33V across 200 ohms is 165mA which is dead nuts for the tuner current draw on AM and usually 170mA for FM. Something else going on...
990db 9090db F-2619 Power Supply Board is Roasted
- Thread starter motorcityguy
- Start date
Have two here, both with the remote mount resistor. neither one gets warm after hours of run time. Something is pulling really hard on that power supply. Any chance you pinched a wire when you removed the tray or left that wire off the underside you have to unplug to remove the tray?
After looking over some schematics it seems like there are a few different resistor combinations out there. The main schematic floating around shows Main power supply R01 as 10 ohm 3W, with a pair of 270 ohm resistors on the RF Supply. An alternate schematic shows 57 ohm + (2) 300 Ohm resistors. Most of the units that I have worked on actually have a 27 ohm 5W resistor on the main PS and (2) 270 ohm resistors on the RF Supply. Wondering if I should be changing R01 on the main PS when swapping to the 200 Ohm resistor?
Another factor that I did not consider is that I did the dolby cal and tuner alignment all with the driver card unplugged Perhaps I am feeding the RF supply an elevated voltage by not having the driver card in circuit.
Another factor that I did not consider is that I did the dolby cal and tuner alignment all with the driver card unplugged Perhaps I am feeding the RF supply an elevated voltage by not having the driver card in circuit.
Last edited:
I used a 135 Ohm 25w which is the stock 270 in parallel. It gets a bit warm, but not "hot" mounted on the tuner tray. Perhaps get some self-adhesive heat sinks to help dissipate some heat. Clean the surface, stick 'em on, and clamp them on over night so the adhesive gets a good grip. People tend to skip the clamping step, then wonder why they fall off.
There is also a black ground wire on all the ones I did underneath the tray. All but one were connected with a spade term, and the other one was soldered. In 2 of the units, someone "forgot" to reconnect that ground wire. I have no idea if it caused a heat related problem or not because I reconnected them before trying anything. Although they did have severely burned boards at the resistors.
Update,
Driver card rebuilt and reinstalled - No Change. I put the original 270 ohm resistors back in and they drop 22V across themselves. This feeds the regulator section with about 30V instead of the 18V when using the 200 ohm aluminum clad resistor. it would be interesting to know the voltages before and after the 200 ohm resistor on someone else's unit. I have about 53V feeding the resistor with approx. 18V on the other side after the drop.
I also unplugged the secondary connector on the bottom of the board which feeds the dolby board, no change with dolby circuit disconnected.
Driver card rebuilt and reinstalled - No Change. I put the original 270 ohm resistors back in and they drop 22V across themselves. This feeds the regulator section with about 30V instead of the 18V when using the 200 ohm aluminum clad resistor. it would be interesting to know the voltages before and after the 200 ohm resistor on someone else's unit. I have about 53V feeding the resistor with approx. 18V on the other side after the drop.
I also unplugged the secondary connector on the bottom of the board which feeds the dolby board, no change with dolby circuit disconnected.
After reading back through some of the various posts on the relocated resistor it seems like some say that it runs very hot and dissipates a great deal of heat to the tuner chassis while other say that it runs fairly cool. An objective measurement is probably best here. I think my resistor is fine. The regulator circuit is working normally and the pass transistor heat sink is barely warm which is the goal when changing from (2) 270 ohm resistors to a single 200 ohm resistor. I verified the temperature at the body of the aluminum resistor with two different thermocouples. After being on for about 1 hour, the temperature is stable at about 150 degrees F. The resistor is dropping 33V which means about 5.5W of dissipation. That is plenty of safe headroom for the resistor. I believe that the reason my tuner fin is maybe perceived as hotter than others is because I used thermal compound during mounting and perhaps am transferring more heat to the fin.
It would be great to compare temperature and voltage drop characteristics with others that have made this modification.
It would be great to compare temperature and voltage drop characteristics with others that have made this modification.
Blue Shadow
Waiting for Vintage Gear from this century
When I worked on Spinedoc's 9090DB, I measured the feed voltage and calculated the proper dropping resistors to get what was needed. I installed the resistors on the chassis as others have. Haven't heard anything but good about that unit since he got it back years ago.
The design of the tuner power supply is just wrong it seems, supplying way too much voltage. Surprised the caps didn't pop.
Scott Muni
Well-Known Member
I added a 200 ohm to the chassis with thermal paste. Voltage drop is 33.2, measured temp of resister, 155 (yikes!) and the surrounding chassis is 140. Also added a stick on heat sink.
Sure does build up a lot of heat with the wood cabinet on, they should have vented the cabinet over the board. Not a good design IMO.
Sure does build up a lot of heat with the wood cabinet on, they should have vented the cabinet over the board. Not a good design IMO.
Attachments
Scott, Thank you fo much for this reference. Your voltage drop and temps are right in line with what I am seeing here. I am going to button this one up. May I ask where you sourced that heat sink?
Scott Muni
Well-Known Member
Amazon heat sink. I have used these on other projects, so far they stay stuck.
https://www.amazon.com/Easycargo-40...56768706&sprefix=heat+sink,aps,73&sr=8-4&th=1
https://www.amazon.com/Easycargo-40...56768706&sprefix=heat+sink,aps,73&sr=8-4&th=1
Blue Shadow
Waiting for Vintage Gear from this century
They should have designed the power supply to give that board a lot closer to the needed voltage and way less in the way of dropping resistors. Having to burn up 35 V or so is making everything in that area too hot.
I am thinking about taking a shot at designing a replacement board based around a modern voltage regulator IC. That board just needs to output 12.5V for the tuner and dolby card and provide the voltage for the stereo indicator as far as I know. I work on these enough that it may be worth it to order a bunch of boards.
Scott Muni
Well-Known Member
Interested in this, if you do make up a bunch of pcb's. So much heat comes off that resister.
larrystevick
New Member
I would buy one.
tnsilver
Stereo Puppy
IIRC there's already a regulator on the stock PCB. The problem, as I see it, is a 30VAC power supply coming from the transformer, having to be reduced to 12.5V. Is there any nifty way to burn off the difference without dissipating heat?
My understanding is that the board comprises a discrete voltage regulator, the large device on the heat sink is just a BJT acting as a pass element. The circuit that actually feeds the board is something like 56VDC so getting it down to 12.5V is definitely a chore, the original design seems to have accomplished part of this with the cement resistors and the other part with the transistor on the heat sink. The big 20-25W chassis mounted transistor mod relieves the stress on the pass transistor so that the board does not overheat but the relocated resistor still cooks. The next time I have an 8080DB or 9090DB on the bench I'll try to pull out a breadboard and do some experimentation.
