Servicing an old Rat?

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JSett
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Servicing an old Rat?

Post by JSett » Fri Sep 23, 2022 10:51 pm

Has anyone here done service work to their old Rat? My early 90's one has started popping a little when switching so I suspect one/some of the electrolytics have gone bad (but I will change the switch again first, just in case)

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From what I'm reading here I should be mainly looking at C7 and C10 (and possibly C5 & C6) as those appear to all do DC blocking in some way? Or should I just do all of them now it's 27 years old?

Any advice would be rad

JS
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Re: Servicing an old Rat?

Post by øøøøøøø » Sat Sep 24, 2022 10:47 am

I don't see a switch on the schematic.

I'm not a Rat expert--is this a mechanical bypass switch? Buffer amp?

Pull-down resistors to and/or from switch?

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Re: Servicing an old Rat?

Post by JSett » Sat Sep 24, 2022 10:51 am

øøøøøøø wrote:
Sat Sep 24, 2022 10:47 am
I don't see a switch on the schematic.

I'm not a Rat expert--is this a mechanical bypass switch? Buffer amp?
Yeah, it's mechanical (true) bypass. DPDT switch but and early form of millennium switching for the LED
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Re: Servicing an old Rat?

Post by øøøøøøø » Sat Sep 24, 2022 10:55 am

The components surrounding the switch are the most important for us to see to diagnose your problem (this includes associated circuitry of the LED indicator).

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Re: Servicing an old Rat?

Post by øøøøøøø » Sat Sep 24, 2022 11:04 am

In any case, I'm not sure I'd be looking at a failed electrolytic capacitor as a suspected cause of switch pop, but it generally doesn't hurt to replace them.

I wonder about the switch itself if we're sure the circuit was formerly working fine without popping. Sometimes an old switch can "bounce" just enough to act like a capacitor for a split second and "charge up" a tiny touch of offset voltage.

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Re: Servicing an old Rat?

Post by JSett » Sat Sep 24, 2022 1:52 pm

øøøøøøø wrote:
Sat Sep 24, 2022 10:55 am
The components surrounding the switch are the most important for us to see to diagnose your problem (this includes associated circuitry of the LED indicator).
Here you go.

Image

It is a very particular setup (and quite advanced for its year)
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Re: Servicing an old Rat?

Post by øøøøøøø » Sun Sep 25, 2022 5:43 am

Interesting.

I think I’d try replacing the switch.

This is a bit out of my area of expertise (try asking on GroupDIY) but if the switch was “bouncing” a lot I wonder whether it may develop a charge through that 22M resistor

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Re: Servicing an old Rat?

Post by øøøøøøø » Sun Sep 25, 2022 5:47 am

One other thing I thought of just now—try disconnecting the LED by lifting one leg.

Does it still pop?

If not, try replacing C11 and C13.

If the LED were to draw a sudden bit of current, it’s not inconceivable that it could cause a pop if the power supply wasn’t handling the sudden draw well

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Re: Servicing an old Rat?

Post by JSett » Sun Sep 25, 2022 6:15 am

I did the switch this morning, still popping. I was short two of the right size caps so have ordered them up and I'm going to change all of the electrolytics in one hit: C5, C6, C7, C10, C11 & C13. The led is hot glued in place (which gloops over the IC chip too) so I'll try and leave that be if possible and see if the cap swap does the trick. Theoretically, with a new switch and caps then no DC should be getting where it shouldn't. I ran a MM on them all and C7 is letting approx 2.4mV through, a couple of the others are >1mV, the rest more like 0.1mV.

Apart from a previous footswitch swap it's done very well considering I've been stomping it into oblivion for 27 years.
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Re: Servicing an old Rat?

Post by øøøøøøø » Sun Sep 25, 2022 8:04 am

C1 and C10 (actually C9 before) should be blocking any theoretical DC offset from a leaking C7, but it doesn't hurt to swap it anyway.

Let us know how it goes.

I suspect C11/C13 maybe? reduced capacitance could theoretically cause challenges recovering from an LED turn-on.

One question--the pop occurs whether powering from battery or external supply?

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Re: Servicing an old Rat?

Post by JSett » Sun Sep 25, 2022 9:04 am

It occurs off both battery and PSU. I use a battery 99% of the time.
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Re: Servicing an old Rat?

Post by øøøøøøø » Sun Sep 25, 2022 11:53 am

Let us know what happens after changing the caps! I'm very curious about this

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Re: Servicing an old Rat?

Post by MrShake » Sun Sep 25, 2022 1:22 pm

Definitely. I haven't had any trouble yet, but I've got a couple coming up on 30 years old, so this kind of thing may become more common. Thanks for keeping us updated so far!

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Re: Servicing an old Rat?

Post by JSett » Wed Sep 28, 2022 2:45 am

Okay. So today is my day off and I set about swapping all the electrolytics. I just used some audio grade Nichicon UFW caps:

Image

Annoyingly, it still pops :fp: My 1984 White Face does not. My only assumption now is that the 2N5457 transistor is at fault somehow. I have found one example online of someone replacing that in a Rat, along with the caps, and it solving the popping problem
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Re: Servicing an old Rat?

Post by øøøøøøø » Wed Sep 28, 2022 8:19 am

It's annoying to need to replace one transistor if you don't have them on hand already. You either find yourself buying way more than you need, or paying $10 in shipping costs for a $0.20 part, or contriving a bunch of other stuff to buy to make the shipping worth it.

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