I'm finally restoring my inherited Quad 303!
I'm following this guide:
https://www.rs-online.com/designspark/refurbishing-a-classic-quad-303-amplifier
I've replaced all the capacitors and everything went good 😉
Now to the problem:
Adjust RV100 for 33.5 volts between tags 5 and 9
One of the channels gives me an irregular reading of 38-41v with the pot all the way up. With the old pot setup it was reading 60v.
I only had to make minute adjustments to the power supply pot and the other channel.
What do you guys think is the culprit?
I'm a noob with a soldering iron.
I'm following this guide:
https://www.rs-online.com/designspark/refurbishing-a-classic-quad-303-amplifier
I've replaced all the capacitors and everything went good 😉
Now to the problem:
Adjust RV100 for 33.5 volts between tags 5 and 9
One of the channels gives me an irregular reading of 38-41v with the pot all the way up. With the old pot setup it was reading 60v.
I only had to make minute adjustments to the power supply pot and the other channel.
What do you guys think is the culprit?
I'm a noob with a soldering iron.
Did you clean the pot? Sometimes they go bad. Check to see if it's working correctly with your multimeter on ohms (and the power off on the amp).
That seems like the first thing to test, will do that. Would a bad pot create the irregular reading though?
Check the 150k resistor too. They sometimes go high, which reduces the adjustment range.
I always replace those 2k trumpets with multi-turn ones.
You don’t need to break lead connections to measure the quiescent. Just measure the voltage between pins 4 and 6, which you can do at the inboard ends of the current-limiting diodes. Aim for about 5mV.
I always replace those 2k trumpets with multi-turn ones.
You don’t need to break lead connections to measure the quiescent. Just measure the voltage between pins 4 and 6, which you can do at the inboard ends of the current-limiting diodes. Aim for about 5mV.
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I picked it up again today and like magic the problem went away!
It is now reading 33.5v between 5 and 9 in both channels 🤔
The pot that sets the voltage to 67 is almost at its limit though.
ejp, I understood the reading between pins 4 and 6 but not the part about inboard end of diodes.
I tried to read pin 4 and 6 and it gives 2mV in one channel and 0 in the other(the same that was giving me problems)
It is now reading 33.5v between 5 and 9 in both channels 🤔
The pot that sets the voltage to 67 is almost at its limit though.
ejp, I understood the reading between pins 4 and 6 but not the part about inboard end of diodes.
I tried to read pin 4 and 6 and it gives 2mV in one channel and 0 in the other(the same that was giving me problems)
Bad news... I broke the connection to read the quiescent and shorted something in the process... Smoke came out, I believe from the rca 38495
I took the Tr201 out and in diode mode I'm reading 805 and 791 so maybe it is still good?
I saw smoke so something had to give... maybe Tr200?
I'm well above my pay grade here but this was passed to me by my grandfather, I need to restore it and hope not to die in the process xD
Will be more careful in the future
I saw smoke so something had to give... maybe Tr200?
I'm well above my pay grade here but this was passed to me by my grandfather, I need to restore it and hope not to die in the process xD
Will be more careful in the future
Today I removed Tr200, it is an E5458 ti. In diode mode with the negative on the middle pin I get readings in both pins, same as with Tr201.
He doesn't look very good though so I will assume it was the one that burned. From my research I can replace it with BC556B.
He doesn't look very good though so I will assume it was the one that burned. From my research I can replace it with BC556B.
You should check all the low value resistors around the driver and output stage both visually and by measuring. That can initially be done with them in circuit. Make sure the two 0.3 ohm resistors are still intact. If smoke came out it could well be a resistor gone up. Check also the 2.2 ohm ground lift resistor is OK.
I would recommend you use a DBT on this until it is fixed. Also which version is it? is it the one with diode biasing or the one with a vbe multiplier?
I would recommend you use a DBT on this until it is fixed. Also which version is it? is it the one with diode biasing or the one with a vbe multiplier?
My serial number is 54730
Still in the regulator board, I could read some values in circuit but not all of them so I just removed the odd ones.
This is what I got:
What resistor whattage should I replace with?
I'm still not sure what burnt in the accident, could it be R202?
Yeah, I'm building one 😉
That puts it as an early one with diode biasing.
The one on the left looks like an 8k2 (8.2k) if it is grey/red/red. That corresponds to your version. Neither looks burned but in any case the values are in the right ballpark. These are all old carbon type resistors with (by todays standards) poor tolerance and stability.
The one on the right I can make out as blue/grey/red which is 6k8. That corresponds with the above list.
Your best way to fault find on this is simply to try and get it powered up. The DBT prevents catastrophic damage in the event of a fault (such as causing output transistors to fail etc) but it won't prevent small parts from burning as it can still supply more than enough current for that.
If the amp will power up without drama then the best way to fault find on this is by taking careful voltage measurands. That should tell most of what is needed to diagnose what is going.
Although it looks like '68k' its's not, it is 6.8k. The colour code confirms that. Errors and ambiguities are not uncommon in service manuals.
Wattage for those can be low. The 6.8k sees under 0.25 watt dissipation and the other around 0.3w dissipation and so 0.5 or 0.6 watt rating is fine for both. They do read OK though at this point though so no need to replace.
Thank you so much to all of you for your support!
It seems that all my resistors are good so I will put them back in to place. The E5458 doesn't look very good but it seems to be working so I will put it back in too.
The quad is a 45wpc, should I use a 100w bulb?
Will report back once I have the components in place and the DBT built 😉
It seems that all my resistors are good so I will put them back in to place. The E5458 doesn't look very good but it seems to be working so I will put it back in too.
The quad is a 45wpc, should I use a 100w bulb?
Will report back once I have the components in place and the DBT built 😉
100 watt should be fine, you might even get away with a 60 watt. Nothing bad happens with too low a wattage.
The transistor looks OK visually, lots of old devices had coloured dabs on them like that.
The transistor looks OK visually, lots of old devices had coloured dabs on them like that.
wpc is not the same as power draw. The service manual says 40-200w depending on signal level. I’d say start with a 60w and see what happens.
Indeed. Each channel of a working 303 should only draw around 30 to 40 milliamps from the unregulated supply which is only around 3 to 4 watts.
I will go for 60w then 😉
Here are the pics:
The darker zones where already there, not the result of my accident. Still puzzled about the origin of the smoke... any chance it didn't damage anything? I turned it off pretty quick
Here are the pics:
The darker zones where already there, not the result of my accident. Still puzzled about the origin of the smoke... any chance it didn't damage anything? I turned it off pretty quick
No it doesn’t. Diode biasing ceased at serial number 11,500. See the service manual.
60W is sufficient for a DBT for the 303.
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