Hi everyone, after some weeks struggling by myself I found it's time to ask for some help. Luckily I found this very alive forum full of audiophiles like myself! I'm a newbie in DIY repair. I'm giving it a shot on a Mitsibishi DA-U200 amp. It's not rational to put so much time in an old amp, it's purely because of the sentimental value. And I love the warm sound of the late 70's / early 80's amps. My backup amp is a 90's Yamaha Natural Sound (AX-470). And that sounds so flat to me after all those years of warm sound!
Now to business:
So my question is: what could be the cause? And where and how to start measuring? I have a simple multimeter, without "capacitance" function, but it does have a hF function for transistors (don't know how to interpret those readings)
What I did before all this:
Bought this amp third hand, because I used to have a similar one in my teens (and blew it up by attaching different impedance speakers and cranking the volume open...)
After a few weeks of happy youth revival:
Now to business:
- After some repairs on the Power Supply (see below), I still get strange DC readings on the speaker outputs.
- Idle Current is a nice 27-28 mV on both channels
- Output on Right speakers is about 12 mV DC on both A and B speaker outputs
- BUT output on Left speakers (A and B) is only 4 mV DC!
- So I'm "missing" about 8 mV DC on the left channel!
So my question is: what could be the cause? And where and how to start measuring? I have a simple multimeter, without "capacitance" function, but it does have a hF function for transistors (don't know how to interpret those readings)
What I did before all this:
Bought this amp third hand, because I used to have a similar one in my teens (and blew it up by attaching different impedance speakers and cranking the volume open...)
After a few weeks of happy youth revival:
- Left channel "explodes": lots of crackle, very loud, that keeps going on even after I pulled out the mains plug. Luckily my speaker survived...
- I suspected the big 6800 uF 35V cans on the Power Supply.
- Left capacitor indeed drained very quickly compared to right one. After 10 seconds, left one is down to 1,5V, while right one still at 20V
- I replaced the big caps with new ones with same specs: 6800 uF and 35V. Problem remains same...
- I checked the voltages on the Power Supply with my multimeter and got strange results: AC voltage on some parts of the Power Supply while it should only deliver DC. Then I asked my brother in law for help as he is a technician. He tipped me to measure the diode bridge
- Diodes on D21S5VB are ok, resisitance is 0,8 - 1,5 MOhm in the counter-current direction and 0 Ohm in the right direction of the current.
- Then the small ceramic capacitors C11-C14 0,01 uF. C12 & C14 gave suspicious readings so I replaced those and when problem remained replaced C11 & C13 as well.
- Now the Power Supply gives a nice 34V on both channels (so 68V total)
- I measured most resistors, they all gave good values
- I checked the Idle Current, those were way out of margin on both channels
- Set idle current back to the factory settings (28 mV +- 2 mV) by following the instructions in the service manual
- But I still get the same strange readings in the speaker output...!!
- And that's where I'm stuck now.
Welcome to the forum! You'll find lots of help here.
12mV and 4mV DC at the speakers is probably perfectly acceptable. Of course, crackling noises aren't. Possible culprits are intermittent solder joints, leaky transistors or caps, to mention only most likely causes. Left channel only, right well behaved?
Can you post the service manual or a good link?
Good luck!
12mV and 4mV DC at the speakers is probably perfectly acceptable. Of course, crackling noises aren't. Possible culprits are intermittent solder joints, leaky transistors or caps, to mention only most likely causes. Left channel only, right well behaved?
Can you post the service manual or a good link?
Good luck!
Thanks for the reply! Yes it's left channel only. I thought I had attached the service manual. Will edit my post to add them
I forgot some important info:
So maybe I solved a problem but I surely created a new one...
- After replacing the ceramic elcos on the power supply, I tested the amp on cheap backup speakers. They are midrange 8 ohm speakers, no bass. Everything seemed fine at first.
- But when I tested it on my 4 ohm Loewe Konzertboxes there was a low background hum, that wasn't there before. Like when you connect a turntable without grounding it.
