Could it be that you connected the two 6k8 resistors to +/-Vcc unreg instead of to the +/- Vcc Pre Reg ?
If that's not the case, turn the bias pot intil you have ca. 35 Volt on the 150R resistors, because everything makes sense so far.
Between points A and B you should have ca 1 Volt where it is now 0.6Volt (-1.4V and -0.8V)
Hans
If that's not the case, turn the bias pot intil you have ca. 35 Volt on the 150R resistors, because everything makes sense so far.
Between points A and B you should have ca 1 Volt where it is now 0.6Volt (-1.4V and -0.8V)
Hans
Oh my god how silly I was! It was the bias pot! I did try to adjust it earlier today but it didn't change the voltage so I thought the problem was another. In reality I had replaced all the pots with Bourns multiturn ones and installed them set halfway, so I only needed to turn it A LOT more than the previous one which was very sensitive. Thank you again! I will finish all the tests of the slow start and then reassemble the other channel. For the upper boards I still have to wait for a couple of transistors.
Thank you and sorry for the inconvenience.
I now have checked emitter voltages on all transistors and all but one are pretty close (±1mV). One of them however wanders pretty quickly between 8mV and 12mV. Would I be better replacing it?
I recently noticed after changing speakers that with the amp well warmed up (about 8 hours of moderately high volumes) on this channel I got some intermittent noises. When it happened I switched off the amp and let it cool down. I though was some leaky electrolyte capacitor, but could it be a failing output transistor?
It only happened twice as I don't normally play for so long the amp and also quickly switched it off because I didn't want to risk damaging the speakers.
I now have checked emitter voltages on all transistors and all but one are pretty close (±1mV). One of them however wanders pretty quickly between 8mV and 12mV. Would I be better replacing it?
I recently noticed after changing speakers that with the amp well warmed up (about 8 hours of moderately high volumes) on this channel I got some intermittent noises. When it happened I switched off the amp and let it cool down. I though was some leaky electrolyte capacitor, but could it be a failing output transistor?
It only happened twice as I don't normally play for so long the amp and also quickly switched it off because I didn't want to risk damaging the speakers.
That does not sound at all the way it should be.
That it deviates a few millivolt in voltage is still acceptable but it should remain stable and not wander between 8 and 12 mV.
However replacing this transistor isn't that easy because the six transistors have a matched Vbe, that's why you measure 10mV.
To find a transistor with the same Vbe, you will have to buy at least 5 and hope you have luck.
But this transistor is not o.k. and should be replaced.
Hans
That it deviates a few millivolt in voltage is still acceptable but it should remain stable and not wander between 8 and 12 mV.
However replacing this transistor isn't that easy because the six transistors have a matched Vbe, that's why you measure 10mV.
To find a transistor with the same Vbe, you will have to buy at least 5 and hope you have luck.
But this transistor is not o.k. and should be replaced.
Hans
Oh yes there is.
With your multimeter you can measure diodes and see their voltage drop, true ?
The base emitter is also a diode.
So measure the Vbe voltage drop from all six newly bought diodes.
Then measure the Vbe of one the good transistors in place, but first remove one side of the 10R resistor that’s connected to its base.
Select from the six new trannies the one that comes closest to the transistor in your amp and reconnect the 10R resistor.
Hans
With your multimeter you can measure diodes and see their voltage drop, true ?
The base emitter is also a diode.
So measure the Vbe voltage drop from all six newly bought diodes.
Then measure the Vbe of one the good transistors in place, but first remove one side of the 10R resistor that’s connected to its base.
Select from the six new trannies the one that comes closest to the transistor in your amp and reconnect the 10R resistor.
Hans
Oh that easy, great! Although in this case it's probably easier to pull out a good transistor to measure it rather than one leg of the 10R resistor. I hate the way they mounted components on this amp, they all have extremely bent leads and pulling on lead from above there's a very good chance of ripping off the pad. I had a very hard time replacing components even with the bottom of the board accessible.
Dear Hans, updates on the restoration.
I reassembled the right channel main board (the one I tested successfully before disassembling it) and it tested ok, as before I restored it.
Then I reassembled the upper board and tested again. Now I can see that the LS output fluctuates around ±1 V max, for a couple of seconds before stabilizing at 0V. At switch off the LS voltage also fluctuates a little around ±500mV (but with some higher spikes I can barely see on my analog scope, for a couple of seconds and then goes to 0V. Is this normal behaviour?
With only the main board installed the LS voltage stayed still for 1 or 2 seconds then it went to around 1 V and quickly stabilized. With the upper board installed, as soon as I press the power switch the LS moves.
Will try to make a video of the scope.
I reassembled the right channel main board (the one I tested successfully before disassembling it) and it tested ok, as before I restored it.
Then I reassembled the upper board and tested again. Now I can see that the LS output fluctuates around ±1 V max, for a couple of seconds before stabilizing at 0V. At switch off the LS voltage also fluctuates a little around ±500mV (but with some higher spikes I can barely see on my analog scope, for a couple of seconds and then goes to 0V. Is this normal behaviour?
With only the main board installed the LS voltage stayed still for 1 or 2 seconds then it went to around 1 V and quickly stabilized. With the upper board installed, as soon as I press the power switch the LS moves.
Will try to make a video of the scope.
Thanks! hopefully the left channel will turn out as well.
Have to say that with the minimal restoration I did on this channel the amp seems to sound incredibly better, much more extended on the high frequences, much more transparent and "precise"...can't explain why this difference, maybe the panasonic FC replacing the old caps...
Have to say that with the minimal restoration I did on this channel the amp seems to sound incredibly better, much more extended on the high frequences, much more transparent and "precise"...can't explain why this difference, maybe the panasonic FC replacing the old caps...
Stefano,
Admittedly mine is a 27 so half the power, but it never really gets more than cozy warm. In fact, it often had me wondering if it was biased properly since it runs so cool, especially compared to several of my other amps. Can you elevate the heatsink and provide adequate convection?
Admittedly mine is a 27 so half the power, but it never really gets more than cozy warm. In fact, it often had me wondering if it was biased properly since it runs so cool, especially compared to several of my other amps. Can you elevate the heatsink and provide adequate convection?
Yes I know reassembled the right channel on the chassis, let’s see how hot it gets. My 23.5 used to run cool at the most, but then I found out bias was set pretty low. Once I set it right it was much hotter, but the rest I did earlier with the channel laid on the towel was too hot.
I also noticed now that I switch the amp on from completely cold that I heard some quite electrical noises from the speaker for just a fraction of a second, which didn’t happen before.
I also noticed now that I switch the amp on from completely cold that I heard some quite electrical noises from the speaker for just a fraction of a second, which didn’t happen before.
Air has to circulate verically along the cooling fins to get the heat removed.
On a towel there is no circulation at all, so heat is building up.
Just make a final check when everything is mounted together to measure the bias current.
Hans
Have been playing the amp (right channel) for a few hours this morning and temperatures seem to be like before. It still is hot but I can touch it comfortably, considering that have been playing at very high volumes (since I now have only one channel I would not play this loud with both channels). So I think everything is under control.
For a final definitive adjustment of the bias how many hours should I leave the amp on? Should it be playing during these hours or just idling?
For a final definitive adjustment of the bias how many hours should I leave the amp on? Should it be playing during these hours or just idling?