The fisher 250TX with a power supply overheating transistor

[havols]

Hi guys. I Have one fisher 250 TX, in excellent condition. I replaced all electrolytic caps from power supply. I replaced too one diode and one resistor. Now, the receiver power on. Phono and aux inputs is normal. FM and Am inputs isn't function perfectly, with a noise. The tune - o - matic function perfectly. When TURN ON (aux, phono or tuner) the receiver, 5 seconds after this, the transistor (TR1001- with circular heatsink) from power supply over heating... too much hot.... Thank you a lot if you can help me.

 

[larryderouin]

What is the serial # of your 250-T / 250-TX? Below 73895 is 250-T. Above 73896 is 250-TX.

Q951 (schematic designation) powers the Tuner board, along with the preamp and tone control board. I would start checking voltages, mainly on the tuner board (2021-9 or -10). It sounds like the tuner noise and the transistor overheating is related. You may have a shorted part on the tuner board. Q951 will get hot (hence the heat sink) but it shouldn't be so hot as to cook an egg.

Fairchild KSA1220AYS would probably be an up rated transistor, but you need to fix the overload problem 1st. And you'd have to fabricate a heat sink to fit it.

 

[havols]

Thank you Larryderouin. Yes, the Q951. This model is 250TX, with wood veneer in front panel. I can't find the serial number.... I Find 2021 Board. Is write E2021BX. When i bought it, i listened for a hour, and one diode (CR952) and one resistor (R953) burned. I replaced they, and now i have this situation. The Q951 is overheating. With multimeter, i measure the transistor, and OK. Is still original. Should i recap all tunner board? It is with original caps. Thanks for help. Regards

 

[larryderouin]

Recapping that board is a GOLD PLATED Hijo de puta! If/when you do recap it, check the resistors for out of spec while you are in it, and check all of the transistors. Take your time and go slow.

If you replaced R953 and CR952, You've got one hell of a CURRENT LOAD that is blowing the Resistor and Zener to the Tuner B+ on Pin 4E and pin 4C of the 2021 board. My suggestion is to take a voltage reading on the 2021 board at pins 4E, and 4C. If either or both are lower than the spec by 5% or so, start following the schematic and board drawings and locate the suspect on the 2021 board. These are the only places that the Power Supply outputs to from the DD and EE points.

Serial # is stamped on the back panel I believe on the right side as you are facing the back. I have a late model 250-T that looks identical to the 250-TX. Had to differentiate it by the Serial #. Mine is in the 68000 group so it's a "T".

 

[havols]

Thank you larryderouin..... I will see the serial number, e post after checking the tensions. To check, i place the multimeter in DC mode, and fixed terminal (+) on 4E, and negative on the chassis? After, the same thing on 4C?. Yes, the recap of 2021 is... hijo de puta... after i will do. Regards

 

[larryderouin]

You got it correct! Insulate the + POS Probe with tape or heatshrink tube to the very end. You just want the tip of the probe to be exposed. This way the cursing will not be available if it slips, and does not short on anything it's not supposed to touch.

 

[havols]

Larriderouin, great help. I'll do it tomorrow. I will isolate quietly .... After post here the answers ... would appreciate if you can help again ... thanks for the help ... I am very curious to put this SS to play ... Regards

 

[havols]

Hi Larriderouin. I checked the tensions. 4C tension measured was +11,6v. 4C In the schematic +12v. 4E tension measured +15,8v. 4E In the schematic +15v. What should I do now? The serial number is this: 83455. Thanks. Regards.

 

[larryderouin]

Voltages are ok. After further thought on it, It may just be that the transistor on the power supply, is running at it's normal temp. Mine runs really hot compared to the others, which is why the heat sink. It's part of the voltage divider network that drops the 60V from the rectifier down to 15v and 12v. As for the noise, See if you can localize the noise source. Get a can of compressed air and flip it upside down.

