Again President Grant L37 runs bloody hot and smells

[13EL121]

Hi all, again a President Grant PC999-AE on the bench which works quite well but L37 runs so hot that the wax begins to cook and smells burned with magic smoke.
I replaced both 560pf caps with 300v silver mica types. The original caps were checked and seem to be working OK. I also replaced L37 with a spare part from a scrap Galaxy Pluto.
The radio is working fine on TX with 10w AM-FM and round about 22w SSB. But during longer overs in FM, L37 runs so hot that it isn't normal. The output begins to drop down after a while. After cooling down the power is back.

Any ideas where to look? Thanks for any input.

73
Ralf

Screenshot_2022-09-12-19-58-37-70_99c04817c0de5652397fc8b56c3b3817.jpg

Screenshot_2022-09-12-19-59-47-68_99c04817c0de5652397fc8b56c3b3817.jpg

[ch25]

Parasitic oscillation.
L37 is 52MHz trap. If it gets hot, 2nd harmonic is really strong.

[13EL121]

Thank you Chris, the Radio came in with a burned L37. It looks not screwdrivered at the first look. So I replaced L37 at first. Maybe I'll check tomorrow for the harmonic at 52-54 MHZ. Maybe the bandpass filters are screwed...
But I couldn't imagine that L37 will run so hot even the filters are out of alignment.

[Edd200sx]

Check the pass caps first I would maybe those micas

[13EL121]

C183 and C185 are replaced. The old ones were checked OK within specifications. Now there are 300v types in.

[Edd200sx]

Not sure then. I’ve seen these cooked by having the power
Set too high but not under normal conditions

[The Collector]

Not that I know ANYTHING about radio guts, but could a dodgy or counterfeit output chip cause an issue?

[cb4ever104]

Never seen this one . How much RF would you even need to fry that inductor ? A lot I would imagine . A spectrum analyser would probably be handy with this fault .

S.

[Nodens]

Check driver and finals biasing if its ok.

[13EL121]

Hi all,

Bias is OK for sure. For best results I set driver bias at 0.73v / a bit more than 60mA.
Final Bias is at 0,65v at 60mA each final. This should be not an issue.
Output is set to 10w AM-FM and 22w SSB which is the maximum output the radio delivers.
Finals and driver are 100% original, no fakes.

I set L37 now for minimum output at 50mhz. Seems to be better now. Will have an eye on it.

[The Collector]

Perhaps drop it down to 7 or 8 watts if it still gets too warm. 10w max output might be straining it?

[cb4ever104]

To be honest all my twin final radios are set at 7/8W . I know they'll do more , but I'd rather leave them slightly below so as not to overdrive anything . There's little to no difference anyway on the RX'ed signal .

[14CS06]

Hello Ralf

Most common faults on GRANT and SS 3900

If the L37 winding is charred (classic GRANT failure) you must also change C185 and C183
If the winding has melted, there is no other way than to recover another core from a wrecked TX and rewind the 4 turns on it.

The capacitors of 560 PF (sometimes 820 PF) are in C 185 and C183
FYI C185 is in series with the L37 winding (so all the HF power that goes to the antenna passes through this capacitor (in SSB when leaving 25 w it ends up with about 50 volts HF at its terminals)!

https://spectrum26.pagesperso-orange.fr ... antes.docx

https://spectrum26.pagesperso-orange.fr/

@+
Claude

[cb4ever104]

Most common faults on GRANT and SS 3900

If the L37 winding is charred (classic GRANT failure) you must also change C185 and C183

He has already changed those capacitors Claude

C183 and C185 are replaced. The old ones were checked OK within specifications. Now there are 300v types in.

[13EL121]

According to Claudes post, L37 seems to be an issue. I had this problem years ago with another Grant. But all these Grants were black faced. Never had a problem with old silver faced Grants. The older PC999-AB and AC silver faced Grants seem to have more maximum power with up to 30w PEP SSB. All black ones I had were maximum 25w PEP. No big difference I know, but maybe there was a slightly difference at the first AE boards?