Hi guys,
I'm recapping a Hitachi for a friend. The amplifier was working fine so I took it on. After recap I went to adjust and found the old bias pots are faulty so I replaced them. But now although the bias has dropped from 330ma to around 170ma. The service manual states 200ma.
The problem is when I adjust it, it doesn't make any difference! I checked a few resistors around the pot and a few Zener diodes and they are fine.
Any advice would be greatly appreciated.
I'm recapping a Hitachi for a friend. The amplifier was working fine so I took it on. After recap I went to adjust and found the old bias pots are faulty so I replaced them. But now although the bias has dropped from 330ma to around 170ma. The service manual states 200ma.
The problem is when I adjust it, it doesn't make any difference! I checked a few resistors around the pot and a few Zener diodes and they are fine.
Any advice would be greatly appreciated.
Attachments
Collector voltages for Q705 and 704 are given, and they set the bias. Are they in range?
The fusible resistors in that LTP section may get tired, and increase in value. If they do, the bias current will drop. Meter check them?
The fusible resistors in that LTP section may get tired, and increase in value. If they do, the bias current will drop. Meter check them?
Hi, I checked the dark black resistors x 3 and they were fine!
I will have to remove the transistors you mentioned and test them in the multi tester I got. I will report back shortly! Thanks
I will have to remove the transistors you mentioned and test them in the multi tester I got. I will report back shortly! Thanks
Thats pretty much same circuit as Hitachi lateral mosfet data sheet.
Bias current is checked by removing B+ fuse and putting multi meter across the fuse on mA setting. I am pretty certain it was 100mA bias current.
100mA is standard standing current for a lateral mosfet.
200mA would be for 2 pairs.
Bias current is checked by removing B+ fuse and putting multi meter across the fuse on mA setting. I am pretty certain it was 100mA bias current.
100mA is standard standing current for a lateral mosfet.
200mA would be for 2 pairs.
remove fuse??
I'm literally about to pull out the transistors.
The service manual does not say to remove the fuse!
I'm literally about to pull out the transistors.
The service manual does not say to remove the fuse!
I only now understand your question. I will put back the transistor and test the voltages
Yes, it's Sampei's schematic. It even found its way into IEEE papers in the 1970s; and it was used in large numbers of amplifiers for years after that.
100mA is approximately the Zero-TC current, so the idle current should preferably be a little higher than that. The ±0.8V bias voltages suggest that they really did mean 200mA, so I would stick to that.
Measuring the bias voltages is the first step.
Maplin used the Hitachi circuit too.
I found the setup info and bias current is set to 50mA not 100mA as I said earlier.
I found the setup info and bias current is set to 50mA not 100mA as I said earlier.
Yes, I did try the Maplin circuit, but whoever laid it out ignored some of the most important precautions that Hitachi stipulated - so I had to do my own.
Specifying <100mA idle only confirms that they did not have a proper grasp on the design.
Specifying <100mA idle only confirms that they did not have a proper grasp on the design.
I did a rework on the Maplin circuit too.
Some modules had trouble with hum so I added RC decoupling on front end.
I also added DC offset trimmer.
I also added option for TO3 or TO247 outputs.
Some modules had trouble with hum so I added RC decoupling on front end.
I also added DC offset trimmer.
I also added option for TO3 or TO247 outputs.
Wowzer this thread was getting very complex!
Its fixed! must have been a bad solder joint!!! put transistors back and touched up a couple of bad looking connections and measure again just to make sure I didn't do more damage and it was fixed!!
Thanks a lot for the very helpful input!
Its fixed! must have been a bad solder joint!!! put transistors back and touched up a couple of bad looking connections and measure again just to make sure I didn't do more damage and it was fixed!!
Thanks a lot for the very helpful input!
Brand New elcos are also leaky. They need a few hours at least to polarise fully, and then the leakage calms down. I assume you have used a bipolar elco?
Given that it's only 4.7u, you could easily switch this one to an MKT{POLYESTER, MYLAR} or MKP. Even low cost ones will be much better performance than elcos, and all have effectively zero leakage. OTOH, Small size elcos get stale and leaky in no time.
Given that it's only 4.7u, you could easily switch this one to an MKT{POLYESTER, MYLAR} or MKP. Even low cost ones will be much better performance than elcos, and all have effectively zero leakage. OTOH, Small size elcos get stale and leaky in no time.