• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

C4 preamplifier dc fluctuation

H

Hifibythekg

Super Member
My C4 is sounding great but despite replacing the trimpots with Bourns the dc fluctuates quite energetically between -170mV and +180mV. Occasionally it will even stray further by another 100 or 200mV.
Thoroughly checked all solder joins, switches cleaned etc.
My question: are there any particular transistor or capacitor locations that I should test out of circuit?
I realise that the DC range is close to the SM spec, but hoping that I could calm it’s volatility.
 
Register to hide this ad
Check your voltages on TR175.. That would be my prime suspect.
 
Cheers Avionic.
And.. many thanks for your help with my A-1 restoration. It’s working and sounding very very special. Exceptionally good on revealing vocal qualities and character I think. Your advice on replacing the original transistors at TR 138 and TR138 were the final piece of a jigsaw that allowed adjustment of the idling current having put TC and TA004’s in the Drive Board and replacing TR 139/140 on the Electrolytic Cap Board. Boy, what a journey but on ‘Disc‘ the sound with vinyl is incredible. Do you know of anyone successfully getting a direct feed (not via power amp and attenuation) out of the A1’s preamp section yet?
Anyway, thanks again.
 
Yep.. Out of circuit. It is a dual field effect transistor ( FET ). A transistor tester will probably tell you it is OK even though it doesn't work properly in circuit.

BTW the only sure fire test is to swap it out with a different one and see if the DC offset settles down.
 
Last edited:
Swap the left channel with the right channel and see if the problem follows the swap.
 
Thanks Avionic.
I should add that this energetic ranging up and down activity is happening on both channels.
So if I swapped them over I imagine it’ll just continue in a similar manner.
Happy to test them though.
But is there anything else that could be causing this and worth checking over while I’m there?
 
Thanks.
If, in your experience, they don’t often/ever go wrong, should I be looking out for bad connections to it?
Also, if I don’t have the equipment to properly test it I suppose I can only gamble on a replacement?
Have I got this right?
But there aren’t any other suspects that would contribute to this energetic ranging of DC almost within the SM parameters?
 
It could be anything in that amplifier circuit. Bad solder perhaps. In both channels-:idea: .. Slim chance but anything is possible.
 
There's a reason Yamaha adjustment parameter is 0 +/- 200 mv.. I get it close to the ballpark and forget about it. But thats me..
 
Not that I can think of.. Other that don't get the stuff in your eyes.
 
Thanks, yep, good tip.
I’ve run into a problem with my lovely A1.. I’ve posted on the other thread but it’s this.. please could you advise? I didn’t replace any caps at all in the A1.
“A-1 has been playing music loudly and beautifully for a couple of days but .... eek!

Just had a random burst of ‘static-ish’ sound through the left speaker of my main sound system. Can’t risk these huge vintage speakers for a minute so have disconnected the amp.

What’s the most likely culprit?

The sound was like two pieces of soft cardboard being scraped but amplified.“
 
Bizarre..
Cold spray applied to area around TR 180. Also TR175. A few extreme readings occurred around 1000mV. But all has settled down and now the range is around +/- 50mV on each channel and much less volatility even an hour later. Surely this action can’t have fixed the issue? Will power down and check again later on..
 
After powering up, the same problem returns.
Freeze spray on TR195 settles the problem for at least 30 mins. DC reads +/- 25mV without volatile fluctuation.
TR195 is a 2SA673A. (SM spec says 2SA673A,C, or D.
What's the best substitution in this location?
KSA1013YTA?
 
You must log in or register to reply here.
Back
Top Bottom