Hafler DH220 problem

[Reveler]

I have a Hafler DH-220 with PC-19c circuit boards. One channel (left) has very low output that is severely distorted. I am looking for help in troubleshooting the problem. I have looked at the following so far:
1. The outputs from the preamplifier to the amp are good.
2. The speaker fuses are good.
3. The voltages to terminals 3 and 10 of the PCB are as specified in the manual for both channels, ~65 v. Thus, the power supply looks to be good.
4. The 4 output MOSFETs on the bad channel are cold to the touch. However, when they are put into the right channel, they work fine.
5. There are no obviously burned components or bad solder joints, but I have not resoldered.
6. I have tapped on various components to try to isolate a bad component, to no effect.

I can take a series of voltage measurements but would greatly appreciate any suggestions as to the best place to start. Thanks very much.

[eyoung]

Hola Y'all...

The manual w/schematics and test point voltages on the board are available at the Hafler website . A good reference is the DH200 thread on the forum. I had a similar problem and it was a dead bias transistor...That being said start at the begining.

Good luck...Elwood

[Reveler]

Well, I have measured the voltages at all the transistors for both good and bad channels, shown in the attached chart, which includes the expected values from the instruction manual. There are clear abnormal values on the bad (left), but I am not sure how to interpret the numbers; bad transistors or failed other components, or both. Any help with interpretation would be greatly appreciated.

Also, relative to a prior post re a different DH220 problem, the voltage across R21 on the bad channel is zero.0 (0.87 on good channel); across R30 zero (0.87); across R33 zero (1.7).

I realize there are lots of numbers on the chart, but hope that someone can spot the pattern. Thanks.

[jaycee]

There was no chart attached.

However, from what you mention in the post, I would say that the problem could be Q10 or Q11. There is no current flowing in the VAS stage and this would explain the outputs being cold as there would be no bias.

I'd check Q7-Q11 for damage. It might also be worth checking the drivers Q12 and Q13. Also check the protection diodes D7-D10 - it's possible teh protection went off somehow and damaged one of these diodes.

Fortunately all the transistors seem to be available still - STmicro still make them. Q9 can be replaced with an MPS2222A

[Reveler]

Thank you for the suggestions. I will check those components. Hopefully the chart will attach this time.

[Reveler]

I am checking Q7-Q13 in circuit with a digital VOM. The protection diodes D7-D10 were checked and are good.

Also, what does a NEGATIVE collector voltage on Q1 indicate?

Thanks for all suggestions.

[megajocke]

It looks like the positive tail current source isn't doing it's job. Check R4 and the connections in that area of the circuit. An open circuit there is likely to be the problem.

[jaycee]

Agreed, but also check the DC offset pot P1. Make sure it's centered for now. I was thinking this may cause a fault if it went open but the diodes D7 and D8 would shunt around it.

If the collector of Q1 has this low a voltage then it is not functioning - the lower diff amp (Q5 Q6 and associated parts) will not function and this explains why there appears to be no current through Q9 Q10 Q11.

My guess is it will either be Q1 itself or the resistor R8. If the diodes or other parts base side were not working, youd have a fault at Q2 as well.

I'd be tempted to say replace all the 2N5401/2N5550 devices, especially the ones in the diff amps. These tend to go noisy with age especially if some fool whose owned the amp prior to you has overdriven the inputs. They are dirt cheap and easy to get.

[Reveler]

R4 is OPEN, as predicted by megajocke. Could this alone be the problem? If so, many thanks for the tip.

All other resistors were checked, including P1 and P2, and they are ok. All diodes were checked and are OK as well.

I removed Q1 and checked it with a VOM. It checks as expected; does this mean it is functioning normally? I will replace it anyway and may replace all the 2N5401/2N5550 devices as suggested by jaycee. How important is this?

I also plan to replace the electrolytics, C8, C12, and C14, since I am in the thick of it. Is there a preferred replacement brand? C13 is discolored showing what might be an early crack, so I will replace it too.

Maybe I am getting close to listening? Thanks again to all.

[megajocke]

Hi,

I'm glad to see the tip was useful Have you got it working? It might be a good idea to replace the resistor corresponding to R4 on the negative side too.

The reason I suspected R4 is that an open there would cause the current source to stop working and also cause negative voltages at the input.

That the current source is not working can be seen from the base-emitter voltages for the PNP diff amp transistors.

If R4 doesn't supply current to the offset null circuit it won't work correctly and apply a negative voltage to the input of the amp.

Also, I've seen high-value resistors used for biasing input stages go open in other amps. Especially the NAD-304 has this problem... I've repaired 3 of that model if I remember correctly - same problem in all of them.