Just finally 'finished' an Exposure VI/VII combo that I've 'restored' from a rotten/non-working state.
Now, I have to remove both boards to get down to LH Channel board🙁
Suppose I should've tested each board before building back into casing...DOH!!
I'll check my soldering, and look for any dry joints....but would appreciate any other ideas/pointers please!!
- Removed all Tant. beads, and replaced with correct electrolytics.
- Updated remaining small electrolytics.
- Tested VI (PSU), and made up 2 x 5pin Domino/Dice Din leads to connect to VII Pre...all tested good (to me!)
- Powering on...Line inputs all present, but Disc (MM) input was initially intermittent (LH)...then LH stopped, leaving only RH.
- After a few minutes....high pitch audio tone can be heard on LH....regardless of input selection.
Now, I have to remove both boards to get down to LH Channel board🙁
Suppose I should've tested each board before building back into casing...DOH!!
I'll check my soldering, and look for any dry joints....but would appreciate any other ideas/pointers please!!
"High pitched tone " ------oscillation = instability .
Easily located if you have an oscilloscope ?
If not connect a wide-band AC millivoltmeter at the output to verify level of signal then trace backwards .
While I don't like tantalum either they do have some positive features that your Exposure has been designed to use.
Its possible that they were used to cut down the signal paths lengths thereby lessening insipid oscillation.
Easily located if you have an oscilloscope ?
If not connect a wide-band AC millivoltmeter at the output to verify level of signal then trace backwards .
While I don't like tantalum either they do have some positive features that your Exposure has been designed to use.
Its possible that they were used to cut down the signal paths lengths thereby lessening insipid oscillation.
No oscilloscope ....but I know a man who has one!!
I have recently been in touch with the designer of that combo....and while Tants were used originally....I've been recommended by him to drop 'em.
Could failing silicon contribute to instability???
If you mean active devices -IE- BJT,s /Mosfets /etc not in my experience I still have 40 year old mosfets in my amps +drivers etc still going strong .
"Failing " usually applies to passive components like capacitors /resistors which do fail due to overheating / leakage/rise in esr / increase/decrease in value over the years .
If you can get a loan of one connect it to the output both without and with a simple resistive load --no signal input and check the output signal strength and frequency.
You should see HF waveforms either pulsing or steady state .
Feedback and compensation capacitors should be checked they are usually small values like 100pf/50pf/ or lower, even 5pf depending on design .
If you could post a circuit diagram I could direct you to the vital ones .
"Failing " usually applies to passive components like capacitors /resistors which do fail due to overheating / leakage/rise in esr / increase/decrease in value over the years .
If you can get a loan of one connect it to the output both without and with a simple resistive load --no signal input and check the output signal strength and frequency.
You should see HF waveforms either pulsing or steady state .
Feedback and compensation capacitors should be checked they are usually small values like 100pf/50pf/ or lower, even 5pf depending on design .
If you could post a circuit diagram I could direct you to the vital ones .
Thanks Duncan2....think this is just what I need!!
Have attached 'theoretical' circuit diagrams for the Exposure....Line and Phono sections.
Component ID's wont necessarily match those on the boards in front of me...
I've linked to a pic of board:
https://i.imgur.com/5TDou5m.jpg
I should add that, when acquired...BOTH boards had burnt out R77...and 4 x shorted Tants on EACH board. Repair work, for me, has been to replace R77, all 10uf Tants (with electrolytics)...and renew the remaining electrolytics.
The board you can see is fine....but Sod's Law....the lower LH channel is the one with the Phono problem. (There is also some quietly audible 'Phutt-putting' which I haven't mentioned yet.....a couple of minutes later...the high tone starts....)
Have attached 'theoretical' circuit diagrams for the Exposure....Line and Phono sections.
Component ID's wont necessarily match those on the boards in front of me...
I've linked to a pic of board:
https://i.imgur.com/5TDou5m.jpg
I should add that, when acquired...BOTH boards had burnt out R77...and 4 x shorted Tants on EACH board. Repair work, for me, has been to replace R77, all 10uf Tants (with electrolytics)...and renew the remaining electrolytics.
The board you can see is fine....but Sod's Law....the lower LH channel is the one with the Phono problem. (There is also some quietly audible 'Phutt-putting' which I haven't mentioned yet.....a couple of minutes later...the high tone starts....)
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In the line PDF the compensation capacitor is a standard placement of design its =C23 value 39pf ,try replacing with 60pf (TEMPORARY !!) and see if the oscillations are reduced /disappear.
In the phono --same value -39pf =C13 replace (TEMPORARY !! ) with 60pf .
I like the bunched -- BC 560C,s, at the input they are one design idea to drastically reduce noise but they are high gain =420Hfe (minimum ) so while you amp is sensitive it doesn't take much to cause instability--no don't change them I like them.
That noise is what we call in the radio repair world "motorboating " I can guarantee its caused by -----yes--- instability ,
just verifies it to me.
In the phono --same value -39pf =C13 replace (TEMPORARY !! ) with 60pf .
I like the bunched -- BC 560C,s, at the input they are one design idea to drastically reduce noise but they are high gain =420Hfe (minimum ) so while you amp is sensitive it doesn't take much to cause instability--no don't change them I like them.
That noise is what we call in the radio repair world "motorboating " I can guarantee its caused by -----yes--- instability ,
just verifies it to me.
Unfortunately...not in a position to substitute alternative cap values...as I don't have any to hand🙁
When small value....20pf, 39pf age.....do measured values drift up...or down please?
The compensation capacitor --as opposed to the FEEDBACK capacitor/resistor to the base of the second input controls the stability and also the ultimate HF bandwidth of the design too high a value and the bandwidth drops --too low and --oscillation .
Okay then I am a practical person without changing values as you don't have them then try the old school "trick " put your finger on the legs of the BJT that the 39pf is attached to and listen for a reduction in noise .
What you are doing is applying BODY capacitance ,don't make fun of it I could name some well known audio designers who have (in the past ) done the same thing to give a quick check on stability.
Go to car-boot sales buy old (but good quality ) non working amplifiers and use the parts in them as a standby, even charity shops .
For American viewers car-boot= TRUNK.
Its a case of when you make changes to an audio design in a high sensitivity amp oscillation can occur , I usually fit high quality Polystyrene low values ( as per JLH ) in that position, I leave others to fit what they think fit .
Okay then I am a practical person without changing values as you don't have them then try the old school "trick " put your finger on the legs of the BJT that the 39pf is attached to and listen for a reduction in noise .
What you are doing is applying BODY capacitance ,don't make fun of it I could name some well known audio designers who have (in the past ) done the same thing to give a quick check on stability.
Go to car-boot sales buy old (but good quality ) non working amplifiers and use the parts in them as a standby, even charity shops .
For American viewers car-boot= TRUNK.
Its a case of when you make changes to an audio design in a high sensitivity amp oscillation can occur , I usually fit high quality Polystyrene low values ( as per JLH ) in that position, I leave others to fit what they think fit .
Thanks....but I'm not confident that I'd be picking the correct 39pf...there are 5 or 6 of them...and all close to a BJT.
I don't intrinsically know the 'geography' (layout) of the board I'm afraid.
I don't intrinsically know the 'geography' (layout) of the board I'm afraid.
My problem is explaining how to trace through an audio amplifier circuit on a PCB.
I realise its easy for me but not so easy for you, tell you what have you got a photo of the PCB close up even the copper rear side maybe would help ?
I realise its easy for me but not so easy for you, tell you what have you got a photo of the PCB close up even the copper rear side maybe would help ?
I can see where you have a problem ,the two schematics don't tie up with the numbering of components nor layout of the picture you provided which I take is the actual board you have.
.....mentioned that in post #5 too😀
The picture is actually one of my boards....not a library pic!!!
If that is actually one of the boards in your amplifier , is that the one you changed (uprated ) the capacitors on?
If you look at that board you will see small polystyrene capacitors ,go round the board and touch one end leg of each slowly to see if you hear the noise reducing .
If you look at that board you will see small polystyrene capacitors ,go round the board and touch one end leg of each slowly to see if you hear the noise reducing .
Completed exactly same repair on both boards....so both are about as identical as you can get.
Thanks...will try that with the small polystyrene caps!
Thanks...will try that with the small polystyrene caps!
...An update!
As time progressed...couldn't actually replicate the high tone...and realised that the Disc input was now totally dead....which then took me to the LM317/LM337 regulators for Disc stage.
Found LM337 to be 'open circuit'..... 🙂
Replaced all 4 regulator on both LH and RH boards for good measure...and
....Hey Presto....all working now.
Can only assume that the whining tone was brought about as I witnessed the death throws of the LM337 in question!!??
Many thanks, in particular to Duncan2....this Forum is great😀
As time progressed...couldn't actually replicate the high tone...and realised that the Disc input was now totally dead....which then took me to the LM317/LM337 regulators for Disc stage.
Found LM337 to be 'open circuit'..... 🙂
Replaced all 4 regulator on both LH and RH boards for good measure...and
....Hey Presto....all working now.
Can only assume that the whining tone was brought about as I witnessed the death throws of the LM337 in question!!??
Many thanks, in particular to Duncan2....this Forum is great😀