Sansui 5050 goes into protection mode when volume turned up

[YapLiMin]

Hello AK'ers. Been getting a lot of good information from the forum and appreciate the wealth of knowledge being shared here.

Working on restoring a Sansui 5050 with a F-2701 Power Supply Board. Without getting into too much detail at this time, I'd like to describe the big picture problem I'm having. For the most part the unit was received intact and original except for the F-2647 driver board which was recapped and most of the transistors were missing. So I did the following:
1) Installed new transistor equivalents where missing.
2) Replaced DC bias trim pots with new multi-turn ones.
3) Original Sanken TO-66 2SA765 & 2SC1445 were checked out of circuit on a DMM and were good.
4) Recapped the F-2701 power supply board and the 6800 uF main filter caps.
5) Replaced fusible resistors on F-2701 power supply board.
6) Checked DC offset at speaker terminals are they are just a few mVs (no DC offset trim pots in this unit).
7) Checked and set DC bias to 15 mV per service manual.

So far I've only powered up the unit on a DBT. Unit powers up, speaker relay clicks in after a few seconds and I can tune in FM stations. Rail voltages are close to SM values and unit works fine on both L & R channels and sounds good at low and normal listening levels. However, once I start turning up the volume where it starts getting loud (approx. 1/4 turn or more of volume knob), the DBT increases in brightness and the speaker relay clicks off. When volume is turned down, the DBT goes dimmer, speaker relay clicks on and sound comes back. This happens on both speaker A & B outputs either with 8 ohm dummy loads or 8 ohm speakers attached.

I just so happen to have a Sansui 6060 up next for restoration. Haven't worked any on it yet but it was easy to pop out the F-2647 driver board from it. This board is all original and I swapped it out with the 5050 F-2647 just to see if the issue was with the new electrolytic caps or the new transistors. Alas no difference and the volume issue is still there.

Need suggestions on what to do next to troubleshoot but these are my thoughts:
1) Output transistors malfunctioning during operation even though they check out good on DMM. Replace?
2) Leaky original electrolytic caps on F-2646 tone control board (volume knob here). Recap?

Thanks in advance.

 

[EastPoint]

What value bulb is in the dim-bulb tester? This is pretty normal—the more volume, the more power it draws, so the bulb will light and the relay will click.

 

[YapLiMin]

It's a 60 watt bulb. The 5050 is 30 watts per channel driven into 8 ohms per the SM. If this is normal should I even try the unit without the DBT? I'm hesitant to do that tho.

 

[DougBrewster]

If the bias and DC offset are stable why would you not run it on direct mains? Note that if you've set the bias on DBT to specification then it will be somewhat high without the DBT. Need to turn it down a bit.

 

[petehall347]

yes plug it in and set bias again

 

[MIJ_Pioneer]

Rail voltages are close to SM values and unit works fine on both L & R channels and sounds good at low and normal listening levels. However, once I start turning up the volume where it starts getting loud (approx. 1/4 turn or more of volume knob), the DBT increases in brightness and the speaker relay clicks off. When volume is turned down, the DBT goes dimmer, speaker relay clicks on and sound comes back.

Hi welcome to AudioKarma! Nice Sansui and nice job documenting, using a DBT, getting this 5050 restored

Your DBT is doing exactly what it should. When it is fully bright it's pulling the max current the bulb can supply (to indicate a short, in this case you are purposely asking for more power with the volume increase) - you're choking it. Either increase the bulb wattage or give it full AC. Many do not play music on the DBT, just to note - however I still do although very low and very short to confirm it's still functioning

Careful, the bias will change when you give it full AC, as well as other voltages. However since your bias is where it should be on the DBT, it shouldn't drastically change to the point of damage, just be sure re-adjust it.

It's a 60 watt bulb. The 5050 is 30 watts per channel driven into 8 ohms per the SM. If this is normal should I even try the unit without the DBT? I'm hesitant to do that tho.

Close, it's not that simple though

On the rear the 5050 is specked pull 210 watts at full power

 

[YapLiMin]

Thanks for all your replies and advice. Much appreciated!

I did not go full AC because I was afraid I would smoke something. Just being cautious and not wanting to cause unnecessary damage especially since I understand the output transistors are no longer in production. In the past and on other units I have worked on , I do not recall seeing the issue where increasing the volume would trip the speaker relay.

So based on all of your advice, I'm first going to get a 100 watt bulb and see what happens. If all is well then I will go full AC and continue on ....

Thanks again and I will report back.

 

[YapLiMin]

Changed the DBT bulb wattage from 60 watts to a 150 watt flood and adjusted the DC bias back down to 15 mV (was only about a mV higher). The DBT is almost off at 0 volume, then flickers with the signal output as the volume is turned up. At the 10 o'clock volume position, the DBT goes bright and flickers more and the speaker relay clicks off and on according to the signal. Same behavior as at the 9 o'clock position when originally using 60 watt bulb.

So this is normal behavior and not an issue as I originally thought?

 

[MIJ_Pioneer]

Yes go full AC, check bias. DBT jobs done, no shorts.

 

[YapLiMin]

@MIJ_Pioneer thanks for the confirmation. Guess I need to brush up on my DBT theory!

 

[Super Noob]

Back your bias down (all the way) after you check it on the DBT and see it rise. Then go full mains (120 AC) and dial up your bias.

FYI... bias trimmers don’t need to be multi-turns. Offset trimmers are better for utilizing multi-turns.

You should look to recap the tone board... maybe those small transistors on that board too if they fall into the category where it is prudent.

Best of luck.

 

[Danny Miller]

the tone board has 1 fusible resistor and the 1313 transistors should be replaced also

 

[YapLiMin]

Thanks to all the AK'ers who responded and advised.

The 5050 has been running on full AC mains for the last 1 hour and no issues whatsoever and working as expected. I realized I did not fully understand DBT operation and so I learned something new today. Many thanks for the lesson!

Now I feel comfortable continuing on and re-capping, deoxiting and cleaning this unit and restoring it to its original condition. Thank you all again.