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Old 28th January 2009, 03:51 PM   #1
KP11520 is offline KP11520  United States
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Default Supro S6616 Trojan Tremolo Help 1965

Hi Vintage Guitar Amp Fans,

I picked up an old Supro S6616 from Craigslist for a friend in an other state. (1965)

The Tremolo works but it is stuck on one speed, so I told him I would fix it before shipping it to him. The Pot/SPST Switch has been changed but it tests fine. It is a 500K and it is a Linear Taper and its values change appropriately across the rotation. The amp is point to point wired so I traced all the wires from the Pot/Switch to the corresponding component. All in order. I also checked every value on all the resistors and caps in the Tremolo circuit, all within spec. I would think if there was anything wrong with the 12AX7, the whole circuit wouldn't work.

The schematic shows a 500 ohm pot but all the other Supro amps with the same circuit (especially the S6622), all show a 500K pot for speed in the Tremolo circuit.

Here is a resource for Vintage Amp Schematics to see: http://www.schematicheaven.com/bargainbin.htm

If you want to see the schematic, unfortunately it is too large to load here so please use the above hyperlink to see the S6616 and S6622

Anybody have any Ideas? I would be Eternally Grateful!


Regards//Keith

BTW, Hi to all of you who I haven't seen in a while! (sorry, playing guitar, of course)
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Old 28th January 2009, 11:54 PM   #2
KP11520 is offline KP11520  United States
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This is the amp!

Hopefully someone can make a suggestion of what to check next! Thanks!

Regards//Keith
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File Type: jpg '65 supro s6616 w.jpg (35.0 KB, 104 views)
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Old 29th January 2009, 10:03 AM   #3
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I agree, it's obviously a 500K pot - and there's almost nothing that could cause it not to work. Basically there's the pot, the 100K resistor in series with it, or the connections to them both.

Check with an ohmmeter from where the 100K meets the .03 and .01 (actually test from one of the capacitors) down to chassis. As you vary the control the resistance should change between 100K and 600K.
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Old 29th January 2009, 02:54 PM   #4
KP11520 is offline KP11520  United States
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Quote:
Originally posted by Nigel Goodwin
I agree, it's obviously a 500K pot - and there's almost nothing that could cause it not to work. Basically there's the pot, the 100K resistor in series with it, or the connections to them both.

Check with an ohmmeter from where the 100K meets the .03 and .01 (actually test from one of the capacitors) down to chassis. As you vary the control the resistance should change between 100K and 600K.
Hi Nigel,

Now this is funny (not really). If I test from the ground (wiper) to where the pot connects to the 100K resistor I get from 1.9K to 490K. Then I go to the other side of the 100K resistor and all I get is 104K. The actual value on the resistor is 104K, so even going through the pot and the resistor, all I get is the value of the resistor. But the pot by itself reads fine when standing alone across its range.

?????? It Boggles my MIND!

Thanks for your help!

Regards//Keith
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Old 29th January 2009, 04:41 PM   #5
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Measure actually on the 100K resistor, what does it read to ground from either side, and do either of them vary as you turn the pot.
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Old 29th January 2009, 05:32 PM   #6
KP11520 is offline KP11520  United States
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One probe to ground (chassis), the other to the far side of the 100K resistor, 104K. When I jump over the 100K resistor to the wire that runs to the outer lug on the pot, at that moment, 403K. every time I moved the pot to a new value, it changed on the lug wire. When I jump across the 100K resistor in series with nothing in between, always 104K.

Freaky?????????

The sum of the parts add up to less in this situation!

In fact, always 104K when in series regardless of what the value is on the pot.

Thanks!

Regards//Keith
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Old 29th January 2009, 06:40 PM   #7
KP11520 is offline KP11520  United States
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So just to eliminate a variable and be sure, I went to RS and bought a 500K pot as a test device.

Sure enough, after the change, measuring from ground to the far side of the 100K pot, I get from 104K to 580K.

Who would ever think. The pot in a stand alone test tests fine but in series acts like a piece of wire.

Maybe the wattage was too low????? but when tested with a piece of test equipment in series doesn't stress it?

That answer is done! Now to find an appropriate pot/switch for this 44 year old amp hopefully with the correct knob.

One last question:

Us Yanks use 120 Volts AC. This amp came with a 2 prong/wire AC plug. I want to use a 3 prong/wire setup with ground to lessen the chance of hum, this is what I think is the right configuration (want to be sure on this vintage stuff). Ground to chassis, Neutral to fuse side and Hot to switch side!

Any thoughts on that format?

Thanks Nigel!
Pints to you for bringing me to the next level thought on troubleshooting! I was stumped!

Regards//Keith
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Old 29th January 2009, 07:41 PM   #8
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Quote:
Originally posted by KP11520
Us Yanks use 120 Volts AC. This amp came with a 2 prong/wire AC plug. I want to use a 3 prong/wire setup with ground to lessen the chance of hum, this is what I think is the right configuration (want to be sure on this vintage stuff). Ground to chassis, Neutral to fuse side and Hot to switch side!

Any thoughts on that format?
Yes, that's sounds fine - although personally I'd like to see the fuse as well as the switch in the live (hot) side (for that matter I'd like to see a double-pole switch). Perhaps it's done that way because it's a non-polarised plug?.

All UK sockets are three pin, earthed, and have a fused plug - it's been that way for decades now - although preceding that there where many different types of plugs, both two and three pin.
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Old 29th January 2009, 08:11 PM   #9
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Quote:
Originally posted by KP11520
[B
. Ground to chassis, Neutral to fuse side and Hot to switch side!

Any thoughts on that format?

[/B]
I always put the fuse in the hot line, before the power switch. This seems to be the common commercial practice as well. (Check out some schems at schematicheaven for commercial samples, if you haven't found your way there already.)That way, if the fuse blows, you don't have anything 'live' on the chassis beyond the fuse holder. With the fuse in the neutral (white) line, the chassis (and the guitar plugged into it) could be hot even after the fuse blew (assuming that the breaker in the household/venue didn't trip.)

Cheers
John
PS (edit) While you are at it, I'd consider replacing the .01 cap that's across the AC line with a new X2 type cap X2 'Safety' Caps (or eliminating it altogether).
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Old 29th January 2009, 10:19 PM   #10
KP11520 is offline KP11520  United States
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Thank you both for making me aware of better ways to make 44 year old technology safer.

Funny thing is that .01 Ceramic Disk cap that the schematic shows across the AC line is actually between (before) the fuse to the chassis.

Any thoughts on handling that if I put the Hot side through the fuse? Leave it there or replace it with a safety cap or move it to the Neutral side or with a safety cap there or a safety cap on both (hot and neutral) to chassis ground or lose it and put a safety cap between Hot and Neutral?

I might as well get this right!

Thanks fellas!

Regards//Keith
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