2220b Amp Section Question

[Jonstigator]

I picked up a 2220b recently for $20 dollars at a yard sale. Got it home and it was weak and horrible to listen to. opened it up and on the amp section pins J716-J720 J714-J710 J721-J717 J711-J715 all jumped with diodes pic included.
Bias and offset cant be adjusted.
Why would anyone do this? Any ones two cents would be great. I think it is because the outputs are blown.

Thanks

It has been quite some time since I logged on. I really like the changes.

 

[wlhd1610]

Those are flyback diodes and look to be ok.

Bob

 

[petehall347]

i was intrigued by this and think i found the answer here ... https://www.tubecad.com/2006/10/blog0084.htm

 

[Jonstigator]

so fly back diodes protect my outputs from inductive kickback? I have never seen these on any of my other 2220b or 2225 receivers. Was this done as a repair? Could this configuration cover for a blown output?

 

[Jonstigator]

The diagram shows two channels and only two fly back diodes. Mine has four diodes

 

[petehall347]

it wont affect anything unless there is a fault like inductive kickback .
yours is 2 channels the diagram just 1

 

[Jonstigator]

oops! Typo! I can set bias but no offset. any ideas?

 

[petehall347]

explain where you are checking ..there might be errors in the manual .. it happened a lot .

 

[Jonstigator]

Yup, no spring chicken there. A well known "typo" is the bias adjustment. I measure bias in between pins J714/716 and J715/717. For off set I go right to the speaker jacks.

 

[petehall347]

now try offsets at L701 and L702 to rule out anything after that point .

 

[Jonstigator]

OK. I was not sure how to measure there so I tried across them with no response then I measured from one end of the coils to ground and the voltage varied from 15/16 mv down to 1.5/.3 mv respectfully.

 

[petehall347]

looks good offsets then .. do you have a scope ? no bother if not .. your meter on AC volts at those points whilst sending a signal to inputs should suffice .

 

[Jonstigator]

Should I go from both sides of coil? will music suffice as a signal?

 

[petehall347]

music should do yes .. just use one input source rather than left and right to save confusion .. if you have a mobile phone you could use an app to use a fixed frequency like 1khz .

 

[Jonstigator]

OK, I tried with a CD player in monitor 1. both sides of coil about 270 mv ac and side of coil to frame the same.

 

[petehall347]

maybe bad connection then in the relay .try tapping it or quickly turning up the volume to break through the corrosion

 

[Jonstigator]

tried that, cranked it all the way up trying to change reading. I have a new Omron I can put there but don't think that's it But I will do it if you think so.

 

[petehall347]

will rule it out if nothing else it will be an upgrade , cranking vol doesn't always work

 

[Jonstigator]

what if I checked at J724 and the relay side of L702? It would check the relay contact I think.

 

[petehall347]

what if I checked at J724 and the relay side of L702? It would check the relay contact I think.

yes good idea