QUAD 303 unstable power supply. - UK Vintage Radio Repair and Restoration Discussion Forum

Arendtallie · 16th May 2005, 11:44 am

I have recently worked on my QUAD 303 which had some problems in the power supply. I replaced the transistors and some damaged resistors, then readjusted the power supply output, plus the standing voltage and bias on the amplifier boards.

The sound has quite improved, but I still can't enjoy the improvement longer than a minute because at any burst of low frequencies it seems that the power supply drops completely, then comes up again, drops, and so on. Even when the music signal is stopped, the oscillation stays and unless I am fast enough to switch off the mains supply, the 68 Ohm resistor near transistor TR201 goes up in smoke.
I think that this is called 'motorboating', which is apparently more of a risk in tube amplifiers. It is nice to have a name for it, but that doesn't lead to a solution

.

Does anyone have an idea what the problem could be, or even have direct experience with this issue? Could it be the 2200pF reservoir capacitors needing replacement also? I noticed that transistor TR200 becomes quite hot, especially after adjusting the bias.

I live in China for a few years, so it is not a real option to send the amplifier to QUAD...

Andrew_Keith · 16th May 2005, 3:18 pm

Hi There, The 68ohm resistor you mention is an emitter resistor for T201 if this is smoking it suggests a short or leaky output transistor or driver. Or the bias is set far too high. If you go into google search and type in Quad 303 circuit diagram, you can get a full schematic for the 303 online. Also check the work you have done in the power supply as this SHOULDNT affect the output stage if all is o.k there. However a short or leaky driver/output transistor would pull down the H.T rail.

Hope this helps.

Welcome to the forum

Arendtallie · 16th May 2005, 4:03 pm

Dear Andrew,

Thanks for your reaction and clear suggestions. Before repairing the power supply, the fins became quite warm even without music, but after replacing transistor TR3 the temperature stays nicely on room temperature. That was at least one powertransistor not working well!
I will replace the other four powertransistors too and check the results.
I just hope it is not the driver transistors, because for TO39 packaging types I can only find obscure local brands here in China, with very limited datasheets.
About the bias, I adjusted it to 18mA (6mV over resistor R124 / R125), and it stays stabile, at least without an audiosignal. I do notice that during bias setup, the temperature of TR200 increases from handwarm to quite hot, which I think is not a good sign.
Would there be any special reason for the power supply to become unstabile with low frequencies?

kind regards,
Arend

Mike Phelan · 16th May 2005, 4:04 pm

Make sure all five transistors are OK regarding shorts on emitter and base pins where they contact the heatsink, and that the regulator and bias presets are smooth and not burnt.
2200s are 2200mfd, not pf (!). Worth checking the two in the power supply.
HTH

Mike Phelan · 16th May 2005, 4:08 pm

Arend - your post was before I posted mine! It is looking like 2200mfds

Andrew_Keith · 16th May 2005, 4:15 pm

Hi Arend, You suggest that you have done a fair amount of work in the power supply, do check for man-made faults as it can easily happen!. I too the same as Mike notice you mention PF not UF for the reservoir caps

I trust it was a typing error

Although your initial post suggests you have not replaced these. They are worth checking though.

Arendtallie · 16th May 2005, 4:53 pm

Dear Mike,

Posting my question on this forum could be one of my smartest moves so far for this problem!
Your comment about the presets is very to-the-point. They had become heavily oxidized and at least one of them had become an open, and I suspect that they are the root cause but the issue now is what other parts have been damaged in the process.
I have put good quality replacements on all five positions. Actually I have replaced them three or four times before I found good ones. With most types the settings would change unaccpetably the moment the screwdriver is pulled out, making adjusting more of a gamble than precision work.

I will follow your advice for the checks on the other four power transistors. The capacitors I have for replacement are 2200 microFarad 200V that I got at a vintage audio shop here in Nanjing, China. Would you have guessed that vintage tube equipment is the big thing here? The shop has brands like QUAD, Mackintosh, Marantz, Dynaco and so on. The main difference with Europe is that apart from electrical restaurations they do nothing to make it look good and leave all the dust and dirt on, otherwise the customers will suspect that the things are fakes since anything gets copied here. So mint condition stuff would not sell here!

kind regards,
Arend

Mike Phelan · 17th May 2005, 7:57 am

Hi Arend
Hmmm..... it sounds as if you need to be wary that the large number of replacement components are out of spec for various reasons. I have worked on a few 303s - they are an example on how they should be designed! Generally, I have found that if one of the output pair goes s/c, nearly all the transistors on this side follow, together with a few other bits!
However, from what you say, I think the motor-boating may be just the power supply.

