Heathkit S-99 distortion after operation

[qualityten]

Earlier this year, I restored a badly rusted S-99, replacing all the capacitors, nearly all the resistors and the sockets on the power board. After members here helped me to catch a mistage on the bass control, I thought I had it all buttoned up, and I stored it away in a mostly clean and dry garage.

On returning to listen to it, I find that it has very mild distortion that gets worse after operating for an hour or two. I would describe the distortion as a burr in the sound.

Square wave testing on all the audio frequencies shows a clear trace, but this gets blurry at the edges as distortion sets in.

Because it is so awkward to trace a signal on the preamp board, I would be grateful for any thoughts on what might be causing this. Thank you.

David

[snowman_al]

Both channels?
Pre-amp or power amp? Feed a signal to the 'tape record' sockets, that eliminates the pre-amp section. If you still get distortion look into the power amp. If it works okay look into the pre...

Measured any voltages yet? Start with the HT and see if it drops after a time. If it is the power amp monitor the cathode volts on the ECL86s (pin 7), do they change?
Alan

[qualityten]

Thank you for these helpful divide and rule questions Alan. I'd not thought about whether the HT might drop over time (and why).

I did measure the voltages a few days ago, but didn't write them down. Will do so tomorrow.

[mole42uk]

Nicely said Alan.

David. Do you have the Heathkit manual that goes with the amplifier? I’ve owned and rebuilt four of these S-99 amplifiers and found the ‘book’ an invaluable aid…..

[qualityten]

Yes, I do have 'the book' and used it regularly during the restoration in winter. I did check the voltages of the power amplifier against the schematic, but will need to do so again for all valves.

I did a quick check on the power amplifier as suggested by Alan and did not hear any distortion, so it's probably the preamp, which is much more inaccessible for measuring voltages).

[Diabolical Artificer]

I find that it has very mild distortion that gets worse after operating for an hour or two.

That suggests a thermal issue. First try the easy stuff,try swapping valves if you can, nextt try cleaning the valve bases by squirting in a drop of switch cleaner into the holes or onto a valves pins, then shove a valve in and out of said base.

No improvement? As Alan says check voltages but do so after 5 minutes then after an hour say, compare the two. Your looking for an increase on a cathode or grid and maybe a drop in anode voltage. If a valve is getting biased more hot, EG a higher Vgk = higher Ik = bigger drop across a resistor = higher possible distortion. One trick you can do if you can't get easy access is to clip your DMM black lead to chassis/ground then lift a valve just enough so you can get to the pins with a probe. This needs care and a sanity check to find pin one which is the one on the right of the gap BTW, always foxes me at first till my brain gets orientated.

Andy.

[snowman_al]

You did not say if it is one channel or both?
There is only one valve that is common to both channels in the pre-amp that is V3 a and b an ECC83.
V1 (EF86) and V2 (ECC83) are individual to each channel.

Alan

[Valvepower]

Hi,

As a total shot in the dark seeing this is assembled on PCB's could there be leakage on the PCB when it warms up?

I did have a S99 but abandoned it long ago as the power amplifier PCBs were basket cases plus it had a faulty mains transformer.

One of the output transformers helped revive a MA12 EL84 amplifier though.

Terry

[qualityten]

Thanks for these further helpful observations. Alan, I think it is common to both, so your point about V3 is well taken.

I was not able to get to this today, but will do so over the weekend. I'll temporarily remove the screen behind the preamp PCB (which uses the same bolts holding the stand offs of the power PCB). Having had grief with the chassis layout of an American Heathkit (the AA50), I do wonder why Heathkit did not give more attention to access for faultfinding in these kits intended for amateur builders.

[qualityten]

Finally got back to this, and with proper voltage checks the picture is different. The preamp voltages all check out.

