help troubleshooting Fisher 440T - Page 2 - 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 3rd March 2013, 02:19 PM   #11
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Quote:
Originally Posted by Ian Finch View Post
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.
Yep I've been able to get the scope thing working, thanks, it's fun 'though my display is slightly rotated. Will check all the Vbe's.

Quote:
Originally Posted by Ian Finch View Post
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.
Will do my best
Quote:
Originally Posted by Ian Finch View Post
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.
Yeah I replaced them right off the bat, went to 1800 uF.
  Reply With Quote
Old 3rd March 2013, 02:43 PM   #12
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
I can check the transistors in place, yes?
  Reply With Quote
Old 3rd March 2013, 08:18 PM   #13
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Yes, that's the plan and we check this base-emitter voltage simply because it's about the only consistent voltage indicating a transistor's condition.
__________________
Ian
  Reply With Quote
Old 3rd March 2013, 11:18 PM   #14
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
All but one of the transistors have good vbe, mostly .7V a couple at .5 on the preamp. Q14 which I think is associated with the FM section has vbe=0. Moving on to checking other voltages...
  Reply With Quote
Old 4th March 2013, 12:45 AM   #15
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Strange update. After checking all the vbe's I checked all the power supply voltages, they all looked good. Hooked up some scrap car speakers to start tracing signal and both sides are now working. The only thing I can imagine is jiggling around some of the small black transistors while checking it rubbed through some oxidation. Should I perhaps pull out all the socketed transistors and deox their leads and sockets?

There's still some hum, not awful, but I haven't yet recapped the preamp. I have some silmic 4.7 uF I can use for the 4uF's shown (C103 & C104), nothing for the mylar .33's so I'll leave them, and the closest I have for the 160's (C107 & C108) are some 220uF nichicons, do you think that's close enough in that position?
  Reply With Quote
Old 4th March 2013, 04:04 AM   #16
diyAudio Member
 
Join Date: Jul 2012
Location: SF Bay Area (east bay), California USA
Replaced the electrolytics in the preamp. Amp is sounding pretty great, still has the slight hum in one channel, even when the volume is all the way down.

FM stereo is soft in one channel, suspect this is about the lack of vbe on Q14.

I have two or three things I'd like to solve before returning this amp.
1) Get Q14 some vbe.
2) Quash the hum
3) Set bias - I don't have a procedure for doing this.\\

Last edited by mmmalmberg; 4th March 2013 at 04:17 AM.
  Reply With Quote
Old 4th March 2013, 08:16 AM   #17
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Hi again
Great that you have found your way around a little better. I didn't realize the transistors were socketed and that is a definite trap. Ideally, they would be removed and all new transistors soldered in directly but if it works and you are certain the leads and socket surfaces are clean and bright, who's to be concerned for the short term, at least?

Q14 I can't locate (Is it Q401?) but it may not be the problem you suspect. Voltages at RF frequencies don't show up on a DMM or on a low frequency 'scope even, as they can be capacitively coupled so you can't measure a DC voltage. However, I'm no expert so I'll leave that to someone familiar with discrete RF design.

The bias adjustment is shown as "BAL ADJ" on the schematic next to the variable resistor or trimpot (R64,65). You adjust this according to the tiny voltage marked and measured across R80 or R81. That's 40-50 mV. Though probably unused since it was built, these tend to be low quality gizmos and may need replacement if you can't get a steady voltage after messing with them. If the voltage is OK to start with, don't touch.

BTW, what was the DC voltage at R80, 81 WRT ground?
__________________
Ian
  Reply With Quote
Old 4th March 2013, 08:55 AM   #18
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Default FM levels

I had a think about the FM stereo and since it is derived passively with a diode bridge type detector, there is little to question about the signal balance there, if the stereo indicator is coming on appropriately when signal level is good and the bridge diodes are working correctly. Use your scope to look closely and compare levels at both left and right outputs from the Multiplex Decoder board. Deselect the connection to the preamp when doing this, so there is no load interfering with the measurement. If you have a dual channel or dual beam 'scope, it's a good opportunity to try that out. Without knowing the brand, 'scopes usually have a "trace rotate" or beam rotate control either on the front panel or internally. Watch yourself around high voltages, though - EHT in larger CRT scopes can punch holes in you.
__________________
Ian
  Reply With Quote
Old 4th March 2013, 09:46 AM   #19
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Quote:
Originally Posted by Ian Finch View Post
.... You adjust this according to the tiny voltage marked and measured across R80 or R81.......BTW, what was the DC voltage at R80, 81....
Oops, that should read R81, R82 in post #17
__________________
Ian
  Reply With Quote
Old 4th March 2013, 10:52 AM   #20
diyAudio Member
 
Join Date: Jul 2003
Location: Dona paula, Goa
There is one more way of testing a transistor, in circuit other than measuring Vbe.
You have to short base to emitter. This will switch off the transistor and full Vcc will appear on the collector.

Gajanan Phadte
  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 06:59 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