Hi All,

 

My early Hitachi radio functions nearly 100% except for the audio output stage.

I get a speaker thump when turning on but no static or sound. The volume and tone controls make no changes to the silence.

I just replaced all caps and no change.  I found the cap C38, which decouples the power amp transistors from the speakers, to be toast and the PO seems to have hooked up two speakers to the output.

I am now starting to investigate deeper. The voltages on the power transistors are normal however I am getting ~230ohms across the input winding of the audio transformer T7 feeding the power stage from the Hitachi FA-6007T op amp.

I'll check against my working radio.

 

Here are some pictures of cap replacement to help others.

 

I also included the service manual  (thanks to @hr369 and @JDMjunkies.ch) ?

Sadly there are no AM radio stations in this area so trouble shooting RF stages will be is tough. Hopefully it is just a problem in the audio stage.

 

 

 

Electrolytic Capacitors:

 

Crusty C38 Cap

 

 

 

 

hitachi_tm-1081-zb_all_transistor_auto_radio_1970_sm.pdf

Nice work! 3 decades ago when I used to design /build / fix amps (long story), your best friend was a signal generator and an oscilloscope. Do you have access to at least the latter? It’s the only way to truly troubleshoot a circuit like that. Having said that you are making great progress.

I would say if two speakers were connected, the final stage amp would be fried and the Darlington transistor or pre-power stage op-amp would also be toast. Very often when a power amp / transistor is overloaded, it pulls way too much current on its collector and effectively shorts out what ever is feeding it.

Watching with interest.

Now don’t swear at me but I replaced all the old electronics with new in mine; with microswitches behind the FM/AM buttons so everything looks totally stock and all the knobs switches function as they should but I get 20W RMS x2, half decent sound and connection to my iPhone so I can hear the Satnav over the exhaust [emoji12]. Somehow sad that all that lovely design and beautiful engineering you see nowadays fits on a chip or two.

Good luck and well done for the great trouble shooting to date! It really isn’t as complex as it looks [emoji106][emoji106]

Thanks for the great advice and support!  I still have a scope.(I used to have access to signal generators 30 years ago too). For this, I have just been lazy and have not pulled the scope out yet.  I'm lucky and have another radio that works so I should be able to compare voltages and signals through the system.  If  I can find an FM to AM converter then I will have a 24/7 input source as there are no AM stations here anymore. ?

I think I found a supplier for the very old Hitachi Op Amp but I will scope out rather than throw a grenade and replace. The voltages on the Darlington array match the schematic so I am hoping it is ok. 

Please post a link to your radio upgrade to this thread to help others!

 

You’re most welcome and I would gladly have popped over to help if you were more local ;)

Great that you can still get the original opamp!! At least you can preserve it.

I didn’t wish to hijack your thread so I didn’t post the link before but since you asked ...

https://zclub.net/community/index.php?threads/stock-radio-modern-internals.20247/

It was all done in a bit of a hurry in small increments of time so I didn’t do as good a job as I would have liked, but it works.

This little bad boy sits totally invisibly under my passenger seat and kicks out some serious bass for it’s size. On about 1/4 to 1/3 of its volume setting it perfectly “fills in” what the stereo lacks without being wide-boy, as we say in the UK, read “red-neck”.

https://www.caraudiocentre.co.uk/product_m-in-phase-car-audio-usw10_p-25817.htm

Thanks for the link. Fantastic work. Nice job!

Thanks buddy, you’re too kind.

 

 

Oh, just a thought - I didn’t have the heart to bin the original parts. If I can find them, i can either send you the main board or mail you the part you need if you wish? Let me know.

 

Thanks AK.  I should be good but someone else reading the thread may wish to have them.

btw your UK website is great. It often comes up in searches and @HS30-H has posted superb information on that site for years.  I love his S20 rebuild thread.

btw your UK website is great. It often comes up in searches and [mention=1359]HS30-H[/mention] has posted superb information on that site for years.  I love his S20 rebuild thread.

Thanks dude! HS30-H (aka Albrech) is our site Yoda! I am forever in awe of / STUNNED by his depth of knowledge (and kindness in helping people out with rare parts).

1 hour ago, 240260280 said:

I love his S20 rebuild thread.

Where is it?

6 minutes ago, HS30-H said:

Where is it?

I thought this was yours: https://www.gtr.co.uk/forum/236658-1969-nissan-s20-twin-cam-engine-rebuild.html ?

Well the 2SB337 power transistors work fine.  I swapped modules and the problem did not move. 

Next will be the OP-Amp.

Good news!  It was the op-amp Hitachi FA-6007T  that died. I'm not sure if there are any other Hitachi radios using this early bird?

FYI a quick and dirty test of the audio section is to simply touch pin 1 and the radio should hum.