Guidance for Forte Audio Model 40 Preamplifier DIY power supply

[sixorfour]

Zen Mod directed me to open up a separate thread for this one.

Basically,

I have a Forte Model 40 with no power supply. I've included pictures of the on board regulators (7815/7915 types) and of the power adapter cable that is hardwired to the preamplifier (Tajimi 3 pin). These seem to share a bit in common with Threshold units of the time although I don't think this particular one was designed by Mr. Pass if my research is correct.

I can't seem to locate the proper Tajimi connector so I assume I'll likely have to get a different type for this project. I really want to come up with a power supply that is at least as good as the original. Any help/guidance would be greatly appreciated!

 

[Zen Mod]

you can build it from scratch, using some veroboard, or you prefer ready made option - I mean pcb and all ?

practically all you need is nice ( plastic or metal ) box, nice xformer and Graetz bridge and caps

input wires - decipher ( using buzzer ) :

one connected with preamp GND ( RCA GND is handy probe point) - that's GND lead

one connected with most left pin of 7815 ( they;re form left - In, GND, Out) - that's Positive lead

then third is for negative lead .... can't remember which pin is In for 7915, and lazy to check

 

[sixorfour]

a ready made option may actually be best.

I had been considering something like this:

Dual Polarity Symmetrical Power Supply For Audio Amp FULLY ASSEMBLED | eBay

Do you think this would work? The Diodes that he's using would effectively be the bridge in this setup, correct?

Sorry for the dumb questions

 

[Zen Mod]

good enough

you need nice xformer (EI or Toroid or what else - irrelevant) , 36Vct (that's twice 18Vac with common point, as GND) , starting from 20VA upwards

so, caps don't really need to have more than 25V, more is OK

say that 4700uF as cap per rail is good to go
more, the merrier

 

[sixorfour]

So I found a power supply from I believe a Threshold FET 10e. It's got the same connector, fits perfectly. My research tells me this thing should be plug and play with my Forte.... before I plug this bad boy in and render both pieces useless, anybody care to tell me whether I'm on track or off track here?

 

[gary s]

Not much too that PSU,transformer, rectifier, 30,000uF electrocaps, resistor feeding a LED and the NP trick of back to back diodes and 10ohm resistor to chassis from GND.

 

[sixorfour]

Not much too that PSU,transformer, rectifier, 30,000uF electrocaps, resistor feeding a LED and the NP trick of back to back diodes and 10ohm resistor to chassis from GND.

Gary! I was hoping you may reply... I actually purchased a used power supply from ebay (the threshold unit seen above). Do you think it is compatible with my Forte 40?

 

[sixorfour]

a closer look makes me think I may be mixing you up with a different Gary, sorry about that.

 

[gary s]

Yeh, I think you have me mixed up with another member. A quick email to Jon Soderberg should get the answer you want.

Around +/- 24VDC should be right - you just need to check pin polarity on the Threshold PSU matches the connector on the Forte 40.

 

[sixorfour]

So i was able to determine the pinout on the cable. I pulled back the sheathing at the connector end and checked the preamp PCB for markings. I've never actually tested a power supply or checked for voltage before. what would be the best way to check this? a good multimeter should do okay right? sorry for the cringey noob questions

 

[Zen Mod]

GND of supply is showing several ohms to case

GND of preamp connector is in shrort with audio GND (any RCA )

so, you need just GNDs to confirm, other one is +

 

[elwood625]

So I found a power supply from I believe a Threshold FET 10e. It's got the same connector, fits perfectly.

The Tajimi connectors are available from Mitsumi USA. They have warehouses in Illinois and California. They have a good online catalog for ordering these connectors. I use them on any of my projects that have a remote power supply.

 

[sixorfour]

I have to agree, these connectors are very nice!!! I actually am under the impression that I should be able to use this threshold power supply directly, no need to harvest/replace any connectors in this case. I'm about 98% sure the wiring is even the same but I'll need to wait for my dad to bring my multimeter back before I can check

 

[rayma]

On the supply output connector, it appears that red is positive, blue is negative, and black is common.
Verify the voltages with the meter. Then compare with the Forte wiring and make sure that everything matches.
Very likely, it will.

 

[Zen Mod]

GND of supply is showing several ohms to case

GND of preamp connector is in shrort with audio GND (any RCA )

so, you need just GNDs to confirm, other one is +

silly me , I did forgot that Treshold/Forte preamp is having bipolar supply

then , you already posted PSU pic - just post proper pic of premp inside, and we'll see it

though, check Rayma's procedure with ohmmeter, it's 99% as he wrote

 

[sixorfour]

THANK YOU EVERYBODY! I LOVE YOU ALL, AND I LOVE MY NEW FORTE PREAMP!!!

This is a huge improvement over my old preamp. I am currently falling in love.

A special thanks to Zen Mod. You have the patience and kindness people only hope to have!!! 🙂

 

[Zen Mod]

I wasn't only one replying here 🙂

 

[rayma]

It looks real clean. What was your old preamp?

 

[sixorfour]

It looks real clean. What was your old preamp?

I am embarrassed to admit it, hahaha... it's an old Kenwood Basic C2. I used it as a practice piece for desoldering and fixing a bad switch (successful) but it has plenty of other problems not to mention it's not a fantastic piece to begin with. All that said though, I think I will continue to practice on the Kenwood. Maybe it'll be my first recap project 🙂

PS thank you for confirming the color scheme - it was the last bit of confidence I needed. Thank you!!

 

[rayma]

Keep hacking on the Kenwood. The experience will be very helpful, and
maybe at some point it will work well enough to sell.