Working on a smoked VOX AC15 guitar amplifier - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: August 22, 2019, 05:40:26 pm
content: Hi guys,Recently I got a VOX AC15 tube amplifier which was smoked and I wrote about in the following link.https://www.eevblog.com/forum/repair/burned-pcb-of-a-tube-amplifier-(vox-ac15)-help-with-track-repair/msg2542545/#msg2542545Just a summary of what happened.After removing the pcb I discovered that R72 was completely fried and there was visible blackening between terminal TT5 and TT7 and a conductive path was formed between the two terminals. There was a trace inbetween the terminals which was connected to the ground pour which showed signs of pitting.What I didI removed all the blackened pcb bits between the terminals and completely cleaned it and I filled the gap with epoxy. There was no conductivity between the terminals. I also removed the copper trace between the terminals as I couldn't understand any reason for it to be there. I replaced the blown resistor and also the HT fuse (250mA). Got fresh new tubes.What happenedI used a 100W bulb in series to power up the amp with an 8ohm dummy load connected. I started with no tubes and then continued adding tubes one by one starting from the pre amp tubes. Each time powering down the amp and powering it up again. There were no shorts, as the light bulb remained dimmly lit. The voltage measurements taken from different points also seemed to be in check. However after a little while the light bulb seemed to go just 'slightly' bright I assumed that's because with all the tubes in and with the normal temperature that should happen.So with no smoke or apparent short I powered it up from the mains and connected the speaker. Straight away there was a hum at about 100Hz (mains frequency here is 50Hz), turning the volume pot had no effect on the hum so I assumed bad filter caps. I injected a tone to the input and the output was amplified.After maybe a minute or so I saw a wisp of smoke coming from the TT7 terminal area, I quickly turned off the amp. The resistors R72 and R73 was boiling hot.What I discovered afterwards.There were some black coloration near the TT7 terminal. I thought it might have been left from before, so just to make sure I dig out the epoxy and used a dremel to remove slightest bit of black bits. In the process of removing the pcb I noticed that connecting point to chassis ground SG1 had a high resistance reading measured with other grounding points and also it looked very tarnished so I used sand paper to clean out that point on the chassis which gave a low resistance reading to other ground points.Since the Pcb was out I checked all the filter caps and everything seemed ok :S. I checked other components and nothing seemed out of place.Without filling the gap between TT7 and TT5 I soldered back all the connection and powered up the amp following the procedure as before, this time I was measuring the current draw from the supply as well. This time I didn't see the slight increase in bulb brightness and it was lightly glowing all the while, and then I tried powering from the mains with the speaker connected. Current draw was about 200mA there was no smoke but the hum was still there. With a signal applied the current draw didn't vary much from the 200mA mark. I didn't want to run the amp too long with the hum so I turned it off and checked the temperature of R72 and R73 which was slightly warm to touch which I believe is normal right?I am hoping that the smoking issue is fixed with theproper ground connection. Could it have been the culprit? Because the owner of the amp said that he saw some sparking and heard something like gunshots from the speaker before it start smoking before bringing it to me.Now my biggest concern is the hum- the filter caps checks out ok on the multi meter removed from the circuit. However on the ESR meter I got some erroneous value, either way I have ordered new caps and waiting for them to arrive. What I worry about is since I removed the copper trace between TT5 and TT7 which was connected to the ground pour is causing the hum due to parasitic capacitance or something?I'm going to fill the gap with epoxy again once when I'm soldering on the new caps.I was hoping that you could shed some light on this matter and point me to somethings to look forThanks so much for the help in advance.P.S I have included some photos and the schematic

author: Renate
author_rank: Super Contributor
timestamp: August 22, 2019, 08:16:25 pm
content: Getting rid of that charred area of the ground was the right thing to do.The loss of that copper won't affect anything.It's more important to make sure that all the carbon is gone.If you can, you should check the ripple on the two big caps.If you don't have a scope you can check with a DVM on AC and a blocking capacitor in series.If you fixed the short, the only possible cause of too much current would be a bad tube or bad bias on the tube.The plates of the tubes should not be glowing (obviously).

author: Yamin
author_rank: Frequent Contributor
timestamp: August 22, 2019, 08:36:31 pm
content: Here is the full schematic

author: Yamin
author_rank: Frequent Contributor
timestamp: August 22, 2019, 08:46:20 pm
content: Thanks @Renate,I haven't got a scope at my place. Interesting point you made about measuring ripple with a multi meter, and a blocking capacitor. Could you explain on how to do thisThe amp is fixed bias isn't it(sorry a bit unsure whether I am saying it correctly)?Wouldn't the improper ground issue cause a higher current draw as well?

author: Renate
author_rank: Super Contributor
timestamp: August 22, 2019, 09:15:44 pm
content: Check for your ~12V on the cathodes, W15, W27.A shorted/leaky C29 or a burnt/lower resistance R38 would make that too low, causing more current through the tubes.Gnd --- [black DVM red] --- [minus Capacitor plus] --- 400V supplyI'd use a large, high voltage cap.It would be safer to use a non-electrolytic 0.1 uF or as large as you have.If you must use an electrolytic watch out for polarity and safely discharge after testing!