Hi, On my schematic, in the bias section, there are those two diodes array. The STV-4H are represented by 4 diodes and the STV-3H is represented by 3 diodes. Is it really the case, if yes, what kind of diodes could have approx. the same temp variation. Or could it be replace with another thermistor having a PTC (positive temperature coefficient) that could be glue to the heatsink. If I replace them with standard diode like 4148 or 914, do I have to put 4 and 3 of them in serie. Thanks for any advice.
Hi, thanks for the reply, well this amp is near 30 years old so you can imagine. The wires connected to those diodes are single solid wire so as soon as you touch it, they almost break everytime. They are from the bias section in a Pioneer Spec 2. I have trouble adjusting the bias as per the service manual (30mV) . Fully clockwise the pot give me only 15mV to 20mV max. at TP. I have succes restoring them by grinding off some material with a dremel to get leads long enough to put a very small connector and glue it in place with epoxy.Still trying with the bias, they made so many errors when they print the circuit board, I must check for polarity every time a put back a component .
1N914's or 1N4148's stacked and put together in thermally conductive epoxy should be a good replacement. for the STV-3 use 3 914's in series, for the STV-4, use 4 of them. lay them out flat, side by side on a flattened STV-3 clip, and cover them with thermal epoxy and let it set. the 4 junction stack should take care of your current being on the shy side, since it gives you another 0.7V diode drop across your driver bases for bias. or you can do what Peavey and other manufacturers have done, and goop the diodes with heat sink grease and somehow set them in physical contact with the heatsink (the best being to use a screw and transistor spring clip, just don't overtighten and crush the glass diode bodies). Peavey actually runs the diodes through holes in the heatsink, and gooped with HS grease. but if you want to get as close as possible to original, making your own diode stacks in epoxy is probably the best.
Pioneer MA22
Hi, I have a pioneer MA22 which seems to have the stv-4h. I had replaced it with 4 1N4148 but without any epoxy. I had checked the forward voltage of each diode with a forward current of 2mA and found the closest pairs so that I got a total of 2.35 forward voltage drop when all 4 are connected in series. But the problem that I face is that the bias voltage starts fluctuating after some time after turn on. It is a class A amp and the heatsinks do heat a whole lot. (around 80 degrees, cant touch it after 15mins). Any leads on the fluctuating bias voltage. Thanks in advance.
Regards.
Hi, I have a pioneer MA22 which seems to have the stv-4h. I had replaced it with 4 1N4148 but without any epoxy. I had checked the forward voltage of each diode with a forward current of 2mA and found the closest pairs so that I got a total of 2.35 forward voltage drop when all 4 are connected in series. But the problem that I face is that the bias voltage starts fluctuating after some time after turn on. It is a class A amp and the heatsinks do heat a whole lot. (around 80 degrees, cant touch it after 15mins). Any leads on the fluctuating bias voltage. Thanks in advance.
Regards.
the diodes need to be thermally coupled to the transistors they're biasing, this is usually done by putting them in GOOD thermal contact with the same heatsink the outputs are on. Peavey does it by putting the diodes through the heat sink with a gob of thermal grease on them. there are 8 holes in their heatsinks for this (one hole for the diode body, one for one of the diode leads to back to the PC board). Sansui and others had the ST-V stacks in thermal epoxy on a mounting clip. without good thermal conduction, the bias will be all over the map. the diodes maintain bias by providing a diode drop that matches the B-E drop of the transistor, which changes with temperature. it's a fairly predictable property of silicon junctions.
Hello, I have mentioned earlier that I have used 4 4148 to replace an stv-4h diode stack. But I checked that I have put only 3. On further digging (through some old notes about the repair activity done by me on this M22 earlier) I found that the specs of the STV-4H are met by using 3 diodes instead of 4. I had a few (around 20) 4148s and checked the forward drop at 27 deg C and 2ma forward current. I found the 3 closest matching diodes with a combined Vf of 1.98mA at If=2mA. (The values mentioned in the STV-4H spec sheet are 1.89 to 2.22V with If=2mA). My question is that why have you used 4. Have you done the excercise of checking the diodes for their VF/IF ratings? I think STV-4H should be replaced by 3 TESTED 4148s cos the 20 diodes that I checked had a sizable variation in VF/IF characteristics. Kindly advice.
Best Regards.
Akshay.
Best Regards.
Akshay.
I may have used 3. I repair over 15 power amps a week for over 15 years. I can't recall every detail. Dont be too concerned with too much theory. If the original has 3 diodes then use 3. No need to do any testing of the diodes. Almost all my customers are DJs, bands and sound contractors. They all pound the life out of their systems here in the Caribbean. In the olders amps I have done this for years, including integrated amps. MOUNTED ON THE HEATSINK OFCOURSE. Those amps are still funtioning.
Which is the best in circuit capacitance tester?
Which is the best in circuit capacitance tester?
Sencore is pretty good, but pricey. it's difficult to get good capacitance readings in-circuit. actually the best in-circuit cap testers are ESR testers. the best ones of those are 100khz and up for test frequency. the 50khz ones are easily "fooled" by parallel resistances and inductances. if you get an ESR tester, don't forget to test the cap for DC shorts as well (some ESR testers do this as well, and they also discharge the cap before testing).
Thanks for the quick reply. Besides the pricey Sencore LC102, Does sencore have any other that measures esr. How abou the LC101 and the older models.
I recently bought the CapAnalyzer 88A for $199. I havent had a chance to use it.
It does not seem straight forward like reading a meter. This is unproductive when you have 25 sets to repair in one week.
What do you guys think about the Leakseeker from Electronix. Check it on ebay. Seems like it will save a lot of time.
Does anyone know of a good book on pro audio troubleshooting. I have never seen one.I mean for the workbench. Dealing with everyday problems. Not the books that are written by engineers who never owned a repair shop.
Echo, you seem to know a lot. How about we write one and split the millions.
I recently bought the CapAnalyzer 88A for $199. I havent had a chance to use it.
It does not seem straight forward like reading a meter. This is unproductive when you have 25 sets to repair in one week.
What do you guys think about the Leakseeker from Electronix. Check it on ebay. Seems like it will save a lot of time.
Does anyone know of a good book on pro audio troubleshooting. I have never seen one.I mean for the workbench. Dealing with everyday problems. Not the books that are written by engineers who never owned a repair shop.
Echo, you seem to know a lot. How about we write one and split the millions.
good book on pro audio troubleshooting?
"Does anyone know of a good book on pro audio troubleshooting. I have never seen one.I mean for the workbench. Dealing with everyday problems. Not the books that are written by engineers who never owned a repair shop.
Echo, you seem to know a lot. How about we write one and split the millions."
I'd buy one! Almost all of the books on repair seem to think that explaining circuit theory is somehow the same as telling HOW to fix that circuit. Almost as if every repairman is intent on taking his hard-won knowledge to the grave? One excellent book from the '50s is "Receiver Troubleshooting and Repair" by Ghirardi, but it focuses on vacuum tube consumer-grade equipment.
"Does anyone know of a good book on pro audio troubleshooting. I have never seen one.I mean for the workbench. Dealing with everyday problems. Not the books that are written by engineers who never owned a repair shop.
Echo, you seem to know a lot. How about we write one and split the millions."
I'd buy one! Almost all of the books on repair seem to think that explaining circuit theory is somehow the same as telling HOW to fix that circuit. Almost as if every repairman is intent on taking his hard-won knowledge to the grave? One excellent book from the '50s is "Receiver Troubleshooting and Repair" by Ghirardi, but it focuses on vacuum tube consumer-grade equipment.