Super-sonic 22 preamp acting funny..

[52 Bill]

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

With an ohm meter and a power supply.

The coil is connected to pins 1 and 16, so they will be at one end of the case. Check this by reading the resistance across the two pins. There should be some reading there or the coil is open.

There are two sets of switch contacts the first set is connected to pins 4, 6, 8. Pin 4 is the switch, pin 6 is the normally closed (NC) contact and pin 8 is the normally open (NO) contact. Use your meter to read the resistance across pin 4 and pin 6. It should be nearly zero ohms. Apply a voltage across the coil and check the resistance across pin 4 and pin 8. It should be nearly zero ohms.

If you don't have a power supply try a nine volt battery or two nine volt batteries in series to get 18 volts. That should be enough to switch the contacts for this test.

Repeat the tests with pins 13 and 11 (NC) and then pins 13 and 9 (NO).

[Mick Bailey]

Referring back to post #26 and making sure of the fault condition;

You know the coil is OK because the 'burn' mode works fine and this is with the coil energised. You then also know the 'burn' contacts in the relay are good.

This leaves the clean 'vintage' mode, which is enabled with the relay in its non-operated condition. If the coil was open-circuit, it wouldn't matter one bit to the clean sound and so the coil can be ignored. The relay is split into two contact sets; K1-A and K1-B. With relays there are usually two failure modes with contacts; they arc up under excessive voltage/current and weld together, or they get oxidised and go high resistance.

We know they're not welded as the amp switches to 'burn'. Again, it wouldn't matter to the clean sound if they were. So this leaves the question of what happens if either of those contacts goes high resistance.

With K1-A, no signal is passing through the contacts. It's merely grounding one end of R5 and if you trace this through there's no connection at all off the wiper of R31. So high resistance in those contacts wouldn't make any difference to the clean sound.

So we're now left with looking at what happens if K1-B goes high resistance, or even opens up completely. If it goes open, then there's no signal at all getting from the volume pot wiper to R45/R14 and therefore no signal getting to the grid of V1-B. There could be some capacitive coupling via adjacent tracks on the PCB that may give a ghost signal, or even between triode halves, but it wouldn't be much.

If the contacts go high resistance, then this resistance will form a voltage divider with R45. The volume and tone controls will still operate, but the signal could be greatly reduced.

The only feasible failure mode of the relay that could affect the clean sound is K1-B's contacts in the NC position, with two outcomes - no sound or reduced sound. You get sound, but the controls don't work. This doesn't quite fit with my view the relay failing unless there's some coupling with the circuit that can't be seen from just looking at the schematic.

One check would have been to solder a temporary bridge between K1-B pins 4 & 6 and re-test to prove/disprove the relay.

EDIT: References to K1-A and K1-B are reversed - see later posts. I've left the text as it stands so that later posts still make sense.

[Telekaaster]

Thanks, Bill and Mick for all that knowledge.. Most appreciated!
Hooked it up to my laptop power 19.5 vdc, and I'm happy to announce that the relay shows defect. Mick, I believe you got the A and B mixed up. But you are right, it is your K1-b, actually the K1-A that has the problem. With no power on there is no connection between 4 and 6. With power, the switch works and there is connection between 4 and 8. The Burn..
Other side is good..

With my limited knowledge, I can't for my life understand how this could bypass my controls and give a volume boost,though. I mean, where does the signal travel instead....?

[Mick Bailey]

Mick, I believe you got the A and B mixed up. But you are right, it is your K1-b, actually the K1-A that has the problem. With no power on there is no connection between 4 and 6. With power, the switch works and there is connection between 4 and 8. The Burn..

You're right - I mixed up the K1-A and B between flipping over to view the schematic and typing.

The thing about signal paths and a schematic is the difference between the map of the London Underground and the actual layout in real life. That neat circuit layout on paper doesn't show the proximity of adjacent leads, PCB tracks, unintentional capacitive and inductive coupling, or signal leakage back through the power supply. It's surprising that you got a reasonable signal level - I would have expected it to be just about audible like when a series FX loop contacts go high resistance.

Anyhow, It's excellent that you've got it sorted out.

[Telekaaster]

I'll be back with a report as soon as parts arrive, to bring the thread to a conclusion.
In the meantime I'll fiddle with less complicated matter, a 5F6a kit rebuild....

Gentlemen, thanks a million again for your time and expertise. If you ever find yourself up in Oslo, be sure to come and collect that pint

[Telekaaster]

Greetings, ampheads..
The relay took forever by Usps, 3 1/2 weeks.... Nice.
Anyhow, I plugged it into my allready installed dip socket(great idea, Mick). The amp was back, and have been stable since.
Hope others will benefit by my journey through this Mordor of a circuit. I'll assume these relays are in abundances in newer Fenders...
Thank you all!
Case closed.