With the unit ON and in FM, Aim the straw at each transistor on the tuner board, and give it a short burst. Less than a second. Listen to see if the noise goes away. If it does on any of them, replace in kind. With the can upside down it becomes a freezing spray, which cools the transistors down. If there is a transistor with a heat related breakdown on the board you'll hear it. Once you get the noise fixed, then recap it if you aren't fed up with it yet.

 

[havols]

The ransistor q951 is good then? It heats up even to the point of not being able to hold your finger on it? OK. Is it possible to turn off the power that goes to the board in 2021 starting the power suply, and use the SS only in phono or aux?? I will try to better understand what you wrote me ... and try to do ... I can change the electrolytic capacitors from 2021 or is not necessary for now? Thank larri

 

[havols]

the problem may be in the AM section?

 

[larryderouin]

Q951 is GOOD! It will HEAT UP as it's dissipating a lot of heat from the voltage drop from the rectifier. Turning off the FM part not possible without upsetting the rest of the power supply. Go ahead and use in Aux and phono, as the power supply will be making DC power to all circuits whether it is used or not. Use the Freezing spray and replace the transistors as above, and recap it while you are replacing transistors. There are a couple of op-amps in between the towers on the 2021 board. Do the same on them as the transistors. If the sound changes you will need to replace them.

Here's some or the bad news. FISHER used in-house (proprietary) numbers on the earlier transistors and it's very hard to figure out what will replace a certain transistor or opamp. They got better by the time the 250-tx came out but there are still some marked like the TR1001. They have to be researched and then located. In some cases they are germanium and very expensive. If you can find another 250-TX or 250-T or 400-T the boards are interchangeable. The 250-T and 400T use an older Opamp, and transistors but will drop right in. Then you may have to have it aligned. You are doing good right now. Just go slow and be methodical in your work on it.

Larry

 

[havols]

Hey larry. I'm living in Manaus, in the Brazilian amazon (Brazil)... hehehehe ... I guess I do not have any more ss or tube fisher here ... but in other parts of Brazil have ... I'll keep an eye .... Very grateful for your help ... I'll use it a bit to listen to some LPs Jazz and blues .... then you tell how things are going ... this is very difficult some help in this regard ... and I will make the process compressed air .... thank you larryderouin

 

[fred soop]

Calculating from the schematic, Q951 will have about 20 V across it with 10 V across R954. Since the load is constant at approximately 90 mA, why not change R954 from 120 Ω to 250 Ω, 5 watts. This will cut the power dissipated by Q951 in half. The resistor will be better suited to take the constant thermal cycling than the transistor.

 

[larryderouin]

Thanks Fred. Good to know. I'll give that a try on mine also.

havols; Like Fred said, change R954 to a 250ohm 5W resistor. That will cut down on the heat and not affect the voltage to the accessory loads. Nothing like living out in the middle of nowhere. I looked up on Manaus on Google Earth last night. And I thought West Virginia was in the sticks!

Larry

 

[havols]

Hey guys ... larry and fred ... Thanks a lot for help ... I will arrange the purchase of the resistor ... I think meeting here .... will replace and will post the result. I'm hoping it works out ... Yes .. in the middle of nowhere ... or in the middle of everything hehehe ... I Forester engineer... and here there is a lot of forest and life ... These valved and first transistor are a passion, a hobby. Best Regards...

 

[havols]

Another question... Can i replace axial caps from power supply by radial caps equivalents (maybe in paralelo)? Thanks

 

[larryderouin]

Sure can (yes). You may have to add an extension wire to one of the leads. Better to make the - (neg) lead longer. Then either stand it up(the cap) if it's short or lay it down and glue to board if a long one (gotta clear the bottom panel).

 

[fred soop]

Additional note on R954, if possible, mount it on a terminal strip and run wires to the circuit board. That will keep the heat away from the board. At least, leave the leads long and space the resistor from the board as far as possible.

 

[havols]

OK... fine... How can I clean the covers potentiometers buttons ... are oxidized .. Thanks