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:

Ian Finch

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2010-04-11 4:22 am
Coffs Harbour, NSW
'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.
 
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...
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.
 
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?
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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).
 
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...
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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.
 
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.

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:)
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.
 
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?
 
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.\\
 
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Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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 Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
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. :eek: