Sick pioneer SA6200

[colin.hepburn]

Hi All
Some time ago I started a thread cant remember which forum it was t but it was named What was your first Amp well I had a pioneer SA6200 Well last week on eBay I came across one the seller said it was in need of a little TLC and At £22 quid or make an offer I though what the hell bit of nostalgia and fun sortning it out so made an offer and got it at £15 quid he described the fault as muffled and intermittent
Well I got it today and popped the lid for a look around didn't see any burnt resistors or blown Cap proceeded to wire up the pre amp only to an old compute amp and speakers connected a CD player and wired it up so now both channels are working fine on to the main amp and no Right Channel
Checking the cables around the back noted a squire box fixed on the panel opening this up revelling two 1.5A fuses right channel fuse blown replacing the fuse and both channels now working well pleased thinking there's nothing wrong with the amp and it was sounding better than I can't remember it sounding back in something like 78 but thy didn't have cds then but it has warm but not woolly so just kept an eye on it for a bit then hit repeat and took the dog out
On my return I began to noted a hiss from the right speaker between tracks like a background hiss like a cheap phono stage can be but at a lower midrange and soft sounding then fading off a little and fading back in with an intermittent sound like it's about to start motor boating but fail to do so then repeats the process over and over turning the balance all the way left don't do anything turning the volume off and the hiss stays
OK t got the DMM out and did some checks
DC offset at switch on bounce up to 28mA with rapped drop back to 0mA
Idle Current
DMM set to 200m left channel @ 13.4mV *0.5R = 6.52mA
Right Channel @11.5mV*0.5R = 5.525mA
The serves Manual don't say what the total current is apart from two places on the schematic adding them together we get 102.3mA but still not sure what the idle current t should be set at
So any ideas of the fault lads could it be a noise emitter resistor or bad Cap see Sch

[Mike H]

I like the way the top channel is drawn "upside down"

How about the bias adjuster presets? Can get pretty grotty with age especially if the open type, not enclosed

If so you could memorise the positions, spray on switch cleaner, twizzle lock-to-lock several times, put back in the right places, try that. (?)


 

[Mike H]

Bit of confusion ~ you say

DMM set to 200m left channel @ 13.4mV *0.5R = 6.52mA
Right Channel @11.5mV*0.5R = 5.525mA

Where is "0.5R" ? Is it the output transistor emitter resistors?

I would hazard a guess those currents are a mite too low, 10mA is probably more like it


 

[colin.hepburn]

Bit of confusion ~ you say

DMM set to 200m left channel @ 13.4mV *0.5R = 6.52mA
Right Channel @11.5mV*0.5R = 5.525mA

Where is "0.5R" ? Is it the output transistor emitter resistors?

I would hazard a guess those currents are a mite too low, 10mA is probably more like it

Hi Mike
Yep those Preset are incased by the looks old white cap
And the heatsink is only just warm
I Am going to recap all the Cap so can change the Presets a well may do the old carbon 5% resistor too it's just a bit of solid state fun easy to work on
R35 0.5 2W test pin 19

[Mike H]

I think that's a good idea.

I've just realised what's wrong with the calculations, you've got e.g. 13.4mV *0.5R = 6.52mA

That's wrong way round should be V/R = I

13.4mV / 0.5 = 26.8mA, which is more like it

"R35 0.5 2W test pin 19"

Ah!! Yes.


 

[colin.hepburn]

I think that's a good idea.

I've just realised what's wrong with the calculations, you've got e.g. 13.4mV *0.5R = 6.52mA

That's wrong way round should be V/R = I

13.4mV / 0.5 = 26.8mA, which is more like it

"R35 0.5 2W test pin 19"

Ah!! Yes.

AH Right thanks mike now why does that not came as a surprise to me

[colin.hepburn]

AHH I am running from its pre amp out into the NAD power amp and I have just lost the right channel volume dropped by half its level as if I swung the balance over to the left

[Mike H]

How does it connect together? Are there like plug-in boards? Back in the '70's there was quite a bit of an obsession with plug-in boards and edge connectors.

Could be dirty pots? Switches?


 

[Mike H]

AH Right thanks mike now why does that not came as a surprise to me

Easy done, was something similar I was trying to do a while ago all back to front, but can't remember what it was lol



 

[colin.hepburn]

How does it connect together? Are there like plug-in boards? Back in the '70's there was quite a bit of an obsession with plug-in boards and edge connectors.

Could be dirty pots? Switches?

NO All Boards are hardwired but they didn't use solder back then just larger pins and twisted the wire around the pins checking it and no one's been in there before all clean a tidy still in factory condition
Switches seem OK the volume control still has that nice tight and smooth feel too it but started to become noisy only after the right channel lost power the sound from the right speaker has lost its top end and the hiss is higher in level than the music I Will try it later as just a power amp to see if the amp part is OK and stick the DMM on it see what the other voltages are I thought if I could be s duff transistor but they ether work ornot don't thay or can they go noisy

[Mike H]

Arrrghh wire wrapping pins! My cassette deck is full of them. That was the next "fad", I forgot

Could be a noisy semi, good plan to try to use the power amp alone

Treble's gone off, deffo sounds like a dodgy contact. It's a mystery alright


 

[colin.hepburn]

Arrrghh wire wrapping pins! My cassette deck is full of them. That was the next "fad", I forgot

Could be a noisy semi, good plan to try to use the power amp alone

Treble's gone off, deffo sounds like a dodgy contact. It's a mystery alright

Well I have just switched it on in same configuration as last night and it working fine from Cold I will leave it running see how long it works for before trying its main amp odd
Heres The Pre-Amp Sch

[colin.hepburn]

Arrrghh wire wrapping pins! My cassette deck is full of them. That was the next "fad", I forgot

Could be a noisy semi, good plan to try to use the power amp alone

Treble's gone off, deffo sounds like a dodgy contact. It's a mystery alright

Well I have just switched it on in same configuration as last night and it working fine from Cold I will leave it running see how long it works for before trying its main amp odd
Heres The Pre-Amp Sch

Well that lasted two and a half hours

[Mike H]

Heres The Pre-Amp Sch

Ah a classic design.


I see some switches there. Are they sliding type?

So only does it after had a good while to warm up. To be honest I'm starting to suspect the wire wrapping "connectors"


 

[colin.hepburn]

Heres The Pre-Amp Sch

Ah a classic design.


I see some switches there. Are they sliding type?

So only does it after had a good while to warm up. To be honest I'm starting to suspect the wire wrapping "connectors"

Switch are the button push types a couple of them not required in my opinion like loudness and a 20db cut switch will run the power amp up see how that fairs and then take some voltages and check the wire wrapping "connectors