Can alternatives be used?

[Mick Bailey]

Doesn't have to be Fender, so long as it has similar specs. Watch the coil current - most 24v dip relays are much higher than the Fender part.

[g1]

Did you try resoldering it? If you have to replace it you want to make sure it has the exact same footprint. That is why using the actual Fender part is the simple solution.

[Telekaaster]

I have tried to resolder it, did't do anything. I'm pretty sure it is blown. It got a temperature rating up to 85°, and it is placed right next to V1. The day it happened the amp was left on for a whole day with tilt back legs. Got hot up in there...

Anyways, I'm gonna track down that Hasco, slip it in and hopefully it'll work.

I will be back with what I hope will be the conclusion..

[Telekaaster]

Uhm... Not so easy to get this Hasco relay. Could not find it in Fenders online inventory, I've sent them a mail though.
Two distributors shows up with google, Online components and Chris electronics. One has a 100$ minimum order plus shipping, the other has a 200$ shipping fee.... These guys does not deal with smalltimers.

There must be a small operation somewhere who keeps a Hasco CS212DC24 in a drawer.... Anybody to the rescue?

[Telekaaster]

Forget that, my 2010 version is equipped with hascos, but as Mick points out above Fender now use the yellow NAIS relays. I ordered them at Amplified Parts now....

I'll be back with the result when package arrives.

[Mick Bailey]

I'd make 100% sure of the fault condition before hunting down a part. Not saying it isn't the relay itself, but I like to know what the exact nature of a fault is if it isn't a cheap part or if I have to order something in.

So, is the coil going open circuit, or are the contacts sticking, or showing high resistance? I have a little compass taken from a key fob, glued to a plastic stick. When a relay coil operates the needle rotates. Cheap. I always intended to get a 'magnet stick' but the little compass works just fine. So that tells me whether coil is working (or not). With most amps the relays operate with the amp in standby, so a resistance reading can be taken to see what's getting switched or not once the coil is eliminated.

It doesn't have to be a Hasco. Could be a Panasonic or any other brand so long as the pinout, voltage and current rating are correct. DIP relays usually follow the same convention for pinouts. I'd install an IC a socket and plug in the new relay. A dab of hot-melt will prevent it from coming loose, though many prefer to solder the replacement back in.

[Telekaaster]

Thank you for more great tricks of the trade. I do have a little compass for my pickup winding, I'll glue it to a stick..

I took out the relay allready, and I rather not stress the points more than I have to..

However that socket idea is brilliant, and gives me an opportunity to understand the nature of this IC. Have I done proper research when I find a DIP 16 socket to be appropriate? Just remove the legs not used?

[Mick Bailey]

Ah, I see my previous post was a little too late. Anyhow, you've got the relay supplier sorted.

Yes - a standard DIP 16 matches the pin spacing. Just snip off the redundant pins to match the board. That way if the relay ever fails again you can unplug it, rather than re-stressing those oh-so-fragile pads.

[52 Bill]

Unless you think that you have damaged the relay by removing it from the pc board, why not test it out of the amp?

[Telekaaster]

Unless you think that you have damaged the relay by removing it from the pc board, why not test it out of the amp?

Well I would really like to, but how do I do it?