01-12-2016, 12:29 PM
Hi Mark, my workshop is a bit small, but as you say it's light. It's nice in Summer as I can look up and watch buzzards or red kites. Bloody cold in winter though, but I just put the big Tek 585 scope on - toasty. I don't have kids upstairs, but have pidgeon's wearing lead boot's. They scare the crap out of me. I'm sure they hang round the corner and wait till i have my hands inside some lethal EHT device, then they jump on the roof with a BANG!! giving me a near heart attack.
Having taken some measurements yesterday of the PSU, I'm getting 180v no signal, which drops to 170v at about 170w. So that should be ok. Thanks for the advice re tfmr, I wasn't looking forward to taking it apart or adding turns etc.
I've put some HS's on the 78/79 regulators, see pics. I had to use a low profile one on one PSU as a standing up type interfered with the fan.. I canabalise PCB's for everything from caps to heatsink's so have a reasonable stash and rarely have to buy them. I did buy some massive jobbies out of a welder that I was going to use for a DC load. After having looked at your dummy load I've used one for mine, see http://golbornevintageradio.co.uk/forum/...p?tid=6049.
I have the amp on test at present, only one channel though as one is overheating. I started this channel off at 180w, it's dropped to 165w ( dropped another volt RMS whilst writting - 35v) ; been testing for over an hour. 1khz @1.5v RMS in, into 8 ohm load. I briefly cranked it up to full volume and got 300w, same in/out etc. I keep getting different OP power, this may be down to some of my test gear being off calibration.
The channel that is overheating is a puzzle. I had it on test for about three hours yesterday at about 100w OP, it was only towards the end that it started overheating. Today I checked the quiescent voltage across TP1, it was still 300mV, so changed it to 270mV, it got hotter quicker. This is the channel that blew the mains tfmr. I rewound the old tfmr and got only 50w OP, I think it's in that first video. While testing all transistors, my probe slipped and took out a pair of OP Q's. Fitted a pair of pulls, tested it and got nearer 250w, weird.
I'll do some more checking on the faulty channel, but wonder if it's the "new" Q's I've fitted having a higher hfe, than the old ones. the pair that blew were OP Q's, I put the drivers in their place and used the "new" Q's as drivers. I did check to see if there was any HF instability on the OP, but could find nowt.
I think your right about the Marconi TF2331, the manual as I read it is a bit vague and doesn't give a thd "range". I got the bit about test gear needing to be 10 times better than the DUT thd figure from an article in ETI I think. I really need to read up a lot more on the subject and have a few PDF's of "testing amps" books. I have that much to try and read up on though, it's finding the time.
Thanks again for all your advice Mark, Andy.
Having taken some measurements yesterday of the PSU, I'm getting 180v no signal, which drops to 170v at about 170w. So that should be ok. Thanks for the advice re tfmr, I wasn't looking forward to taking it apart or adding turns etc.
I've put some HS's on the 78/79 regulators, see pics. I had to use a low profile one on one PSU as a standing up type interfered with the fan.. I canabalise PCB's for everything from caps to heatsink's so have a reasonable stash and rarely have to buy them. I did buy some massive jobbies out of a welder that I was going to use for a DC load. After having looked at your dummy load I've used one for mine, see http://golbornevintageradio.co.uk/forum/...p?tid=6049.
I have the amp on test at present, only one channel though as one is overheating. I started this channel off at 180w, it's dropped to 165w ( dropped another volt RMS whilst writting - 35v) ; been testing for over an hour. 1khz @1.5v RMS in, into 8 ohm load. I briefly cranked it up to full volume and got 300w, same in/out etc. I keep getting different OP power, this may be down to some of my test gear being off calibration.
The channel that is overheating is a puzzle. I had it on test for about three hours yesterday at about 100w OP, it was only towards the end that it started overheating. Today I checked the quiescent voltage across TP1, it was still 300mV, so changed it to 270mV, it got hotter quicker. This is the channel that blew the mains tfmr. I rewound the old tfmr and got only 50w OP, I think it's in that first video. While testing all transistors, my probe slipped and took out a pair of OP Q's. Fitted a pair of pulls, tested it and got nearer 250w, weird.
I'll do some more checking on the faulty channel, but wonder if it's the "new" Q's I've fitted having a higher hfe, than the old ones. the pair that blew were OP Q's, I put the drivers in their place and used the "new" Q's as drivers. I did check to see if there was any HF instability on the OP, but could find nowt.
I think your right about the Marconi TF2331, the manual as I read it is a bit vague and doesn't give a thd "range". I got the bit about test gear needing to be 10 times better than the DUT thd figure from an article in ETI I think. I really need to read up a lot more on the subject and have a few PDF's of "testing amps" books. I have that much to try and read up on though, it's finding the time.
Thanks again for all your advice Mark, Andy.
