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Old Technics amp gain has gone mental after changing some parts

Anolsos_1974

Anolsos_1974

Active Member
Hi all, it’s a technics su c04. The volume pot was shot so I swapped it for the same spec pot. Stupidly I didn’t keep track and after replacing some bad caps (the two main caps, and the one in the power supply) I noticed the gain on the main volume knob has jumped - now it reaches full volume in about one quarter turn of the volume control. My limited experience is wondering if there is a mistake with the resistance of the VR as I had a similar experience replacing VRs in old speakers where I couldn’t find the exact spec VR so all the recognisable change in respective driver volume occurred in a half turn rather than a full turn, if that makes sense.

I don’t want to waste anyone’s time but if there is something obvious I’m doing wrong I would appreciate it!

I attach two pictures, the second shows heat damage around the power supply section which prompted the change of that capacitor
 

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Anolsos_1974 said:
I noticed the gain on the main volume knob has jumped - now it reaches full volume in about one quarter turn of the volume control. My limited experience is wondering if there is a mistake with the resistance of the VR as I had a similar experience replacing VRs in old speakers where I couldn’t find the exact spec VR so all the recognisable change in respective driver volume occurred in a half turn rather than a full turn, if that makes sense.
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Did you replace an original audio taper (log taper) pot with a linear taper one?

The full throttle at 1/4 turn is a classic symptom of this.

Good luck!
 
Anolsos_1974 said:
Yes I have this! Long discontinued unfortunately. I have a number of a uk company that does audio stuff so they might have something. I guess linear pots are what most folk need.
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So the pot is a linear taper one? Then the issue is elsewhere.

I would start by making sure all the wiring to and from the pot is correct.

Good luck!
 
Mr Boochie said:
In general, most volume controls are audio (logarithmic) pots. When you substitute a linear pot for an audio pot, you get a super-sensitive control that maxes out at 10 O'Clock.
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I didn’t know this so thank you. I’m trying to learn on this less important amp so I don’t make the mistakes on my proper amp which has similar, and intermittent faults
 
Anolsos_1974 said:
I didn’t know this so thank you. I’m trying to learn on this less important amp so I don’t make the mistakes on my proper amp which has similar, and intermittent faults
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See my edited post. I believe you have a linear pot. So, the issue lies elsewhere. Sorry!

Have you downloaded the Service Manual? That is a complicated unit. Good luck!

:thumbsup:
 
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Hello again,

Anolsos_1974 said:
Hello, yes according to the service guide it is a linear pot so the issue lies elsewhere! Strange though because the previous one was just crackling and popping
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Hi, I just put back the original volume pot (which according to the service guide is linear as you mentioned) and the volume works normally again - so is the guide wrong? When i stick a voltmeter on the terminals of both VRs they both look linear. Are there different types of linear?!

I also tried adjusting the dc balance to get 2mV (as specified in the service guide) but the voltage was jumping all over the place. I thought bad pots there maybe so replaced them both but it made no difference - could these two be related?

I also replaced pretty much all the electrolytic capacitors as I had already ordered them. I am actually sat listening to the amp and it sounds ok so I think I am getting somewhere!

But if you have any idea about these niggles I would love to hear!
 
Anolsos_1974 said:
Hello again,


Hi, I just put back the original volume pot (which according to the service guide is linear as you mentioned) and the volume works normally again - so is the guide wrong? When i stick a voltmeter on the terminals of both VRs they both look linear. Are there different types of linear?!
Click to expand...
Thanks for the update!

No, I believe that Technics designed the volume circuit to have a LINEAR pot. While a majority of manufacturers design circuits to use AUDIO (or logarithmic) volume pots, this is not 100% true.
 
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the letter codes for linear vs log are different on Japanese vs US gear. I don't happen to know what the Euro standard is. a US "A" pot is audio/log, in Japan thats linear. US B pots are linear, in Japan thats audio/log.

either way its not real difficult to figure it out if its at all functional. Put the pot at mid travel and measure from each end to the wiper. A linear pot will be half the resistance from either end to wiper, +/- how accurately you set the pot. A log pot will be roughly 85% pot value from one end to center and 15% from the other end. The exact value varies somewhere between about 80/20 and 90/10 but its very obviously not 50/50.
 
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