Fisher 59A amp modification

:scratch2:Hi Fisher friends

I found 4 NOS RCA 1N2326 diodes used in the conversion of the bias pots on my hybrid 59A amp.
Larry was great and emailed me the specs for the conversion of the 49A. However ....when I crossed refereced the listing for the resistors in that circuit that the modification called out to REMOVE, those reistors are of different values between the 49A and the 59A.
Would that indicate that the resistors that are called out to INSTALL for the coversion actually different for the 49A and the 59A. Both conversions use the RCA 1N2326 diode.
Does anyone have a Service Manual for the 59A that has the "Modification"
listed on the page for the amp?
Boy... I do not want to make a mistake and take out all of the germanium transistors!!!!
Everything is working beautifully now....should I just leave well enough alone?
Thanks
George

The diode will be the same, but as the 59A has 8 transistors, vs the 49a's 4 transistors each of the Diode-Resistors are handling different amounts of current. I haven't seen a 59A manual that has the mod for it. I've got a spare 59a amp that does have the mod. I'll look at that for the resistor values.

Here's the pertinent parts of the schematics, and the BIAS MODS in the 49A manual.





These are taken off my spare working amp. This is a 59A from a 1965 Ambassador.
I marked up the parts in the 1st picture. There are 3 more sets like this one for a total of 4.

Hi Larry
Thanks for the photos.
I posted three pictures of my 59A with the original components around the bias pots.
There is the original gray resistor marked 3xI or 3x1. Then there is a small resistor that does not go from the transistor emitter but is located on the
terminal strip. Also the large resistor is colored large red, red, brown,gold.
On your photo's that resistor is orange, gray, orange.
Now that I have banged my head repeatedly on the work bench!!!! I think I will go to bed.
Thanks
George

Carefully rotate the gray (emitter resistor. I think you'll find it's a .5ohm 3 watt 5% resistor.

The Red Red Black gold is a 22ohm 1/4w 5%. It bypasses the pot.

The new set up is as follows;
1.) Remove the pot and install the 1n2326 to the same leads. Terminals 1 & 2
2.) remove the 22ohm 1/4w(Red Red Black) resistor from Terminal 1 and 2.
3.) Install the 10oHm 1/4W resistor from the emitter on the transistor to Terminal 2 on the strip.
4.) Remove the original .5ohm resistor from the emitter and Terminal 1 and replace it with the .33ohm 3w 1% resistor. repeat 3 more times for the other output pairs. I prefer 1% resistors in circuits like this, although 5% won't hurt anything.
Get 10 of each and match them with your ohmmeter. This way the circuits are as close as possible and all will run the same.

That BIG Resistor (looks like Red Red orange or brown???) should be a 220ohm red red brown gold). Lift one leg from the terminal and ohm it out. If it's right around 220 leave it in. If it's more than 5% out of spec (ok range is 209 to 233ohms) replace it with a 220ohm 2W 5%. Metal film or Metal Oxide is fine. Doesn't have to be carbon comp or carbon film. That's R21.

Larry

Hi Larry
I think I finally have it with your help. The 1N2326 RCA diodes arrived in the mail today and look fine. They even have the "red dot" that the instructions on the 49A call out.
I hope that this research and disscussion will help someone out in the future.
The Fisher site is just one of the best.
Thanks for all your help. I will post the final pictures when I finish the project.
Thanks again
George:thmbsp:

George; On the schematic, Following from the emitter from the transistor, you have in line, the gray emitter resistor to Terminal 1, the diode between Terminal 1 and 2 (Red Dot on TWO) then the LARGE 220ohm resistor. This matches both the schematic and the pictures. (I checked the amp too.).

Larry

Hi Larry :thmbsp:
I prayed to the audio Gods and they heard my prayers.
I finished the conversion in the exact manor that you directed and the amp work beautifully!!!!
There are a few photos of the completed conversion for reference in case someone else should need the information.
Again....thanks for all your terrific help.
George

Looks Good George! Hell you did all the hard work and got the ulcers for it!

Now we have verification of a documented Factory mod for the 49A and 59A amps that will help with Bias Stability! Can you put the info on the 1n2326's (who you got them from ) in the COMMON PARTS for FISHER's Sticky thread.


Larry



Note to FUTURE Owners. These diodes are NO LONGER AVAILABLE from the normal sources, and hence quite expensive. They were kind of hard to find and will only get harder as time goes by. It's up to you whether you want to do this modification or not on your 49A/59A amplifier. (65-66 Custom Electra's and 65 FUTURA and 65 Ambassador consoles. )

Hello
will these work?

https://www.ebay.com/itm/1N2326Temp...191764?hash=item261791da14:g:8vAAAOSwiBBckE5L
Thanks
Tom

They should. I'd get 2 and replace both if you have to replace one of them. Or is this a shotgun (replace everything under the sun) rebuild?

I purchased a Futura VI, mostly because I really like the looks of it, I have never worked on radio equipment before but am pretty handy at fixing things, So I thought I would give it a shot.
The Futura was DOA, the first thing checked were the fuses the four 2amp were all blown. I replaced them turned them on and three immediately blow.
so I have replaced most of the capacitors ( I am still waiting for C1,C2,C3, C4, C25, and C29 to arrive in the mail) but mostly all of the rest have been replaced.

while I am waiting for the remaining capacitors I got impatient and tried it out again, 3 of the 4 fuses immediately blow.
after some research on this site - I figure its most likely bad transistors - I log on ebay and purchased 8 RCA SK3015 transistors (which cross with NTE121MP) So I think they will work.
but after further research on the site- maybe there are four bias pots are bad, which cause the transistors to go bad, which cause the fuses to blow?
I am fighting the urge to pop in the new transistors because I don't want to destroy them.

so my thoughts were to do this modification first?

what do you think?
thank you for your help
Tom

Do the basics 1st. Verify operation and values of the current parts. re-do the caps, and what ever resistors are more than 5% out of spec. Test the tubes to make sure they aren't shorted. Test all the transistors using the 6 way test here....... https://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/ Disconnect one lead of the pots, mark the case and the knob, then connect a meter to the center tap and one of the outer taps. operate the knob SLOWLY and check for smooth migration of the values up and down. If you get any opens (O/L usually on DMM's) the pot is bad. replace that pot and modify the circuit as above.

Get it running 1st then do the mods.

For initial testing, those power amplifier circuits can be run as low as ±5 volts for checking that the voltage is evenly distributed across the 4 transistors. At 5 volts, the possibility of damage to new transistors is low. It will even produce sound at that voltage but power will be greatly limited.

How would I test at 5 volts? I tested all the transistors including 2 of the 8 new ones, I attached my results, I am too inexperienced to understand what it means. Any advice would be helpful.

Thank you Tom

I'm talking about disconnecting the internal power supply from the power amplifier and powering it with ±5 volt rails with an external power supply with the transistors in circuit. You could accomplish the same with a single 12 volt supply. Connect it across all 4 output transistors with + to + and - to -. At idle, you can then simply connect one meter lead to the negative of the 12 volt supply and touch the other lead to each of the 4 transistor cases. They should read approximately 3, 6, 9, and 12 volts, indicating even distribution. The output rail using a single supply will be floating at 6 volts so if you connect a speaker, add a 1000 µF capacitor in series. Maximum output would be approximately 1 watt with this setup.

If everything is good, and the supply is variable, you can then increase the voltage and recheck the distribution across the 4 transistors.