FET 10 /HL is making a "squealing" sound on start up. Help!

[Eric M]

I have an FET 10 / HL I just picked up. For the first 3 minutes of operation it will have a high pitched "squeal" from the right channel. Not 60 or 120 Hz hum, this is a higher frequency. It does not change with input or input selection, volume, balance or any of the signal distribution (tape deck) or signal mode (stereo, mono, reverse) settings. I've been reading several threads here on the FET 10/HL and plan to do a cap swap but wanted to take care of this first. Attached are a few pictures as it seems there are a few variations of this unit.

Hear the "Squeal" here:

Any ideas to remedy this would be appreciated.

Also, if anyone has a schematic of this particular FET 10/HL, I'd love to get a copy. I've downloaded a few from here but none seem to match.

Thanks for the help.


5/19/23 (THE FIX IS ON PAGE 2 OF THREAD)

 

[Zen Mod]

change places of channel modules

inform - does squeel follows module or it stays in same channel

be sure that all screws situated at contact points (motherboard, modules) are properly tight

 

[Eric M]

I swapped the modules and checked the contact point screws at the bottom of the board as well (Loosened and then tightened). Same squeal on the same channel.

I'm assuming this rules out both of the modules. Other than 2 resistors under each module, the other components are to the side of the modules. The topology is mirror image, top to bottom, not left to right.

If the issue is in that area, any idea if the top or bottom portion is the right channel?
And any suggestions what I should look at next?

See above, I will post a video of the squeal

(I was planning on changing the four 4.7 uF power supply caps once this was worked out. Could old electrolytic caps in the power supply cause this sort of noise?)

 

[Zen Mod]

it looks as each channel is having own voltage reg on motherboard
can you confirm that?

 

[Eric M]

I was thinking the same thing but without a schematic, it's hard to tell, I'm not good enough yet! Each channel has 12 resistors, 10 transistors, 5 caps and 2 pots.

Attached is a schematic for the Threshold E-Line preamp power supply, it's not the same but component count is close. It's not a very scientific way of answering your question, but I think you may be correct, each channel has it's own voltage reg on the mainboard and that is what we are looking at to the side of the modules.

There was a member, Gary B, who kept popping up when I was searching FET 10 / HL who seemed to know quite a bit about these. Some of the posts went back to 2010. Is he still around here?

Thanks

 

[GaryB]

Hi - I'm still alive and kicking. I'm out of town visiting relatives and will write more when I have some time.

 

[Eric M]

LOL, good to hear! Whenever you have the time,

Thanks,
Eric

 

[GaryB]

What power supply are you using? And have you tried it with the inputs shorted?

 

[Eric M]

I just tried it with shorted inputs at the selected inputs, still a squeal on the right channel.
The power supply is almost identical to the stock unit, just everything is beefed up. I have another Threshold preamp that came without a power supply so built this to use with it.

Thanks

 

[Zen Mod]

it seems that there are separate voltage regs for left and right modules, situated on left side of MoBo

you can easily check that with continuity test of DMM

visual check for soldering, wiggling parts - that can give you some clues

if you have located actual reg for squeaky channel, there are so many parts that even if you go with blind replacement of all semis , it's easy and straightforward

I did check my files - none of them are adequate for your actual preamp

 

[Eric M]

That might be the best way to go. I'm the sort of person who if I do one channel, I'll do the other. Considering shipping will be the most expensive cost of the replacement parts, I'll probably order all the parts for the power supply regulator for both channels.

I guess I really don't need to identify which area is the problematic parts on the board if I do it this way!

I was going to skip the resistors at this point. Don't those usually show signs on the external if there is an issue? Tell me if I'm just being lazy, I've identified all the parts on the entire unit and resistors are cheap. There are 26 resistors but only 5 values. Talk me into it if you think I should order these as well.

Thanks,
Eric

 

[GaryB]

Have you convinced yourself that the squeal comes from the preamp and not the amplifier? Swapping the left and right channel cables from preamp to amp should cause the squeal to move to the left channel if the preamp is the source of the squeal. In other words, have the right channel from the preamp drive the left speaker and left channel drive the right speaker.

 

[Eric M]

Yes, I did the left and right switch with the preamp, amp, cables and speakers. The issue always stays with the right channel of the preamp and goes away when the power supply is off for the preamp.

I'm now noticing the left channel squeals also. This is new. It goes away after a few minutes as the right does (Right does take longer to go away).
I may start with resoldering the board, maybe it's a aged / cold joint, this unit is 30 plus years old.

I'll try to do that tonight and will post up.

Thanks,

Eric

 

[indra1]

Try replacing the 4 electrolytics near the zeners before messing with semis.

 

[Eric M]

That's what I was thinking. I'll resolder the board first, if that doesn't do it then replace those 4 caps, then the balance of caps in the regulator section. If still no luck I'll replace the two MJE15030 and two MJE15031 and lastly I'll replace all the MPSA42 and MPSA92 transistors. That's everything .... it better work after that!

Can anyone suggest a different order of replacing component's?

Thanks

 

[Zen Mod]

caps are always first, if you are sure they're old ....... or simply unsure they're young enough

 

[Eric M]

I resoldered the main board and ran it for 3 hours, no squeals. Then turned it on this AM and the squeal was back in the right channel, the left is now dead quiet. I suspect the faulty part(s) work when warmed up and the heat from the resoldering did just that.

I'm going to order parts and start replacing on the main board (power supply regulators).

I was wrong about the split of the regulator, its is not top and bottom but left and right, see the pictures below, I have drew a line dividing the left and right channel regulators and have labeled the modules left and right as well. There are 2 tantalum caps right in the center. They go from 0V to +V and -V (Independent of L and R).

I'll post back up once I get into swapping parts.

 

[Zen Mod]

replace electrolytics

if those small yellow are tantalums, replace them too, with solid caps

 

[Eric M]

Got every part on the main board, except resistors, on order. For the small yellow, I'm replacing one with
Vishay Multilayer Ceramic Capacitors MLCC
The other with
Vishay Tantalum

Good choice or try again?

Thanks,

Eric

 

[Zen Mod]

if value allows (size), always take solid cap instead of Tantalum
even if nowadays tantalums are different breed, solids are generally better