input selector switch replacement help

[1108]

I’m replacing the input selector switch on my Knight amp. It’s the 3 wafer type. I’m not sure how to wire a new switch. As an example looking at the schematic (attached) point 12 on all 3 wafers have something connected to it. Do I just connect all three of those to point 12 on the new switch?

Thanks

 

[rojoknox]

I’m replacing the input selector switch on my Knight amp. It’s the 3 wafer type. I’m not sure how to wire a new switch. As an example looking at the schematic (attached) point 12 on all 3 wafers have something connected to it. Do I just connect all three of those to point 12 on the new switch?

It sounds as if you're trying to use a single-deck switch to replace the three-deck unit; is that the case? The new switch must be exactly the same (three-wafer) as the original (unless you are doing a major mod, which is a different story). "Point 12," to use your example, has a different function on each wafer. On Section 1, it is the input to the 7025 preamp V1A (left channel). On Section 2, it is the right-channel input to the Stereo/Mono and Normal/Reverse switch. On Section 3, it is part of the right-channel phono equalization circuit. Connecting these three points together would not be good!

The good thing here is that it's not a specially-made switch with custom connections; each wafer is a two-pole five-position switch. The schematic notes "SECT #[x] REAR" which means you look at the switch with its shaft facing away from you. The switch probably won't have numbered terminals; you can use a felt marker to mark them as the schematic shows, which will help in rewiring the switch.

This looks like what you need, but it's spendy (nearly $150!):

D7C0306N-60 Electroswitch | Mouser

How bad off is the original switch? Maybe it just needs a good cleaning.

Take care,
--
J. E. Knox "The Victor Freak"

 

[Eli Duttman]

Hi Guy!

Part of the complexity of the OEM part is associated with the ability to switch between RIAA and NAB equalization, within the low level mag. preamp section.

Assuming you purchased, as I previously suggested, an inexpensive Lorlin brand part, the complexity gets swept away. Directly wire the phono I/P jacks to the low level section, which gets configured exclusively for RIAA. The O/P of the low level section, along with the remaining jacks (for line level sources), gets connected to the rotary switch. The wipers of the new rotary switch get connected to the Normal/Reverse slide switch. What was complex becomes simple.

BTW, if you plan on recording LPs in some other format, the low level preamp section needs additional work. The ability of 12AX7s to drive a load, like a recording device, is WRETCHED.

 

[1108]

Hi Eli,

I thought I'd get started on some of this other stuff while I wait for the PT.

Yes I ordered the switch you suggested.

This sounds simple enough, but I can't positively identify "the low level section" or "The O/P of the low level section". Can you point them out for me?

Not being able to record isn't a problem, but can it handle tape playback ok?

Thanks

BTW, I ordered the tubes and parts from Jim McShane and you're right, fair prices and he doesn't whack you on shipping.

 

[Eli Duttman]

R15 and R44 are the low level section O/Ps. Notice that those parts connect to the source selector switch. Each will continue to do so, but only to 1 rotary switch position.

That low level section employs active equalization and obtains multiple curves by switching the parts in the NFB loop. You will have to identify the parts employed in the phono lo position and retain them. The other parts in the K1 and K4 PECs are disconnected. Personally, I'd discard the PECs and install precise, modern, discrete parts of the correct value. Frankly, I'd discard the whole low level section and install the tweaked passive EQ RCA setup whose schematic I've uploaded. Among its virtues, the passive setup comes close to perfection in matching the RIAA curve and it can drive a recording device.

Modern tape decks are line level devices and listening to tape is not a problem. In the early days of stereo it was common to find units containing a stereo playback head and only mono electronics. Pre-recorded stereo tapes could be listened to by connecting the playback head to tape head I/Ps, while bypassing the internal mono electronics.

 

[1108]

Thanks Eli. I'll look this over and I'm sure I'll have more questions for you.

 

[1108]

Hello Eli,

I’m sorting through this and want to see if I understand you correctly.

Plan “A” replace the switch
- connect the low level output(R44) and the other inputs to the switch
- connect the phono input to K4

“You will have to identify the parts employed in the phono lo position and retain them“. How would I do that?


Plan “B” replace the whole low level section (I like this plan)
What else besides K4/K1 would you circuit replace?


Thanks

 

[Eli Duttman]

R44 is the right channel phono O/P. Don't forget to connect R15, which is the phono O/P of the left channel. Make sure you don't get left/right signal source wires criss-crossed or incorrectly sequenced between the 2 groups of lugs on the new switch.

The junction of R6 and R7 is the left phono I/P. The junction of R35 and R36 is the right phono I/P. Notice the use of shielded wire. If what's currently in place will not reach between the RCA jacks and the "guts", go to a nearby "Rat Shack" and buy a small spool of shielded wire.

Phono Lo on the OEM selector corresponds to 1 of contacts 3-4-5 (left) and 9-10-11 (right) on the back of the switch's 3rd deck. You have to make the identification. Whatever wires "survive" get connected to the wire at contact 6 (left) and contact 12 (right).

Plan B is a big job and requires a level of experience/knowledge you seem to lack. Look before you leap! If you can't figure out, on your own, what to do, don't undertake the effort at this time.

 

[1108]

So plan A it is.

I've build a few amps and understand connecting the switch, etc. But I've never did a rehab like this other replacing a few caps.

Thanks for all your time and help.