2270 weak left channel

[Vehto]

I have another 2270 this unit was really dirty older model. I guess thats why I got it cheap off e-Bay

All the lights were burned out. face plate was dirty some letter wearing off
cabinet is worn well scratches. Knobs were dirty bits of unknown stuff here and there. Dial string real loose

I Replace all the lights with LEDs. Power switch was bad took it apart and cleaned it. All the knobs cleaned up well,replaced the vellum

I checked the offset and Bias and both channels came in per spec.

But to my surprise I have a weak left channel:scratch2: I checked the power transistors and they are fine.

Does anyone have any Ideas before I go checking each transistor?


This amp has seen some good times I gather, sort of feel nostalgic working on this beauty.

Any help appreciated.

 

[airtime]

check the front end. Like the headphone jacks, etc...

I had a similar problem that turned out to be the cabinet screws shorting out the headphone jacks.

sounds like the amp board is OK. trace it from the amp board out.
charles

 

[catrafter]

The tip about foot screws is a good one, seen this problem many times.
If that's not it, try cross patching the pre-main jumpers and see if the problem switches to the right side.

Tom

 

[Vehto]

Ok will try it thanks for the tips

 

[Vehto]

Ok I checked the screws and jacks are ok.

But switching the pre amp jumpers did narrow done the problem. ( Thanks Tom)
weak channel switch to right side so I guess my problem is in the preamp section?
which I never have worked on. first I need to find the board. Then test.
does any one know good test points? I will pull up the schematic. What should I look for

I will read up on it here in this forum.

Thanks again for the tips

Fred

 

[catrafter]

Before you jump in with both feet, have you cleaned and exercised all the front panel switches and controls? Don't forget the switches built into the front panel dubbing in jacks.
The preamp board is indicated by the arrow in the picture.

Tom

 

[Vehto]

Yep I cleaned a the switches and Pots cleaned the dub & phone jack and so on.
I'm reading previous post you & EW ecluser contributed to some time ago.
( Thanks again and again for the wealth of knowledge)


I guessing a recap is in order for the PreAmp board.
If I have a 4.7uf 25v cap laying around I plan to replace C408 on the preamp bd
Plan on replacing the trans to 2sc1000 & 2SA493
still looking for a replacement on 2SA493

Wish digikey had substitution guide of some sort

Fred

 

[catrafter]

2SA992 and 2SC1845 should work OK for those.
Mouser has the Fairchild versions (KSA992 and KSC1845).

Tom

 

[mattj0582]

I had a similar problem on a 2275 and turned out to be the caps on the tone board. Works lovely now.

 

[Vehto]

yeah Im in the process of ordering the caps. I wonder if I should replace the round disk ceramic caps (pf) along with the Electrolytic?

 

[catrafter]

yeah I'm in the process of ordering the caps. I wonder if I should replace the round disk ceramic caps (pf) along with the Electrolytic?

Not unless they are physically broken. They last darn near forever in this application.

Tom

 

[Vehto]

Ah good I can save a little ....On order

Thanks

 

[Vehto]

Update:

I replace the caps and trans on the Left channel only on the preamp board and ,,,dang same problem weak left channel. :thumbsdn:

I did a resistor check on both sides of the board and the results compared the same. Left vs Right

So I noticed when I pushed in the "R" mono switch the left channel comes in stronger (P600 BD)

Nothing happens when I press the "L" mono switch (with the speaker on the bad left channel)

So I dunno where to go from here? I noticed the sound gets worse as I let the unit run longer. any suggestions appreciated.

 

[Smwilli]

Almost sounds like the problem is related to heat as the unit warms up. Maybe try some freeze spray on possible bad components?

 

[Vehto]

Yes I ran out I will buy some today..Thanks

 

[Vehto]

More testing today
preamp board J401 I measure .4-6mv J402 1.9mv with volume all the way up.

Another test
While speaker hooked up on the weak left channel pressing Hi Blend switch boost the sound a bit
Right channel Hi blend does nothing. (cleaned all switches pots and Jacks)

Question could I have a dirty speaker relay? :scratch2:

Any help welcomed.

 

[sregor]

Question could I have a dirty speaker relay? :scratch2:

Any help welcomed.

Remember when you switched the signal at the premain and it went to the other channel? Means it's not the speaker relay!

Another way to check the speaker relay - connect a speaker between the two (left and right) positive speaker connections - if the relay is okay then the signal should get louder if you turn the balance control to either extreme. Safest to do at low volume levels.

 

[Vehto]

mmm ok Thanks Sregor

 

[Vehto]

More testing today I swapped wires on the Preamp bd J401 J402

the problem affected the right channel. I checked resistance on pots r005 & 6
They seem to be fine. So the problem is before the Preamp Bd.

I took it the front of the unit apart half I did not see any broken wires on the switches or pots including the selector switch. This one has me stumped.

The left side did sound better with the new caps on the preamp bd once I figure out the problem I will fix the right side.

The hi blend switch does affect the left channel for some reason,:scratch2::scratch2:
I have not taken the switches apart.

Anyone have any clues? or point me to some info?

 

[sregor]

Not a lot left. Signal goes selector switch, Tape monitor switches, mono switches, then volume control and balance. The hi blend is just before the volume. Have you tried the tape inputs instead of through selector? (This would eliminate or point to the selector switches) There is also the dubbing jacks as mentioned by Catrafter earlier. (the signal goes through interrupter type connections in the jacks) Scope would be real handy - you may be able to signal trace with a DMM if it is sufficiently sensitive. Use low frequency signals ac mV scale. Good luck