If I select tape and short the inputs, noise stays the same.
Noise level is not affected by the volume pot at all, except when it’s turned all the way to max, then it suddenly goes quiet.
It follows the preamp output, the unit has wire running between preamp and power stages so easy to disconnect. When I cross connected it problem followed preamps left channel.
Took it out and measured 164pf on my meter, so I think it’s ok. Put it back and soldered in place but no change
thinking about this it is very strange indeed. Maybe I should try replace it anyway. Could it be a matter of total value that makes it work when I add another 100pf?
I’ll have to see what I got in my parts bin and experiment some more with this, any ideas what could be the reason this suddenly need to be bigger?
They measure slightly over 1k in circuit, so should be fine I think
Just tried without C513/514 for fun and this strangely works too, have yet to try installing new ones, but I find it a little odd both paralleling and removing completely works equally well… have to chew on that one for a moment
Were the voltages ok around Q503 ( b 5.8V , c 12V , e 5.2V ) and does putting a finger on the transistor case also stop the noise .
Well since you asked I decided to measure again today, and the answer is actually no. They measure 11.5V 5.2V and 4.8V. This slipped my attention last night, sorry. Q504 measure OK.
Today I also found out that increasing C513/514 does not work, if I remove them amp seems fine. If I put them back in and add another 100pf on the pcb track close by, amp also seems fine.
Today I also found out that increasing C513/514 does not work, if I remove them amp seems fine. If I put them back in and add another 100pf on the pcb track close by, amp also seems fine.
11.5V collector together with 4.8V on the emitter is a bit of a puzzle . What are the voltages like on Q505 ( b1.8V , c 3.4V , e 1.2V ) as they share the same rail through R559 ( 3.9K ) .
The slightly lower voltages might just be due to R559 being of higher resistance than 3.9K I have looked at the various currents and they suggest a possible value for R559 of 4221 Ohms which is still within 10% tolerance of 3.9K , I dont know what the resistors actual tolerance is . What I was trying to discount was any extra current draw thro R559 and the currents thro Q505 are all ok as they are thro Q503 .
You seem to be drifting away from suspicion of Q503, but IMHO all of your experiments suggest local oscillation around Q503--- namely finger touch susceptability, added cap at base cures, removing C513 cures. Emitter follower stages tend to be susceptible to oscillation.
I think simply removing C513 is the easiest remedy. It presents about 44K reactance at 20kHz and the source impedance it sees is highly variable as R529 and R531 are changed, as well as the Loudness switch/tape monitor; point being is that its corner is well above 20kHz, varies wildly with control settings, and is so far down the signal path that it offers little benefit re EMI suppression. IMO, C513 is problematic rather than beneficial.
I appreciate the instinct to not subvert the designer's original intent, but something has to change in order to cure the problem. Replacing Q503 might help, but the original transistors are obsolete, so uncertainty reigns.
I think simply removing C513 is the easiest remedy. It presents about 44K reactance at 20kHz and the source impedance it sees is highly variable as R529 and R531 are changed, as well as the Loudness switch/tape monitor; point being is that its corner is well above 20kHz, varies wildly with control settings, and is so far down the signal path that it offers little benefit re EMI suppression. IMO, C513 is problematic rather than beneficial.
I appreciate the instinct to not subvert the designer's original intent, but something has to change in order to cure the problem. Replacing Q503 might help, but the original transistors are obsolete, so uncertainty reigns.
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No drifting away from Q503 just trying to nail down the actual fault and not trying to mask a fault by removing C513 as the noise has not always been there. C513 is not far down the signal path if you are using the tape monitor input . Hey I can suggest what I like but the choice is up to the OP .
Suggested next steps to draw my input to a conclusion :-
1 check resistance of R559 to confirm assumption in post 35
2 Check correct orientation of C531 and make sure it is electrically in circuit
3 Swop over Q503 and Q504 to see if the noise /oscillation moves with the transistor . BC 549B would be a replacement should it be required .
1 check resistance of R559 to confirm assumption in post 35
2 Check correct orientation of C531 and make sure it is electrically in circuit
3 Swop over Q503 and Q504 to see if the noise /oscillation moves with the transistor . BC 549B would be a replacement should it be required .
Sorry for the little delay, really appreciate you guys taking time to help me out but yesterday I just had to do something about current snow situation, we got about 30-40cm last week and I was a bit behind on the shoveling.
Back at it now I measured R559=4.15k so a your assumption is right about that one.
I have messed around with C531 first I replaced it with another 1000uf I had within reach, this seemed too fix the problem so I tried changing C532 to same type, and noise came back, only thing I have achieved with this is a few seconds turn on delay on Q503-506. However during this delay everything is of course completely silent.
will get at those transistors next and I should also mention I got a new usb cable for my little picoscope which luckily brought it back to life, so now I have a scope to play with too.
Back at it now I measured R559=4.15k so a your assumption is right about that one.
I have messed around with C531 first I replaced it with another 1000uf I had within reach, this seemed too fix the problem so I tried changing C532 to same type, and noise came back, only thing I have achieved with this is a few seconds turn on delay on Q503-506. However during this delay everything is of course completely silent.
will get at those transistors next and I should also mention I got a new usb cable for my little picoscope which luckily brought it back to life, so now I have a scope to play with too.
I might consider this eventually if I can't cure the problem otherwise, but if I do I have to be sure that removing C513/514 really takes care of the problem and not just masking it somehow, hopefully my little scope can be helpful in this case.