Jump to content

240260280

Hitachi TM-1081-ZB Radio Repair Thread

Recommended Posts

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:

cap replace3.jpg

Cap replacement.jpg

 

Crusty C38 Cap

C38 Bad.jpg

 

cap replace2.jpg

 

caps 3.jpg

 

caps.jpg

caps 2.jpg

 

hitachi_tm-1081-zb_all_transistor_auto_radio_1970_sm.pdf

  • Like 4

Share this post


Link to post
Share on other sites

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 . 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

  • Like 1

Share this post


Link to post
Share on other sites

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!

 

  • Like 1

Share this post


Link to post
Share on other sites

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


2777db438c9038119d415c0f51ebf04f.jpg

  • Like 1

Share this post


Link to post
Share on other sites

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.

 

  • Like 1

Share this post


Link to post
Share on other sites
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).

Share this post


Link to post
Share on other sites
1 hour ago, 240260280 said:

I love his S20 rebuild thread.

Where is it?

Share this post


Link to post
Share on other sites

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.

 

  • Like 1

Share this post


Link to post
Share on other sites
7 hours ago, 240260280 said:

Yes but what's the connection with zclub.net?

From here:

8 hours ago, 240260280 said:

btw your UK website is great. It often comes up in searches...

They are two - completely unconnected - clubs and forums.

Share this post


Link to post
Share on other sites
15 hours ago, 240260280 said:

Pin 1 is on right left side

Uhhhh.... That might make sense to someone, but I'm not one of them.  LOL

Out of curiosity, do you have a datasheet for that part?

Share this post


Link to post
Share on other sites

LOL Sorry. I corrected entry but left the wrong right in. <<< makes sense right? Fixed now.

 

Sorry. No I do not have data sheet. I would love to see it too  I think the IC is more than just an op-amp as it feeds back to the auto tuning circuit. See Pin 7's output below.

I will guess that the op-amp function can be substituted with a compatible op-amp but you will loose the auto tuning without developing the associated circuitry to trigger an amplitude lock on a signal or maybe it is a mute while tuning control?

Feedback.jpg

Share this post


Link to post
Share on other sites
13 minutes ago, 240260280 said:

 the wrong right in. <<< makes sense right?

Yup! I Got it!  Haha!!

So not that it really matters, but that's an AF (Audio Frequency) amp, not an op-amp.

Good luck with the repairs. I paid my way through a couple years of college fixing audio-visual equipment. One of the things I learned is that 95% of all failures are blown output stages. Even if the problem does not appear to be a blown output, it might be anyway.  LOL

  • Like 1

Share this post


Link to post
Share on other sites

I bravely swapped bad unit's output stage to working unit and it continued working.  I put working units output stage in bad radio an no change. 

 

I then moved good AF SIP to bad radio and it worked... risky but easy.   I did verify all voltages in power transistors were the same before swapping so low risk in the end.

  • Like 1

Share this post


Link to post
Share on other sites

Cool. Neat find. based on the pin numbers and similar use in the schematics, I would concur that there are a lot of similarities.

Good luck with the repair. Sounds like you're almost done!

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use. We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.