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Exposure IV...Dual Mono I believe...any Circuit diagrams/schematics out there??

OK...with some help (Thanks David)...been in there now.

Right channel working....probs on the left. It seems that the O/P transistor on the -ve rail is 'full on' (excuse my paraphrasing!) all the time. A bit of fishing around on the PCB found that one of the small transistors (BC182C I think...(but might be MPSA56) ...although testing OK, was found to have an unacceptably low hfe value.

So...plan is to replace BC182C's...and MPSA56's for a start.
What are the currently available equivalents for these please??
UPDATE...can see that Langrex have MPSA56, but BC182B, and BC182L instead of the BC182C I'm wanting.....will either/both/none suffice??)

The 40 year old Res. caps....were tested, and although working, had dropped massively from 10000uf spec.....so new caps too!
Tempted to replace relays too.
 
bc182B is readily available. The "B" refers to a gain band but in practice they will all have much higher Hfe than the standard requires so will replace a "C"
There's also a BC182L which is unashamedly a higher gain version. Either will do.

MPSA56 is still fairly readily available even though manufacture recently ceased. Be careful where you buy but it shouldn't be hard to get if you want a real one. BC556 is pretty much identical if you want to play completely safe.
 
colasblue said:
bc182B is readily available. The "B" refers to a gain band but in practice they will all have much higher Hfe than the standard requires so will replace a "C"
There's also a BC182L which is unashamedly a higher gain version. Either will do.

MPSA56 is still fairly readily available even though manufacture recently ceased. Be careful where you buy but it shouldn't be hard to get if you want a real one. BC556 is pretty much identical if you want to play completely safe.
Click to expand...
...just also spotted an MPSA06 on the board too...
 
Arkless Electronics said:
A, B, and C are the gain banding bur beware that the L means a different pin-out.
Click to expand...

...so I need to stick to the same banding...C...?
Sitting here at the moment going through the RS site....there's a branch here in Leeds, and I can pick-up, and save postage costs!
Trying to find an equivalent....like the BC546...where I can buy 10 or so...and not 500, 1000 etc
Proving difficult!
Wondering if there's a ZTX variation that would work?...and they come in 10's...! :)
 
Don't worry about the banding. Years ago transistors were batch tested for gain and the better ones sold for more. These days they pretty much all exceed the highest band but are untested. They just call them all "B".

Another BC546 would almost certainly substitute a BC182 so you could just buy BC546 and 556
 
Tonewheelkev said:
...so I need to stick to the same banding...C...?
Sitting here at the moment going through the RS site....there's a branch here in Leeds, and I can pick-up, and save postage costs!
Trying to find an equivalent....like the BC546...where I can buy 10 or so...and not 500, 1000 etc
Proving difficult!
Wondering if there's a ZTX variation that would work?...and they come in 10's...! :)
Click to expand...

You have the main Farnell in Leeds which will often sell in smaller quantities. They show BC182B as in stock in min quantity of 5 at 13p each

First I must point out that blanket replacement of components in the hope that amongst them you get the faulty one is no way to go about repairing anything! Exposure will no doubt repair it for you as can I.

I would say some of the other advice upthread is a little cavalier...
 
Unfortunately ...Farnell doesn't have a counter in Leeds anymore...closed a few years ago :(
Found small Q's at Ebay/Langrex....but still wanting a couple of MPSA06 for completeness.

Realise what you're saying about blanket replacement...but I'm not really doing that, having now found at least 2 small transistor faults...!

Re-checked...counter in Leeds been closed for 5 years now....
 
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That diagramme certainly can't be right since it shows a complimentary darlington output stage and the Expo uses the symmetric darlington circuit. BC556's also certainly aren't suitable candidates for the output transistors.
 
Hi !

Has any of you Exposure IV owners been able to determine whether the original power transistors are regular type or Darlington? I just can't figure out how the TO92 drivers (MPSA06 and MPSA56) can be adequate to drive non-Darlingtons. Most pictures I see of those amps that have had their output transistors replaced have MJ15003 and MJ15004 in place of the originals.
 
Jean Landry said:
Hi !

Has any of you Exposure IV owners been able to determine whether the original power transistors are regular type or Darlington? I just can't figure out how the TO92 drivers (MPSA06 and MPSA56) can be adequate to drive non-Darlingtons. Most pictures I see of those amps that have had their output transistors replaced have MJ15003 and MJ15004 in place of the originals.
Click to expand...

Just email Exposure directly. They’re very helpful.
 
Jean Landry said:
Hi !

Has any of you Exposure IV owners been able to determine whether the original power transistors are regular type or Darlington? I just can't figure out how the TO92 drivers (MPSA06 and MPSA56) can be adequate to drive non-Darlingtons. Most pictures I see of those amps that have had their output transistors replaced have MJ15003 and MJ15004 in place of the originals.
Click to expand...
The original power transistors are regular. The Exposure IV 1987 year of production has a normal mid power transistors on this places SM2178A, SM2177A.
 
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