- Popping and cracking started again on left speaker after 5 min., so I immediately ended the test
- The new ceramic elcos are 103J instead of 103M (0.01 uF +/- 5% instead of 0,01 uF +/- 20%).
- Voltage from power supply is now 34V on each channel, 68V in total. While according to service manual it should be 28,8V on each channel...
So maybe I solved a problem but I surely created a new one...
Attachments
Thanks! How can I check if that transistor is functioning properly / is faulty? -> Newbie question: is there a tutorial somewhere for testing transistors?
It needs to be tested for noise, which won’t happen with rudimentary test equipment. I just pull them and put in a pair of PNPs that can handle whatever supply voltage is being used and if it quits sounding like thunder and lightning the problem is solved. This is one of a handful of types that throw up a red flag whenever I see them.
An expendable speaker is a good tool since it helps to "hear" the problem, but I'd add a series 100 ohm to give the speaker a better chance at surviving.
You could poke around with a wooden chopstick or similar. Sometimes you get lucky and the defect responds to tapping.
wg_ski advice is good. Intermittents and leaky transistors or caps can be vexing because the offending part is often inside the amp's feedback loop. The still functioning parts tend to suppress/obscure the defect. Despite negative feedback, I did once find the culprit with a scope when the collector waveform seemed suspicious re its base. My guess was correct and swapping fixed.
Good luck.
You could poke around with a wooden chopstick or similar. Sometimes you get lucky and the defect responds to tapping.
wg_ski advice is good. Intermittents and leaky transistors or caps can be vexing because the offending part is often inside the amp's feedback loop. The still functioning parts tend to suppress/obscure the defect. Despite negative feedback, I did once find the culprit with a scope when the collector waveform seemed suspicious re its base. My guess was correct and swapping fixed.
Good luck.
Nothing usually goes wrong with those ceramic caps. Pretty hard to kill except maybe mechanical damage. If one was shorted, you’d know it. Some types will drift in value, but never anything serious.
The purpose of those caps is to suppress ringing in the transformer caused by the rectifier diodes. It doesnt even happen all the time (heavily load dependent) so it’s hard to tell if they are even needed. If you do get transformer ringing, it manifests itself as a low level “bizzzzzzzzz” sound coming from the speaker. And you can’t get rid of it any other way than properly addressing it (With the caps on the rectifier).
The purpose of those caps is to suppress ringing in the transformer caused by the rectifier diodes. It doesnt even happen all the time (heavily load dependent) so it’s hard to tell if they are even needed. If you do get transformer ringing, it manifests itself as a low level “bizzzzzzzzz” sound coming from the speaker. And you can’t get rid of it any other way than properly addressing it (With the caps on the rectifier).
Thanks. Then I'll try to replace the 2SA798 with two transistors first and than see if the buzz is still present. I've found the threads discussing how to "make" a dual transistor to replace the old one.
Update: I tried the tapping the offending transistor first, combined with cleaning its legs with rubbing alcohol.
Amp has been playing for an hour now without distortion... fingers crossed
Amp has been playing for an hour now without distortion... fingers crossed
Update: I was not looking forward to make my own dual transistors, so I tried @TonyTecson 's cleaning tip. Et voila! Since I cleaned the legs of the 2SA798's I've had no more "explosions" or other mishap. - fingers crossed! -
Also a shout-out to @Toothbrush and many others for persevering in their quest with similar amps. I learned a lot!
Also a shout-out to @Toothbrush and many others for persevering in their quest with similar amps. I learned a lot!
But, as @east electronics sternly but fairly pointed out: this is most probably a temporary solution... After 40 years this dual transistor has metal fatigue.
Maybe during Christmas holidays I'll try to replace the 2SA798's
Maybe during Christmas holidays I'll try to replace the 2SA798's
Update: Still haven't replaced the 2SA798 transistors, but the "cleaning session" seems to have prolonged their lives. The amp still plays noiselessly since I rubbed the transistors with isopropanol.