Marshall Reverb 75 (5275) - Hum issues

[turk 182]

plugs are males jacks are females, you stick a plug into a jack....don't giggle thinking about that...just check the input jack,please!

 

[decap]

Main input has "shorting" type of jack socket. The measured resistance across is 0.5 ohm, which can be considered as pure short.

 

[turk 182]

ok that's good...so now we have to look elsewhere...

 

[decap]

Yep, I have cleaned the shorting contacts on all jacks with deoxit.

 

[turk 182]

i'll wait for pic's of the solder side, i'll wager there's either a broken control, funky solder or something still amiss with the send return jacks..


after that confirming that the low voltage supply is ok would be next on my list.

 

[decap]

Ok, will shoot some detailed pics of the solder site.

Send/Return were cleaned (only return has shorting jack), but not resoldered.

Low voltage power supply was checked and partially rebuild (new 100uF caps, paired 15V zeners, approx +/- 16.2 V on both rails).

 

[turk 182]

i'm stopping for a scotch break....

 

[decap]

i'm stopping for a scotch break....

Thank you and enjoy your scotch.

 

[decap]

Solder side photos

 

[turk 182]

some of the photos are not well focused but one thing that stands out is some of the traces have been jumpered, not a good sign!


i take it the jack board on the back of the amp is still installed?

 

[decap]

Do you mean the "jumper" near the red LED? That is my work. The track was cracked, scraped down the lacquer and bridged it.

The back jacks are still connected.

 

[decap]

Maybe if I resolder everthing it would be faster then remote photo solder joint examination .

 

[decap]

The amp is on the bench again, so I can perform test or take additional photos.

 

[turk 182]

just be careful...as someone who does a lot of repair work blanket solutions means that you never learn the details of the fault and that would make me lose sleep and drink more scotch...


considering some jumpers have been employed in previous repairs is it possible the board was flexed enough to develop a crack (try backlighting it see if you notice any cracks in the phenolic)
and a magnifying glass does help in exposing faulty solders.

 

[decap]

Ok, going to inspect the board with "backlighting".

 

[decap]

The PCB seems to be good. No cracks.

---

Regarding the E.Q. push/behaviour without the E.Q. engaged the input has higher sensitivity. Another indication that the issue is in the preamp part.

 

[decap]

@turk 182 what jumpers exactly are we speaking here?

For example the jumper between c33 leg and L6 point is there from factory.

 

[turk 182]

ok i stand corrected on the jumpers.


so now it's down to signal tracing stages and confirming local grounds eh.


quick question if you connect to the fx loop return and short the cable do things go quiet?

 

[decap]

Well that will be "fun", because currently I do not have oscilloscope. And my audio probe small active pc speakers were not sensitive enought to track the hum (already tried that).

So maybe the first step will be confirming local grounds.

 

[turk 182]

did you ever get around to checking the rear board?