help troubleshooting Fisher 440T - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 2nd March 2013, 10:47 PM   #1
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Default help troubleshooting Fisher 440T

Trying to help out a friend with his Fischer 440T. He said it had a hum and hadn't used it for a couple of years. I figured it probably needed p.s. caps and told him I'd take a look at it. Took a look inside and at least a dozen caps were leaking badly. Ordered up p.s., coupling, and a bunch of other electrolytics figuring "in for a dime, in for a dollar".

De-ox'd the pots to the best of my ability. Very hard to get a shot at the balance and tone pots. Replaced the p.s., coupling, and the other majorly leaking caps. Powered it up, one channel is dead and the one that's not has a hum.

I've tracked the dead channel down to the tone board. Signal is there at input, not at output.

The hum is present when the volume is down, regardless of any switch selections or pot settings.

Any ideas or pointers re. troubleshooting either of these would be greatly appreciated - thanks!

-Mark M.
p.s. pdf of the manual with schematics here: http://www.markmalmberg.com/440T/440T.pdf

Last edited by mmmalmberg; 7th March 2013 at 03:20 PM. Reason: fix link
  Reply With Quote
Old 2nd March 2013, 11:33 PM   #2
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
'Nice simple design.
There are a couple of 1uF electrolytics in each channel on the tone board that could be open. Also check Vbe is ~0.6V on each transistor.

If you have a small, sensitive amplifier lying around, you can use this with a cap of ~100nF in series with a lead to the input to probe the signal in the DUT. Use a shielded lead, like a mono RCA connect and connect the braid or outer connector to ground. I occasionally use an old battery operated PC speaker for this but any simple small speaker arrangement - even an AM or FM radio can be used.

It's not unlikely that every electrolytic is in dire need of replacement but don't touch the tuner yet. 'Hope this is helpful.
__________________
Ian
  Reply With Quote
Old 2nd March 2013, 11:55 PM   #3
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Hi Ian. Very grateful for your thoughts. Can I equally use a scope to trace the signal? I just bought a funky old one that seems to work, to the limited degree I know what to do with it Or I could probably come up with something like your describing, 'though I'm pretty sure I don't have a 200nF cap around.

So I'll check the 1uF caps, I should be able to find the signal at each end of each of them? They look like coupling caps on the inputs and outputs of the tone control board yes?
Thanks...
  Reply With Quote
Old 3rd March 2013, 01:20 AM   #4
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Sure, a 'Scope is better generally - I don't say so in case guys just don't have one or plan to for the little use they may have. Hearing is believing, too. You could use any cap from say, 0.1-10 uF if necessary

Yes, those are just coupling caps but if they don't couple, it sure will be hard to hear anything. They can also form a path for hum and noise. You could use film capacitors to advantage there. Even polyester or MKT types will be OK and better than small electros. - quite in keeping with the vintage too, if they can be coaxed to fit with their minimum 5 mm lead spacing, that is.
__________________
Ian
  Reply With Quote
Old 3rd March 2013, 01:46 AM   #5
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
OK so I proved myself wrong, going back over things more carefully I have input and output from the tone control board. I just realized I measured the transistor base voltage to ground rather than to the emitter, will have to redo that; got readings of .7 and .9V.

But I guess now I'm on to trying to trace the path into the preamp from the volume control, yes? Or is the tone control board considered part of the preamp?

I'm assuming the hum's not coming from the tone board since it's not affected by the volume control or any other controls, yes?

Last edited by mmmalmberg; 3rd March 2013 at 01:59 AM.
  Reply With Quote
Old 3rd March 2013, 01:55 AM   #6
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Does seem like it would be reassuring or maybe informative to actually listen to the output of the tone board through another amp, can I run that directly into a power amp, or into a line in?
  Reply With Quote
Old 3rd March 2013, 03:51 AM   #7
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
You can connect to another amplifier as detailed but you need to use an isolating capacitor somewhere in line or DC currents will flow and cause a short. Ground connection will need to made to complete the signal path too.

The capacitor must be large enough to permit bass frequencies at the impedance presented by the amplifier. Whilst 0.22 uF is enough for the Fisher amplifier, others may require up to 10 uF input capacitance because of their low impedance. Obviously, the voltage rating must exceed the circuit voltages and any DC differential with the other device. Film caps are safest to avoid polarity mistakes and general audio quality issues, where they are still cost effective.

As the hum may still be in the amplifier section, turn the volume control right down to isolate the amplifier input and measure Vbe of the transistors to check the fault is not with blown transistors. This could easily happen with failed caps. Don't have speakers connected when any tests are performed as the load will just confuse readings and likely cause more damage anyway until you isolate the problem(s).
__________________
Ian
  Reply With Quote
Old 3rd March 2013, 05:07 AM   #8
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
OK thanks Ian I'll see what I can come up with...
  Reply With Quote
Old 3rd March 2013, 05:32 AM   #9
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Quote:
Originally Posted by Ian Finch View Post
As the hum may still be in the amplifier section, turn the volume control right down to isolate the amplifier input and measure Vbe of the transistors to check the fault is not with blown transistors. This could easily happen with failed caps. Don't have speakers connected when any tests are performed as the load will just confuse readings and likely cause more damage anyway until you isolate the problem(s).
Are the power transistors the only ones I should check, or is it just as likely to be a problem with one of the smaller ones? I actually suspect the power amp transistors will prove OK, as in bumping around with probes and such I've seen some little pops make it out to the dead channel's speaker.

I might try injecting a signal from my android phone (I have a couple of signal generator apps) at low level into the power amps if I can find the correct point, and see if I can get something to come out of both sides. I have both dummy loads and junk car speakers to test with...
  Reply With Quote
Old 3rd March 2013, 07:56 AM   #10
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Check all transistors for Vbe. There is no point in only being 2/6 certain or whatever. When you have some assurance that all are OK there, then consider a signal again in the normal way. We know something is still wrong so don't connect speakers or loads yet, use your 'scope (Google a general user guide to set up AC connection with a x10 probe setting and voltage range of about 5V/cm to start with), headphones or spare amplifier to monitor the output stage.

Unusually, this has a transformer coupled voltage amplifier stage. I can't say as I've seen this before in this size amplifier but the overall operation is similar. Verify the voltages marked on the schematic and follow the the service guide, crosschecking channels for certainty. Take care not to slip with probes and short parts that were OK but possibly now irreplaceable. Also check the DC voltage at the output nodes: (btm of R81 or R82) This should be close to half the power supply voltage or around 22V, I think. Check the idle current as shown for good measure too. Don't adjust if it seems way off - something else is wrong and needs fixing first. Adjusting to suit an error condition just loses the correct setting.

I guess I don't need to ask if you replaced the output caps C29,30. If they are lame, nothing will sound too good but you could double them to 2,200uF with some improvement.
__________________
Ian
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Fisher 500c Rucyrius Tubes / Valves 1 30th November 2011 03:24 PM
Fisher 302 myron0707 Analogue Source 3 6th August 2011 09:37 PM
fisher pa301 dizmaloner Solid State 0 25th August 2010 03:26 AM
Fisher 500C troubleshooting mholz Tubes / Valves 6 28th April 2010 02:47 AM
fisher x100a draget Tubes / Valves 16 16th June 2005 02:09 AM


New To Site? Need Help?

All times are GMT. The time now is 11:06 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2