Failed thermistor or cutout in a power amplifier transformer. Soundcraft Gigrac. - Page 1
Forum section: electronics / repair

author: Yamin
author_rank: Frequent Contributor
timestamp: November 02, 2021, 11:07:10 am
content: Hey everyone,I have got a Soundcraft Gigrac which was in protection mode and the output was muted. After troubleshooting I found out that the safety component inside the transformer was open, which leads the LTP bias to be not there etc...also the fan comes on at full speed.The thing is I'm not sure whether its an NTC type component or a thermal cut out which inside the transformer. I used a 10ohm resistor to jumper across the terminal, the amp sprang back to life. The fan however on at low speed.I was 80% sure that it should be a thermal cut out. I removed the connector, stripped back the wire and measured the resistance across the leads. I get a very high resistance reading to start with and sometimes I get something in the range of 500kOhm and it goes back into the mega ohm region which leads me to think could it be a NTC type of component? I can't remove the covering of the transformer to take it out.How would you suggest on me repairing this? By looking at the schematic should it be a thermal cutout or NTC type component?Should I bypass it with perhaps a 10ohm or lower value resistor? I know bypassing safety components is not ideal, but with this particular model there are quite lot NTC for measuring the temperature and fuses etc. Normally I don't see any thermal cutout type safety measures in other transformers on these kind of gear.Thanks in advance.P.S with the connector left open the LTP bias voltage is around 75V. When I jumper using the 10ohm resistor I measure something in the region of 60-65ishV

author: KevinA
author_rank: Regular Contributor
timestamp: November 03, 2021, 06:33:19 pm
content: Service data here;https://elektrotanya.com/soundcraft_gigrac_300_600_mixer_sm.pdf/download.html#dlIf the fan is running, there is nothing wrong with the mains transformer; look at the output stage.

author: Yamin
author_rank: Frequent Contributor
timestamp: November 03, 2021, 10:28:31 pm
content: Quote from: KevinA on November 03, 2021, 06:33:19 pmThanks KevinA, the pdf page I attached is from the SM. The amp did not work with the open thermal cutout coming from the transformer. When I bypassed it the amp works. The service manual does not say what kind of protective component is inside the transformer and I am left guessing.

author: bob91343
author_rank: Super Contributor
timestamp: November 03, 2021, 11:49:04 pm
content: I can't speak for this design but generally those inside-the-transformer devices are safety items to pass some government regulation. They are like fuses and once open, the transformer is supposed to be scrapped. So I would assume it's just a thermal cutout that can't be reset.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 04, 2021, 08:29:24 am
content: The EV P range of amps uses a similar device.On the block diagram page 5 it seems to be marked as an NTC, but I can't find it anywhere in the diagrams.From memory it normally measures around 100 ohms.They used to fail, a fixed resistor wired in place got it back out of protect.https://elektrotanya.com/electro-voice_p3000_excellent_quality_sm.pdf/download.html#dlLet me know if you find the circuitHaven't seen one of these in years, they were very good amps.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 07, 2021, 10:11:02 pm
content: Not seen NTC's used before to regulate bias from transformer temp.Maybe there was a severe design problem they had to implement, or maybe the product just overheated with thermal runaway left out in the sun or something.The EV P3000 uses these NTC's probably as a small, cheap (and conveniently buried in a transformer) and resettable temp cutout instead of a switch.The circuit is bottom left of page 19, Q1 and Q2.My 100 ohms memory at room temp is probably in the right ballpark for an NTC, the circuit will trigger Protect between 600 and 800 ohms or so.

author: Yamin
author_rank: Frequent Contributor
timestamp: November 12, 2021, 06:53:53 am
content: Thanks guys for the replies. I was going to replace the faulty component with a resettable thermal cutout placed against the transformer (since I can't open up the transformer to place it inside). I had already emailed the transformer manufacturer asking about what was inside and they finally did reply. Its a THERMIC SM1-130 (thermal cutout).Quote from: Audiorepair on November 04, 2021, 08:29:24 amThose are absolute beasts right, I remember seeing one in a club during my college years.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: November 12, 2021, 08:20:49 pm
content: Just for educational purposes, I was working on an old Dynacord Powermate 1000 today, and found a similar thing on the schematics.Only they call this one a PTC. (PTC-XFOR bottom left on the mains TX, circuit top right)Haven't looked at the circuitry it drives, but it is here is anyone is at all interested.Edit:Seems Q16 triggers the overtemp.Normally held off by the PTC.