One the power amp, the voltages of V5a and V9a are all much lower than they should be. The grid voltages are 14V instead of 28V. The anode voltages are 238V instead of 250V. And the cathode voltages are 21V instead of 28V. The HT2 supply to this part of the circuit is also lower, 257V instead of 270V, even though the capacitors and resistors supplying this are correct.

Any thoughts on the possible reason for these lower voltages? Once warmed up the distortion is clearly audible in both channels, though subjectively worse in one of them.

(As a side note, it was easy to remove the screen behind the preamp PCB. Apologies to the Heathkit designers!)

[snowman_al]

Lets go back a step.
Please confirm there is or is not distortion if you feed the power amp(s) with the signal via the tape socket.
Even pull V3 out just to ensure the pre-amp is not injecting into the power section.
The power amp voltages may not be what you expect, but pointless chasing them at the moment if the pre-amp has the distortion.

[qualityten]

Rechecking signal in at the tape socket: there is much more volume when V3 is pulled out and little or no audible distortion. It's hard to be exact, because putting the signal in there also highlights the poor tracking of the balance pot. (Where can I find a 500k reverse log potentiometer to replace this?)

So it seems as if there is a problem with the preamp. I don't know where to go next with this. Does the effect of removing V3 give a clue?

The assembly manual (p47) just attributes distortion to: "... faulty valves, a leaky coupling capacitor or resistors that have changed in value due to overheating during assembly. An ohmmeter will prove helpful in checking for leaky capacitors and resistors that are out of tolerance."

[snowman_al]

At least try another ECC83 V3a/b. Lets eliminate it.

Reverse log or anti log pots are scarce. Usual dodge is to make a 'fake anti log pot' from a linear pot.
Find a 500kΩ dual linear pot that fits the S-99 hole and knob.
Use a 47kΩ (or 68kΩ) resistor to connect the centre tag (out to vol control) to the input tag (from the Normal / Reverse switch). One for each half of the pot.

It will work better than what you have (had). Have a look at https://sound-au.com/project01.htm#s5 Better Balance Control fig 8.

[qualityten]

I have tried two other good ECC83s in V3 position. The result is the same. Now that I'm attuned to it, the distortion is audible immediately, though it does seem to get worse after a time. I also observe that there is no increase in volume when V3 is pulled, just absence of distortion.

I'll order a decent quality 500k dual linear pot to deal with the mistracking balance pot.

[qualityten]

I have found a suitable pot in my parts box and have replaced the defective balance pot as per post #13. Thanks.

[joebog1]

Qualityten, you have all ( or most ) of the bits needed to make a quality ten. Also part of the Playmaster series. By most parts, I mean the iron which is vitally important and hard to get today in decent quality.
I beleive that the Heathkit stuff used Hammond made transformers and it was in fact very nice quality.
Its not the super best, but highly capable. This article might fancy your tickle.

Joe

[joebog1]

Last two pagelets.
Also the pics of my set of iron. These were made by Redline in Melbourne to match the "selected " version of the amp. I will try find the adds for the same.
Basically its identical to the article exactly as presented, but came in kit form with Polystyrene coupling caps, high stability plate resistors, and supposedly selected EF86's made by Radiotron is Australia.

It will be in an upcoming thread, but I just wanted to show that you have all the bits needed to make a FINE quality ten.

Joe

[qualityten]

Thank you for sharing this information Joe. I'll enjoy reading this documentation, here and in your 'old' hifi thread.

Using the Heathkit transformers to build a different amp is a possibility, but I haven't yet given up on sorting out the distortion on the S99.

[wd40addict]

Heathkit UK normally used good quality Hinchley transformers.

[PJL]

Where are we at? I am a bit concerned about the subjective test of distortion as it could be anything from the source to the speakers (and hearing!).

Are the output cathode voltages OK? v4(7)=V5(7)=10V
Is the concertina OK? voltages across R38=voltage across R40?

NOTE: The V5(1) grid voltage of 28V on the schematic is plainly wrong and the loading of your meter will further lower the measured value.