Arendtallie · 19th May 2005, 4:08 pm

Dear Mike and Andrew,

At this very moment I am enjoying the astonishing sound the 303 is giving on the ESL57's. I found the issue with the unstability of the power supply, so I thought I'd let you know.
In retrospect, I used a too small soldering iron for connecting the two wires to the power transistor, and the resulting solder was a bad soldering joint. It must have had a high resistance, and at least it was mechanically not good. Redoing it with a better iron resulted in the driver transistor not overheating anymore, and a perfectly stabile power supply under all conditions. The lesson I learned: don't be so motivated that the work continues into the wee small hours, especially if the result is well hidden out of sight like this soldering joint...
I didn't replace anything on the audioboards or the power transistors of the audio circuit, so it is still a 100% authentic 303 amplifier. I am considering to place back some of the original parts I replaced on the power board in my attempts to find the root cause, just to keep it as original as possible. I checked the 2200uF elcos. These are working fine so I will not replace these either.
The only thing I will still do is take a scope and signal generator with me from work and optimize the bias settings; it would be a pity not to get the best of the best after all this work. Even with the problems, I am so impressed with the improvement that I am moe than happy to have gone through the process. Thank you so much for your advice and support, it really made a huge difference!

kind regards,
Arend van Dam

mudlark · 20th Jul 2005, 2:33 pm

Hi,
I'm not advertising, but NETaudio do a replacement power supply that is acknowledged to work well. They also provide caps and output transistors.

It is possible to get hold of the Quad service notes off the net.

good luck.

16th May 2005, 11:44 am	#1
Arendtallie Retired Dormant Member Join Date: May 2005 Posts: 4	QUAD 303 unstable power supply. I have recently worked on my QUAD 303 which had some problems in the power supply. I replaced the transistors and some damaged resistors, then readjusted the power supply output, plus the standing voltage and bias on the amplifier boards. The sound has quite improved, but I still can't enjoy the improvement longer than a minute because at any burst of low frequencies it seems that the power supply drops completely, then comes up again, drops, and so on. Even when the music signal is stopped, the oscillation stays and unless I am fast enough to switch off the mains supply, the 68 Ohm resistor near transistor TR201 goes up in smoke. I think that this is called 'motorboating', which is apparently more of a risk in tube amplifiers. It is nice to have a name for it, but that doesn't lead to a solution . Does anyone have an idea what the problem could be, or even have direct experience with this issue? Could it be the 2200pF reservoir capacitors needing replacement also? I noticed that transistor TR200 becomes quite hot, especially after adjusting the bias. I live in China for a few years, so it is not a real option to send the amplifier to QUAD... Last edited by Darren-UK; 4th Jun 2007 at 7:10 pm. Reason: Spelling error in title corrected.

16th May 2005, 3:18 pm	#2
Andrew_Keith Retired Dormant Member Join Date: Nov 2004 Location: Salford, Greater Manchester, UK. Posts: 150	Re: QUAD 303 unstabile power suppply Hi There, The 68ohm resistor you mention is an emitter resistor for T201 if this is smoking it suggests a short or leaky output transistor or driver. Or the bias is set far too high. If you go into google search and type in Quad 303 circuit diagram, you can get a full schematic for the 303 online. Also check the work you have done in the power supply as this SHOULDNT affect the output stage if all is o.k there. However a short or leaky driver/output transistor would pull down the H.T rail. Hope this helps. Welcome to the forum

16th May 2005, 4:03 pm	#3
Arendtallie Retired Dormant Member Join Date: May 2005 Posts: 4	Re: QUAD 303 unstabile power supply Dear Andrew, Thanks for your reaction and clear suggestions. Before repairing the power supply, the fins became quite warm even without music, but after replacing transistor TR3 the temperature stays nicely on room temperature. That was at least one powertransistor not working well! I will replace the other four powertransistors too and check the results. I just hope it is not the driver transistors, because for TO39 packaging types I can only find obscure local brands here in China, with very limited datasheets. About the bias, I adjusted it to 18mA (6mV over resistor R124 / R125), and it stays stabile, at least without an audiosignal. I do notice that during bias setup, the temperature of TR200 increases from handwarm to quite hot, which I think is not a good sign. Would there be any special reason for the power supply to become unstabile with low frequencies? kind regards, Arend

16th May 2005, 4:04 pm	#4
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: QUAD 303 unstabile power suppply Make sure all five transistors are OK regarding shorts on emitter and base pins where they contact the heatsink, and that the regulator and bias presets are smooth and not burnt. 2200s are 2200mfd, not pf (!). Worth checking the two in the power supply. HTH __________________ Mike.

16th May 2005, 4:08 pm	#5
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: QUAD 303 unstabile power suppply Arend - your post was before I posted mine! It is looking like 2200mfds __________________ Mike.

16th May 2005, 4:15 pm	#6
Andrew_Keith Retired Dormant Member Join Date: Nov 2004 Location: Salford, Greater Manchester, UK. Posts: 150	Re: QUAD 303 unstabile power suppply Hi Arend, You suggest that you have done a fair amount of work in the power supply, do check for man-made faults as it can easily happen!. I too the same as Mike notice you mention PF not UF for the reservoir caps I trust it was a typing error Although your initial post suggests you have not replaced these. They are worth checking though.

16th May 2005, 4:53 pm	#7
Arendtallie Retired Dormant Member Join Date: May 2005 Posts: 4	Re: QUAD 303 unstabile power supply Dear Mike, Posting my question on this forum could be one of my smartest moves so far for this problem! Your comment about the presets is very to-the-point. They had become heavily oxidized and at least one of them had become an open, and I suspect that they are the root cause but the issue now is what other parts have been damaged in the process. I have put good quality replacements on all five positions. Actually I have replaced them three or four times before I found good ones. With most types the settings would change unaccpetably the moment the screwdriver is pulled out, making adjusting more of a gamble than precision work. I will follow your advice for the checks on the other four power transistors. The capacitors I have for replacement are 2200 microFarad 200V that I got at a vintage audio shop here in Nanjing, China. Would you have guessed that vintage tube equipment is the big thing here? The shop has brands like QUAD, Mackintosh, Marantz, Dynaco and so on. The main difference with Europe is that apart from electrical restaurations they do nothing to make it look good and leave all the dust and dirt on, otherwise the customers will suspect that the things are fakes since anything gets copied here. So mint condition stuff would not sell here! kind regards, Arend

17th May 2005, 7:57 am	#8
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: QUAD 303 unstabile power suppply Hi Arend Hmmm..... it sounds as if you need to be wary that the large number of replacement components are out of spec for various reasons. I have worked on a few 303s - they are an example on how they should be designed! Generally, I have found that if one of the output pair goes s/c, nearly all the transistors on this side follow, together with a few other bits! However, from what you say, I think the motor-boating may be just the power supply. __________________ Mike.

19th May 2005, 4:08 pm	#9
Arendtallie Retired Dormant Member Join Date: May 2005 Posts: 4	Re: QUAD 303 unstabile power suppply Dear Mike and Andrew, At this very moment I am enjoying the astonishing sound the 303 is giving on the ESL57's. I found the issue with the unstability of the power supply, so I thought I'd let you know. In retrospect, I used a too small soldering iron for connecting the two wires to the power transistor, and the resulting solder was a bad soldering joint. It must have had a high resistance, and at least it was mechanically not good. Redoing it with a better iron resulted in the driver transistor not overheating anymore, and a perfectly stabile power supply under all conditions. The lesson I learned: don't be so motivated that the work continues into the wee small hours, especially if the result is well hidden out of sight like this soldering joint... I didn't replace anything on the audioboards or the power transistors of the audio circuit, so it is still a 100% authentic 303 amplifier. I am considering to place back some of the original parts I replaced on the power board in my attempts to find the root cause, just to keep it as original as possible. I checked the 2200uF elcos. These are working fine so I will not replace these either. The only thing I will still do is take a scope and signal generator with me from work and optimize the bias settings; it would be a pity not to get the best of the best after all this work. Even with the problems, I am so impressed with the improvement that I am moe than happy to have gone through the process. Thank you so much for your advice and support, it really made a huge difference! kind regards, Arend van Dam

20th Jul 2005, 2:33 pm	#10
mudlark Diode Join Date: Jul 2005 Location: Cumbria Posts: 8	Re: QUAD 303 unstabile power suppply Hi, I'm not advertising, but NETaudio do a replacement power supply that is acknowledged to work well. They also provide caps and output transistors. It is possible to get hold of the Quad service notes off the net. good luck. Last edited by Darren-UK; 4th Jun 2007 at 7:07 pm. Reason: Activated link to Net-audio.